LIVE MAP (ABOVE) ... SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1142 PM CST Mon Feb 20 2023
Valid 211200Z - 221200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
ACROSS PARTS OF THE NORTHERN MID ATLANTIC AND LATE TONIGHT ACROSS
PARTS THE SOUTHERN GREAT PLAINS...
...SUMMARY...
A few strong thunderstorms with gusty winds and small hail are
possible across parts of the northern Mid Atlantic this afternoon,
mainly across eastern Pennsylvania into New Jersey. Additional
strong thunderstorms posing at least some risk for severe weather
are possible across northwest Texas and southwestern Oklahoma by
early Wednesday.
...Synopsis...
Blocking will remain prominent within the mid/upper flow across the
mid-latitude eastern Pacific, with the northern branch of the
evolving split flow becoming increasingly amplified across much of
the Pacific into western North America during this period. This
will include building mid-level ridging to the north of the high,
and the evolution of a large deepening trough near and inland the
U.S. Pacific coast, as an intense north-northwesterly jet digs
offshore.
Farther downstream, strong mid-level height rises are forecast
across much of the eastern U.S. by early this evening, as
large-scale ridging builds within the westerlies, to the north of
persistent subtropical ridging centered over the Caribbean. Before
the ridge builds into the Northeast, mid-level troughing is forecast
to rapidly shift east-southeast of the Great Lakes/Upper Ohio Valley
and offshore of the northern Atlantic Seaboard.
In higher latitudes, a deep mid-level low of Arctic origins,
initially digging across Hudson Bay, likely will begin to turn
eastward toward northern Quebec. However, associated cold surface
air, which has worked its way toward the Canadian Rockies, is
forecast to surge across the international border through much of
the northern U.S. Great Plains and Upper Midwest, while substantive
lower/mid-tropospheric warming occurs across the southern Great
Plains, lower/middle Mississippi and Ohio Valleys. By 12Z
Wednesday, the primary strengthening low-level baroclinc zone will
extend across the central Great Plains into the Ohio Valley and Mid
Atlantic. However, in the wake of prior cold intrusions into the
Gulf of Mexico, and beneath warm and dry layers aloft, low-level
moistening to the south of the front will remain modest and strongly
capped across most areas.
...Northern Gulf States into Middle Mississippi/Ohio Valleys...
A rather large area with a 10 percent risk for thunderstorms remains
indicated. It is possible that a corridor of weak thunderstorm
development may linger beyond 12Z this morning across parts of the
northern Gulf Coast states/Tennessee Valley vicinity. Otherwise, as
mid-level inhibition increases with warming aloft, the risk for
thunderstorms may be rather negligible until early Wednesday, across
parts of the lower Ohio Valley, in response to elevated moisture
return and warm advection, to the north of the stronger capping.
....Northern Mid Atlantic...
In the wake of an initial frontal wave migrating offshore, models
indicate that boundary-layer warming and mixing with insolation
along and east of the Allegheny Mountains, coupled with modest
residual low-level moisture, may contribute to weak destabilization
ahead of an occluding surface low migrating east-northeast of the
lower Great Lakes. Beneath very cold mid-level air (including 500
mb temps at or below -30 C), it appears that the environment may
become conducive to scattered low-topped thunderstorms capable of
producing small hail/graupel. Some of this activity, mainly across
parts of eastern Pennsylvania into New Jersey, may also aid the
downward transfer of momentum associated with 40-50+ kt
west-southwesterly flow in the 850-500 mb layer, contributing to a
risk for damaging surface gusts during the mid to late afternoon
hours.
...Southern Great Plains...
Models indicate a narrow corridor of weak boundary-layer
destabilization (including CAPE to around 500 J/kg) across northwest
Texas into southwestern and central Oklahoma as early as the 06-12Z
time frame late tonight. This is forecast to occur near/just ahead
of a weak surface cold front accompanying the leading edge of
stronger mid-level cooling, associated with the perturbation
emerging from the subtropical eastern Pacific. In the presence of
intensifying south-southwesterly deep-layer mean flow (including 50+
kt in the lowest 6 km AGL), it appears that forcing for ascent may
become sufficient to overcome relatively warm, dry and well-mixed
air in the 850-700 mb layer, and support increasing thunderstorm
development. If this occurs, strong deep-layer shear may contribute
to an organizing line with potential to produce severe wind gusts.
..Kerr/Wendt.. 02/21/2023
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