Thursday, July 7th, 2016 high heat/humidity induced thunderstorm possibilities.

As we push through the first week of July real sultry summer weather has bestowed upon us with temperatures climbing well into the 90's with dewpoint temperatures well into the 60's to even lower 70's.  While the duration of the high heat/humidity will be on the brief side (only a few days), its presence will certainly be felt.  Also, as typical with this time of year when we run into bouts of high heat/humidity, we also run into the potential for afternoon showers and thunderstorms and as we move through the next couple of days we will deal with that potential.  For right now, however, we will take a look at Thursday.

While widespread showers and thunderstorms and widespread severe weather is not expected tomorrow, a few isolated thunderstorms are expected to develop.  Not every town is going to experience a thunderstorms (in fact only a handful of towns may be so lucky) but for the towns that do the storms could be on the strong side.

For Thursday, we are expecting to see numerous locations (especially away from the immediate coastline) experience their second consecutive day of 90+ degree temperatures.  This, combined with dewpoint temperatures into the 60' to perhaps lower 70's will yield a moderately unstable airmass and this will provide the fuel necessary for the development of thunderstorms.  In fact computer forecast models are indicating as much as 2000-3000 J/KG of surface-based cape across the region with as much as 200-2500 J/KG of mixed-layer cape.  Typically it is difficult to get mixed-layer cape values these high without the presence of 6.5 C/KM or higher 700-500mb lapse rates (they will only be on order of about 5.5 C/KM tomorrow), however, given the presence of upper 60's to lower 70's dewpoints this will help compensate a bit for weaker lapse rates:

   The question is will we have any source of lift to generate showers or thunderstorms..the answer to this question is it appears so:

Now with such high instability values and a lifting mechanism moving overhead, you would perhaps expect more in the way of widespread thunderstorms or even a bigger threat for severe weather, however, we are lacking a few key ingredients.  Below is a forecast sounding for Windsor Locks, CT tomorrow afternoon.  I have highlighted a few different features:

white:  weak wind shear:  The winds throughout the troposphere tomorrow will be very, very weak.  In fact, up through 20,000+ feet winds will only be around 5-15 knots.  That is very weak.  Winds this weak will make for storm organization quite difficult and also prevent sustained updrafts.  This will mean anything that gets going will quickly collapse and weaken.

blue:  drier air:  Notice the spread between the dewpoint temperature (solid green line) and the temperature (red solid line).  This indicates the presence of some very dry air.  Now, dry air in this level of the atmosphere is a good thing, so long it doesn't mix to the levels below it.  In tomorrow's case, conditions will allow for some of this drier air to mix down into the lower levels which could also end up dropping dew points a little bit, thus lowering instability values.

Purple:  warming:  There are two areas here, the solid purple line next to the temperature line and then circled below on the bottom left with "700-500mb lapse rates".  This warming in this level yields a temperature change between about 10,000ft and 18,000ft of only 5.5 C/KM.  This is very weak.  What this means is as rising air parcels encounter this level, this will rise rather slowly and with weak shear storms will struggle to really penetrate this level and when they do it will only last very briefly which also indicates short-lived (or pulse storms).

Putting all this together what can we expect?  Most towns stay dry tomorrow, however, given the presence of high heat/humidity and an approaching system, atmospheric ingredients will be in place for the development of a few thunderstorms, especially along the east sides of the Berkshire/Worcester Hills.  With weak shear in place and high instability, any storm could quickly become quite strong, however, with the weak shear will be short-lived.  The presence of the drier air aloft and dewpoint depression at the surface (surface temperatures in the 90's minus the dewpoints in the upper 60's to lower 70's yielding a depression ~20F) will result in the potential for some strong wind gusts.  With high moisture content this could yield a wet microburst (heavy rain/strong winds) which could result in some wind damage.  Not everyone sees a storm tomorrow but whoever does could see a nasty thunderstorm.  Thunderstorms will also be slow movers so they could drop a lot of rain in a short timeframe resulting in some flash flooding, even despite the dry ground.

July to Open with Threat for Showers and Thunderstorms.

As we move into the first day of July, a shortwave trough will be digging into the upper mid-West/Great Lakes region and sliding eastward towards New England pushing a cold front with it.  As the cold front/shortwave trough push towards New England, the system will be encountering a fairly juicy and unstable airmass.  This will set the stage for the development of some afternoon showers and thunderstorms, some of which could be strong to locally severe.  While the greatest likelihood for any stronger to severe thunderstorms will be confined to NY/PA, during the late afternoon, evening and overnight hours some of this activity will work into New England.

During the morning hours a warm front will be pushing northward and will be accompanied by clouds and a few showers.  As the warm front pushes through dewpoints will begin to climb well into the lower to middle 60's making it feel a tad uncomfortable.  With cloud cover expected to break as the morning progresses, ample surface heating is expected with temperatures climbing well into the lower to perhaps mid-80's away from the coastal plain.  While mid-level lapse rates (change of temperature with height through the troposphere) aren't expect to be very steep (on order of 5.5 C/KM to 6 C/KM) and will result in the presence of stronger instability, the combination of dewpoints in the 60's and temperatures into the 80's will still yield as much as 1500-2000 J/KG of surface-based cape, 1000-1500 J/KG of mixed-layer cape, and lifted index values between -2C and -4C.  All these values are suggestive of a fairly unstable airmass and this will provide the fuel for showers and thunderstorms to develop in the afternoon back across PA/NY closer to the cold front.  

In addition to a fairly unstable airmass, winds aloft will increase in response to the approaching shortwave trough.  Computer forecast models indicate as much as 30-40 knots of wind at 500mb (~18,000ft ASL) with an area of 50-60 knots across western PA by early afternoon:

With 700mb winds (~10,000ft ASL) expected to be around 30-35 knots as well these values will provide sufficient shear for sustained updrafts and storm organization.  Not only will winds aloft be on the stronger side tomorrow, but some computer forecast guidance is indicating the possibility of some directional wind shear in the lowest several thousand feet of the troposphere tomorrow.  If we look at a forecast sounding for near Poughkeepsie, NY we can see this (encircled in white):

Notice the wind barbs and how they change direction (from SE to SW in only a matter of a few thousand feet).  This is something to keep an eye on tomorrow across NY/PA and perhaps western New England as if any thunderstorms that develop are able to become strong enough, they could utilize this shear and we could see the possibility of some rotating (supercell) thunderstorms.

As for tomorrow, the greatest potential for showers and thunderstorms (including any threat for strong to severe thunderstorms) will mainly be confined to NY/PA and VT during the afternoon hours.  If any thunderstorms were to become strong to severe they could pose a risk for strong to damaging wind gusts, some large hail (especially if any thunderstorms take on supercell characteristics), and any storm rotation would have to be closely watched.  While the majority of the activity during the afternoon should be west of New England, people across western MA and western CT should keep an eye on the sky.  Given sufficient height falls associated with the shortwave through, we can expect scattered to widespread shower/thunderstorm activity to occur as the afternoon progresses with the storm mode being mainly clusters/line segments, however, a few discrete supercells will be possible.  

During the evening and overnight this activity is expected to work into New England.  While the loss of daytime heating will tend to stabilize the atmosphere, mid-level lapse rates are expected to increase on order of 6.5 C/KM to 7 C/KM thanks to cold air advection in the mid-levels of the atmosphere.  This will help to maintain some instability into the overnight so a strong to perhaps marginally severe thunderstorm will be possible through the overnight.

Not everyone will see showers or thunderstorms tomorrow but for the people that do, some of the storms could certainly pack a punch.  In addition to the threat for winds/hail, frequent cloud-to-ground lightning will be possible along with torrential downpours.  

Showers/T'storms Across New England Tuesday, June 7th, 2016

A trough is expected to dig and amplify through the Great Lakes region and push towards the New England states during the day on Tuesday.  Associated with the amplifying trough will be a strong cold front sliding east along with a rather potent piece of shortwave energy which will move through the middle levels of the atmosphere.  As this potent piece of shortwave energy approaches it will be accompanied by some rather anomalous cold air in the mid-levels of the atmosphere (500mb temperatures...~18,000ft ASL about -14C to -17C!!!).  As the system approaches we may see conditions in place which will favor the development of showers and thunderstorms and some of these storms could be strong to severe:

Tomorrow's setup is quite intriguing across New England as the anomalous mid-level cold temperatures will provide some very steep mid-level lapse rates across the region (meaning a quick decrease of temperature with height through the troposphere) on the order of 6.5 C/KM to 7 C/KM.  While there will be some cloud cover and perhaps some AM showers or even a thunderstorm to deal with, there is a possibility that we break through the clouds as the morning progresses, allowing for sunshine and ample heating to occur.  If enough sunshine breaks out some areas could see high temperatures push into the lower 80's or so.  

Outside of some uncertainty regarding cloud cover and how much sun we see, lies questions with how much low-level moisture we will have to work with.  Computer forecast guidance is all over the place with regards to the surface dewpoints with some guidance mixing dewpoints into the upper 40's to lower 50's while other pieces of guidance keeps dewpoints well into the 50's to perhaps even lower 60's.  The extent of the thunderstorms and threat for severe thunderstorms will be highly dependent on what happens with the dewpoints.  Basically the higher the dewpoints, the greater likelihood for severe thunderstorms. 

Given the potential for temperatures ranging from the mid-70's to low 80's, and dewpoints either in the 50's or 60's, when coupled with the steep mid-level lapse rates this would yield at least a modestly unstable airmass (if we see more sunshine and temps closer to 80F or into the lower 80's and are able to keep dewpoints in the 60's we would be looking at a much more unstable airmass).  This would provide enough fuel for shower and thunderstorms to develop ahead of the cold front.  

Winds aloft are also expected to strengthen throughout the afternoon in response to the amplifying trough and approaching shortwave.  500mb winds are expected to strengthen to as much as 50-60 knots region wide with 700mb winds expected to be around 30-35 knots.  These values will yield 0-6km shear values on order of 35-45 knots which is more than enough for not only organized thunderstorms but for some of the storms to become strong to severe:

 All in all tomorrow will feature the development of showers and thunderstorms in the afternoon across New England.  Depending on the degree of instability we are able to realize (all dependent on degree of sunshine and how high dewpoints are) some of these thunderstorms could quickly become strong to severe and pose a threat for large hail and strong to damaging wind gusts.  Given the degree of wind shear aloft, with enough instability we could see some supercell thunderstorms develop which would enhance the potential for larger hail and any rotation would have to be closely watched as well.  

Showers/Thunderstorms Possible Ahead of Cold Front Friday, April 22nd, 2016

Our next weather maker looks to occur on Friday as a shortwave trough and associated piece of shortwave energy drives a cold front east through the region.  As the cold front approaches from the west, there are some indications the front could be accompanied by some showers and perhaps even some thunderstorms.  

When dealing with the potential for thunderstorms, the biggest question we will face is how much instability develops ahead of the approaching cold front.  With some cloud cover and increasing clouds, temperatures are only expected to reach the lower to perhaps mid-60's (cooler along the coast and higher elevations) and dewpoints are expected to climb into the lower to mid-50's.  This combination of temperatures/dewpoints isn't favorable for the development of high amounts of instability, however, with the approaching shortwave and shortwave trough, an area of steeper mid-level lapse rates is expected to move overhead thanks in part to cold air advection in the mid-levels of the atmosphere.  This will work to yield some modest instability values ahead of the cold front.  The GFS computer forecast model is projecting a plume of 700-500mb lapse rates exceeding 6.5 C/KM to 7 C/KM moving overhead early to mid Friday afternoon:

The GFS computer model at this time is forecasting surface-based cape values on the order of 500 J/KG with most-unstable cape values between 500-1000 J/KG.  These values are indicative of some very modest instability and while they aren't overly impressive, when combined with some wind shear aloft, these values may be sufficient enough to not only produce thunderstorms but perhaps a few stronger to marginally severe thunderstorms:

With the approaching shortwave trough, we will also see an increase in the winds aloft.  While the winds aloft aren't expected to be overly impressive, they should be sufficient enough to further enhance the degree of lift across the region, thus increasing the potential for showers and thunderstorms to develop ahead of the cold front.  While the upper-level jet around 200-300mb isn't forecasted to be overly strong, there will be some enhanced support from the mid-level jet at 500mb.  In fact, parts of southern New England, particularly south of the MA Pike will briefly enter the left-front quadrant of a 50-60 knots mid-level jet streak passing just to the south of CT.  This is a favorable quadrant for enhanced lift.  Bulk shear values from the surface to 700mb are forecasted to be around 25-35 knots with bulk shear values from the surface to 500mb forecasted to be 30-40 knots.  These values are sufficient enough to yield the possibility for organized thunderstorms if any develop:

What can we potentially expect on Friday?  For Friday we can expect winds to increase from the southwest as the morning progresses and the front approaches.  We'll see a mixture of sun and clouds as well with clouds increasing heading into the afternoon.  Expect showers to develop as well during the early afternoon hours.  If enough instability materializes, we could see a line of heavier downpours and thunderstorms develop ahead of the cold front.  Given the potential for fairly steep mid-level lapse rates and adequate shear aloft, some of the thunderstorms could become strong to marginally severe and pose a threat for hail and strong winds.  This potential though is highly dependent on the degree of instability that materializes.  Showers/thunderstorms should clear by early evening and temperatures at night will drop into the 30's with afternoon temperatures on Saturday and Sunday right at seasonal levels.  

Thursday, April 7th, 2016 Rain Storm

April has gotten off to a rather active start weather wise and this will continue with yet another storm system impacting the region on Thursday.  However, this time we will see the area of low pressure track well to our west meaning warmer and more moist air advecting into the region and rain as opposed to snow.

Looking at this morning's run of the GFS forecast model, we see that the trough will be amplifying (deepening) well off to our west.  This will allow for heights and subsequent ridging to build into southern New England and also allow for the developing area of low pressure to track to our west:

As the system approaches southern New England it will begin to strengthen and we will see an increase in the winds aloft and also an increase in warm air advection as well as moisture advection.  The strengthening of the winds aloft and approaching trough/cold front will also work to rapidly increase the degree of lift (upward vertical motion) across the region.  The combination of these factors will lead to widespread rainfall, some of the rain which will be extremely heavy at times:

Computer forecast models strengthen the upper-level jet (250mb winds) and mid-level jets (500mb and 700mb (not shown)) and low-level jets right over the region.  The increase in upper/mid level winds will work to strongly increase the degree of lifting aloft and the increase in the low-level jet from the south will transport a great deal of moisture into the region.  This is evident by precipitable water values (PWATS) exceeding 1'':

The result will be widespread rainfall on Thursday lasting well through the evening and into the overnight hours.  Expect rainfall to be quite heavy at times.  Rainfall amounts of 1-2'' should be fairly common with even a few of the favored upslope locations perhaps getting as much as 3'' of rainfall.  Localized flooding will be a possibility as well.

Once the trough and cold front pass we will once again see another strong shot of cold-air advection across the region and some more cold temperatures for the weekend.

More Accumulating Snow in the Forecast

We are going from one accumulating snow event right into another as yet another strong piece of shortwave energy and associative low pressure is projected to dig southeast across the Great Lakes and track either right over CT or just south of Connecticut.  The exact track will be critical as to how much snow falls and where the cutoff is between accumulating snowfall and a mixture of precipitation.

As the shortwave energy tracks over the Great Lakes it will acquire moisture and the low pressure will begin to strengthen rapidly due to a very tight baroclinic zone (very tight temperature gradient in the lower levels of the atmosphere).  This tight baroclinic zone will contribute to a swath of very intense precipitation, whether it be snow or rain:

Both the NAM and GFS forecast models (as well as others) suggest we will see some pretty intense 700mb vertical velocities with the low pressure as it tracks across the area.  As these intense vertical velocities move overhead this will yield a several hour period of intense precipitation (rain or snow) associated with one or several bands of precipitation:

With the possibility for very strong lift associated with bands of precipitation this also means there will be subsequent areas of sinking or or subsidence.  This will actually prevent precipitation from falling to the ground across some areas.  We can actually see this by looking at bufkit.  Looking at this morning's run of the GFS at Windsor Locks, CT notice those blue contours?  These indicate areas of sinking motion.  What we can take from this is if we do indeed see some intense bands there will be areas of sinking motion and that would cut away at potential snowfall totals for several areas:

Keeping with bufkit we will also illustrate just how strong the lifting may be and how this is indicative of the potential for a several period of extremely heavy precipitation.  We will look at Windsor Locks, CT and Worcester, MA to show this.  The following parameters are overlayed; snowgrowth (the purple and yellow contours), omega (with red contours indicating upward motion - the more contours and more negative the value the stronger the lift and blue contours indicating sinking motion - the more contours and more positive the value the stronger the sinking motion), and temperature (the faint yellow/brown contour).  For snowgrowth you want this zone to be between 12,000ft and 18,000ft and want the -15C contour to cross this zone.  When these conditions are met you stand a great chance of generating a great deal of dendrites and near perfect snow growth.  In the case of this system that criteria will be met.  Now to maximize and realize this snow growth you want very intense lift to run through this zone.  Notice how the NAM is showing some very intense upward vertical motion in this zone while the GFS is a little less aggressive  (at least at Windsor Locks):

With this all said what can we expect?  For one, the AM commute tomorrow is going to be quite hazardous as we are looking at the timing of precipitation from around the pre-dawn hours lasting until late morning or even early afternoon (later start and end times the further east you go).  Expect a very slow commute with numerous delays.  Areas across southern CT through southern RI and into SE MA will begin as mostly rain, however, may see a transition over to snow towards the end of the event as temperature begin to quickly crash.  Across inland locations as much as 3-5'' of snow will be possible with higher amounts possible across the higher elevations.  Initially, snow will have trouble sticking to the pavement, however, with heavier intensities of snowfall allowing to further cool the surface, snow may begin to stick to paved surfaces.  At the height, snowfall rates could approach 1.5'' to even 2'' per hour.  Below is a map of what I expect:

Vigorous Shortwave Energy to Bring Snow Squalls to Region Overnight

If you're a weather enthusiast you are going to enjoy the timeframe from say about 3:00 AM to 9:00 AM Sunday morning.  If you aren't a weather enthusiast well hopefully after reading this post you'll have a whole new appreciation for weather and just how crazy it can be at times.

For a good while our long-range computer forecast models have been extremely consistent at hinting at a major, major cooldown just after the first of the month.  In fact, some of the cold project has been near or at record cold type stuff.  Too put this into perspective, the high temperatures for Monday and Tuesday (at least) will be as much as 15-20F below average!!!  That is pretty remarkable.

Let us take a look at the big picture and then break everything down on what we can expect overnight tonight into Sunday morning.  Looking at this morning's run of the GFS forecast model we can see a vigorous piece of shortwave energy associated with an amplifying trough digging out of southern Canada into the Great Lakes.  The image on the left hand side is the forecast position of the shortwave about 8:00 PM EDT this evening and the image on the right is the forecast position of the shortwave about 5:00 AM EDT Sunday morning.  In addition to focusing on the track of the shortwave, note the back contours and the numbers associated with them.  These numbers are the height of the atmosphere in meters where the 500mb level is located.  These numbers are VERY low for this time of year and indicate that the 500mb level is very low in the atmosphere (we will actually discuss this a bit more further down):

What does a shortwave (and in this case a vigorous one) have to do with our weather and what we can expect?  Well several atmospheric variables (ingredients) will be in place for the generation of numerous snow squalls across the region from say about 3:00 AM EDT to 9:00 AM EDT.  Some of these snow squalls are going to be extremely heavy, be associated with high snowfall rates, and be accompanied by strong winds and perhaps even thunder and lightning!  We will explore these variables below.

The first variable we will look at are the forecasted mid-level lapse rates (change of temperature with height).  The steeper the lapse rates (indicating a stronger decrease of temperature with height) the more unstable the airmass (great for enhancing upward vertical motion and also increasing thunder potential).  Computer forecast models are forecasting mid-level lapse rates to be anywhere from 7.5 C/KM  to as high as 8.5 C/KM (meaning a 7.5C to 8.5C decrease in temperature per kilometer of the atmosphere).  Those values are very impressive, especially for our region:

The next variable we will look at is the degree of lift.  Now, it's one thing to have lift and strong lift but if there is no moisture to lift nothing will happen.  While we will not look at it, there will be plenty of moisture available with this system.  Computer forecast models are forecasting a period of VERY intense upward vertical motion moving across portions of southern New England during the overnight period.  This intense lift will be responsible for the development of snow squalls and also be responsible for any of the heavier snow squalls with very heavy snow fall rates.  We will also explore this more further down:

We will now look at a few bufkit forecast soundings for a few select locations across the region.  We will do this to further explore the degree of lift and also look at something very cool regarding the tropopause and the stratosphere.

I've selected three locations (Windsor Locks, CT, Worcester, MA, and Falmouth, MA to illustrate further the intense lifting being advertised by computer forecast models.  The images are from bufkit and overlayed is the area of best snow growth zone (the purple and yellow contours) and omega values (with the red contours indicating upward vertical motion and blue contours indicating sinking or downward vertical motion).  Since the red contours indicate upward vertical motion, the more red contours you have and the more negative the value the stronger the upward vertical motion.  Given how we are looking at values as high as 40-45 -ub/s that is indicative of some exceptional vertical motion!  Given how these values also intersect with the area of best snowgrowth this is a very good indication that we will not only see great snowgrowth but very large and fluffy flakes...the kind that accumulates very quickly:

One very cool aspect of this system will be how low the stratosphere will be or how close it will be to us here at ground level.  Typically around these parts we see the stratosphere up around 30,000-35,000ft or so.  Well given the extreme low heights associated with the shortwave, the stratosphere is actually going to descend to right around or perhaps just below the 500mb level and in this case that may be as low as 15,000ft to 18,000ft AGL!!!  Given how this will in fact occur well below the upper jet stream this is known as a tropopause fold and is something that doesn't happen very often.  Sometimes when these occur you can get a period of exceptionally strong wind gusts (>75-80mph).  Below is an example of how low the tropopause will be (note: the stratosphere begins just above the tropopause):

All in all what can we expect?  As the shortwave approaches and lift increases we can expect numerous snow showers and snow squalls to develop across the region between 3:00 AM EDT and 9:00 AM EDT.  Given the degree of lift and instability that will be present, some of these snow showers and snow squalls will produce periods of very heavy snows with snowfall rates perhaps exceeding 2-3'' per hour, especially if there is any thunder or lightning involved!  Now the one thing to remember is we are not talking about a widespread precipitation shield but snow squalls (think of it as those heavy passing downpours we would see in the summer).  This mean not everyone may see snow and not everyone will see accumulations.  For those who do see accumulations, as much as 1-3'' will be possible, however, there is even opportunity for a few locations to perhaps see as much as 5''...6''...even 7'' of snow!!!  If that occurs it will be extremely isolated but someone may very well receive this.  Don't worry though, despite the cold tomorrow this will melt very quickly with a much higher sun angle and length of day now in place.

Once the shortwave passes we are looking at very strong winds with damaging wind gusts across the region.  We are looking as sustained winds of 25-35 mph with wind gusts as high as 55-65 mph possible.  Winds of this magnitude will be enough to cause isolated to scattered tree damage and power outages.  Sunday is definitely going to feel more like late January/early February than early April!

After tonight's event our attention turns to overnight tomorrow into Monday where in fact more snow will be possible but we'll touch upon this later!  At this time it appears a track will favor more in the way of rain but we will watch.