231 
ACUS01 KWNS 281252
SWODY1
SPC AC 281251

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0751 AM CDT Tue Sep 28 2021

Valid 281300Z - 291200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE MID-ATLANTIC REGION...

...SUMMARY...
Strong to severe gusts and hail are possible this afternoon and
evening across parts of the Mid-Atlantic region.

...Synopsis...
The mid/upper-level pattern over the CONUS will be progressive and
highly amplified this period, featuring mean troughs over the East
Coast and just inland from the Pacific Coast.  In between, ridging
will amplify as it shifts eastward from the Plains States to an axis
from the western Gulf of Mexico to Hudson Bay.  A shortwave trough
-- embedded in a broad field of cyclonic flow around the mean trough
-- was evident in moisture-channel imagery over the upper Ohio
Valley.  This perturbation will pivot southeastward to eastward
today across WV, southern PA, VA, MD, and the Delmarva Peninsula,
moving offshore between 18-00Z.

In the West, the synoptic trough will shift eastward across the
northern Rockies and over the Great Basin by 12Z tomorrow.  A
formerly cut-off low now over the San Juan Mountains has a broad but
mostly weak field of associated cyclonic flow, covering the Four
Corners States and central/southern High Plains.  The low will
devolve to an open-wave trough amidst larger-scale height falls
preceding the progressive Pacific system.  By 00Z, the opened trough
should extend from central CO across eastern NM and far west TX.  By
12Z, it should weaken further, with an axis near a CYS-LHX-MAF line.
By then, convectively generated/enhanced vorticity lobes may exist
in downstream southwest flow, across north and central TX.

The 11Z surface analysis showed a low over eastern NY, with cold
front southwestward over central parts of OH/IN/IL, becoming a warm
front over northeastern MO, central NE and western SD.  The eastern
part of the front will move southeastward across New England and the
Mid-Atlantic region through the period, reaching the Carolinas and
northern GA by 12Z.

...Mid-Atlantic region...
Widely scattered to scattered thunderstorms are expected to develop
near the mid/upper trough and along/ahead of the cold front,
offering occasional damaging to isolated severe gusts, and sporadic 
hail.

Favorable lapse rates aloft near the 500-mb height/thermal troughs
will overlap a diurnally heated boundary layer, containing 60s F
surface dew points, with PW around 1/25 to 1.5 inches. Nearly
unidirectional and not particularly strong low/middle-level wind
profiles will maintain modest shear and support assorted discrete to
clustered multicellular modes.  Forecast soundings and planar progs
reasonably indicate that similarly favorable/prefrontal diurnal
destabilization and moisture as farther south will extend into the
preconvective boundary layer across more of southeastern PA and NJ,
so the 15% area has been expanded north somewhat, while being
trimmed on the west end in central VA due to concerns over lack of
greater lift/convective coverage farther behind the mid/upper
trough.

...Southern Plains...
Widely scattered to scattered thunderstorms should develop this
afternoon in an arc across northwest/west-central/southwest TX in a
zone of relatively maximized low-level ascent near the dryline, and
diurnally weakened MLCINH.  Isolated severe hail and gusts are
possible from the early, relatively discrete stages (mainly
multicellular).  The afternoon/preconvective environment will be
characterized largely by 60s F surface dew points beneath favorable
low/middle-level lapse rates for MLCAPE around 1500-2500 J/kg,
locally/briefly near 3000 J/kg.  Though strong veering of flow with
height is expected, modest speeds should keep hodographs small and
effective-shear magnitudes generally under 35 kt.  Some potential
exists for upscale aggregation this evening into a loosely organized
MCS near the eastern Hill Country and/or I-35 corridor of central/
south-central TX.  Though low/middle-level winds and low-level lapse
rates each will be weak over the area by that time, the wind
probabilities have been shifted east somewhat over the area, to
account for potential of isolated marginally severe gusts related to
cold-pool-driven processes.

..Edwards/Jewell.. 09/28/2021

$$