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:
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