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Tuesday, August 13th, 2013 severe threat


weatherwiz

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I'm really struggling to think of any other significant hail reports for GYX this year. Tough to do with freezing levels consistently at 14 kft. Other than that, there were a couple isolated big wind events. One in Grafton County, NH and another up by Rangeley, ME.

 

North of your CWA, but there was quite the windthrow event last month in northern Piscataquis - began near the Arm of Chamberlain and extended into the NW portion of Baxter, where the worst damage occurred and with some blowdown patterns suggesting rotation. I've seen the airphotos and would guess that several hundred acres were flattened or severely damaged. If CAR has posted an evaluation of that event, I missed it.

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I think there could very well be some isolated/scattered reports, but this doesn't look like a widespread event due to cloud-cover. Who knows, maybe it will over-perform if we break out. MAV MOS has noticeably been lowering high temperatures though. Just looking at BDL for example, it's going from 85, 86, 85, 84, 79, 78 over the last few runs. (Euro/NAM 2-m temps have been consistently in the mid to upper 70's for highs)  If we get socked into clouds and 70's, that may not bode very well. Also MLLR's are very meh. Again, just looked at one station, but NAM was 5.3°C/km for BDL.

 

On a site note, I've re-worked my severe weather equation to use the following factors in generating severe thunderstorm potential:

Mid-level lapse rate, cloud-cover, MLCAPE, bulk shear, helicity and ML LCL. 

It used to use two composite parameters for tornadoes and supercells, but after careful consideration, those terms were redundant considering that they're heavily influenced by shear and helicity, and the overall equation was lacking a focus on buoyancy, as well as cloud-cover, which can affect low-level lapse rates and overall instability.

 

The setup for tomorrow is decent when it comes to wind shear. Helicity is actually pretty good, as well as some lower LCL's. It's CAPE that's a wild card and as we discussed, lapse rates are meh.

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12z GFS is very lackluster. Shows very little instability in eastern sections of SNE with only slight improvements further west. SBCAPE stays below 1000 Jkg-1 and mid-level lapse rates are between 5.0 and 5.4 Ckm-1. All that junk in the morning may really hamper this event's potential, as has been the case so many times this "severe weather" season.

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Tomorrow looks like a few light storms, but nothing impressive.

We may actually have the best shot in the morning into midday...could see some marginally damaging winds here and there. Maybe a brief-spinup, but even that isn't terribly likely.

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Wonder if there's some convective feedback again. 18z NAM brings a 50kt LLJ into parts of southeastern Massachusetts tomorrow afternoon as low pressure deepens. It also eliminates any meaningful instability across the interior ahead of the cold front.

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This reminds me a bit of the last ordeal...  awesome looking directional shear, so-so looking speed shear...bulk is thus a tad on the high side.  Seems good, but CAPE genesis will be too challenged with all the clouds around; and now the 18z goes so far as to lift another ball of strata rains right at the wrong time of day -- very similar fashion as the last failed event.  

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The NAM is going nuts with the LLJ tomorrow. I really don't see much of a TOR threat really.

Looks like isolated wind gusts for the most part. There could be a very slight tornado risk from GON-PVD-TAN and SE? If the low was going to develop a couple hundred miles further north, it would be more interesting.
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I wonder if we see a 7/10 split somewhere with the Philly stuff clipping Cape Cod and the other stuff in wrn SNE. Maybe it does develop a little further east like it did last week.

Have to see where and how quickly that Sfax low develops and how quickly it strengthens. PWATS aren't as high as last week so I don't think we see 4-5" totals but some spots could get 2" or so

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Hi res imagery suggests some improvement in sky coverage may take place as the afternoon runs out, and it would be a race of sorts.

 

HPC analyzes the warm boundary into S VT/NH now.  

 

71/70F along Rt 2 and already there are some insidious looking black bases with tendrils of dark scud hanging unusually low ....consistent with low LCLs.   I do not believe with generalized tendency for convergence amid near-by sfc low PP, and theta-e pooling, it would take a whole helluva lot of trigger to get something to develop, but the CAPE insert needs to happen.     

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Hi res imagery suggests some improvement in sky coverage may take place as the afternoon runs out, and it would be a race of sorts.

HPC analyzes the warm boundary into S VT/NH now.

71/70F along Rt 2 and already there are some insidious looking black bases with tendrils of dark scud hanging unusually low ....consistent with low LCLs. I do not believe with generalized tendency for convergence amid near-by sfc low PP, and theta-e pooling, it would take a whole helluva lot of trigger to get something to develop, but the CAPE insert needs to happen.

sun in and out here
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sun in and out here

 

 

Yep, back edge of denser exhaust from MCS S of LI just came over rendering our skies as partly mid level, with scattered lower level scud and early towars whisking by from the SSW.   SFC breeze is SSE in the trees, and that mid level I mentioned is moving from west to E  -- this is visually quite the helicoidal flow.   

 

Not sure what the convective temperature is just yet, but if that is breached, I feel there could be a spinner or two (as we've collectively come to call our EFO jobs..).  Thing is, we are S of a warm front, and E of a cold front, amid the cage of a low pressure, where convergence is not really going to be in short supply.  If anything pops it will be interesting to follow... 

 

But again... CAPE race.

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