Strong/Severe T'storm Threats Tuesday, September 10th-13th, 2013

As we move through August and into September strong/severe thunderstorm possibilities across New England begin to decrease very rapidly.  This is due to the fact that as we move through this time of year we begin to transition from a summer weather pattern toward an Autumn weather pattern and it's much more difficult to get higher amounts of instability to develop across the region.  While it becomes much more difficult, it certainly isn't impossible and for it to happen you need a really special setup.  For the past several days now, medium/long range computer forecast models have been very consistent with this "special setup".  When referring to this "special setup" we are talking about the elevated-mixed layer.

When talking about the potential for severe weather and higher event significant severe weather events, having an elevated mixed-layer in place is extremely critical in order to achieve a major severe weather outbreak consisting of high end significant severe weather.  High end significant severe weather refers to widespread wind damage, typically with winds in excess of 65-70 mph, widespread large hail reports exceeding 1.5''-2'', and tornadoes, typically with at least one strong/violent tornado (EF2 or greater).

Elevated mixed-layers are areas of the atmosphere, typically between 800-600mb of extremely warm and dry air.  This lead to very steep mid-level lapse rates on order of at least 7.5-8 C/KM and can even exceed 8.5-9.5 C/KM.  This combination, especially when coupled with very high surface temperatures and lots of low-level moisture (high dewpoints) leads to extreme amounts of instability which is the main source of fuel for t'storms.  Elevated mixed-layers are located across the Inter-mountain west region across the southwestern United States and can/usually gradually shift east into the Great Plains region.  While it's very rare for elevated mixed-layers to advect all the way to New England, they can do so given special atmospheric setups.  If you happen to remember June 1st, 2011, the day of the Springfield tornado, that was associated with an elevated mixed-layer.

When looking for elevated mixed-layer advection into New England, what you want to see is a very strong 700mb ridge centered across the southeastern US with 700mb temperatures in excess of +10 to +12C over your region.  You also want ridging at 500mb and to be on the crest or top of the ridge along with an amplifying trough to your northwest.  This allows you to be closer to the cooler 500mb temperatures and also allows you to be closer to the much strong mid-level jet.  Here are some images from today's 12z GFS run for 48 hours out, meaning the following images are for 8:00 AM eastern time Tuesday morning.

During the morning hours of Tuesday a warm front will be lifting northward through the region as well, and in fact, will be the leading edge of the elevated mixed-layer air which will move into the region.  As the warm front lifts northward humidity will be on the sharp increase as dewpoints will climb to near 70F and the low-level airmass will warm as well and to the point to where surface temperatures could reach the mid-80's to even near 90F in spots if full heating potential is realized.  

On Tuesday, severe weather could be a definite possibility across northern New England first in the AM associated with the warm front and then again during the afternoon as a cold front slowly slides east and a piece of short-wave energy moves through aloft.  Late Tuesday evening/overnight then could be interesting for central/southern New England as we could be looking at the potential for strong to severe t'storms during the overnight hours.  

As we move into Wednesday and perhaps even Thursday, as a cold front slides east severe weather may be a possibility both days as well.  What we will have to see is whether we can hold the elevated-mixed layer or at least steeper lapse rates both days as if we can, both days could be extremely active with severe weather.

At this point it's a bit too early to get into specifics for Wednesday or Thursday but as far as Tuesday is concerned it could be very active across central/northern NY and then into VT/western NH/northwestern MA during the AM/PM.  We could be looking at the possibility for damaging winds, large hail, and perhaps the threat for isolated tornadoes depending on whether or not the low-level jet is strong enough and hodographs can become enlarged.  

As far as Tuesday night is concerned for the rest of MA and down into CT/RI, this will all depend on how much forcing is present.  We definitely will have to watch the Tuesday night period for some severe weather across these areas with large hail/damaging winds being more of a threat.

As we move through the next 24-48-60 hours, details should continued to be ironed out.