Active Severe Weather Wednesday

The ingredients are coming together for a formidable outbreak of thunderstorms across Connecticut this afternoon. In fact this morning while letting the dog out I noticed the sky was covered in altocumulus-castellanus clouds. Say what???? The clouds known as ACCAS are frequently seen on the plains before big severe weather days. The clouds indicate strong mid level instability.

Heat and humidity, coupled with somewhat steep mid level lapse rates (see ACCAS picture above), will produce substantial instability across southern New England. Take a look at the 12z OKX sounding – wow!

How do you like that? MUCAPE near 4000 j/kg! Part of that is a function of steep mid level lapse rates (700-500mb 7.7 C/km) with what appears to be a remnant elevated mixed layer plume. These steeper lapse rates will move offshore later today so instability will remain quite high but likely short of extreme levels.

High amounts of boundary layer moisture (dew points at the surface above 70!) should provide plenty of juice even with mid level lapse rates becoming less impressive. Mixed layer CAPE values should easily approach or exceed 2000 j/kg.

While the 6z NAM has MLCAPE values generally just shy of 2000 j/kg the 3z SREF plumes show many members with MLCAPE values above 2500 j/kg! An impressive instability display that is no surprise given this morning’s 12z OKX sounding.

3z BDR SREF plume diagram. y-axis is MLCAPE in j/kg while x-axis is time in GMT.

The largest negative for today’s event, that will prevent this from becoming a widespread significant severe weather outbreak, is the modest deep layer shear. We certainly have the instability but the shear is a bit unimpressive.

This is the NAM 500mb wind speed forecast for this afternoon and the state is covered by winds of 30-40 knots. 0-6km bulk shear at this time is likely around 30-35 knots for the most part. That’s adequate for severe storms (remember shear helps organize convection and updrafts) but likely not enough for significant severe.

Of course these things can change in time. If we wind up with stronger mid level winds than progged and can increase the amount of deep layer shear the threat will increase.

Storms will fire today as CIN is removed from increasing low level moisture, strong heating, and heights falls associated with advancing shortwave. Weak front should provide some low level convergence to kick things off as well. We’ll be primed to go by late morning so I wouldn’t be surprised to see things start as early as noon today.

The other threat for today’s storms is the potential for flash flooding. The atmosphere is loaded with juice and there may be multiple rounds of thunderstorms in any given location. Something to keep in mind. The threat for tornadoes looks low today with relatively weak low level winds. Still, you can never rule out an isolated spin up.

Twitter is the place to find me today for updates – @ryanhanrahan.

Significant Severe Weather Threat Later Today

1630z SWODY1

After two bouts of severe weather last night with a spectacular lightning display, gusty winds, and large hail in Litchfield County the real show may not be until later today.

The Storm Prediction Center has upgraded portions of southern New England to a “moderate risk” which means a 45% chance or greater likelihood of severe weather within 25 miles of any given point.

Much like last Wednesday (during the Hampden County tornado) the instability in the atmosphere is extreme. When CAPE exceeds 3000 j/kg instability is high when it exceeds 5000 j/kg it’s extreme and that’s what is in place just to our west thanks to high dew points and a remnant elevated mixed layer aloft.

16z SBCAPE (Courtesy: SPC Mesoanalysis)

Though there will be moderate to strong shear in the atmosphere capable of producing rotating updrafts (and subsequently large hail) the tornado threat is low. The wind shear today is mostly speed shear as opposed to directional shear. Last Wednesday winds turned rapidly with height (directional shear) which is favorable for tornadoes particularly when that change happens in the lowest mile or two of the atmosphere.

Today’s severe weather event will likely bring an organized squall line capable of producing destructive winds. Out ahead of the squall line any storms that organize will be capable of producing large hail and potential a brief/isolated tornado but nothing like last Wednesday.

12z NAM 6 Hour Forecast 500mb Heights and Vorticity

The atmosphere will remain capped until 2 or 3 p.m. and then we’ll see things explode to our northwest. The cap will be weakened as a disturbance moves in from the west resulting in large-scale ascent. As 500 mb heights fall you generally see the removal convective inhibition (known as CIN and pronounced “sin”)  and thunderstorms start to fire.

I expect the significant severe weather threat to exist down to the Connecticut shoreline today as well. These are the kinds of events that pose a severe weather risk for the entire state.