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2021 Mid-Atlantic Severe Weather - General Discussion


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Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1226 AM CDT Thu Jul 29 2021

   Valid 291200Z - 301200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER A PORTION
   OF THE MIDDLE ATLANTIC REGION...

   ...SUMMARY...
   Thunderstorms associated with wind damage and a risk for a tornado,
   will be possible from the Ohio Valley, central Appalachians and
   Mid-Atlantic states on Thursday.

   ...Northeast through Middle Atlantic area...

   Upper pattern in this region will be characterized by a broad trough
   today. MCV/shortwave trough embedded within the synoptic trough will
   move from the lower Great Lakes to the Middle Atlantic. At the
   surface a warm front will move northward through the Middle
   Atlantic, while a cold front advances southeast, reaching PA and the
   central Appalachians by evening. Thunderstorms should be in progress
   at the start of the period in association with the MCV over the
   southern portion of the Great Lakes. Other storms may be ongoing in
   the warm-advection regime north of the warm front over a portion of
   the Middle Atlantic. Widespread multi-layer clouds may precede the
   warm front. However, the warm sector should become moderately
   unstable during the afternoon with 1000-2000 J/kg MLCAPE supported
   by the advection of richer low-level moisture and pockets of
   diabatic heating. Thunderstorms are expected to develop ahead of the
   cold front in association with ascent attending the southeast-moving
   MCV as well as along the southeast-advancing cold front. This area
   will reside within belt of stronger mid-upper flow associated with
   the synoptic trough supporting 40-45 kt effective bulk shear.
   Therefore, some supercells may evolve along with line segments with
   bowing structures as activity develops southeast during the
   afternoon into the evening. Primary threat appears to be damaging
   wind and possibly some hail, though low-level hodographs will be
   sufficient for a few tornadoes, especially if sufficient instability
   can develop in vicinity of the warm front.  
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3 hours ago, yoda said:

So about the 00z NAM soundings across the region from 21z to 03z...

00z NAM sounding for 00z tonight at KIAD = :yikes:

Yes, I know the usual caveats... but you don't see those everyday around here

I know there's a lot of CAPE, but I honestly have no idea how to read what's in the rest of the soundings...

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One thing to remembers, the CAMS yesterday were way off on the placement and severity of the line in Wisconsin. The activity developed further SW than progged so we'll have to watch how thing play out in OH, WV, and western PA around midday. Still, this is looking like a solid SLGT/ENH day. Good shear and a strong fropa in the middle of summer, not something you see every day.

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This was the early morning disco from LWX about today FWIW

NEAR TERM /THROUGH TONIGHT/...
Latest analysis depicts a weak stationary front straddled
northwest to southeast across the area. A stronger front and low
pressure is located over the Great Lakes, with the low heading
east and the front progressing southeast. Aloft, a weak
shortwave is passing to the east with a second stronger
shortwave over the Great Lakes. This is all embedded within a
dominant northwest flow as a longwave trough remains to our
northeast, and a ridge sits over the central Plains.

Showers early this morning associated with the departing
shortwave should dissipate shortly. The clouds and increased
moisture over the region have kept temps in the higher 60s to
lower 70s. This relatively high starting point will give us a
boost, but clouds moving in from the MCS to our northwest will
start overspreading the area this morning. This MCS likely will
greatly affect our severe weather threat today as the strong
shortwave and cold front moves into the region. If the MCS
impact is minimal, with cloud cover relatively thin and brief
and minimal if any showers, then our odds of a significant
severe weather outbreak will be quite high thanks to ample CAPE
combined with the high shear moving overhead. However, if the
MCS holds together more and we have extended cloud cover and
more showers/weakening storms, the high CAPE component will be
harder to come by, which could potentially significantly damper
the severe weather potential. Will need to watch progress of the
system closely. Right now, areas least likely to be affected by
the morning MCS appear to be the Baltimore area and points north
and east, where an enhanced risk of severe weather is now in
place from the SPC. This risk tapers to slight and then marginal
further south and west, where the MCS influence is likely to be
more substantial. Highs likely reach the mid-upper 80s, perhaps
with spotty 90s, with higher temps of course more likely to fuel
stronger storms later today. Best timing for storms looks to be
very late afternoon and evening across the metro, assuming the
MCS doesn`t disrupt CAPE too much.
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23 minutes ago, Eskimo Joe said:

DY1 ENH...and it doesn't looks like *too* much cloudiness this morning. LFG!!!

really? pretty cloudy/foggy up my way. we did get some early morning rain so that could be why

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Check out the morning visible satellite loop on weather.cod.edu. You can see the low level flow is S - SW, while it's NW - N aloft. Going to be an interesting day for some, definitely would rather be from Baltimore SW into Fredericksburg/ Winchester for this.

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3 minutes ago, WxUSAF said:

In OCMD this morning so I guess I’ll miss my house getting slabbed this evening :(

didnt you say that would happen? go on vacation and we get severe weather? (i could be confusing you with someone else)

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10 minutes ago, mappy said:

really? pretty cloudy/foggy up my way. we did get some early morning rain so that could be why

I'm in New Windsor today. I woke up to fog, but it's since burned off and we have nothing but blue overhead. I'm not worried about the low level stuff along I-95 today, it's the mid/high level leftover crapvection that's near Pittsburgh. If that can die off then we're golden.

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Yeah, I flew out around 6pm that day.  Must have just missed it.  We had an actual tornado?

Around Washington-Liberty, yeah it was. Touched down near the Chipotle off 29. Anyways, seems like you might’ve had a decent microburst near you. Just a guess from these photos, but Nottingham took a beating.

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Looking at the sounding here on the 3km NAM, that dry layer and the respectable low level lapse rates would seem to signal a damaging wind threat. DCAPE gets up over 900 for a time. Also some low level directional shear, thus a slight chance of a tornado. The most impressive parameter with this threat are the increasing wind speeds with height. The CAPE looks a tad skinny, but decent. My non-expert observations. Also predicting a fail for my yard.

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The FV3, 3k NAM, RGEM and even the ICON are focusing much of the convection in extreme NE MD, N DE and SE PA with what looks to be the track of the MCV. Other storms fire to the south and southwest of that low, but more scattered.

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13 minutes ago, CAPE said:

The FV3, 3k NAM, RGEM and even the ICON are focusing much of the convection in extreme NE MD, N DE and SE PA with what looks to be the track of the MCV. Other storms fire to the south and southwest of that low, but more scattered.

MCS events have a tendency to be progged by short term guidance too far NE of where they occur. Last night was a perfect example of this. The 00z NAM had convection going over the UP of Michigan and Green Bay, yet the initiation and main body of the storm was well to the west.

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4 minutes ago, Eskimo Joe said:

MCS events have a tendency to be progged by short term guidance too far NE of where they occur. Last night was a perfect example of this. The 00z NAM had convection going over the UP of Michigan and Green Bay, yet the initiation and main body of the storm was well to the west.

I’d feel better if any run of any CAM was showing something interesting for the DC area.  Haven’t found one yet.

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