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Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0752 AM CDT Thu Jul 21 2022

Valid 211300Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
NORTHEAST AND SOUTHEAST...TODAY THROUGH THIS EVENING...

...SUMMARY...
Scattered severe thunderstorms capable of producing mainly damaging
wind gusts appear possible over parts of the Northeast and
Southeast, today through this evening.

...Synopsis...
In mid/upper levels, the prevailing large-scale pattern will persist
with an anticyclone anchored over the AZ/NM region, and a mean
trough over the Eastern States, anchored by a co*plex/elongated
cyclone over northern ON and southern Hudson Bay.  However, the
northern-stream pattern will beco*e slightly more zonal through the
period, with..
1.  Height falls/dampening mean ridge over the northern Rockies and
vicinity, ahead of a strong/eastward-moving shortwave trough now
offshore of the Pacific Northwest, and
2.  Slight net height rises beginning late in the period over the
Midwest, Great Lakes and Ohio Valley, beco*ing more prominent into
day 2.  This will occur after a trough -- apparent in
moisture-channel imagery from James Bay to WV -- moves northeastward
over QC, Lake Ontario and NY today, while deamplifying somewhat.

At the surface, the 11Z analysis showed an occluded low over
northeastern ON, with occluded/cold front arching across western QC,
to near BUF, LEX and MEM, beco*ing quasistationary across southern
AR to a low over southern OK, then curving across northwest TX and
southeastern NM.  This boundary should move back northward across
OK, TX and NM through 00Z, while advancing to western New England,
central VA, northern AL, and northern MS.  A prefrontal surface
trough was analyzed over eastern NY and NJ, and should move little
before being overtaken by the front later today.  By 12Z the front
should be offshore from New England, then extend southwestward
across the Carolinas and northern GA, beco*ing quasistationary and
weakening across AL/MS/AR.  Another cold front, analyzed initially
from eastern Lake Superior across southern MN to western NE, should
move southward today, decelerate and then move northward as a warm
front over NE, IA and SD tonight. 

...Northeast...
Widely scattered to scattered thunderstorms are expected to develop
from midday through the afternoon along and especially ahead of the
front, moving east-northeastward to northeastward across the outlook
area.  Damaging to locally severe gusts and isolated large hail are
possible, with a marginal tornado risk as well.

Favorable moisture remains over and upstream from the outlook area
in low levels, with widespread mid 60s to low 70s F surface
dewpoints anticipated for storm initiation.  Despite patchy areas of
low- and middle-level clouds seen this morning in satellite imagery,
sufficient diurnal heating should occur in that moist boundary layer
to strip away already-small MLCINH values evident in modified RAOBs
and objective RAP analyses across this region.

The strongest large-scale lift should stay north of the
international border.  Still, with the heating underlying subtly
increasing ascent ahead of the ejecting shortwave trough, bulk
steepening of low/middle-level lapse rates will occur, as well as
strengthening of mid/upper-level flow.  The result should be a plume
of 1000-1500 J/kg MLCAPE, perhaps reaching near 2000 J/kg in
southern areas where moisture and diabatic heating will be greatest.
 Even without particularly large low-level hodographs, such an
environment supports supercell potential, as well as organized
multicells and perhaps short lines with bow/LEWP formations.

...Southeast...
Widely scattered to scattered thunderstorms should develop through
the period in a lengthy swath along/ahead of the front.  Damaging to
severe gusts will be the main concern, with only a marginally severe
and isolated hail threat.  Cyclonic flow aloft associated with the
larger/mean trough will still extend as far south as the southern
Appalachians through the period, with strong difluence across much
of the Southeast as well.  Farther west, the front, later outflows,
and differential-heating boundaries will be foci.

Over the Carolinas, GA and portions of AL, a prefrontal outflow
boundary will continue to be generated and propagate for at least a
few more hours from ongoing convection/precip over the TN/NC/GA
mountains.  This boundary should be the main focus for development
over the midday to midafternoon time frame, with resulting
convection potentially arranging into at least a loosely organized,
southward arc of mainly wind-producing thunderstorms.  Some westward
backbuilding into very well-heated/unstable air also is possible,
leading to some net southwestward turning.  Similar behavior may be
noted from any other convective clusters that can organize via
aggregated cold pools in the hotter, more deeply-mixed environment
of the Arklatex/east TX/LA region, though that threat appears more
conditional and uncertain closer to the mid/upper ridge, and in
weaker ambient deep-layer flow.  Rich low-level moisture and intense
diurnal heating will support 2500-3500 J/kg MLCAPE east of the
Arklatex, decreasing westward into both residual cloud cover from
morning activity, and a generally drier boundary layer.

...Central Plains to IA...
Widely scattered to scattered thunderstorms are possible in
multiple, possibly spatially overlapping episodes this afternoon and
tonight.  Isolated severe gusts are possible area-wide, with hail
over the eastern lobe.  This outlook area has been expanded to
better reflect the diversity of potential convective initiation/
propagation regimes from this afternoon through the overnight
period.  This afternoon, development may occur near the western limb
of the front, as well as the nearby part of the lee trough to its
south, over a strongly heated/mixed boundary layer supporting gust
potential from any sustained convection that can develop.  Forecast
soundings suggest 300-800 J/kg MLCAPE, but weak low/middle-level
winds/shear, with high-based, pulse/multicellular storms and poorly
organized clusters possible.

Farther east and from this evening into overnight, convection may
develop as moist advection occurs, with the southwesterly LLJ
impinging on the elevated frontal surface, leading to an LFC amidst
step midlevel lapse rates.  MUCAPE of 500-1500 J/kg is possible,
45-60 kt effective-shear magnitudes beneath the southwestern fringes
of the stronger northwesterlies aloft.  This will support hail
potential.  A few downdrafts may accelerate through a deep,
relatively dry low-level profile below the lifted moist layer, then
penetrate the shallow near-surface stable layer at or near severe
levels.

..Edwards/Mosier.. 07/21/2022