SPC AC 201735
Day 2 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
1235 PM CDT Fri Jul 20 2018
Valid 211200Z - 221200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
THE SOUTHEAST...AND ALSO NORTHEAST MT...
CORRECTED FOR GEOGRAPHIC DESCRIPTION IN SUMMARY
Severe thunderstorms are possible Saturday, mainly during the
afternoon and evening, across parts of the Southeast through the
southern Appalachians and adjacent portions of the Piedmont. Severe
thunderstorms will also be possible during the afternoon and evening
across northeast Montana.
An anomalous deep-layer cyclone is expected to move slowly
southeastward into the Ohio Valley on Saturday. A surface boundary
will progress into portions of the Southeast, with other
convectively-induced boundaries likely to be present during the day.
An embedded vorticity maximum off of the Carolina coast will be
moving northeastward up the Atlantic Coast through the period,
resulting in the development of a compact coastal low that will be
approaching southern New England by Sunday morning. Further west, an
upper trough and associated surface low will move eastward across
the southern Canadian Prairies, with a trailing cold front expected
to push into the northern High Plains.
...Southeast into the Ohio Valley...
Widespread intense convection during the D1/Friday period casts a
considerable amount of uncertainty regarding the distribution of
instability on Saturday across the Southeast into the Ohio Valley.
However, with seasonably strong northwesterly flow and some remnant
of the EML likely to be in place, pockets of moderate-to-strong
instability and effective shear are expected to develop by Saturday
afternoon across portions of the Southeast. A few supercells capable
of large hail and localized damaging winds will be possible with
initial development, followed by the potential for one or more
upscale-growing clusters with a more organized wind threat. Due to
considerable uncertainty regarding the coverage and location of
potential redevelopment on Sunday afternoon, a relatively broad 15%
area is maintained in this outlook.
...Northern High Plains...
A conditionally favorable environment for supercells is expected to
develop on Saturday afternoon across eastern MT into northeast WY
and perhaps SD, as low-level southeasterly flow advects moisture
underneath steep lapse rates aloft, and effective shear increases
with the approach of stronger midlevel flow ahead of the trough.
Large-scale forcing is expected to largely remain north of the
border, but at least isolated thunderstorms are possible along a
surface trough in eastern MT, and also developing across the higher
terrain of eastern WY/western SD. Any storms that develop in this
region would pose a risk of large hail and severe wind gusts, and
perhaps a tornado or two.
...Coastal Mid Atlantic into Southern New England...
As the coastal low moves northeastward, low-level hodographs will
lengthen substantially across portions of the Mid Atlantic coast
into southern New England by Sunday morning. Surface-based
instability will negligible until very near the end of the period,
but some brief strong wind gust and/or tornado threat may
materialize Sunday morning across portions of the NJ/DE coast into
Long Island and coastal southern New England.
...MAXIMUM RISK BY HAZARD...
Tornado: 2% - Marginal
Wind: 15% - Slight
Hail: 15% - Slight
NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 0600Z