196
ACUS01 KWNS 270600
SWODY1
SPC AC 270559

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1259 AM CDT Mon May 27 2024

Valid 271200Z - 281200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
SOUTHEAST STATES INTO THE MID-ATLANTIC AND LOWER GREAT LAKES...

...SUMMARY...
Severe thunderstorms are expected this afternoon and evening across
parts of the lower Great Lakes, Mid-Atlantic, and Southeast. More
isolated severe thunderstorms are possible across much of central
and eastern Texas, as well as southwest Oklahoma this afternoon and
overnight.

...Synopsis...
Strong to severe thunderstorm potential today and tonight will be
focused along a composite cold frontal/outflow boundary draped from
the Great Lakes region into the Mid-Atlantic/Southeast and westward
into the southern Plains. 05 UTC surface observations reveal a
mature surface low over the upper Great Lakes. A diffuse cold front
is noted extending from the mid-MS/lower OH River Valleys (where a
severe MCS is ongoing) into the southern Plains, with an additional
cold frontal surge analyzed across parts of the Midwest. These
features should migrate to the east/southeast over the next 12-24
hours as the synoptic surface low moves into the lower Great Lakes
region by late tonight, driven primarily by large scale ascent ahead
of a low-amplitude upper wave (observed over the mid-MS/lower OH
River Valleys at around 05 UTC).

Although only modest deepening of the surface low is anticipated
over the forecast period, mid to upper-level flow will likely remain
fairly strong (40-60 knots between 700 to 500 mb) ahead of the upper
wave. This will support moderate to strong deep-layer shear over
much of an expansive warm sector stretching from the Southern Plains
into the Southeast/Mid-Atlantic and northward to the lower Great
Lakes. Thunderstorms developing along the aforementioned cold front
and/or outflow boundary will likely see some degree of organization
with an attendant severe risk.

...Pennsylvania and western New York...
Northward moisture return is ongoing across PA and NY in the wake of
a decaying QLCS. Although lapse rates across the lower Great Lakes
into PA will remain fairly marginal, increasing low-level moisture
and diurnal heating should erode weak MLCIN by around 18 UTC.
Concurrently, a weak mid-level perturbation, most likely associated
with the remains of the ongoing MCS across the OH River Valley, will
overspread the region around early afternoon resulting in scattered
thunderstorm development. While buoyancy will be meager (500-1000
J/kg MLCAPE), 40-45 knot flow above the surface should mix down
within stronger downdrafts, supporting a damaging/severe wind
threat.

A second round of thunderstorms is possible during the late
afternoon/evening hours across western PA as a secondary cold
frontal surge pushes east. The intensity of this activity will
likely be dependent on the degree of overturning and air mass
recovery in the wake of the early afternoon thunderstorms. However,
a severe wind threat may materialize if sufficient recovery can
occur.

...Mid-Atlantic...
A moist air mass is currently in place across the Mid-Atlantic with
upper 60s/low 70s dewpoints observed from the Carolinas into MD. 6-7
K/km lapse rates are also noted in 00 UTC soundings, which should
support 2000-2500 MLCAPE by mid-afternoon across the region. The
approach of the upper wave should promote troughing in the lee of
the Appalachians. It remains unclear whether convection will
primarily be focused along the lee trough or along a residual
outflow boundary emanating from the ongoing MCS to the west.
Regardless, sufficient buoyancy and deep-layer shear will support
organized convection, including the potential for supercells with an
attendant risk for large hail, severe wind, and a few tornadoes if
sufficient low-level SRH along and ahead of the boundary can be
realized, as hinted by some model solutions.

...Southeast...
The ongoing MCS is forecast to gradually decay as it moves into
central/southern MS/AL/GA by 12 UTC, though embedded segments within
the line may pose a severe threat at the start of the forecast
period. Regardless, an outflow boundary should become apparent
somewhere across the Southeastern states by mid-morning, though
guidance varies on the exact placement. Weak capping is noted in 00
UTC soundings from across the region, suggesting that diurnal
heating, aided by weak ascent along the boundary, should foster
thunderstorm development by early afternoon. MLCAPE upwards of 2000
J/kg combined with around 30 knots of deep-layer shear should allow
for some initial storm organization, including the potential for a
few supercells. Ambient low-level vorticity along the boundary may
support a brief tornado threat with initial convection. However,
mean west/southwesterly winds should yield poor boundary-normal
deep-layer shear, which suggests storm interactions along the
boundary may promote upscale growth by late afternoon with an
increase in damaging to severe wind potential.

...Midwest...
A mid-level perturbation is noted in water-vapor imagery across
northern MT. This feature is forecast to reach the Midwest region by
peak heating this afternoon. Cool temperatures aloft over a somewhat
dry air mass should promote modest (500-1000 J/kg) MLCAPE.
Deep-layer wind shear is expected to be marginal, but lift ahead of
the vorticity maximum should promote isolated to scattered
thunderstorm development with the potential for strong to severe
winds - especially if more consolidated/organized clusters can
become established.

...Texas into southwest Oklahoma...
Surface observations show the cold front slowly moving south across
central TX, but this boundary should become increasingly diffuse
through the day amid strong diurnal heating across much of TX.
Thermally-induced surface pressure falls to the west of a dryline
(noted from west-central TX to the Big Bend region) will maintain
east/southeasterly low-level winds through the day. This will not
only maintain an influx of rich Gulf moisture (dewpoints in the
mid/upper 70s), but will support hodograph elongation given 40-50
knot westerly flow aloft. Forecast soundings suggest sufficient
heating will erode MLCIN by early afternoon, but weak confluence
along the dryline and neutral to rising heights aloft may limit
convective coverage. Consequently, confidence on storm coverage and
location is low with considerable spread noted in 00z CAM solutions.
However, this appears to be a low-probability, but potentially high
impact scenario with the potential for significant hail (2 inches in
diameter or larger), severe wind, and perhaps a tornado where storms
can develop. Elevated convection appears possible well into the
overnight hours across northern TX into southwest OK as isentropic
ascent atop the stalled frontal boundary increases with the onset of
the nocturnal low-level jet. This activity should primarily pose a
severe hail/wind threat that may persist into the early morning
hours Tuesday.

..Moore.. 05/27/2024

$$