Unlike last week when we were expecting a medium/high impact severe weather event across the state tomorrow’s severe weather potential is quite a bit lower. There’s a few reasons for this.
First, let’s take a look at the setup way above our heads halfway through the troposphere around 11 a.m.
We have a fairly powerful upper level trough moving through (notice the dip in the black lines) across the northeast. At the same time, a disturbance known as a shortwave is overhead in southern New England racing to the northeast. Ahead of this shortwave is where air will typically rise and behind it is where air will typically sink. With the early arrival of this shortwave we can expect to see sinking air/subsidence during the midday and early afternoon following a period of morning rain and even some thunder.
So – where’s the severe weather threat? In the morning (during the AM commute) we could see a pretty could downpour and lightning show in a few towns. Behind this is where the severe weather threat exists – but it’s far from a certainty. You can see on our computer models a very large discrepancy in how the atmosphere will look late Wednesday afternoon. Below you will find forecast soundings for Hartford from the GFS and the NAM.
The GFS (top) would indicate little if any severe weather potential. Much drier air filters in from the west resulting in a dramatic drop in instability. The NAM, however, keeps a fair amount of moisture around in the lowest 3km of the atmosphere which results in a much more unstable profile. To visualize that look at the difference between the yellow line and the red lines above.
CAPE is defined as the area between a parcel and the environmental temperature (think integral – calculus fans!) The more area between the yellow line (parcel) and red line (environmental temperature) the more instability. This makes sense because air parcels warmer than their environment will rise – and it will rise more quickly the greater that difference is.
So where does that leave us tomorrow? The NAM solution is intriguing because it generates a sufficient amount of CAPE and the NAM also has some substantial wind shear which can help organize thunderstorms and also favor supercells. More than 30 knots of deep layer shear along with impressive low level veering (winds south at the surface and westerly 3km up) is concerning.
My gut feeling is that the GFS solution – the uneventful one – will be most correct. As that morning shortwave moves through we’ll be left with subsidence that should limit afternoon storm coverage. Additionally, our short range ensembles (SREFs) which I find useful in the warm season, show very meager CAPE during the afternoon – generally <1000 j/kg.
So there you have it. At this point I’m expecting a low impact event which would means isolated coverage for strong storms – and of the storms that form borderline severe weather rather than high-end severe weather would be favored. If the more vigorous solutions (like the NAM) come to fruition then we will have to reevaluate tomorrow morning.