Tracking Jose...Does Southern New England See Any Impacts?

With regards to (at the time of this writing) Tropical Storm Jose, I personally, may end up eating my words.  This week during my broadcast at school, I briefly touched upon Jose and I stated Jose would pose no threat to southern New England.  Well over the past several days really computer forecast models have been further indicating that not only will the beaches and coast see impacts from high surf but a good chunk of the region may see strong winds and heavy rains from Jose.  While this probability is on the table there is still a great deal of spread within the computer forecast models with regards to the exact track and there is still that possibility that Jose moves on out to sea with no impacts with the exception of high surf and swells.  In retrospect, do I, or should I regret my choice of words and be so quick to dismiss the possibility of an impact from Jose?  I guess the answer to this is yes and no.  These days in weather there is so much hype and this hype comes from numerous sources.  You have hype from some TV stations and this hype is generated to increase their ratings (higher ratings means more money), you have hype from hobbyists and people who study weather.  This is done to draw attention to themselves and get more "followers" on their social media platforms.  Then you have people who just base forecasts off each and every single model run.  This right here is not forecasting.  Forecasting is more than just looking at each model and each individual model run.  Forecasting is understanding persistence, understanding atmospheric patterns, understanding biases within each forecast model, understanding climatology and understanding what types of patterns can correlate to certain outcomes.  It is very difficult to get tropical systems to directly impact our region and there is a good reason for that.  It takes a very special weather pattern and a very specific interaction of numerous ingredients for these scenarios to happen.  Anyways, I'll end the long rant and we will look into all the specifics and what we need to watch for over the coming days with regards to Jose and where Jose may track.

The 5:00 PM advisory from September 15th by the National Hurricane Center (NHC) has Hurricane Jose just past the threshold between a tropical storm and a category 1 hurricane with maximum sustained winds of 75 mph (A tropical storm is classified as a hurricane when maximum sustained winds reach 73 mph) and well north and east of the Bahamas.  The cone of uncertainty indicates well...a great deal of uncertainty with the potential track.  The potential track ranges from a very close call with the east coast of the United States to a track well east of the United States.  Notice as the forecast position of Jose changes with each day the cone of uncertainty grows larger.  This is not all that uncommon, however, it is uncommon to see such a large spread.  This is due to the fact that the computer forecast models are really struggling with the track of Jose.  The reason for this is due to how the upper air pattern evolves over the next few days and this is something we will explore within this post.  Also notice that the NHC does expect Jose to re-strengthen back into a hurricane as denoted by the "H" within the track but they also forecast it to weaken back to a tropical storm as denoted by the "S":

 Exploring why the cone of uncertainty is so wide we will look at the latest ensemble means from this morning's runs of the GFS and European computer forecast models with the GFS on the left and the European on the right.  Each of those individual tracks are different solutions projected by each member from within that computer forecast model.  These maps are often referred to as "spaghetti" plots because they resemble that of spaghetti:

If these look messy well it's because they are.  What we can gather from this, however, is that there is rather strong consensus that Jose will come relatively close to the east coast, the question just is how close and what happens to Jose when nearing the east coast.  Some members bring Jose close enough to provide some impacts while other members just turn Jose right out to see, likely with minimal impact.

Before we explore further, you're probably wondering if we do receive direct impacts from Jose will it be a hurricane?  The answer to this question is it is very unlikely and there are some reasons for this.  For tropical storms to become hurricanes and become strong hurricanes they need 3 key ingredients; they need to traverse over very-warm ocean waters which have sea-surface temperatures of at least 80F or higher, they need to be in an area and move into an area of relatively weak wind shear aloft (strong wind shear tends to disrupt the circulations of tropical systems), and they need to be in an area of high moisture content (if dry air is drawn into the system the system will weaken).  We will first look at sea-surface temperatures (in degrees C) as of yesterday across the western Atlantic.  I have circled two areas.  The area circled in red will be waters in which Jose will move through.  Correlating the colors to the color bar at the bottom we are looking at water temperatures which range from about 27C to 29C or 80F to 84F.  With what I mentioned above, these water temperatures are certainly supportive for Jose to not only to keep its current strength but are supportive for some further strengthening.  The second circled area is water temperatures off the the Carolina coast extending northward along and just off the New England coast.  Water temperatures here are in the 20C to 22C range or 68F to 72F.  Obviously these temperatures are far below the 80F threshold.  This is one factor which will inhibit Jose from being a hurricane if it were to make landfall or come very close to our area:

    
We will now explore the wind shear over the western Atlantic.  On the map below, the green contours indicate areas of favorable (or weaker) wind shear (favorable meaning increased likelihood that a tropical system will strengthen) while red contours indicate areas of unfavorable (or stronger) wind shear (unfavorable meaning an increased likelihood for weakening).  I have circled where Jose is positioned currently.  Notice how Jose is in an area of favorable wind shear.  Also notice, however, Jose will be moving into a widespread area of much stronger wind shear:

Thus far we see that not only will Jose be lurking into much colder waters but Jose will also be lurking into an area composed of much stronger wind shear.  Two factors which will prevent Jose from being hurricane status if it were to come close to this region.

Finally, let's explore moisture content over the western Atlantic and to do this we will look at a current snapshot of water vapor imagery.  I have circled 3 areas where drier air exists.  The biggest area of interest is drier air working over much of the continental United States and some drier air east of Florida.  There is also some drier air to the east of Jose.  There is a possibility that some of this dry air could work into Jose and if that happen further weakening would likely occur:

Now let's discuss the upper air pattern and how the upper air pattern may evolve over the United States and Atlantic Ocean over the next several days as this right here is what will ultimately determine what path and track Jose will take.  The first thing we will look at is the projected forecast of what is known as the Pacific-North American teleconnection pattern (PNA).  The evolution of this teleconnection pattern strongly correlates to the development and evolution of the upper air pattern over North America.  This index has 3 phases; positive, negative, and neutral.  When the PNA is positive, this correlates to a trough (dip in the jet stream) across the eastern Pacific Ocean extending into western North America.  The result from this is a ridge (northward build in the jet stream) across the eastern United States extending into the western Atlantic.  The negative phase is just the opposite with ridging across the eastern Pacific extending into western North America with the result being a trough over the eastern United States extending into the western Atlantic Ocean.  When the signal is very weak the PNA is deemed to be neutral.  Currently (as shown in the image below) the state of the PNA is just all so slightly negative, however, it can be considered neutral.  Over the coming days, however, as ridging builds into the Gulf of Alaska this will result in a trough amplifying (or strengthening) across the western United States.  This will result in ridging building into the east coast and the PNA will become further negative:

This is typically not something you look for when talking about the idea of a tropical system working up the coast.  What you preferably want to see if this trough out in the west amplifying as it moves through the central part of the country and really amplify in the vicinity of the Ohio Valley region.  You also want the trough to begin taking on what is called a negative tilt (we won't go into details on what that means now since this post is already long enough) as the system is beginning to take a turn up the east coast.  When these conditions are met the trough does what is called "capturing" the system and enhances the likelihood for the storm to either hug the coast or make landfall along the coast.  However, there are other ways to get a track close to the coast or get a landfall along the coast.  While this section scenario is a bit more difficult it can happen.  This will be explored with below graphic.  The below graphic is looking at the 500mb height anomalies from this morning's run of the GFS for Monday morning.  The blue shadings represent where a trough is positioned and the red shadings indicate where a ridge is positioned.  I have placed 5 numbers across features in which their evolution will play crucial to what happens with Jose's path:

1) This is the ridging which is building into the Gulf of Alaska and this is a feature of the negative PNA state.  This promotes a trough into the western United States and we see this with #2.

2) The result of that ridging building into the Gulf of Alaska helps to promote and shape a trough digging into the western-tier of the United States.  The result from this is to build ridging into the eastern United States and we see this with #3.

3) The result of the trough digging into the west is for ridging to build across the eastern United States.

4) A trough is positioned across Greenland and this is indicative of the negative phase of what is called the North Atlantic Oscillation (NAO).  Sandwiched between #3 and #4 is that strong ridging across the eastern United States.  This also creates what is called a "block".  This block is something that could prevent Jose from turning out to sea and the only possible track would be right into the east coast with a potential landfall anywhere from the Carolina's to the New England coast.

5) This is a cut-off low positioned right over the central Atlantic Ocean and the configuration with what was explained in #'s 3 and 4 further enhance this blocking.

If you look closely between the #4 and #5 notice the area of "white".  This could be a focal point as well.  This would represent a "weakness" or a gap inbetween that trough across Greenland and the cut-off low.  This could act as an escape route for Jose.  If that trough in #4 is not as strong as advertised or the cut-off in #5 is weaker or displaced further east, this opens the gates for a sharp turn out to sea.  If, however, that trough in #4 is stronger or that cut-off in #5 is stronger or further west we now have a higher probability for a track towards the coast with a landfall somewhere.  How these features evolve over the next several days will be extremely telling as to what scenario we can expect.

If we were to see any impacts here in southern New England what can we expect?  Depending on what happens with the overall track and intensity of Jose the possibility exists for some strong winds, either with the system itself or due to a rather strong pressure gradient which may develop.  The latest forecast from the NHC yields anywhere from a 20% to 30% likelihood of tropical storm force winds (sustained winds >= 39 mph) across southern New England with 30% to 40% probabilities just to the south and east of Nantucket.  This could certainly yield the potential for not only some wind damage but some power outages as well...especially since the trees are still fully leaved.  In fact, if we do see a scenario where we are looking at sustained winds greater than 40 mph we could see an enhanced likelihood for wind damage and power outages:

In addition to the potential of strong winds the potential would also exist for some heavy rains and flash flooding.  This, aspect, however, will also depend on exact track and overall strength.  What is a given though will be the high surf and swells along the coast.  Entering these waters is not recommended.  Coastal flooding will be very possible too, especially during high tide cycles.

Finally, even if we do get the worst of Jose but Jose is a weak tropical storm this does not mean we can't get torrential rains, some flash flooding, or strong winds.  In fact, tropical storms alone can produce just as much rainfall as hurricanes.  Some of our biggest flooding events have occurred with tropical storms or remnants of tropical storms.  This isn't saying we will see this scenario its just a way to make it understood that tropical storm conditions can still pack a punch.

Over the next few days the details should begin to become much more clear as to what track Jose will take and what type of impacts we will see from Jose.  Stay tuned!

What's the Deal with Hurricane Irma?

Much of the news story over the past few weeks has centered on the state of TX as Hurricane Harvey brought catastrophic flooding to much of southeastern TX, substantial damage thanks to intense winds and flooding, and also numerous tornadoes.  Not very far past Harvey, attention has shifted out towards the deep tropical Atlantic where yet another intense hurricane lies.  Hurricane Irma has blossomed into a dangerous category 3 hurricane and continues to strengthen as it makes a push towards Cuba and the Islands.

 If you follow social media or even some news outlets you may have encountered postings or have heard about computer forecast models striking the east coast with Irma or even perhaps a hit here in southern New England.  In my opinion, posting these graphics so far out is very irresponsible.  While of course a hit along the east coast is possible or even a hit here in southern New England is possible, we're talking about something which is 7+ days out.  While we can forecast in this timeframe with rather decent confidence at times, when it comes to tropical system the forecast confidence decreases drastically and there is pretty much no confidence at all.  In my mind, the goal for a forecaster, whether that forecaster be a degreed meteorologist or a hobbyist is to inform the public in a responsible manner, not scare the public.  The later is something that is easily done nowadays as social media has blossomed and this is because so much information gets posted and when this is done irresponsibility the wrong idea gets out.

While we are still several-plus days away from any potential landfall along the east coast there are numerous factors which will come into play which will determine; 1) If Irma makes landfall along the east coast and 2) where Irma would make landfall. Within this post we will look at all of those factors.

Over the past few days there has been rather strong agreement between the computer forecast ensembles have been in rather strong agreement in bringing Hurricane Irma very close to the east coast of the United States.  It is likely this reason alone which has led many to be discussing the possibility of a hit along the east coast.  Below are ensembles from both the GFS and European computer forecast models showing the cluster of potential tracks.  The GFS ensemble mean is from this morning's run of the GFS while the European ensemble mean is from last night's run (as of this writing this morning's run was not yet out):

While both computer forecast models show a great deal of members showing a landfall somewhere in the southeastern United States, the European model has several members which actually take Irma and curve it out to sea.  While a landfall somewhere along the east coast is possible, a re-curve out to sea is just as possible.  The million dollar question is; what determines whether Irma makes landfall or whether Irma re-curves out to sea?  This is where we explore everything that will play a factor in what the ultimate outcome will be.

Believe it or not, one of the key pieces to all of this is thousands, and thousands of miles away well out in the western Pacific Ocean in the form of Typhoon Sanvu.  Extensive research has shown that re-curving typhoons out in the western Pacific play a substantial role in the evolution of the synoptic weather pattern across the eastern central/eastern Pacific ocean and across North America.  More times that not, tropical systems are quite large in nature.  Within these systems a great deal of condensation occurs and the process of condensation releases what is called latent heat.  When you have these large systems with a great deal of condensation, you're going to get a great amount of latent heat release.  In fact, so much latent heat can be released that it actually shapes the weather pattern across the areas mentioned above.  This can be illustrated by looking at this mornings run of the GFS computer forecast model.  Looking at the jet stream at 250mb (almost 40,000' off the ground)) notice the extensive ridging (massive northward build in the jet stream) which develops across Alaska, western Canada, and the western United States over the next 5-days:

The significance of this ridging developing is it typically correlates with a trough (dip in the jet stream) either across the central and/or eastern United States.  Looking back at the image, notice the troughing develop and amplify (strengthen) between the central and eastern United States.  The significance of this feature is, when dealing with land-falling tropical systems across the east coast, and especially southern New England, this feature is nearly a prominent feature in many land-falls.  However, there is also more to this.  In order to increase the likelihood of a land-fall with this sort of scenario, you typically want the trough to amplify and become what is called negatively tilted as the tropical system is beginning to make the turn up the coast.  When this all occurs, the negative-tilt of the trough 'captures' the system and tugs it right into the east coast.  A great example of such a scenario would be with what occurred with Hurricane Sandy.

The next feature to focus on is across the northern Atlantic Ocean.  Again, using this morning's run of the GFS computer forecast model we will focus on the large area of high pressure which becomes established over a large portion of the north Atlantic:

  The importance of this feature is it could act as a block and actually prevent the tropical system from re-curving out to sea.  In this scenario, the only possible track would likely be into the United States.  Like with the troughing scenario, however, there is also more to this.  If the extent of this high pressure cell shifts eastward, this now opens up an 'escape' route for the system to make a re-curve out to sea.  How this high pressure center evolves over the next several days will be extremely critical in what Irma does as far as track is concerned.

The latest forecast track of Irma shows Irma potentially making striking Puerto Rico, the Dominican Republic, and Haiti regions:

  Not only would this yield substantial destruction across portions of these areas but the topography, especially of Haiti would drastically alter the strength and organization of the system.  When tropical systems go over land, they weaken significantly and rapidly as they need the ingestion of warm and moist air in order to keep going.  Also, Haiti is home to some rather large mountains.  If a tropical system goes over a mountain, if the mountain(s) are large enough, they can vastly disrupt the circulation of the tropical system which causes it to weaken substantially.

Some other factors to consider with regards to Irma are the degree of wind shear out ahead of Irma as well as available atmospheric moisture.  Tropical systems tend to strengthen when they reside in areas with weak wind shear and high amounts of available moisture content.  Assessing current wind shear across the Atlantic Ocean and just off the east coast of the United States we have low wind shear present around Irma and across the majority of the projected path of Irma.  However, across the eastern United States and just off the east coast we have rather strong wind shear:

The forecasts are for this region of stronger shear to remain in place for at least the next several days.  If this holds true, if Irma does take a turn towards the east coast or comes up the east coast, Irma could weaken quite a bit because of this.

Current water vapor imagery does show some drier air out ahead of Irma.  This may not be all that significant, however, if Irma does ingest some of this drier air this could act as a weakening factor for Irma:

 At this juncture its still extremely too early to say for certain what the likelihood is for a landfall across the eastern United States or if we have to worry about a landfall here in southern New England.  At this time, however, my best guess would be either a landfall off the southeast coast (perhaps SC/NC/N FL) or either a sharp re-curve out to sea once Irma nears the east coast.  However, the uncertainties are still rather high and any possibility remains on the table at this point.  All the latest data will continue to be reviewed and over the coming days we'll have a much better idea as to what solutions are more likely and which aren't.

Friday, August 18th, 2017 Thunderstorm Potential

A rather vigorous piece of shortwave energy currently across the upper mid-West will continue pushing off to the north and east into Canada over the next 24 hours (although this energy and the system will weaken) and as it does so a warm front will approach southern New England by Friday morning with a trailing cold front pushing through late Friday evening.  As this system approaches we will see the potential for showers and thunderstorms increase.  As the warm front begins to approach early tomorrow morning this will be accompanied by showers and perhaps even a few thunderstorms.  This initial activity will clear out by late-morning.  While the theme will be for extensive cloud cover to persist, there are indications that we could see some breaks in the clouds.  With temperatures and dewpoints increasing with the passage of the warm front, any breaks of sun will be rather crucial towards the afternoon potential for showers and thunderstorms as more heating would mean a more unstable atmosphere and more fuel for storms to work with.  While thunderstorms aren't expected to be too widespread during the afternoon the thunderstorms that develop will have the potential to become strong to perhaps severe.

With the main piece of shortwave energy weakening as it moves northeastward into Canada mid-level wind fields aren't expected to be all that impressive, however, computer forecast models are showing 35-40 knots of wind at 500mb (~18,000' off the ground) and 25-30 knots of wind at 700mb (~10,000' off the ground).  These values aren't overly impressive, however, they are adequate enough for any thunderstorms that do develop to become organized:

One of the biggest questions will be how unstable does the atmosphere become?  With mid-level lapse rates rather poor (on order of 5.5 C/KM to 6 C/KM) this will limit how unstable the atmosphere will become, however, with dewpoints expected to climb through the 60's and perhaps into the lower 70's and surface temperatures warming through the 70's and maybe even lower 80's if enough breaks of sun can occur the atmosphere should become unstable enough to support the potential development of thunderstorms as well as the potential for these thunderstorms to become strong to severe.  There is a quite a bit of uncertainty, however, with exactly how unstable the atmosphere becomes.  A look at a sref plume for various computer forecast models and their output of mixed-layer cape values (a parameter used to access atmospheric stability) for Windsor Locks, CT shows this well.  Each of these "lines" is an output from these different computer forecast models with the dark black line indicating the mean of all the models.  Notice how you have some models forecasting much higher mixed-layer cape values while others offer not much at all.  This indicates that there is a great deal of uncertainty for tomorrow with regards to the actual degree of instability:

There are some indications though that we could see numerous breaks in the cloud cover as the afternoon progresses thanks to the still strong August sun and some drier air working into the mid-levels of the atmosphere.  (We will look at this on the image below the next paragraph).

If we are to see thunderstorms tomorrow afternoon what hazards could we potentially be dealing with?  In addition to torrential downpours and lighting, these thunderstorms could contain strong to damaging wind gusts, perhaps some hail, and we do have to watch for any rotating thunderstorms.  The 18z NAM bufkit for Windsor Locks, CT for late tomorrow afternoon features a rather interesting look.

1) Circled in white are forecasted cape values through different levels of the atmosphere.  This particular computer forecast model indicates that by 5:00 PM tomorrow afternoon there could be as much as 350-400 J/KG of 0-6km cape.  While not overly impressive this is certainly enough to help fuel any thunderstorm which develops and enhance the potential for these storms to become strong to perhaps severe.  With just over 300 J/KG of hail cape being forecasted as well, this indicates the potential for hail within the stronger thunderstorms.

2) Circled in red is what is called a hodograph.  This provides a visual of how the winds are behaving as you ascend in the troposphere.  This asses both wind speed and direction with height.  Just above the origin notice how the line begins sort of straight but eventually begins to curve off to the right?  This is indicating that not only are winds changing direction with height in a portion of the  troposphere but you have a portion of the atmosphere where winds are strengthening with height.  This indicates that if any thunderstorms do become strong enough they could develop some rotation.
3) The value output for cape just over 1100 J/KG shows that this computer forecast model indicates we could see greater than 1000 J/KG of mixed-layer cape develop which is certainly supportive for any thunderstorms to become strong to severe.  The value output of just over 200 helicity (a value which measures that change of wind speed with height and change with wind direction with height) indicates there is at least enough of a change of wind speed with height and change of wind direction with height that could warrant the potential for stronger thunderstorms to begin rotating.  The value output for shear of 28 m/s indicates that there is enough wind shear present (speed/change in speed with height) that storms could utilize this wind energy which could further enhance the potential for any thunderstorm to become strong to severe, especially if there is enough instability present.

4) The red and green lines circled on the thermodynamic diagram are showing temperature (red line) and dewpoint (green line).  When these two lines are very close together this indicates a nearly saturated atmosphere and when they are further apart this indicates a much drier atmosphere.  From about 925mb on up see how these lines becomes further apart?  This indicates the presence of drier air which indicates we could see sufficient breaks in the clouds.  This would yield more surface heating and greater instability and a higher threat for thunderstorms and strong to severe thunderstorms.

All in all, as of now its a bit difficult to fully assess the potential tomorrow given the uncertainties with how unstable the atmosphere can become.  With the main shortwave weakening, only modest height falls, poor lapse rates, and potential for a good deal of clouds, we may not see widespread thunderstorms tomorrow but for any storms which develop or move into the region they could certainly become strong to severe.  This potential will last well through tomorrow evening.

Strong to Severe T'storms Possible Thursday, July 13th, 2017

The active weather pattern continues as we move through the week and on Thursday we will once again be looking at the possibility for showers and thunderstorms and these may arrive in two rounds; one during the early morning hours and the second round during the afternoon/evening hours.  While a strong thunderstorm can't be ruled out Thursday morning the focus of attention will be on the afternoon/evening timeframe.

A surge of theta-e air will push into southern New England over the next couple of days and with this we will see an increase in precipitable water values (PWATS) upwards of 1.5'' to 2+'' at times and dewpoint values approaching the lower to mid-70's, especially come Thursday:

While mid-level lapse rates aren't expected to be overly steep (700mb-500mb lapse rates in order of perhaps 6-6.5 C/KM), surface temperatures expected to warm into the 80's combined with the rich low-level moisture will allow for moderate instability to develop Thursday afternoon with surfaced-based cape (SBcape) values perhaps exceeding 2500-3500 J/KG and mixed-layer cape (MLcape) values exceeding 1500-2000 J/KG:

With a frontal boundary to the north and approaching shortwave energy, we will see an increase in the mid-level wind fields as the day progresses on Thursday.  Computer forecast models shows a 50+ knot 500mb speed maximum pushing right through southern New England during the mid-to-late afternoon hours with also an increase in the low-level jet at 850mb.  The increase in shear (especially in the middle levels of the atmosphere will contribute to 30-40 knots of effective shear and 0-6km shear which is plenty for any thunderstorms which do develop to become organized:

This point-and-click forecast sounding for a location in southern CT shows the potential atmospheric conditions on Thursday.  Highlighted with the white circle shows plenty of shear throughout the troposphere and circled in blue shows even the potential for low-level winds to remain a bit backed.  This is evident in the forecast hodograph also circled in blue.  This means there would be the possibility for a thunderstorm to begin to rotate if it became strong enough.  Circled in red also shows strong shear values present through several key layers in the troposphere:

While there are kinks to be worked out with regards to timing, any cloud cover leftover from any morning activity, etc, as it stands right now we are looking at the potential for a rather active Thursday afternoon across southern New England with numerous showers and thunderstorms expected to develop.  Given the ingredients mentioned, several of these thunderstorms could become strong to severe and pose a threat for strong to damaging wind gusts, hail (perhaps even large hail if any storms rotate), and flash flooding given presence of rather high dewpoints.  The details will continue to be fine tuned over the next 24-36 hours.

Isolated to Widely Scattered T'storm Possible Wednesday, June 21st, 2017

Tomorrow is one of those setups which goes under the radar for many.  This is likely due to the fact that computer forecast models aren't spitting out cray high CAPE (Convective Available Potential Energy) values (CAPE is a measure of how unstable the atmosphere is...higher the CAPE, the more unstable the atmosphere)or maybe some other reasons but there will be several ingredients in place tomorrow, including a lifting mechanism which will be the focus for lifting this energy to allow for the development of showers and thunderstorms.  While moisture will be limited (dewpoint temperatures expected to mainly be in the mid-50's) several ingredients will yield the potential for some of these thunderstorms to become strong to even briefly severe with the main threats being strong to damaging wind gusts as well as even some hail.

A rather strong 70-80+ knot 500mb (~18,000') will pass just to the south of CT and Long Island, potentially placing a good chunk of southern New England within the left exit region of this jet streak (a favorable quadrant for enhanced upward vertical motion:

A rather impressive piece of shortwave energy is expected to track across northern New England with some embedded shortwave energy within the 500mb flow tracking across central and southern New England.  This shortwave energy will be the focal point for a lifting mechanism needed.  Also associated with the shortwave will be some rather unseasonably cold temperatures at 500mb with 500mb temperatures projected to be between -13C and -16C.  This will yield modestly steep mid-level lapse rates of around 6.5 C/KM (decrease of temperature change with height between 10,000' and 18,000') which will help to boost some modest instability despite the lack of higher dewpoints:

Using a forecast sounding from central CT from the latest run of the NAM computer forecast model we can illustrate a few key ingredients for tomorrow:

1) The red line on the right is the temperature and the green line to the left is the dewpoint temperature.  Form this we can see the temperature and dewpoint at various levels in the atmosphere (as you ascend from the bottom...which is the surface you are increasing in altitude through the troposphere).  Notice how far spread apart the temperature and dewpoint lines are in the lowest 5,000' of the troposphere or so.  This feature is called an inverted-v (it looks like an upside down v).  This feature is of importance because it suggests that if thunderstorms were to develop and become strong enough they will have a higher potential for producing strong to damaging wind gusts.  This is due to the fact that as precipitation begins to fall from higher up in the storm, the air in the lower levels is so dry that some of this precipitation will actually evaporate on the way down...the process of evaporation cools the surrounding air and this air descends downward much more quickly (remember warm/moist air rises and cool/dry air sinks) towards the surface.  We feel this process as strong winds as this air hits the ground and spreads out in all directions rather rapidly.

2) Rather strong wind shear between 700mb and 300mb (~10,000' to 30,000').  As mentioned above, the strong jet dynamics just to the south will help enhance lifting but these strong winds aloft will also help support the possibility for some organized lines of thunderstorms and also organized updrafts meaning thunderstorms will be able to sustain for a good amount of time.

3) This Dcape value (Downdraft cape) is a measure of the rain-cooled downdraft strength of downdrafts within thunderstorms.  The greater the values, the greater the potential strength of a thunderstorms downdraft.  We are potentially looking at values upwards of 1000 J/KG which indicates the possibility of rather strong downdrafts tomorrow in any of the strongest thunderstorms.

4) Notice from about 800mb to 700mb we see the temperature and dewpoint lines pretty much come together.  This indicates moistening of these levels aloft so if air parcels are able to lift above the dry level, there will be more moisture for any updrafts to work with.

5) These measures of instability aren't all that impressive, however, with strong shear aloft and key points made in the last four bullets they don't have to be for any thunderstorms to become strong to severe.

Below is the area where isolated to widely scattered thunderstorms are most probable.  Any of these thunderstorms will have the potential to become strong to severe with the main threats being damaging winds and hail.  Due to lack of greater moisture availability flash flooding is not expected to be a concern.  The time frame will be from 2:00 PM to 8:00 PM, however, the threat may persist until 10:00 PM for far eastern MA:

Monday, June 19th, 2017 Thunderstorm Potential Update

We continue to monitor the potential for thunderstorms during the later part of Monday, including the potential for a few of these thunderstorms to become strong to even severe.  While the biggest potential for thunderstorms including the risk for strong to severe thunderstorms looks to exist just west of southern New England, we can't rule out the possibility of a few of these storms working into western portions of MA and CT during the early evening hours.  Thunderstorms, however, are expected to quickly weaken as they push through western MA and western CT.

The environment tomorrow across southern New England and back west into NY down through PA and into the mid-Atlantic will actually be quite favorable for not only thunderstorms but for some of these thunderstorms to become strong to severe.  There will be a few limiting factors which will prevent a region wide severe weather outbreak (poor mid-level lapse rates and lack of initial capping being two of these factors, however, several ingredients will be in place to favor isolated to scattered strong to severe thunderstorms.  Once you get into southern New England, however, one major limiting factor will be lack of forcing as the cold front doesn't approach until very late evening and the best mid and upper support don't arrive until close to dark.  While the atmosphere will be prime for strong to severe thunderstorms, the lack of better forcing here will likely suppress this potential, thus keeping just west of our area.

There are two ways we can characterize the atmosphere tomorrow; unstable and highly sheared.  When you put those two together and factor is a source for upward vertical motion (cold front for example) not only can you think thunderstorm potential but the potential for some of the thunderstorms to become strong to severe...so long they can tap into all this energy.  Let's take a look at this plume (a forecast of several different computer forecast models) from the 11:00 AM run of the SREF (short-range ensemble forecast) model of mixed-layer cape for Windsor Locks, CT (left), Chicopee, MA (center), and Poughkeepsie, NY (right):

 The mean forecast from multiple different computer forecast model outputs has mixed-layer cape values ranging from 1000-1500 J/KG.  When taking into account the degree of shear (which we will look at next) these values would be more than enough to support some thunderstorms becoming strong to severe (assuming thunderstorms are able to grow tall enough to utilize this energy).  With surface temperatures expected to push near 80F (perhaps even into low 80's in any spots which see more in the way of sunshine) and dewpoint temperatures around 70F, these values should easily be reached.  the only thing holding them from becoming higher are lack of steeper lapse rates (greater decrease of temperature with height through the troposphere).

Looking at effective vertical shear values for the same selected locations, the mean forecast from several computer forecast models suggest effective bulk shear values around 30 knots.  These values support thunderstorm organization and would increase the potential for thunderstorm updrafts to persist longer (meaning longer-lasting thunderstorms):

Not only will winds aloft be quite strong tomorrow but it is what will be going on with these winds aloft which adds a secondary focal point of interest to any potential thunderstorms tomorrow.  While winds through much of the troposphere tomorrow will be mainly out of the SW (this is termed unidirectional wind shear) there will be a bit of directional wind shear (change of wind direction with height) within the lowest few kilometers of the troposphere as winds towards the surface may be more southerly as opposed to southwesterly.  This will create a bit of a "spin" within the lowest portion of the troposphere.  This is shown well by rather long (indicative of the strong wind shear) and somewhat curve (showing the change of direction of winds within lowest few kilometers of the troposphere) hodographs.  This will be illustrated by looking at bufkit soundings from the 18z run of the NAM computer forecast model for Windsor Locks, CT and Poughkeepsie, NY:

What are we looking at tomorrow?  Numerous thunderstorms are expected to develop back across NY and PA during the afternoon hours and begin progressing towards southern New England towards dark.  While the atmosphere (as described above) will be rather unstable across southern New England the thunderstorms may not push into the region until we get near sunset meaning much of this instability will be loss due to the loss of daytime heating.  However, there are some indications that discrete (individual) thunderstorms could form ahead of the thunderstorms which develop back across PA/NY and push into western MA and western CT between 3-6 PM.  This is not certain, however, and will have to be evaluated as tomorrow progresses.  The strongest of thunderstorms that develop tomorrow will be capable of producing strong to damaging winds in excess of 70 mph.  Freezing levels are expected to be fairly high tomorrow so hail is not a big threat but certainly some small hail can't be ruled out, and the possibility of an isolated tornado exists as well, especially back towards the Hudson Valley across NY.  Flash flooding will also be a concern tomorrow as well as the atmosphere will be quite juicy and thunderstorms could train over the same locations.  As mentioned in the opening, the highest threat for strong to severe thunderstorms will be just west of southern New England, however, some of this potential could spill over into extreme western portions of MA and CT:

Thunderstorms Possible Monday, June 19th, 2017; Some Could Be Strong to Severe

A warm front will continue to slowly push northeastward through southern New England during the day on Saturday and as this warm front pushes through a summer-like feel will return to the region (mainly in the sense of high humidity) as temperatures for Sunday and Monday away from the immediate coast are expected to push near the 80F mark (perhaps even a bit higher on Monday depending on cloud cover) with dewpoints climbing through the upper 60's to perhaps low 70's.  During the day on Monday a weather system will be approaching from the west as shortwave trough pushes east towards southern New England during the day as shortwave energy track northeastward into Canada from the upper mid-West.  This will swing a cold front towards the region.  Out ahead of the cold front the combination of near 80F surface temperatures, dewpoints around 70F, and strong winds aloft will all set the stage for the possibility of showers and thunderstorms to develop ahead of the front and the potential exists for severe of these to become strong to severe.  While scattered severe weather is possible I don't think we will see a widespread severe weather event here in southern New England.  A bit greater of a threat will exist to the south and west of southern New England.  While we do have some ingredients that will be in place to support some strong to severe thunderstorms on Monday we do have several inhibitors which will prevent a more widespread organized event from occurring.  We will analyze all below:

Low-level Moisture/Instability:

Ample low-level moisture will be in place for Monday as surface dewpoint temperatures are expected to be right around the 70F mark which will make it feel quite humid outside and precipitable water values (PWATS) surging upwards of 1.50'' to 2'' indicating a great deal of moisture will be in the air.  Despite rather poor mid-level lapse rates of 5.5 C/KM to 6 C/KM (temperature with height between 10,000'-18,000' only decreasing by 5.5C/KM to 6 C/KM) the degree of low-level moisture combined with surface temperatures around 80F should yield around 1500 J/KG of mixed-layer cape (measure of how unstable the atmosphere is).  The poor mid-level lapse rates will be one big inhibitor for a more widespread organized severe weather event.  The steeper the mid-level lapse rates, the greater the upwards acceleration of air parcels through the mid-troposphere.  Weak mid-level lapse rates can be overdone by very high dewpoints (dewpoints nearing the mid-70's or by exceptionally strong forcing):

These very high PWAT values may also be an inhibiting factor towards a more widespread and organized severe weather event.  While you do want the presence of rich low level-moisture (which comes in the form of high dewpoints) there is a such thing as too much.  When PWAT values begin exceeding the 1.5'' threshold and start nearing 2'' this can yield what is referred to as water loading.  For severe weather, one set of ingredients you want is rich low-level moisture but you want dry air or drier air between 10,000' to 18,000'.  When the moisture profile is rich through the mid-levels you get water loading and this ends up reducing a storm's updraft strength.

Wind Shear:

There will be no shortage of wind shear as rather unseasonably strong wind fields are expected to overspread the region on Monday ahead of the approaching cold front.  Computer forecast models indicate 40-50 knots of shear at 500mb (~18,000') out of the southwest overspreading southern New England by Monday afternoon, 700mb winds (~10,000') of 35-45 knots out of the southwest, and 850mb winds (~5,000') as strong as 35-45 knots out of the southwest as well.  These values are more than enough to help with the generation of organized thunderstorm clusters/lines and also enough to increase the potential for longer and more sustained updrafts and this is indicated by the combination of strong 700mb-500mb winds which could yield upwards of 40-50 knots of 0-6km shear.  Not only do these values indicate the potential for updraft organization/sustainability but they indicate the potential for some mid-level storm rotation as well.  While these values are quite strong we are lacking something here in southern New England and that's enhanced support for the core of the mid-level and upper-level jet stream.  On the 500mb map the area circled in red is where the core of the mid-level jet stream is (called a jet streak) and circled in blue is likely where the enhanced forcing from this jet streak is.  This area will run from portions of northern New England back southwest through parts of NY and down into PA and into the mid-Atlantic.  Enhanced forcing from jet streaks is something I find to be very critical to getting more organized severe weather outbreaks and this is just one more factor I foresee being an inhibiting factor here:

  
Also, notice how we have southwesterly shear from about 850mb up to 500mb?  (This actually rings too virtually from the surface to 250mb (~35,000').  This is going to yield shear which is parallel to the cold front and lead to storm motion which is parallel to the cold front.  This is typically not something which favors widespread severe weather outbreaks either.  This does indicate that storm mode will be linear (storms forming line segments) so strong to damaging wind gusts are the main threat but also indicates storm motion can be rather slow.  Given the high PWAT values, the combination of these high PWAT values and storm motion parallel to the flow/front may lead to not only training thunderstorms (multiple thunderstorms going over a single area) but this could lead to a threat for flash flooding and it's very possible the threat for flash flooding becomes more of a hazard than the threat for strong to damaging wind gusts here in southern New England.

This is sort of hard to show on a single image but one other negating factor will be rather weak height falls across the region, in fact, the heights may remain rather neutral.  Having strong height falls is yet another way to really enhance upward acceleration of parcels.

All in all for us here in southern New England, Monday afternoon into the evening should be rather active in terms of showers and thunderstorms with the main storm mode being multiple line segments.  Several of these thunderstorms could become strong to severe posing a threat for strong to damaging wind gusts.  While hail will be possible, it will be rather isolated due to poor lapse rates and moist mid-levels of the atmosphere.  This potential will rapidly decrease as you near the coast due to stabilizing influences from the cooler ocean waters.  Portions of northern New England back southwest across central/eastern NY and down into SE PA, MD, NJ, and DE greater potential will exist for clusters of strong to damaging winds.