SPC AC 201728
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1228 PM CDT Sat May 20 2017
Valid 211200Z - 221200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
THE RIO GRANDE VALLEY IN TEXAS...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM
SOUTHWESTERN TEXAS TO THE CENTRAL GULF COAST...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE SOUTHERN HIGH PLAINS...
...SUMMARY...
A few strong/severe storms will be possible from southwest Texas to
the central Gulf Coast, with the highest likelihood near the Rio
Grande Valley. Elsewhere, a few strong storms are possible across
the southern high Plains and perhaps parts of the Ohio Valley and
lower Great Lakes.
...Synopsis...
Broad, mid-level cyclonic flow will encompass much of the central
U.S. Sunday, as a closed low gradually weakens and lifts
north/northeast into western Ontario. Meanwhile, along the
southeastern periphery of the mid-level trough, a series of subtle
impulses will translate east/northeast across the Ohio Valley,
Appalachians, and Southeast. Farther west, a ribbon of higher
vorticity within the southern stream will approach the Rio Grande
Valley.
...Portions of southwest Texas to the central Gulf Coast...
A stationary front, modulated by overnight convection and related
cold pools, will stretch from near the Texas Big Bend eastward to
the lower Mississippi Valley. Pockets of storms will likely be
ongoing along/ahead of this front Sunday morning, and the location
of later-day convection will be sensitive to outflow
boundaries/convective overturning from these initial storms.
Nonetheless, within pockets of heating, dew points in the upper
60s/lower 70s will support moderate buoyancy within a modestly
sheared environment. In turn, a few instances of marginally severe
hail and/or strong wind gusts will be possible.
The highest likelihood of severe weather likely exists across parts
of southern/western Texas near the Rio Grande Valley, where stronger
mid/upper flow (associated with the sub-tropical jet) will be
present. Here, effective shear upwards of 45-50 kt will be favorable
for updraft organization/rotation in any semi-discrete cells. These
storms would be capable of large hail and damaging winds, with the
threat transitioning to primarily wind if cells grow upscale. One
caveat with this severe potential is the possibility that early-day
convective initiation across Mexico will reduce destabilization
considerably over the region, resulting in more of a Marginal Risk
of severe storms. However, the potential exists that sufficient
buoyancy develops through the day to support a higher threat, and
the Slight Risk is maintained to account for this scenario.
...Portions of the southern high Plains...
Within a regime of broadly cyclonic flow aloft, modest forcing for
ascent will support isolated/widely scattered convection across
parts of eastern New Mexico, southeastern Colorado, the
Texas/Oklahoma Panhandles, and far southwestern Kansas. Sufficient
effective shear and modest low-level moisture may favor a few
stronger storms capable of isolated large hail and strong wind gusts
during the late afternoon and evening.
...Portions of the middle Ohio Valley...
Along/ahead of a cold front pushing east, pockets of stronger
heating may support weak surface-based buoyancy by afternoon. While
stronger mid-level flow will exist farther northwest, modest deep
shear may be adequate for a few stronger storms across eastern
Indiana, Ohio, and parts of northern Kentucky during the day.
However, the marginal nature of both the forecast instability and
deep-layer wind fields suggest the potential for severe weather
remains too low for probabilities at this time. A Marginal Risk
could be introduced in later outlooks though.
..Picca.. 05/20/2017
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