Severe Weather Likely Monday & Tuesday

There are a few kinds of severe weather setups here in Connecticut. There’s the classic high-end severe weather outbreak that is generally characterized by northwesterly flow aloft, high/extreme instability, and strong wind shear (e.g. July 1989, May 1995, May 2010). There’s the more common borderline instability/shear setup that can favor borderline “pulse” storms or weakly organized line segments. And then there’s the Monday/Tuesday setup.

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The warm season cut-off low can be a prolific severe weather and flooding maker. While high-end or significant severe is unlikely it’s not out of the question. I blogged a bit about this kind of setup last June and a good example of a high-end severe weather day from an anomalous cut-off in July is the 2008 New Hampshire F2.

So what’s the deal with Monday and Tuesday’s severe weather threat? The NAM and GFS both show a strengthening low level jet on Monday and several shortwaves embedded in the southwesterly flow aloft moving through. The 9z SREF shows MLCAPE around 1000 j/kg Monday afternoon.

9z SREF / Courtesy: SPC
9z SREF / Courtesy: SPC

With modest instability and strengthening low level shear low topped supercells are quite possible tomorrow. Damaging winds and tornadoes will be possible. Lifted condensation levels will be quite low as 0-1km shear ramps up – that means we’ll have to watch the radar closely!

If that weren’t enough it looks like we could do it all over again on Tuesday with another low CAPE/high shear environment. The details will come down to mesoscale details that are tough to figure out this far in advance. In addition, flash flooding is a possibility with excessive rain possible. Here’s the WPC/NCEP precipitation forecast for the next 72 hours.

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As I like to say – beware the Upper Level Low. These cut-offs can produce pretty nasty weather in the warm season, particularly in July and August when water temperatures in the Sound and south of Long Island are near their climatological peak.

Here’s something interesting based on this morning’s NAM solution for Monday. The CIPS analog guidance looks for other days with similar synoptic setups and looks at what kind of weather was reported that day (damaging winds, tornadoes, or even snow amounts in winter). Below I’ve posted the probability from CIPS of probability >5 severe reports and also >1 tornado.

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SVRtorn_N1_110km_nam212F036

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