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5/20 MCV Severe Threat


cbelke
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Ohio storms could threaten eastern PA later today. Models not 100% sure with exact placement as it traverses east toward us today. Lehigh Valley, Poconos, NNJ in crosshairs perhaps?

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NWS PHI:

Late this morning/ early this afternoon the warm front will pull
northward in response to the MCV to about/ or just north of I-195 in
NJ. The MCV will then turn more easterly and track along the warm
front. In the warm sector, destabilization is forecast to occur with
MLCAPE values of 1500 to 2500 J/kg. The main possible failure mode
for convection this afternoon will be the rapidly warming low level
thermal profiles. The latest forecast soundings off of the NAM and
GFS all show some version of capping in place across the area. The
overall strength of the CAP looks like it can be overcome given the
strength of the forcing for ascent. The most likely scenario at this
point appears to be convection maintaining itself across central PA
early this afternoon and quickly going linear as anvil-level storm-
relative flow vectors point southwest. The convection is then
forecast to turn more easterly, or into the instability and down the
warm front. Bulk shear vectors with the convection are perpendicular
to the orientation of the convection at about 40 kts which makes
damaging winds the primary threat. An isolated tornado embedded in
the line of convection is also possible given the proximity of the
warm front. The convection looks like it will approach the Lehigh
Valley around 2 - 4 PM and quickly head east towards the NJ coast.
As the convection heads east it will encounter an increasingly
capped environment and begin to decay. The main question here is how
quickly will the convection weaken. In these type of setups,
convection usually weakens but doesn`t immediately collapse. Towards
MD and DE, mostly dry weather is expected as the forcing from the
MCV will be just to the north. Expect high temperatures mostly in
the mid 80s today.
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sneaky severe threat today especially whoever gets the MCV bow echo.  shear is low probably due to weak upper air flow 500 and above, but that may be offset pretty strong winds near 700 which the storms might be able to tap into.  Surface flow is weak but completely backed 180 to the upper level flow above it, look at the surface wind barb below.  Hodograph as a result is curved.

Some capping in place but it appears weak here.  I'd think if a MCV bow echo is bearing down, it will overcome weak capping with ease until getting closer to coast maybe. 

Hi res guidance except HRRR keeps MCV up north towards Poconos/Lehigh Valley.  HRRR blasts it through SE PA after turning SE from central PA.  Upper air flow is more west to east, so it must be something on a mesoscale level that turns it SE against the flow a bit.  not sure...

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MD 838 graphic

 Probability of Watch Issuance...80 percent

   SUMMARY...The severe threat is increasing across portions of central
   PA into MD. Damaging gusts and perhaps a few tornadoes may accompany
   the strongest, longer lived storms. A WW issuance will be needed
   within the next couple of hours.

   DISCUSSION...A remnant mid-level MCV (evident via water vapor
   imagery and 700-500 mb objective analysis fields) is currently
   located just west of the OH/PA border and continues to track
   eastward towards central PA, and is expected to support more robust
   convective initiation in the next few hours. Immediately ahead of
   the MCV center lies a highly sheared low-level environment, with
   recent PBZ VAD data showing relatively long, curved low-level
   hodographs and over 300 m2/s2 of associated 0-1km SRH. At the
   moment, buoyancy remains scant across portions of western into
   central PA. However, modest breaks in the clouds are contributing to
   modest surface heating, with mid 60s F surface temperatures already
   noted across central PA, with low 70s F along the PA/MD border. With
   surface dewpoints around 60F across several locales, RAP forecast
   soundings suggest temperatures need to warm into the lower 70s for
   appreciable buoyancy to develop given the presence of relatively
   poor (5.5-6.5 C/km) low and mid-level lapse rates.

   While buoyancy is expected to initially be marginal (i.e. at or
   below 1000 J/kg MLCAPE), localized deep-layer ascent and strong
   low-level shear associated with the approaching MCV should support
   at least a few strong to severe storms developing early this
   afternoon. Damaging gusts and perhaps a few tornadoes will be the
   primary concerns, though a brief instance or two of marginally
   severe hail cannot be completely ruled out with the longer-lived,
   persistent rotating updrafts. A WW issuance will be needed within
   the next couple of hours to address the severe threat.
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47 minutes ago, susqushawn said:

storms firing over northern WV, farther south than all 12z hi res guidance 

COD-GOES-East-subregional-Mid_Atlantic.radar.20220520.143500.gif-over=map-bars=.gif

Yeah, you knew the models would struggle with placement. Seemed that way earlier (in Ohio) as well. 

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6 minutes ago, mweav067 said:

It seems odd to me to have a Tornado Watch up but without a Severe Thunderstorm Watch coupled with it.

I'm guessing the criteria, lead time, etc. are different between them?

I've never seen both simultaneously issued, I suspect that the Tornado Watch inherently assumes that severe thunderstorms are possible and when the setup is conducive for tornadoes (or is it tornados?) a tornado watch is specified vs svr thunderstorm.  Here's a NWS definition: 

Tornado Watch

A Tornado Watch is issued when severe thunderstorms and tornadoes are possible in and near the watch area. It does not mean that they will occur. It only means they are possible.

Severe thunderstorms are defined as follows:

1) Winds of 58 mph or higher

AND/OR

2) Hail 1 inch in diameter or larger.

 

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2 observations from my end:

  1. Much of the NWS discussion today had predicted the line to eventually stop moving NE and eventually move more eastward. Considering it is already centered around the PA/MD border, I think Philly and south Jersey may be in position to get the brunt of this if it holds together.
  2. The HRRR has not initialized anywhere near that level of intensity over its last few runs, so I think we may have to be in nowcasting mode for the rest of the day.

COD-GOES-East-subregional-Mid_Atlantic.radar.20220520.160500.gif-over=map-bars=.gif

hrrr_ref_frzn_neus_1.png

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On a separate note, if this end up producing SVR warnings all the way into NJ, wouldn't that meet the criteria for a derecho from a distance perspective?

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day1otlk_1630.gif

...Mid Atlantic Region...
   A well-defined MCV is noted on satellite imagery over southwest PA. 
   This feature will track eastward today across PA, northern MD, and
   into NJ, fostering the development of severe thunderstorms. 
   Destabilization ahead of the MCV, combined with strong low and
   mid-level wind fields near the circulation will promote the
   development of supercell and bowing structures capable of damaging
   winds, large hail, and a few tornadoes.  Given the rapid eastward
   motion of the activity, it should move off the NJ coast well before
   dark, ending the severe threat.
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2 minutes ago, susqushawn said:

who knows how to zoom the SPC map into our region?  

Take your 2 fingers and start with pull a reverse “pinching” motion on your screen 🫡 

 

jk jk , I know there’s a way to use google overlay somehow. 

 

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7 minutes ago, susqushawn said:

day1otlk_1630.gif

...Mid Atlantic Region...
   A well-defined MCV is noted on satellite imagery over southwest PA. 
   This feature will track eastward today across PA, northern MD, and
   into NJ, fostering the development of severe thunderstorms. 
   Destabilization ahead of the MCV, combined with strong low and
   mid-level wind fields near the circulation will promote the
   development of supercell and bowing structures capable of damaging
   winds, large hail, and a few tornadoes.  Given the rapid eastward
   motion of the activity, it should move off the NJ coast well before
   dark, ending the severe threat.

Wow

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