Dangerous t'storms possible across New England Tuesday-Thursday

Computer forecast models continue to indicate a very volatile airmass will advect into New England tomorrow morning lasting through Thursday as a highly anomalous late season weather pattern will be developing.  A strengthening ridge across the southeastern United States coupled with an amplifying trough working through the Pacific Northwest and northern tier of the country is allowing the advection of an elevated mixed-layer into New England.  A warm front is approaching New England now and the elevated mixed-layer will be associated with the warm front.  As the warm front moves through the region Tuesday morning, a very strong surge of low-level moisture will work into the region along with an unseasonably warm low-level airmass.  In the mid-levels of the atmosphere a pocket of very warm temperatures around 700mb of +10C to +12C will work overhead, signaling the elevated mixed-layer which will lead to very steep mid-level lapse rates on order of 7.5-8 C/KM.  The combination of rich low-level moisture, very steep mid-level lapse rates associated with the elevated mixed-layer, and strong surface heating will lead to extreme amounts of instability across the region at times over the next few days.  This combined with increasing shear/forcing aloft will lead to the potential for a significant severe weather event to occur across portions of the region.  Let's break this down day-by-day and into details.

Tuesday, September 9th, 2013

As a warm front lifts northward on Tuesday morning, a very strong surge of theta-e air will slide into the region with the highest axis occurring across central/upstate New York, western MA and into VT/western NH where the core of the low-level jet will slide through.  Currently, a cluster of t'storms is ongoing across portions of the upper-Midwest and southern Canada and this complex, or mesoscale convective system (MCS) will continue diving southeast along this boundary and should impact central/upstate NY and parts of MA/VT/NH during the morning hours.  There could be some strong to severe t'storms within this complex during the morning hours with large hail being the primary threat.  Once this complex clears the area this is when things could become very interesting.

Once the complex clears, skies are expected to rapidly clear and the surge of rich moisture will continue working into the region and mid-level lapse rates will continue to steepen as the elevated mixed-layer works overhead.  Dewpoints should climb into the mid to upper 60's and surface temperatures should reach the mid to upper 80's.  Combine these with mid-level lapse rates approaching 7.0-7.5 C/KM will lead to an extremely unstable airmass with surfaced-based Cape values exceeding 3000-4000 J/KG, mixed-layer Cape values exceeding 2000-3000 J/KG, and lifted index values as low as -9C to -12C.
With the warm front still in the vicinity, helicity values are expected to be very high with 0-3km helicity values on order of 300-400 m2s2, and 0-1km helicity values on order of 200-300 m2s2.  Since this region will also be closer to an eastward progressing cold front/upper level trough, wind shear in the mid and upper levels of the atmosphere will be quite strong with a mid-level jet around 40 knots which should yield to 30-35 knots of vertical shear.

These factors will yield to atmospheric conditions extremely favorable for severe thunderstorms including the possibility of tornadoes and the potential would exist for strong tornadoes.  While the atmosphere will be prime for these conditions, it's very unclear whether any storms will develop in the afternoon.  There will be a pretty strong cap across the region thanks to the elevated mixed-layer and there are no clear triggers to break this cap, though there could be a weak shortwave that moves through aloft or orographic lifting could be a source of lift.  If the cap can't be broken then t'storms are unlikely to develop.  However, if the cap can be broken, the potential for severe t'storms would be highly likely, though coverage may be limited, and any t'storm would have the potential to produce very large hail, possibly as big as 2'', damaging winds in excess of 65-70 mph, along with the threat for a tornado and given the high helicity values/Cape values a strong tornado would be possible.  Again, this is all dependent on whether the Cap can break.

If t'storms are able to develop tomorrow, they could converge into another MCS complex late evening and work southward impacting MA/CT/RI.  If we do see an MCS develop and work into these areas, the impacts would be overnight and again this would pose a threat for severe weather, likely with large hail being the main threat though strong winds would also be a possibility.

Wednesday, September 11th, 2013

Wednesday will be highly prime for high end significant severe weather region wide, however, across much of MA/CT/RI the airmass is expected to remain strongly capped so t'storms are more unlikely across these areas, though the potential can't be ruled out.  If any storms do develop they would quickly become severe posing a large hail/damaging wind threat.  Shear will be much weaker here so storm movement would be very slow and given that flash flooding would be a concern as well.
Across upstate NY/VT/NH/western ME it's a much different story.  Shear will be much stronger and there will be a defined forcing mechanism.  If strong surface heating can develop the atmosphere will become extremely unstable once again.  On Wednesday, we could be looking at the possibility of a high end severe weather outbreak across this region with large hail as big as 2'' in diameter, damaging winds in excess of 70 mph, and the threat for tornadoes and possibility of a strong tornado.  The tornado threat will depend on whether or not storms remain discrete or develop into a line.  Discrete would increase tornado chances while linear action would favor damaging winds.  This is  situation which will be closely monitored over the next 34-36 hours.

Thursday, September 12th, 2013

This day would favor eastern NY/southern VT/NH/MA/CT/RI for strong to severe t'storms, however, this potential is a bit more unclear.  The elevated mixed-layer should be east of the region or weakened but lapse rates could still remain steep, up near 6.5 C/KM.  If there is little cloud debris or morning showers, the lapse rates should remain fairly steep and we would see strong surface heating.  This would yield to moderate amounts of instability across the region.  With the front/trough progressing eastward, winds aloft will strengthen and become very favorable for organized t'storms.  As far as Thursday goes we will have to watch how the atmosphere responds to previous day's convection.  We will have more details over the next few days.