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Day 2 Outlook Categorical Probabilistic
Categorical Day2 0700Z Outlook
  

Images courtesy of the NWS Storm Prediction Center
 Forecast Discussion

  
000
ACUS02 KWNS 210500
SWODY2
SPC AC 210459

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1159 PM CDT Mon Sep 20 2021

Valid 221200Z - 231200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS WEDNESDAY
AFTERNOON THROUGH WEDNESDAY NIGHT ACROSS PARTS OF EASTERN 
OHIO...FAR EASTERN KENTUCKY AND TENNESSEE...WESTERN NORTH CAROLINA
AND VIRGINIA...WEST VIRGINIA...WESTERN MARYLAND AND PENNSYLVANIA...

...SUMMARY...
Strong thunderstorms may impact parts of the Cumberland into
Allegheny Plateau/upper Ohio Valley vicinity Wednesday afternoon and
evening.  Some of this activity might pose a risk for tornadoes and
damaging wind gusts, and a risk for strong wind gusts may persist
with scattered lingering storms spreading across and east of the
Allegheny Mountains overnight.

...Discussion...
Output of the various models continues to indicate that the more
progressive belt of westerlies, emanating from the mid-latitude
Pacific, will begin to undergo amplification across the northern
mid-latitude eastern Pacific into western North America during this
period.  It appears that this will include large-scale mid-level
ridging building inland across the British Columbia and Pacific
Northwest coast and downstream troughing digging to the lee of the
Canadian Rockies, accompanied by cyclogenesis across the Canadian
Prairies.

In somewhat lower latitudes, guidance indicates that a blocked
regime will be maintained across much of the mid-latitude Atlantic,
and continue to impact flow upstream, across much of the United
States.  Perhaps most prominently, it appears that elongated
large-scale mid-level troughing, forecast to develop across the
middle and lower Mississippi Valley vicinity by 12Z Wednesday, will
only slowly shift eastward through Wednesday night.  It may broaden
considerably, with models generally indicating that one significant
short wave perturbation will contribute to a deepening embedded
lower/mid tropospheric cyclone.  However, spread among the various
models has been large, with this development taking place anywhere
from the lower Ohio Valley into the lower Great Lakes region.

It appears that this may be accompanied by modest surface
cyclogenesis along a slowing cold front.  Of primary concern is the
potential for this to be accompanied by enlarging, clockwise-curved
low-level hodographs in the warm sector east of the low, where a
seasonably moist boundary layer may include surface dew points in
the mid 60s to near 70F.  Although lower/mid tropospheric lapse
rates may not become particularly steep (with the mid-level cold
core lagging to the west/southwest), the environment might still
become conducive to organized convective development, including
supercells, with the potential to produce a couple of tornadoes and
damaging wind gusts.

This threat seems most likely to become focused in relatively close
proximity to the low, during a period of more substantive deepening
late Wednesday afternoon and evening.  Based on model output
favoring a slower progression of the mid-level low (given the
downstream pattern), including the 20/12Z ECENS/ECMWF and similar
20/12Z UKMET, it appears most probable that this will become
centered across parts of southeastern Ohio, southwestern
Pennsylvania and northwestern West Virginia.  However, some risk for
potentially damaging surface gusts may continue with lingering
convection developing along/ahead of the surface cold front as it
advances into, across and east of the Allegheny Mountains overnight.

..Kerr.. 09/21/2021

$$