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5/16 WESTERN NE Severe Threat Disco


ChrisM

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MESOSCALE DISCUSSION 0826

NWS STORM PREDICTION CENTER NORMAN OK

0536 PM CDT WED MAY 16 2012

AREAS AFFECTED...FAR EASTERN NY INTO PORTIONS OF VT/NH/MA/CT AND

WESTERN MAINE

CONCERNING...SEVERE THUNDERSTORM WATCH 280...

VALID 162236Z - 170000Z

THE SEVERE WEATHER THREAT FOR SEVERE THUNDERSTORM WATCH 280

CONTINUES.

SUMMARY...SEVERE TSTM WATCH 280 CONTINUES UNTIL 01Z FOR FAR EASTERN

NY INTO PORTIONS OF VT/NH/MA/CT AND WESTERN MAINE...WITH DAMAGING

WINDS AND SEVERE HAIL THE PRIMARY CONCERNS.

DISCUSSION...SCATTERED STRONG/SEVERE TSTMS ARE ONGOING ACROSS THE

REGION THIS AFTERNOON NEAR A COLD FRONT...WITH THE MOST ROBUST

STORMS ONGOING ACROSS FAR EASTERN NY NEAR/NORTHEAST OF THE ALBANY

AREA INTO SOUTHERN VT AS OF 2230Z. THIS IS WHERE A WEAK SURFACE WAVE

/1008 MB/ IS ANALYZED ALONG THE FRONT...COINCIDENT WITH A PRE-COLD

FRONTAL AXIS OF LOWER 60S F SURFACE DEWPOINTS WITH MLCAPES ON THE

ORDER OF 500-1000 J/KG. A FEW/MAINLY TRANSIENT SUPERCELLS REMAIN

POSSIBLE IN THE SHORT TERM...BUT OTHERWISE STORMS SHOULD CONTINUE TO

GENERALLY CLUSTER/SPREAD EAST-NORTHEASTWARD ACROSS SOUTHERN VT/NH

AND WESTERN MAINE THROUGH EARLY/MID EVENING. A DOWNWARD INTENSITY

TREND SHOULD BEGIN TO OCCUR BY THE 00Z-01Z TIME FRAME AS THE

BOUNDARY LAYER COOLS...BUT NONETHELESS...ADDITIONAL PORTIONS OF

MA/NH/SOUTHWEST MAINE MAY WARRANT A SPATIAL/TEMPORAL EXTENSION OF WW

280 WHICH IS CURRENTLY SCHEDULED TO EXPIRE AT 01Z.

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It's not the wind direction. If there was ample sun, it would be no problem.

doesn't the south wind inhibit convection, and also help keep the ample sun, since the modified marine layer airmass wasn't eroded? There are many problems for SNE, not just one or two.

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doesn't the south wind inhibit convection, and also help keep the ample sun, since the modified marine layer airmass wasn't eroded? There are many problems for SNE, not just one or two.

Not necessarily. We actually want sfc winds to be more S/SE b/c this helps to enhance the helicity, especially when we have SW/W/NW flows aloft. However, the southerly winds certainly do increase the affects of the marine layer and can clouds in place...this is why getting major events with S/SE wind directions works out during EML setups...That dry air aloft mixes out and erodes the mid-level clouds and that allows for the stronger sfc heating to take place which will then work on mixing out that marine layer.

As for today though, I don't think the southerly component to the wind was strong enough to really play a major factor, it would have for the south coast though I think. Take a look off to our west across PA/southern NY...those locations were right along the front and there was great dynamics there, however, the column was so dry and that dry air mixed down that I think it prevented convection from firing there.

Up north where there was also stronger forcing/dynamics along the front there was also richer llvl moisture and storms were able to develop.

For us here in SNE we were just too far removed from any stronger lift and we did have issues with dry air aloft which probably meant even weak lift wouldn't have been able to do much.

You can see now across SE NY that as the stronger forcing/dynamics near showers are starting to pop up.

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doesn't the south wind inhibit convection, and also help keep the ample sun, since the modified marine layer airmass wasn't eroded? There are many problems for SNE, not just one or two.

I don't think it was really an issue of instability since these kind of numbers were progged. I think the bigger issue was the lack of forcing down here. Models shifted north with the best lift which took areas farther south (NYC, for instance) out of the game.

In fact the 12z models started to show this as a problem with 500mb heights actually rising from POU on south from 18z-00z. Think QG height tendency equation and realize why that equals fail.

Best forcing/QG lift was north and that's where the storms went.

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The late timing, plus dynamics displaced north..then on top of that s winds at sfc? That's a kiss of death for SNE convection in May, even if something rogue were to survive into ORH/BOS/PVD etc. I don't know, just a general observation. Not saying it has played a role, even so far, as we knew the best timing was after 6pm. Hope to be wrong.

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The late timing, plus dynamics displaced north..then on top of that s winds at sfc? That's a kiss of death for SNE convection in May, even if something rogue were to survive into ORH/BOS/PVD etc. I don't know, just a general observation. Not saying it has played a role, even so far, as we knew the best timing was after 6pm. Hope to be wrong.

Yeah timing put everyone east of BDL/BAF out of the game.

In the end the synoptic evolution of the whole system became a bit convoluted. What looked favorable yesterday started to fall apart overnight on the models. What was a favorable looking jet structure went to hell with two separate jet streaks that put most of SNE in an area of subsidence. That, coupled with poor timing, were the main factors that went against this one.

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Ultimate fail here. The instability was there, but the lack of a good trigger (i.e. synoptic forcing) took SNE out of the game. Steel grey skies to my north though from the passing line up near the VT/MA line. Maybe Mother Nature will throw me a bone and I'll get a shower from the cold front.

As for the southerly flow, it was not the killer IMO as the flow is pretty weak across SNE. Right along the coast, yes, but further inland, probably not so much. A stronger southerly flow would advect the marine layer further inland.

The best situation for severe weather here in SNE is a NW flow aloft with a W to SW flow near the surface. The NW flow brings in cold air aloft, while the SW flow brings in heat in the low-levels. The directional shear also creates good helicity. It also keeps the marine influence confined to SE CT, SE MA and S RI. A W to NW flow near the surface will eliminate the marine taint all together. There's a reason why NW flow severe events are the best here in SNE.

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