What Can we Expect from Hermine Across southern New England?

The biggest weather story we have been dealing with the past few days is that of what is now Tropical Storm Hermine.  Hermine made landfall as a category one hurricane early Friday morning near St. Marks, FL and is expected to move northeastward along the eastern seaboard.  It is from here we have to watch closely as several computer forecast models have hinted that the system could move far enough northward to provide some impacts across portions of extreme southern New England.

As of 11:00 AM EDT Friday morning we see the 500mb energy associated with Hermine already well inland across southeastern GA.  From here the system is expected to track right up along the eastern seaboard, however, there are some questions with this.  There is still a possibility that the system (or at least the eye...or what's left of it) could move right back over the warm ocean waters off the coast of the Carolina's.  If this does occur there would be a strong possibility that Hermine could actually one again strengthen and keep it's tropical characteristics (remain a warm-core system).  If, however, the system stays centered over land, Hermine will continue to weaken and eventually become extratropical in nature (it will evolve from a warm-core to a cold-core system) and end up becoming baroclonic in nature (meaning it develops warm and cold fronts.  If this happens the system will occlude and that would promote further weakening all well south of southern New England.

As Hermine works up the coast something rather interesting will happen.  There is an area of high pressure that is sliding off to the east across northern New England.  As this area of high pressure pushes off the coast of ME it will act as a "block".  This "block" will do one of two things; 1) it will prevent Hermine from pushing out to sea and 2) It will actually allow for Hermine to stall off the east coast...the big question is where exactly does Hermine stall and this will ultimately determine how significant of an impact we see.

Over the past day or so computer forecast models have become in better agreement that Hermine will stall somewhere either off the Carolina coast or NJ coast.  This scenario would mean for insignificant impacts for extreme portions of southern New England.  We would be looking at periods of heavy downpours and gusty winds.  While not a major issue by any means we could see some minor coastal flooding across the CT shoreline and RI coastline.  This scenario, however, would spell major trouble for the Carolina and especially NJ coastline up to the NYC coast.  With the system potentially stalling and sitting out there for several days we would be looking at significant coastal flooding across these locations and this would be a very bad situation for anyone along the immediate coasts from the Carolina's up to the NYC coast.

What should we expect across CT/RI/SE MA?  In all likelihood, Hermine will occlude too far south to provide much in the way of significant impacts across much of southern New England outside of some gusty winds, periods of heavy downpours, and minor coastal flooding.  Taking a look at some GFS bufkit profiles for New Haven, CT, North Kingstown, RI, and Falmouth, MA during the day on Sunday the GFS is only advertising wind gust potential as much as 35-45 mph and that is more likely along the immediate coast then further inland where gusts could be as high as 30-35 mph.  Much of winds may be do more to the pressure gradient between Hermine and the area of high pressure to our northeast than with Hermine alone.  This is actually something to watch as well because if Hermine is stronger to our south and the high pressure is a bit stronger as well, this would increase the gradient and could allow for some stronger wind gusts...if this occurs we would have to watch for some isolated power outages.

All in all beginning late Saturday we should see an increase in both clouds and humidity levels with a few isolated showers possible.  Overnight Saturday into Sunday the winds should slowly begin to increase as well with the running risk for a heavy downpour.  These conditions will persist through all of Sunday and perhaps lasting into Monday or even Tuesday depending on how long Hermine sits off the coast and where Hermine stalls.  While major to significant impacts across extreme southern New England aren't looking very likely (remember this will be the case from coastal Carolina's up to NYC coast where significant coastal flooding is looking more likely), we will still receive some minor impacts with the periods of heavy downpours, gusty winds, and minor coastal flooding.  If any significant changes to the track, strength, and forecast occur they will be highlighted in any later blog posts.
  

Strong to Severe Thunderstorms Possible Tuesday, August 16th, 2016

After a one-day break from beyond oppressive heat and humidity a warm front will be approaching Tuesday morning bringing a return to the hot and sticky airmass.  However, its stay will be rather brief as a cold front is expected to push through in the afternoon.  The heat and humidity will not be the big weather story tomorrow, however, as we will be dealing with the risk for thunderstorms and several of these will have the potential to become strong to severe.  In fact, a few computer forecast models are indicating the threat for a few tornadoes as well.

Overnight and into tomorrow morning an area of surface low pressure will be tracking to the northeast well to the west of southern New England.  As the surface low pushes through western PA into western NY and into Quebec, Canada a warm front will surge northward through southern New England where several computer forecast models indicate the warm front could stall in the vicinity of the MA Pike.  South of the warm front the airmass will become very humid.  Surface dewpoints are expected to surge back into the mid 70's with precipitable water values (PWATS) increasing to over 2.3''...indicative of a very moist airmass:

As the warm front approaches we will have to deal with the potential for showers and perhaps even some thunderstorms and we will have to watch this potential rather closely.  If the morning warmup happens to be associated with any thunderstorm activity there will be favorable parameters in place not only for some of these thunderstorms to become strong but perhaps severe and carry the risk for a tornado.  Below is a bufkit profile for New Haven, CT for 9:00 AM tomorrow morning from the 18z 4km NAM computer forecast model.  We see strong moisture advection, good solid cape in the lowest 6km of the atmosphere and very good low-level directional shear and speed shear indicated by nearly 30 m/s of shear.  We also see pretty low lifted condensation levels (LCL's) which mean cloud bases will be rather close to the ground:

The potential for morning thunderstorms would likely be in the 6 AM to 10 AM timeframe.  This is not a given but is something that needs to be watched given the parameters that will be in place.  In addition to the potential for a tornado, storms could contain some gusty winds along with torrential downpours and frequent lightning.  After we wrap up with any morning activity our attention will quickly shift towards the afternoon and evening hours where the potential will exist for a much more potent and perhaps bit more widespread severe weather threat.

With the warm front getting hung up somewhere across central MA, this will enhance the likelihood that surface winds across much of CT, RI, and SE MA will remain "backed" and be more out of the south to even southeast.  This is significant because as you go up a few thousand feet in the atmosphere the winds will begin to transition to a more southwesterly direction with winds then transitioning to a more westerly direction even higher up.  When you have these change of wind directions with height this is what is called directional shear and creates "atmospheric spin" which can allow for a thunderstorms updraft to not only become tilted but begin rotating if the updraft is able to utilize this.  Winds aloft are also expected to increase throughout the afternoon hours as the area of surface low pressure begins to strengthen.  In fact, some computer forecast models indicate that winds about 5000ft ASL will increase to as much as 30-35 knots with winds even higher up increasing to 40-50 knots.  This increase of winds with height is called speed shear.  One way to measure direction and speed shear is through helicity values.  Looking at 0-1km and 0-3km helicity values for tomorrow they are forecasted to be very impressive.  Typically when looking at the possibility for a tornado you want to start seeing 0-1km helicity values approach 125 m2s2 and 0-3km values approach 150 m2s2.  These values, especially 0-3km helicity certainly approach or even exceed those thresholds:

While we know the wind shear will be there the next question is with regards to how unstable will the atmosphere become and how much surface heating we see.  With such a moist low-level airmass in place it will not take much sun for the atmosphere to rapidly destabilize, even with poor mid-level lapse rates (though this will be a bit of a negative for getting even more potent updrafts to occur).  While clouds could be an issue early on, especially if we see lots of morning activity, computer forecast models indicate we will break out into some sun and strong surface heating will commence with temperatures surging through the 80's.  Given the degree of wind shear aloft, if the atmosphere can become quite unstable the threat for several strong to severe thunderstorms will exist and posing the threat for a tornado, strong to damaging winds, and even some hail.

Below is a bufkit profile sounding for Windsor Locks, CT from the 18z 4km NAM computer forecast model for 6:00 PM tomorrow evening and what we see is a very favorable thermodynamic/dynamic environment for any thunderstorm to become strong to severe and pose a risk for a tornado, strong to damaging winds, and hail.  In fact, this sounding would indicate the potential for a strong tornado would be on the table:

As typical with thunderstorms and severe thunderstorm potential across southern New England, we really won't know what will happen or unfold until we are able to see the details iron out during the morning hours.  These are always very complex setups and in a region where severe weather (especially high end severe weather and tornadoes) aren't as common as further west there are typically always red flags.  These setups and how they unfold and produce all depend on the timing of all the main features...does the strongest instability coincide with the strongest wind shear and strongest forcing from the approaching cold front?  These are some of the questions we will have to iron out tomorrow.

Given how these setups don't produce all that often around here its very tough to say for certain as to how much in the way of severe thunderstorms we'll see or even if we'll see any tornadoes, however, right now that possibility is on the table and we will have to watch how the conditions unfold in the morning.

Review of the 8/10/2016 North Haven, CT tornado

On average, the state of Connecticut gets about 1-2 tornadoes per year and on Wednesday, August 10th, 2016 Connecticut recorded its first tornado since July, 27th, 2014 when an EF-0 tornado also coincidentally occurred in New Haven county.  While the setup yesterday afternoon was not a screaming tornado setup, the setup and ingredients in place are known to have produce tornadoes in the past.  These setups are very challenging to forecast and its difficult to really relay the potential to the public but they don't always produce and given how we usually see a few of these setups per summer, the last thing you want to do is hype up each event as potentially producing a tornado.  However. yesterday afternoon the ingredients in place all came together to produce a short-lived tornado in a small section of North Haven, CT which damaged trees and some structures (mainly from flying debris and downed trees).  We will take a look at the setup and ingredients in place and look at some of the clues that were on hand just prior to tornadogenesis.

Setup/Ingredients 

On Monday, computer forecast models indicated the potential for some strong to even marginally severe thunderstorms, including the risk for an isolated brief tornado across CT for Wednesday.  Computer forecast models were advertising sufficient instability values to go along with ample low-level moisture (dewpoints increasing into the 70's), with directional wind shear (winds changing direction as you increase with height through the troposphere), and a source of lift (nearby warm front).  However, as resulting computer forecast models runs sort of began backing off this potential until late Wednesday morning/early afternoon rolled around.

Below is a bufkit model sounding from the RAP (Rapid Refresh) computer forecast model for New Haven, CT for the hours of 1:00 PM EDT (just prior to tornadogenesis and the tornado touchdown) and 2:00 PM EDT (just after tornadogenesis and the tornado touchdown).  By analyzing the forecast sounding we see several parameters which were suggestive (but not overwhelming) of the potential for a brief tornado:

While none of the parameters here scream tornado, the values and parameters are borderline enough to indicate that the possibility of a tornado is certainly in the cards.  You had a very moist low-level airmass in place with surface dewpoints well into the lower 70's.  While there wasn't a tremendous amount of sunshine (although some peaks), the presence of this rich moisture was enough to allow for a few hundred joules of cape (measure of atmospheric stability) to develop.  With the warm front pretty much overhead of this area, this allowed for the advection of the strong moisture and also allowed for winds in the lowest 5,000ft of the atmosphere to become "backed"...indicating good directional shear.  You had winds at the surface in a southerly to perhaps even southeasterly then slowly turning to a more south-south westerly to southwesterly all within the short vertical distance, thus indicating a solid deal of atmospheric spin and you also had strong shear (>20-30 knots of wind).  All you needed was some sort of strong enough updraft to realize the weak instability that was in place and then utilize the wind shear aloft and boom...the recipe for a brief/short-lived tornado was in place.

Also, thanks to the presence of a very moist low-level airmass and the surface temperatures and dewpoints very similar this yielded very low lifted-condensation levels (LCL's).  Having low LCL's is very critical (especially in New England) for tornadogenesis to occur.  When LCL's are low this indicates the cloud bases are not very high up from  the ground and its likely possible that when a storm's updraft utilizes the instability/shear, the rotating updraft located not very high up has an easier likelihood of reaching the ground.  Anyways, LCL's were between about 500-750 meters!!!  That is about as low as you will ever seen them.  You can see this in the attached image below.

As mentioned the degree of directional shear and strong winds aloft (indicating good speed shear) lead to those curved/somewhat elongated hodographs.  Helicity values (which measure directional/speed shear) were more than sufficient for the possibility of an isolated tornado.  For 0-1km helicity values once you get above 100 m2s2 and when 0-3km values approaches and exceed 150 m2s2, that is a sign there is strong enough directional and speed shear in place to warrant the possibility of a tornado.  As you can see in the attached image, both 0-1km and 0-3km values certainly met these thresholds.  Effective storm relative helicity values of 100 m2s2 were also certainly supportive.

Mixed-layer cape values were well over 250 J/KG and in reality were likely closer to 500-750 J/KG which when combined with the degree of wind shear that was present was more than enough to support a strong enough updraft to realize the wind shear and beginning rotating.

The supercell composite parameter was a 4 across southern CT (pretty high for around here!) which indicated that if a discrete thunderstorm developed, the chances of it becoming supercell-like were pretty high.

Too conclude, while the setup wasn't a screaming tornado-producing setup, several ingredients were in place and suggestive that this potential existed...all we needed was a discrete thunderstorm with an updraft strong enough to utilize these ingredients and that's exactly what happened.

There is also more too this!  For a more detailed and scientific look at the radar presentation check-out the blog post done by Emmy-Award winning meteorologist, Ryan Hanrahan of NBC Connecticut.  He did a fantastic write-up on the radar presentation and how dual-pol radar picked up on this very well.  The link to his blog post is below:

http://www.ryanhanrahan.com/2016/08/10/north-haven-tornado/

Friday Soaking In Store?

We need the rain...there is no question about it.  Looking back at the past 90-days we can see, region wide there is quite the rain deficit.  In fact, if we extend this period to the past 6-months we can see the rain deficit is even greater:

With the month of July yielding summer in full-fledged beast mode with temperatures exceeding the 90F mark numerous times and very little (in fact hardly any) widespread rainfall the ground is bone dry yielding a substantial brush fire risk.  With this we really need this rain and it looks like we'll get our wish on Friday as a developing area of low pressure is expected to track northeastward along an advancing cold front providing ample moisture and lift to generate widespread precipitation.

Both the NAM and GFS computer forecast models are indicating the potential for a quite a bit of precipitation to fall, beginning late overnight Thursday and lasting through the first half of Friday.  In fact, both forecast models are indicating as much as 1-2'' of rainfall could fall across a large portion of the region with even potential for isolated higher amounts, especially in any embedded thunderstorms:

As the cold front approaches from the west, a warm front will be surging northward at the surface.  With the area of low pressure expected to track right over southern New England or just south so the warm front will likely not make much progress into southern New England and actually may stall just north of Long Island Sound across coastal CT.  South of the warm front the airmass is expected to become exceptionally moist with dewpoint temperatures rising well into the 70's and precipital water values (PWATS) rising well above 2''...and indication of an exceptionally moist airmass:

One interesting aspect with this system will be the strengthening of the lowe-level jet and winds within the lowest 5,000ft of the atmosphere.  Both the NAM and GFS indicate that winds will strengthen to as much as 30-40 knots and be coming from the southeast.  This flow will not only work to transport copious moisture in off the Atlantic but will actually also work to enhance the degree of directional shear in place.  This will be very important for anyone south of the warm front as if any thunderstorms develop and tap into this shear they could begin to acquire rotation and pose a threat for a brief/weak tornado.  The highest likelihood for this is across NW CJ into the NYC metro area and onto Long Island.  In fact, many of the tornadoes which have occurred in the NYC area and on Long Island have occurred in similar setups to tomorrow.  With the degree of instability isn't expected to be all that great, with the surge of low-level moisture (dewpoints near or above 70f), and rich presence of low-level moisture in the low-levels thanks to the southeasterly winds will create a very buoyant low-level airmass and an unstable layer here.  While 500-800 J/KG of mixed-layer cape is not usually sufficient for severe weather, given how this setup would yield low-topped thunderstorms (thunderstorms which don't grow very tall) and they feed off of the low-level airmass and utilize those conditions.  This combination of shear/cape would be sufficient to enhance storm rotation:

This point-and-click forecast sounding for a location on Long Island shows very good low-level directional shear along with strong winds (encircled area on bottom left) and looking at the hodograph you can see a curved looked indicating the good directional shear present (encircled on top right):

For those been asking for rain it appears Friday will finally deliver the goods and for a widespread across across southern New England where region wide as much as 1'' of rain could fall.  Some localized areas could also see up to 2'' of rainfall with perhaps even a few locally higher amounts!  This would certainly do wonders for the long-term rain deficit we're facing.

As for any severe weather potential, we have to watch where the warm front stalls as just south of the warm front atmospheric conditions could favor some of the thunderstorms becoming strong and posing a threat for strong to damaging wind gusts and even a brief/weak tornado.

Assessing Today/Tonight's Thunderstorm/Severe Weather Potential

The potential exists for numerous shower and thunderstorm activity to develop region wide later this afternoon with the activity lasting well into the overnight hours.  Some ingredients will be in place for a few of these thunderstorms to become strong to severe and have the potential to produce strong to damaging wind gusts and perhaps even some large hail, especially in any rotating thunderstorms.  The next big question is, do these ingredients (which we will look at below) all come together at the right time to produce a bigger severe weather event?  This is a question in which we will slowly begin to answer as the afternoon progresses.  The reason for this is due to the mesoscale (small-scale) factor of the setup.  Often times with convective events, the severity and extent of them all comes down to very small-scale features and it is very, very difficult to forecast/pinpoint these features more than several hours in advance.  

Instability

When dealing with the potential for convection, one of the questions we always face is how unstable will the atmosphere be.  For the past several days, computer forecast guidance has hinted that with the combination of heat/humidity the airmass could become quite unstable...the question was whether or not cloud debris from previous day's convection over the Great Lakes would allow for less in the way of surface heating.  Given the satellite trends this morning, this is not the case.  While we do have some cloud debris moving over the region, strong surface heating is occurring across many places and away from the coast temperatures should soar well into the lower to middle 90's.  With dewpoints in the mid to upper 60's this will yield quite an unstable airmass.  

While typically we see the airmass become more stable as the sun begins to set (with the sun setting and surface temperatures lowering this allows for a stabilizing atmosphere), there are certain ingredients which could keep us quite unstable well into the overnight hours.  These ingredients include; the potential steepening of mid-level (700-500mb lapse rates) thanks to cooling temperatures in the mid-levels of the atmosphere and/or the advection of an elevated mixed-layer plume, and a further moistening of the low-level airmass.  The latest NAM computer forecast models show mixed-layer cape values as high as 1500-2000+ J/KG indicating the airmass will remain quite unstable well into the evening and into the overnight hours:

Wind Shear

As the shortwave energy approaches winds throughout the troposphere are expected to increase and this would help any thunderstorms that develop organize and also enhance the potential for strong to damaging wind gusts.  By early evening we see the nose of a rather modest 35-45 knot 500mb jet punching into southern New England.  While the 500mb winds over southern New England alone aren't that strong, the core of the stronger winds just off to the west can actually help to intensify convection if the thunderstorms are just to the east of this axis.  

We also see an increase in 700mb which should approach/exceed 30-35 knots.  This will help yield vertical shear values of around 35 knots which is supportive for thunderstorms to become organized and also may help with the enhancement of storm updraft rotation.  

850mb winds are expected to approach 30 knots as well which will help with stronger low-level shear values and also enhance some potential for updraft rotation.  The combination of these strong winds aloft will yield a threat for strong to damaging wind gusts with the strongest thunderstorms:

While the shear still looks to remain adequate, latest trends have decreased the degree of wind shear over southern New England for later on.  If this trend does continue the threat for severe weather would be diminished and we would only be looking at a few isolated reports of wind damage and maybe some hail.  Something to keep an eye on as the afternoon continues to progress.

Dry Mid-Levels

While some dry air is good to have in the middle levels of the troposphere, too much dry air is no good as this eats away at storms updraft strength...at the same time it can enhance the potential for stronger winds to mix  down.  Forecast soundings do indicate a quite a bit of dry air in the mid-levels, however, some indications are they could moisten up somewhat.  This is something to keep a close eye on.  If the degree of dry air is too much, this would limit thunderstorm coverage and could decrease the potential for severe weather.  This is a rather difficult signal, however, as one model shows a great deal of dry air and the other model not nearly as much.  Too illustrate we will look at a bufkit sounding for Windsor Locks, CT for around 5:00 PM this evening:

Forcing

Perhaps one of the biggest questions is the degree of forcing.  Latest data has continued to slow down with the eastward progression of the shortwave energy moving through Quebec Provence in Canada.  Forcing is huge as it provides the lift necessary for parcels to rise and generate showers and thunderstorms.  While there are hints at some localized areas of enhanced forcing across the region, it's unclear as to whether that would be enough to spark any activity.  Looking below we see the main piece of energy still well west at 8:00 PM this evening.  in fact, it is entirely possible we see this energy push through tomorrow sparking a threat for thunderstorms, including a few strong to severe thunderstorms:

  

What to Expect:

At the least it looks like we'll see some shower and thunderstorm activity develop later on, however, the coverage and extent of this activity is still not all that clear.  As the afternoon unfolds the situation will become more clear.  At this time it appears the potential foe widespread severe weather may be lowering, however, if a cluster of thunderstorms does develop it could produce a swath of damaging winds and perhaps even some hail.  

Given the slowing down of the shortwave energy, this may be opening up a window for the possibility of thunderstorms tomorrow.  The atmosphere does look to be unstable and with steep lapse rates and strong wind shear, thunderstorms could become strong to severe and pose the threat for strong to damaging wind gusts and large hail.  

There is lots to work out over the next several hours but if today doesn't work out that could just mean increased potential tomorrow.  

Thunderstorm/Severe Weather Potential Friday, July 22nd, 2016 Across New England

A rather intriguing setup looks to take place across New England on Friday as a potent shortwave trough and attendant cold front plow into a very hot and humid airmass.  While there may be some questions regarding how high temperatures climb (we will touch more on this later on), the potential exists for temperatures to climb into the lower to even mid-90's.  With dewpoints expected to soar into the upper 60's to perhaps lower 70's (although they could drop...again we'll touch upon this more later on) the combination of heat and humidity will make it feel quite uncomfortable.  The combination of heat and humidity would also yield quite an unstable airmass and that would be the fuel necessary to spark off some thunderstorms and some of which could become strong to severe.  

Looking at the 500mb pattern in place Friday morning we see substantial ridging across the central and southern portion of the United States placing the northern-tier of the United States and a good chunk of the northeast on the crest of the ridge allowing for a northwesterly flow in the mid and upper-levels of the troposphere:

While not all northwesterly flow events produce severe weather or widespread severe weather across New England, some of our biggest events gave occurred with northwesterly flows in the mid and upper levels of the troposphere.  

Embedded within this northwesterly flow will be two features which will be very big potential focal points towards the development of thunderstorms along with the potential for some severe thunderstorms.  These two features are; a shortwave trough which is expected to push into northern New England along with a subsequent piece of shortwave energy and attendant cold front and the other feature being a potential elevated mixed-layer plume.  The potential for an elevated mixed-layer (EML) plume to traverse over the region would vastly increase the potential for not only thunderstorms and a few thunderstorms but would increase the potential for a severe weather outbreak.  Too save time to further read and understand EML's please refer to my blog post devoted to them:

http://weatherwiz.blogspot.com/2015/06/what-is-elevated-mixed-layer.html

For Friday we are tracking two potential pieces of shortwave energy.  One is expected to push through northern New England during the morning hours.  With increasing dewpoints, low-level moisture, and shear, this feature could be associated with showers and thunderstorms which move through the Great Lakes the night before.  It is this potential which yields questions with how much heating we see later on in the day.  If this feature is associated with a quite a bit of cloud debris and shower/thunderstorm activity, it could take quite a while to clear things out meaning we don't achieve maximum heating.  This would yield lower temperatures and subsequently lower instability values.  However, this could be more important for northern New England and areas north of the MA Pike as the timing of the second shortwave would favor thunderstorm activity during the daylight hours.  Further south across southern New England, the timing may favor thunderstorms after dark, thus the degree of heating is not as important.  While typically, especially across this region, thunderstorms lose their strength as the sun begins to set and instability begins to fade, steepening lapse rates (faster decrease of temperature with height through the troposphere) could help to keep the airmass quite unstable well into the overnight and this would yield the threat for thunderstorms (both strong to severe) to persist well into the overnight for southern New England.

The second shortwave feature is the main player and would be the focal point for afternoon/evening thunderstorms.  This is expected to approach northern New England during the late afternoon hours and be accompanied by modest height falls.  

The next feature is the potential plume of EML air which may traverse over New England.  This feature would be key into yielding the moderately to perhaps extremely unstable airmass and would provide sufficient fuel for thunderstorms to not only become quite strong but reach severe limits:

     

As far as wind shear is concerned, winds throughout the troposphere should be more than adequate to not only help generate thunderstorms but keep updrafts maintained, and also perhaps help with updraft rotation.  The best wind shear appears to be located across northern New England where they will be closest to the stronger 500mb and 250mb winds:

What can we expect for Friday?  Well in the very least, we won't really have a full idea until Friday progresses as as are then able to pin-down the mesoscale (small features which play major roles in these type of events) aspects.  However, right now this is some of the potential that exists...

Some morning shower and thunderstorm activity is expected to push through portions of New England Friday morning.  After this activity passes the question becomes how quickly do we clear out?  Multiple computer forecast guidance indicates we will clear and do so quickly allowing for temperatures to soar into the 90's with dewpoints near 70F.  This combination along with steep lapse rates would yield a strong to extremely unstable airmass for much of New England.  The focus now would be across northern New England where they will be closer to the approaching second shortwave and strongest shear.  Here we will be on the lookout for the development of more showers and thunderstorms and on a widespread level.  Given the potential for strong to extreme instability and strong wind shear several of these thunderstorms could become strong to severe and produce strong to damaging winds, large hail, and given the degree of shear, supercells would be possible yielding the potential for an isolated tornado.  The extent of this potential all depends on degree of instability and timing of the features (does the shortwave time perfectly with the arrivial of steep lapse rates and strong shear?).  If everything times together we could be looking at a major severe weather event with numerous reports of several weather.  If the timing is not perfect than we would be looking at widespread thunderstorms with only a few becoming strong to severe and isolated pockets of damage.  

For southern New England...the watch will be on the activity to the north as this activity could come together to form what is known as a mesoscale comvective complex (MCS) and push through portions of southern New England overnight.  If the airmass remains quite unstable, this complex could produce a swatch of strong to damaging winds and even large hail.  Again this depends on the degree of instability and how the activity to the north evolves.  

Too summarize some of the things to watch;

1) EML/lapse rates...There is not 100% confidence or agreement that we do see an EML-plume/associated steep lapse rates traverse into New England.  Some computer guidance suggests this while others do not.  If we don't see this feature occur the threat for a severe weather outbreak would vastly diminish and the overnight severe threat for southern New England would be very, very slim.  

2) Morning cloud debris associated with shower/t'storm activity...again, this is more of a concern for northern New England but if it takes a while to clear out and surface temperatures do not realize their potential, the degree of instability will be vastly less and the threat for a widespread severe weather outbreak would be lessened.  

3) Surface dewpoints...This is something else to watch.  Some forecast soundings indicate surface and boundary layer winds may become more westerly in nature.  If the degree of low-level moisture isn't very high, or if the high dewpoints only exist up to a couple thousand feet or so, westerly winds would work to mix down some of this drier air aloft and that would drop the surface dewpoints and vastly decrease instability and subsequently severe weather potential.  

All in all Friday has the potential to be quite an active day for a good chunk of New England and we are just coming off of the region's biggest severe weather event this summer (Monday, July 18th, 2016).  While this potential exists, there are certainly alot of questions still to be answered with regards to how the key features evolve and we should get a better idea as new data continues to roll in.  As Friday morning progresses, we will develop a very good idea of what to expect as the mesoscale features are pinned down.  

Monday, July 18th, 2016 Thunderstorm Potential.

Another hot and steamy day is in store for Monday as temperatures ahead of an approaching cold front are expected to soar into the upper 80's to lower 90's with dewpoint temperatures in the 60's and perhaps rising into the lower 70's as the front approaches (thanks to increasing theta-e and theta-e pooling).  The combination of heat and humidity will lead to a quite unstable airmass tomorrow across southern New England.  While mid-level lapse rates may be modest at best (~6 C/KM), dewpoints climbing up to around 70F (or perhaps a little higher) could help to offset the weaker lapse rates and yield as much as 1500-2000+ J/KG of mixed-layer cape tomorrow afternoon:

With such an unstable airmass in place and an approaching weather system, the question becomes do we see showers and thunderstorms tomorrow and if so, what are the chances for strong to severe thunderstorms?  With the main cold front pushing through western and central New York during the late morning hours, a line of showers and thunderstorms is expected to develop back across central New York state during the late morning and early afternoon hours and push southeast  with the storm motion from southeast to northeast.  This line would approach southern New England during the early evening hours.  This would result in some unfavorable timing for our region, however, what we will have to watch for is a pre-frontral trough setting up across eastern New York and sliding through southern New England during the early to mid afternoon hours.  It is this feature along with modest height falls which could provide the focus for the develop of showers and thunderstorms across southern New England during the afternoon hours.  

Severe Weather Potential:  

While a widespread severe weather outbreak is not anticipated, the possibility does exist for several thunderstorms to become strong with a few reaching severe limits.  The degree of instability is certainly not a question tomorrow nor is the degree of wind shear aloft, however, there are some uncertainties with regards to where the best mid-to-upper level forcing is located.  By late afternoon, we see as much as 35-45 knots of wind at 500mb (~18,000ft AGL) overspreading southern New England which is more than sufficient for thunderstorms to become organized.  However, we see the strongest winds back further to the west and subsequently the strongest mid-level forcing as well:

One thing to watch tomorrow is perhaps the degree of low-level directional shear that a few of the computer forecast models are showing.  Computer forecast models are showing southerly winds at the surface, then turning to southwesterly in the lower-levels of the atmosphere and then more westerly in the mid-levels of the atmosphere.  This is indicative of good directional shear and with good speed shear (increase of winds with height through the troposphere), this can allow for strong thunderstorms with have strong and mature updrafts to actually begin rotating which can increase the potential for both larger hail and even a tornado...just something that needs to be watched.  The best potential for this possibility would be the CT River Valley where the surface winds could remain more southerly to even southeasterly:

While the more widespread thunderstorm activity may be confined back across much of New York into PA and into VT along/just ahead of the cold front and closer to the best mid and upper level forcing, the pre-frontal trough sliding through southern New England during the afternoon should provide a focus for showers and thunderstorm development as well.  While the setup really doesn't favor a big severe weather event right now, some of these thunderstorms across southern New England may become strong with a few reaching severe limits.  These thunderstorms will be capable of producing strong to damaging wind gusts, some hail, and while a low potential, an isolated tornado can't be ruled out given the degree of directional shear being advertised.  All storms will produce cloud-to-ground lightning and with precipitable water values approaching 2'' torrential downpours are likely.  The best threat for strong to severe thunderstorms may be confined to NY, PA, VT, western MA, and northwestern CT.  However, if the better forcing pushes east faster than modeled and overlaps the strongest instability and wind shear, then a bigger severe weather threat may materialize.