The biggest driver behind today's severe weather potential is that of an elevated mixed-layer (EML). The elevated mixed-layer is a layer of air (typically found between 10,000' and 18,000' above the ground) which in the United States originates across the Inter-mountain west. As air descends down the Rocky Mountains it warms adiabatically (and drys) This feature is extremely critical to the severe weather outbreaks which occur across the Great Plains. Since this feature is associated with a layer of extremely warm and extremely dry air it acts as an atmospheric cap. What it does is it can prevent clouds and showers/thunderstorms forming and it can allow for extreme instability to develop beneath it (especially during situations where surface temperatures are into the 80's and 90's and dewpoints are in the 60's and 70's). Above the cap you have a very sharp temperature decrease with height and this yields a 700mb-500mb lapse rate which can vary between 7.5 C/KM and 9 C/KM. This only fuels the degree of instability. While these features typically don't make it to our parts they sometimes can and if they are timed with a cold front or strong piece of energy moving through the jet stream aloft we can get widespread severe weather events. Some past examples include; May 31st, 1985 tornado outbreak, July 10th, 1989 Hamden, CT F4 tornado, July 15th, 1995 widespread damaging wind event across NY/PA into New England, May 31st, 1998 northeast severe weather/tornado outbreak, and most recently the June 1st, 2011 Springfield tornado.
Observing mesoanalysis data this morning mid level lapse rates (700mb-500mb...about 10,000' to 18,000') just to the west and southwest of New England are as high as 7-7.5 C/KM with these values expected to push into New England during the day:
With surface temperatures away from the coast expected to push into the 80's with dewpoints climbing well into the 60's under the presence of steep lapse rates the atmosphere is expected to become quite unstable this afternoon with surface-based CAPE values exceeding 2500-3000 J/KG, mixed-layer cape values approaching 2500 J/KG, and lifted index values exceeding -5C to -7C. These values all indicate a significant amount of instability.
In addition to strong instability we will have have very strong wind shear aloft. Mesoanalysis data shows 0-6km wind shear values of 40-50 knots across much of the region which will assist in not only strong organization but in the potential for supercell thunderstorms as well:
One thing to watch as well with regards to the wind shear is any directional component (change of wind direction with height) to the winds. This point-and-click forecast sounding from the overnight run of the NAM forecast model for a location in western CT shows the low-levels winds a bit backed with nearly southerly winds at the surface then becoming more southwest as you ascend with height:
The timeframe for thunderstorms today will be between about 3:00 PM and 9:00 PM this evening. A line of strong to severe thunderstorms is expected to develop across southern NY and PA and track towards CT later on. If the instability parameters mentioned above occur given the wind shear we have in place this line of thunderstorms will likely be accompanied by numerous severe thunderstorms capable of producing widespread damaging winds as well as large hail. We also need to watch any discrete thunderstorms as they will have the potential to produce a tornado. This is expected to be a rather active afternoon across the northeast and parts of New England.
