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:
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