Loaded Gun – But No Trigger

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If you like big storms you’ve got to love a sounding like this. A plume of very steep mid level lapse rates, known as an elevated mixed layer, is streaming east toward the northeast. This sounding shows the EML in place over Windsor Locks Wednesday afternoon. Temperatures in the mid levels of the atmosphere cool very rapidly with height – on the order of 8 to 8.5 C/km between 500 and 700mb – almost dry adiabatic!!!

The result is a tremendous amount of CAPE. This type of sounding is what’s called a loaded gun. If the cap (which is sizable) can be broken the resulting thunderstorms can be explosive. Thankfully, it seems unlikely the cap will be broken. Connecticut will be in the midst of a brutally hot day with a ridge of high pressure cresting overhead.

Mike Ekster and Pete Banacos published a paper about the presence of EMLs in a staggeringly high percentage of severe weather days that resulted in fatalities or injuries (53% and 49%, respectively) in the northeast. Most memorable high end severe weather events in the northeast have occurred with an elevated mixed layer overhead (1953 Worcester tornado, 1973 West Stockbridge tornado, 1985 Pennsylvania super tornado outbreak, 1989 Hamden tornado, 1995 Connecticut hailstorm, 1995 super derecho, 1998 Labor Day derecho and tornado outbreak, 2011 Springfield tornado to name a few). While many large severe weather outbreaks have occurred on an EML day the presence of an EML does not guarantee severe weather – in fact there are many days where the “cap” is too strong to overcome and no thunderstorms result even though instability is sky high.

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The airmass on Wednesday is going to be extremely hot for the time of year. 850 mb temperatures are progged to be hotter on Wednesday than they’ve been all summer! +20 to +22C 850 mb temperatures are forecast across southern New England that should allow the mercury to soar into the 90s. If winds can stay westerly and clouds can stay away 95-98F is possible in the valley!

A few notes about this setup. If thunderstorms can break through the cap upstream we’ll have to watch them closely here in Connecticut. At this point I’m not expecting much but if a rogue storm forms it could be quite severe. By Thursday the EML plume exits to the east and instability wanes while deep layer shear and forcing increases. We could see a round of severe weather on Thursday but at this point the coverage and intensity is still unclear. Worth watching.

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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.

Memorial Day Severe Weather Threat

This is a convoluted severe weather threat developing across the state this afternoon with several things working toward a major, high-end outbreak and several things working against.

There’s no question the atmosphere will be primed for big severe weather later today. As I’ve written about quite a bit on this blog we will have an elevated mixed layer over Connecticut this afternoon with northwesterly winds in the mid levels of the atmosphere. This is an immediate red flag for high end severe weather potential!! The elevated mixed layer is a region of very steep lapse rates (temperatures decreasing quickly with height) 2 to 3 miles up in the atmosphere. When coupled with warmth and humidity near the ground a “loaded gun” sounding develops. If you can tap into that instability – watch out!

The 12z sounding from Pittsburgh shows an elevated mixed layer firmly in place. The OKX sounding shows an EML as well with a lifted index of -4.5C. Not bad for 12z.

So we have the instability – but do we have enough wind shear? Wind shear is important in promoting storm organization and developing rotating updrafts. Without a rotating updraft you’re not getting big hail. Today it appears we have the shear.

While winds at 500mb (or about 3.5 mile up) are only modestly strong there is significant directional shear. Winds near the ground will be out of the south while winds upstairs will be out of the northwest! With enough speed this will develop long and curved hodographs as seen above off the 6z NAM. This indicates supercells are possible. The low level shear especially is quite impressive indicating the potential for tornadoes. Both deep-layer and low level shear is not to the level it was last June 1st, however.

So what’s the negative? What’s working against today’s storm potential is the lack of a solid trigger and large scale subsidence. You’d like to see some type of shortwave or disturbance in the atmosphere with falling 500mb heights. That’s absent today. What this may do is reduce the areal coverage of storms but no necessarily decrease the severity of storms.

While storms may be isolated they have the potential to be quite severe. High-end severe reports and even tornadoes are possible in western New England and New York today. What makes the threat even more difficult to ascertain is the mesoscale convective system that has developed over Ontario and Lake Ontario. Not sure how that will impact downstream development.