SPC Apr 21, 2017 0100 UTC Day 1 Convective Outlook

SPC 0100Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0755 PM CDT Thu Apr 20 2017

Valid 210100Z - 211200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE EASTERN TX PANHANDLE AND WESTERN/CENTRAL OK...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE SOUTHERN PLAINS INTO THE OZARKS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE LOWER GREAT LAKES AND OH VALLEY...

...SUMMARY...
Isolated severe storms capable of producing mainly large hail will
be possible across parts of the southern Plains tonight.

...Lower Great Lakes and OH Valley...
A shortwave trough evident on water vapor satellite imagery across
the Great Lakes and southern Ontario will continue eastward towards
New England and Quebec overnight. Related surface low and cold front
will likewise develop east-southeastward across western NY/PA and
the OH Valley through the end of the period. Several clusters of
strong to locally severe thunderstorms persist across this region
early this evening ahead of the cold front. Weak instability, with
MLCAPE generally 250 J/kg or less, characterizes the warm sector
immediately ahead of the front. The loss of daytime heating will
result in further reduction of instability into the late evening and
will likely foster an overall weakening trend with ongoing
convection. While hail/gusty winds could still occur for the next
several hours with the strongest cores given 35-40 kt of effective
bulk shear, the increasingly isolated and marginal nature of the
severe threat warrants a reduction in probabilities.

...Southern Plains...
As a southerly/southwesterly LLJ strengthens this evening and
overnight across parts of TX/OK, numerous thunderstorms will likely
develop to the north of a slow-moving cold front draped from
northeast to southwest across southern OK into west TX this evening.
This front is not expected to make much additional southward
progress overnight, and steep mid-level lapse rates (7.2 C/km in the
700-500 mb layer) present on the 00Z Amarillo TX and Norman OK
soundings will support MUCAPE of 1500+ J/kg at or above 850 mb.
Effective bulk shear values of 40-50 kt and strong mid-level flow
associated with an approaching shortwave trough from the Great
Basin/Southwest will encourage supercell structures with initial
elevated thunderstorm development, most likely over parts of the
eastern TX Panhandle, north TX, and western OK this evening.
Additional convection may form later this evening/overnight across
parts of northeastern NM and the western TX/OK Panhandles with
upslope low-level flow and the arrival of large-scale ascent
associated with the previously mentioned shortwave trough. Large
hail will be the main severe threat across both regions. Eventual
clustering of convection casts some doubt on a continued severe hail
threat through the overnight hours and into early Friday morning
with eastward extent into central/eastern OK and western AR, but
embedded supercells may still be capable of isolated large hail
through the end of the period across these areas.

..Gleason.. 04/21/2017

Read more

from SPC Forecast Products http://ift.tt/rxAC6Z