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2017 Severe Thread


mappy

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37 minutes ago, yoda said:

12z NAM NEST continues to argue that we get rocked Friday evening into Saturday morning

PW county 0800z SAT

2017062212_NAMNST_044_38.55,-77.24_sever

That low LCL will help try to get a spinny down to the surface. It's what I would look for during high shear/low cape events in the southeast. Basically, the lower the cloud base is to the ground, the "easier" it is for a quick spin-up to happen. I went to a workshop at BMX several years ago and that's what they look for too. So... Yeah. Could be one or two spinnies around the region tomorrow afternoon (for us near the BR *IF* anything fires off the mountains) or as main line pushes through. 

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40 minutes ago, yoda said:

Why is that?  DPs in the mid 70s are certainly possible with the type of environment we would be in with PWATs over 2 inches

Why do I think it's contaminated, or why do I think the contamination is bad?

I think it's contaminated for several reasons: 1) saturated or nearly saturated profile through a deep layer; 2) high omega values (synoptic-scale motion typically comes close to falling within the +/- 10 microbar/sec lines); 3) when I go to that lat/lon and forecast hour in the 18Z NAM3k run, it lands in a blob of simulated reflectivity.

As for why the contamination is bad, it's simply that a contaminated sounding isn't representative of the broader environment. So one has to be beware of applying the usual conceptual models using contaminated parameter values.

It looks like we'll have some screaming 850's so the hodographs might be pretty long in the low levels, I just don't trust those forecast soundings verbatim.

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28 minutes ago, Kmlwx said:

Looks like moderate to heavy rain to me on the sim reflectivity. That does not look like severe or anything close to it. 

      agree.   not even sure we'll have thunder, as there really isn't much instability.  and a few (albeit outlier) solutions don't get everyone wet.

 

 

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1 hour ago, boviscopophobic said:

Why do I think it's contaminated, or why do I think the contamination is bad?

I think it's contaminated for several reasons: 1) saturated or nearly saturated profile through a deep layer; 2) high omega values (synoptic-scale motion typically comes close to falling within the +/- 10 microbar/sec lines); 3) when I go to that lat/lon and forecast hour in the 18Z NAM3k run, it lands in a blob of simulated reflectivity.

As for why the contamination is bad, it's simply that a contaminated sounding isn't representative of the broader environment. So one has to be beware of applying the usual conceptual models using contaminated parameter values.

It looks like we'll have some screaming 850's so the hodographs might be pretty long in the low levels, I just don't trust those forecast soundings verbatim.

 

       It's true that the 8z sounding posted by Yoda is "contaminated", in the sense that convection is underway at that site when that profile is extracted, so you've modified the low-level wind field and moistened the low levels.   In that sense, bovi (can I call u that for short?  :rolleyes:)  is correct that it's not representative of the broader, pre-convective environment we want to assess.    When I pick a sounding to assess convective potential, I try to grab one right before precip arrives at that location.

       All that said, there IS decent low-level shear and modest instability before the convection arrives, and others have correctly noted the low LCLs, so it warrants watching.    It's interesting that the NAM nest keeps us in the low 80's through Friday night which helps us squeak out some cape.    If we cool much more than that, it will probably be tough to get surface-based storms.    At this point, my curiosity level is raised, but I'm not yet in.

    

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I'm out for the late tonight event.   It's still *possible* that we get a spinup or two, I suppose, but the hi-res guidance just doesn't look very impressive.   I see no updraft helicity signatures in the HRRR, and the NAM nest has little precip in DC metro.   (The Hi-res windows at least have precip for DC and points north).   The best chance for SVR might be a couple of isolated cells during the late evening;  it's suggested by the Hi-Res Window ARW, although shear is marginal as the better wind fields haven't arrived.

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Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
300 PM EDT Fri Jun 23 2017

.SYNOPSIS...
Tropical air will envelop the area today. A cold front will
cross the area late tonight into Saturday. High pressure will
return Sunday and persist through early next week.
&&

.NEAR TERM /THROUGH TONIGHT/...
In spite of pockets of insolation, atmosphere within the forecast
area capped, as evidenced by the lack of agitated cumulus on
satpix and just showers on radar. The same cannot be said across
western Pennsylvania down the Ohio Valley, in vicinity of the
frontal zone. Storms in that region shear-driven, and that will
be our concern tonight.

Recent runs of HRRR/RAP both depict some enhancement of the
showers in Virginia. Given 1500-2000 J/kg SBCAPE, expect some
thunder to develop by mid-late afternoon. That area should be
scattered in nature and have a diurnal component to it.

By 21-22 UTC, the line of thunderstorms, and scattered storms
ahead of the line, will be approaching the Potomac Highlands.
With loss of daytime heating and residual inhibition, expect a
weakening trend in the strength of convection. But, the overall
coverage in the Highlands will be increasing. Precipitable
water above 2 inches suggests that heavy rain can be expected
within any thunderstorm.

Latest model guidance (12 UTC cycle) offers a quicker timing.
The southwesterly low level jet will be substantive (60-70 kt at
850 mb), so anticipate that some thunder can be expected tonight
even without daytime heating. As has been discussed the past
couple of days, the question remains whether these storms can
root in the boundary layer east of the Appalachians. If they
can, it will be busy, with low-topped rotating updrafts. While
heavy rain can be expected too, the system overall remains
progressive (quicker than previously modeled), which will
preclude greater issues.

Bottom line... have categorical PoPs crossing forecast area
between midnight and 6 am (likely beginning mid-evening), with
heavy rain possible. With dewpoints in the lower 70s, low
temperatures follow suit.
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10 minutes ago, yoda said:

Good amount of TWs coming out of PIT CWA

Severe Weather Statement
National Weather Service Pittsburgh PA
553 PM EDT FRI JUN 23 2017

PAC051-232200-
/O.CON.KPBZ.TO.W.0009.000000T0000Z-170623T2200Z/
Fayette PA-
553 PM EDT FRI JUN 23 2017

...A TORNADO WARNING REMAINS IN EFFECT UNTIL 600 PM EDT FOR CENTRAL
FAYETTE COUNTY...

At 552 PM EDT, a confirmed tornado was located near Uniontown, moving
east at 40 mph.

HAZARD...Damaging tornado.

SOURCE...Weather spotters confirmed tornado.

IMPACT...Flying debris will be dangerous to those caught without
         shelter. Mobile homes will be damaged or destroyed. Damage
         to roofs, windows, and vehicles will occur.  Tree damage is
         likely.

This tornadic thunderstorm will remain over mainly rural areas of
central Fayette County, including the following locations: Mill Run,
Dunbar, Ohiopyle, Chalkhill and Normalville.
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