SPC AC 101250
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0750 AM CDT Sun May 10 2020
Valid 101300Z - 111200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS A PORTION
OF THE OHIO VALLEY...THE FLORIDA KEYS...AND THE SOUTHERN HIGH PLAINS
TODAY...
...SUMMARY...
A few strong to severe thunderstorms are possible across a portion
of the Ohio Valley, the Florida Keys, and the southern High Plains
today.
...Synopsis...
In mid/upper levels, a strong cyclone initially over the Gulf of St.
Lawrence is forecast to retrograde northwestward across Newfoundland
and Labrador through the period. Associated cyclonic flow will
cover much of the CONUS from the Plains eastward, including
northwesterlies in which a strong shortwave trough currently is
crossing the upper Mississippi Valley. This leading perturbation,
which is quite evident in its comma-shaped presentation in satellite
and composited radar imagery, should pivot across the Ohio Valley
into tonight. In the process, it gradually will phase with an
initially faster-moving, lower-amplitude trough evident in
moisture-channel imagery over ND. The combined trough should extend
from western NY to the southern Appalachians by 12Z tomorrow.
In the southern stream, a shortwave trough initially over the
northern Gulf of California and southern CA is embedded within
larger-scale ridging. This feature should move northeastward to
eastern NM by 12Z. Well downstream, a cluster of weak
perturbations/vorticity maxima over the central/eastern Gulf will
move eastward to east-northeastward over FL.
At the surface, 11Z analysis showed a surface low over southern WI
near MSN, with cold front arching across northern IL, southwestern
MO, western OK, and northeastern NM/southeastern CO. The front will
settle southward across OK and the southern High Plains today before
moving northward again into southeastern CO this evening, with weak
cyclogenesis possible in the latter area overnight. Meanwhile, the
WI low will move/redevelop eastward across Lower MI through the rest
of the morning, to near western Lake Erie around 00Z. By then the
cold front should extend from the low across central/eastern OH, and
eastern portions of KY/TN. By 12Z, the low should reach west-
central NY, with cold front over the Delmarva region and eastern
Carolinas.
...Ohio Valley States...
Once sufficient warm-sector heating takes place to effectively
remove MLCINH for frontally forced parcels, scattered surface-based
convection (showers and thunderstorms) should develop early this
afternoon over portions of IN, in at least a broken arc. This
activity should intensify and become denser in coverage as it
impinges upon continued diurnally destabilizing and somewhat
moistening areas of the warm sector, from near the surface low
southward, while crossing parts of OH/northern KY. The main concern
will be isolated severe (50+ kt) gusts, with widely scattered gust
coverage at subsevere levels still potentially causing minor damage.
Lack of stronger low-level theta-e and overall buoyancy will be a
limiting factor for what would be a substantially more intense event
in this pattern, were there seasonally typical boundary-layer
thermodynamic conditions. Still, strong large-scale lift/cooling
aloft should extend atypically cold, -25 to -27 deg C 500-mb
temperatures over the frontal/convergence zone. This will
contribute to steep low/middle-level lapse rates, even for surface
temperatures only in the 50s F. That, and dew points generally in
the upper 30s and 40s F, yield 100-400 J/kg MLCAPE in forecast
soundings (locally/briefly near 500 J/kg). The inflow layer may
exhibit inverted-v thermodynamic profiles, encouraging strong/
locally severe gusts. With time and eastward extent into higher
terrain of WV and western PA, low-level lapse rates and overall
buoyancy will diminish, though a marginal wind threat may persist
that far east before activity weakens substantially.
...Southern High Plains...
Isolated to widely scattered thunderstorms should form this
afternoon over higher terrain of northeastern NM from near RTN
southwestward to northeast of LVS, as well as in a zone of low-level
convergence, deep mixing and weak MLCINH in southeastern NM.
Isolated severe hail and gusts are possible in the relatively short
(2-3 hour) window of favorable buoyancy for the southern part of the
outlook area. Early hail and a more-prolonged wind threat may occur
in a conditionally longer-lasting scenario involving clustering and
cold-pool development across the area between RTN-TCC.
Already-modest low-level moisture generally will decrease with
northward extent while winds aloft and deep shear increase.
Substantial directional shear will exist area-wide. However, the
corridor of favorable surface-based buoyancy will be very narrow in
east/west extent, with peak preconvective MLCAPE ranging from around
1000 J/kg over southeastern NM (amidst 25-35 kt effective-shear
magnitudes) to less than 500 J/kg north of I-40 (35-45-kt shear
vectors). Evening diabatic cooling will close the severe threat
window quickly over most of the area, though cold-pool forcing may
cause the potential for strong/isolated severe gusts to extend into
parts of the Panhandle/South Plains.
...FL Keys...
Scattered thunderstorms in episodic clusters will occur near a
stalled/residual frontal zone that will persist across the area
through today. As the Gulf perturbations approach, and pass just to
the north, short-lived/small-scale shots of DCVA/large-scale lift
will contribute to modest midlevel destabilization and support for
thunderstorm development in the frontal zone. The boundary layer
will be very moist -- with surface dew points in the 70s F -- but
with weak lapse rates and abundant clouds/precip. Still, effective-
shear magnitudes may reach 40-50 kt this afternoon, aiding storm
organization where pockets of sufficient buoyancy exist for storm
maintenance. Strong to marginally severe, water-loaded downdraft
gusts will be the main concern, especially over the Straits where
MLCAPE will be larger, but perhaps into the Keys.
..Edwards/Goss.. 05/10/2020
CLICK TO GET WUUS01 PTSDY1 PRODUCT
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z
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