Significant Severe Weather in VA/NC on Thursday?

All,

An upper level ridge strengthening over the plains (and extending north into Canada) on Tue/Wed will result in strengthening NW flow aloft downstream over the TN/OH valley and Mid-Atlantic on Wed, and the advection of a modified elevated mixed layer into the region, with H7-H5 lapse rates on the order of 7-8 c/km across NC.  A robust shortwave in NW flow aloft is expected to approach the mountains 12-15Z Thursday, crossing NC/VA Thursday afternoon/evening, with an attendant sfc low deepening to 997-999 mb as it tracks east through VA and the DELMARVA. In advance of the shortwave, strengthening southerly low-level flow will advect rich boundary layer moisture northward under the modified EML, resulting in strong instability by peak heating Thu, with MLCAPE values of 2000-3000 J/kg across central NC.  Additionally, 0-6 km shear is progged to be on the order of 35-45 knots, which would be sufficient for supercellular organization, especially given steep mid-level lapse rates and strong instability.  It appears that the stage is set for an enhanced severe weather event Thu aft/eve, though specifics remain difficult to ascertain given that this is still 72+ hrs out and that there may be ongoing convection that ‘outruns’ the forcing over the TN/OH valley late wed/wed night, potentially crossing the Appalachians Thu morning. Whether or not convection would survive across the mountains is difficult to say, steep mid-level lapse rates will contribute to 1000-2000 J/kg of MUCAPE wed night over central NC, though CIN will rapidly increase within several hours after sunset.  At any rate, it seems like the primary severe weather threat in central NC on Thu would be very large hail and damaging winds, perhaps significant wind given DCAPE values progged as high as 1500 J/kg.  The tornado threat is a bit more difficult to pin down, though it would appear that the best tornado threat would be in VA and the DELMARVA in closer vicinity to the surface low and warm front where the best low-level shear would be present, with increasingly straight hodographs further south in NC.  Additionally, any discrete mode in NC would likely be short lived given strong instability, little or no CIN during peak heating, and high DCAPE that would foster strong cold pools and relatively quick upscale growth into an MCS, though that could result in a significant damaging wind threat, especially in eastern portions of NC/VA.

I see SPC mentioned us in the Day 4-8 outlook, though they didn’t add a 30% prob.  Anyone else have thoughts on the upcoming severe potential on Thu?

-Brandon V.

H7-H5 Lapse Rates Mon-Thu via 00Z (or 06Z) 06/10/2013 GFS

H7-H5 Lapse Rates Mon-Thu via 00Z (or 06Z) 06/10/2013 GFS

00Z (or 06Z) 06/10/2013 GFS/NAM Mon-Thu

00Z (or 06Z) 06/10/2013 GFS/NAM Mon-Thu

 

00Z GFS sounding 06/10/2013 valid 18Z Thu 06/13/2013

00Z GFS sounding 06/10/2013 valid 18Z Thu 06/13/2013

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About bvincentnws

Meteorologist National Weather Service 2003 - Current
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2 Responses to Significant Severe Weather in VA/NC on Thursday?

  1. ryanelliswforah says:

    Nice write up Brandon! I haven’t seen any soundings other than the one you presented so I’m not sure if the soundings change with time but are you at all concerned with how dry the sounding is in the hail growth zone? Certainly there is plenty of fat cape there and shear of 35-45 knots would certainly be enough to maintain up drafts for longer periods of time I just wonder how much moisture will be available there to grow the hail…particularly because the levels below the growth zone are also very dry so vertical moisture transport could be limited as well. Thoughts? I work Thursday evening and its my first one back after three days off so I will be keeping track of how this threat evolves. Thanks for the post!

    • bvincentnws says:

      Thanks for the feedback, Ryan! That’s an excellent question. It’s hard to tell exactly how dry or moist it is just by looking at the GFS sounding I posted, but, it’s actually a moist sounding, the PWAT on that particular sounding is 1.50″, which is climatologically near (maybe slightly above?) normal in central NC for this time of year per the radiosonde-based PWAT climatology. That sounding has a surface dewpoint of 68F (20C), a 925 mb dewpoint of 63F (17C), and an 850 mb dewpoint of 57F (14C), which is pretty good for mid June, especially in the presence of good synoptic forcing (respectable height falls/dpva) and favorable diurnal timing. Additionally, the modified EML (characterized by 7.0-7.5 c/km mid-level lapse rates) expected to be in place is moist in comparison to an unmodified elevated mixed layer, i.e. a typical EML in the central plains where mid-level lapse rates are on the order of 8-9 c/km. I would be much more concerned about capping or dry air entrainment if we had an unmodified elevated mixed layer and little or no synoptic forcing.

      Also, the dry air that is present in the modified EML (and in the boundary layer) significantly helps with regard to DCAPE, promoting more vigorous downdrafts and stronger cold pools, which, in turn, can help force additional convection and foster upscale growth into a larger, more organized cold pool /MCS/ with considerable damaging wind potential. A good example of such a scenario was on July 1, 2012 (during a heat wave and a few days after the June 29 derecho) when isolated convection in central NC grew upscale into am impressive ‘midwest-style’ MCS that plowed into eastern NC with 70+ knot outbound velocities via KRAX in the lowest 3-7 KFT AGL.

      There are a lot of questions still with this upcoming event, the timing and amplitude of the shortwave could change, the modified EML could be stronger/weaker, there could be convection/outflow progressing east of the mountains by mid/late morning, etc. So, it’s certainly not a sure bet that we’ll see any significant severe weather. In the end, about all I can say is that I have pattern recognition goosebumps 🙂

      I’m not sure how much moisture needs to be present for large hail to develop, but, LP supercells form in relatively dry environments in the high plains, they’re typically high-based, and they’re notorious for producing large hail (though little rain).

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