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.

Analyzing Tuesday's Thunderstorm Potential

A summer-like feel has arrived here in southern New England as for the past few days many locations have either flirted or exceeded the 90F mark and we've added a touch of humidity as well as dewpoints have hovered right around 60F as well.  We will have one more rather hot and somewhat humid day on Tuesday before a backdoor cold front pushes through and will yield temperatures a good 10-15F cooler for Wednesday with much less in the way of humidity.  As the backdoor front pushes through there will be the risk for some isolated thunderstorms tomorrow.  With temperatures once again expected to soar through the 80's across much of southern New England tomorrow (and a few spots making a run towards the low 90's), dewpoints creeping a bit higher into the lower 60's, and mid-level lapse rates (decrease of temperature with height between about 10,000'-18,000')  around 6.5 C/KM this will lead to a rather unstable airmass.  While winds aloft aren't rather strong, there is some modest shear (~30 knots) at 500mb (~18,000').  Typically when you have a cold front work into such an environment there is a solid chance for not only scattered thunderstorms but for several of these thunderstorms to become strong to even severe.  This won't necessarily be the case tomorrow.  While I think we will see some isolated thunderstorms with a few potentially becoming strong and briefly reaching severe limits, I don't think storms will be as widespread as several computer forecast models are suggesting and this will be explained below.  

The type of cold front sliding through tomorrow is known as what is called a backdoor cold front.  Typically for our area, we see cold fronts mainly come from the west, northwest, or north but every once in a while we see cold fronts work in from the Atlantic ocean from either the northeast or the east.  These cold fronts are more driven (or are developed) due to the temperature differences between the cooler ocean waters and the warmer land while cold front which arrive from the west, northwest, or the north are driven (or developed) due to different airmass which exist over land.  With this, backdoor cold fronts are typically very shallow in nature and have a lesser slope than regular cold fronts.  So I don't get any more technical here the jest of this is...the less shallow the slope, the less intense the upward vertical motion is associated with the front.  Think of it like cold fronts vs. warm fronts.  This is one reason why warm fronts are usually associated with steady rain as opposed to big time thunderstorms like cold fronts can be.  This is because warm fronts have a much more shallow slope than cold fronts...hence weaker upward vertical motion.  

As for tomorrow, the backdoor cold front will be sliding in from the northeast and we can see this by looking at and analyzing a 2-meter surface projection for tomorrow afternoon from one of the computer forecast models.  The northeast to southwest kink in the 2-meter isohytes indicate the movement of the front will be from northeast to southwest:

There are several ingredients that will be in place tomorrow (some of which was described in the opening paragraph) which do favor some isolated thunderstorms but there are also several red flags (one being the backdoor nature of the front) which indicate the activity may not be all that widespread and the storms will struggle to get very strong.

As stated in the opening paragraph, some computer forecast models are actually hinting that tomorrow will be a rather active afternoon and evening in terms of thunderstorm activity, however, I think this is overdone.  In order to save time and keep this post from being any longer than needed I will use one example to illustrate each flag.  If you look at the GFS computer forecast model's projection of surfaced-based CAPE tomorrow (a measure of how unstable the airmass is.  The larger the cape, the more unstable the atmosphere is said to be) you will see some rather hefty values of 3,000-5,000 J/KG...those values are nearly unheard of here across southern New England...especially anything near 5,000 J/KG:

What could be the cause for this?  The answer...dewpoints...really, really high dewpoints.  Whenever I see CAPE values being forecasted that high around here the first two things I check to see are 1) Are the surface dewpoints being projected to be too high?  2) If not, how steep are the mid-level lapse rates going to be?  The answer here is the dewpoints by the GFS are being forecasted to be way too high.  In fact, the GFS has dewpoints well into the mid and upper 70's across a good chunk of southern New England tomorrow:

umm yeah this is not happening.  There are a few reasons for this but the one reason we really need to look at are the projected direction of winds within the lower portion of the troposphere.  Winds are forecasted to be a more northwesterly direction which typically allows the atmosphere to mix much higher (and the result is mixing down drier air from the upper portion of the lower troposphere and lower portion of the middle troposphere) and is also a downsloping flow for some off of the Berkshire mountains.  We can see this from a point-and-click forecast sounding taken from northern CT:

Severe other computer forecast models also suggest dewpoints reaching the 70F mark or even exceeding it, however, there is just no way that happens given projected low-level wind direction.  The result here...computer forecast models are overdoing CAPE by ALOT and this results in them forecasting widespread thunderstorms with a greater risk for some strong to severe thunderstorms.  This low-level wind flow direction also tends to limit low-level convergence (which leads to rising motion) which is also not good when looking for widespread thunderstorms.  

The next flag we will look at is the lack of much stronger wind shear.  Using the same sounding as an example, winds aloft aren't projected to be rather strong at all, however, winds from 700-500mb are forecasted to increase to >30 knots as the day progresses from north to south and this will work to increase deep-layer shear a tad.  Deep-layer shear is important for both aerial coverage of thunderstorms and how long storms can live as well as enhancing how strong they can become.  When you have weak wind fields, this works to lessen the degree of upward vertical motion and it also prevents any sort of updraft and downdraft separation.  For storms to have longevity you want updraft/downdraft separation so the storm can continue feeding off warm/moist air, otherwise if there is little to no separation, the storm's updraft is dominated by downdrafts/rain cooled air and the storm rapidly weakens.  Given how we won't have really strong shear tomorrow and only modest deep-layer shear, I think this will lead to storms really struggling to get very strong and also mean they are 'pulse-type' meaning they weaken rather quickly:

Lack of a more defined and stronger piece of shortwave energy rotating through the middle troposphere and lack of stronger height falls will also preclude activity from becoming more widespread and prevent activity from becoming too strong.  A stronger piece of shortwave energy really helps to accelerate upward vertical motion as does stronger height falls.  For the height falls, if this map were to animate you wouldn't see much of a southward progression of these isohytes which indicates heights remain rather steady:

   

The end result..I think tomorrow we will see a few thunderstorms pop up as the boundary progresses southward through the region and the greatest chance may be confined to southern CT, southern RI, and far SE MA, especially if a sea-breeze boundary were to develop...this interaction between this boundary and the backdoor cold front could really locally enhance convergence and allow for a few really decent thunderstorms to develop.  All in all I think most towns will stay dry but for those towns who do see some action, there is a possibility the thunderstorms could briefly become strong/severe and pose a threat for strong winds or even some hail but I think this potential is rather limited/isolated at this time.