Day 1 Severe Weather Outlook

Valid 111200Z – 121200Z


Severe thunderstorms are expected to spread across the Mid South and
central Gulf States today. Damaging winds and a few tornadoes are
possible. Localized damaging-wind threat will be noted with
convection across the Ohio Valley and Middle Atlantic.


Intense 500mb speed max is translating through the base of a strong
short-wave trough over the southern Plains late this evening. This
feature should become negatively tilted during the day as flow
increases to near 120kt at 500mb over the OH Valley by early
evening. In response to this feature, primary surface low will track
along a polar front along the OH River into southeast IN. This
should allow modified warm sector to surge ahead of the front into
portions of the OH Valley. Even so, forecast soundings north of the
TN Valley do not exhibit much instability, and convection that
evolves ahead of the strongly-forced short wave should struggle to
produce lightning. Will maintain low severe probs for locally
damaging winds ahead of the intense speed max.

Of more concern will be convection that is ongoing at the start of
the period along a strong front near the MS River. Late this
evening, a well-organized squall line was advancing east across
AR/east TX toward LA. This activity is handled well by 00z model
guidance which surges a squall line into western MS by the start of
the day1 period. Earlier thoughts regarding this convective scenario
remain. Strongly sheared linear MCS is expected to advance across
the central Gulf States during the first half of the period.
Environmental shear strongly favors supercells but the primary
convective mode should be linear along a strongly forced boundary.
Damaging winds are the primary threat with this activity, though
embedded circulations could pose some tornado threat. It’s not
entirely clear how much pre-frontal discrete convection will develop
due to limited heating. Even so, surface dew points rising into the
upper 60s should be adequate for surface-based supercells. Given the
forecast shear, tornadoes are certainly possible, especially if
discrete convection can develop ahead of the squall line.

Downstream across the Middle Atlantic, increasing southerly flow
will allow boundary-layer moisture to advance north across the
Carolinas into VA where 60s surface dew points are expected during
the overnight hours. While warm advection will undoubtedly encourage
showers across this region, the primary large-scale forcing will
spread across the OH Valley into the northern Middle Atlantic.
Forecast soundings suggest modest lapse rates which do not allow for
meaningful instability to materialize across this region. Despite
the strengthening wind fields, will opt to maintain 5% severe probs
due to weaker forcing and meager buoyancy.

..Darrow/Nauslar.. 01/11/2020


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