Day 1 Severe Weather Outlook

Valid 061300Z – 071200Z


Severe thunderstorms are expected to spread from the northeast Gulf
Coast this morning to the Carolinas and perhaps southeastern
Virginia by early evening, offering a threat of damaging winds and a
few tornadoes.

Broadly cyclonic mid/upper-level flow will continue to cover most of
the CONUS from the Rockies to the Appalachians, related to a major
synoptic trough initially near an axis from IWD-FNB-CDS-6R6, then
southward over central MX. The synoptic trough will shift eastward
across the upper Great Lakes and middle/lower Mississippi Valley
through the period. The southern part will take on a more neutral
to slightly negative tilt as a basal shortwave trough — now evident
in moisture-channel imagery over Coahuila and southwest TX — pivots
eastward then northeastward. That shortwave trough should reach MS
and southeastern LA by 00Z, then eject to parts of VA and the
Carolinas by 12Z tomorrow.

As this occurs, the mid/upper height gradient will tighten
considerably east of the synoptic trough across the southeastern
CONUS, with 500-mb southwesterlies reaching 120-130 kt by 00Z, as a
170-185-kt 250-mb jet max crosses the north-central/northeastern
Gulf and FL Panhandle. 500-mb height falls around 150-200 m will
spread over the outlooked area as well.

The 11Z surface analysis showed a cold front from eastern portions
of KY/TN across northern/western AL, southeastern MS, and extreme
southeastern LA, then across the northwestern Gulf. This front
should move to the western Carolinas, northern/western GA and the
central FL Panhandle by 00Z. By 12Z, a longstanding/elongated area
of low pressure should evolve into a better-defined cyclone over the
inland Mid-Atlantic region, with the cold front offshore from the
Atlantic Coast, except across parts of southern FL.

…FL to Carolinas/southeastern VA…
For near-term concerns across the GA/AL/FL Panhandle region through
midday, refer to SPC tornado watch 26 and related mesoscale
discussions. Additional watches are probable through the day and
perhaps into this evening as the severe threat spreads/shifts across
the outlook area.

Numerous (main convective band) to widely scattered (warm-sector)
thunderstorms will move northeastward along and ahead of the cold
front. Pockets of diabatic/diurnal heating will contribute to
boundary-layer destabilization, as will theta-e advection. Some
trajectories across eastern GA and the eastern Carolinas will
emanate from a corridor of relatively lower theta-e over the FL
Peninsula for much of the day, but continued warm/moist advection
from the northeast Gulf should contribute to more-favorable
thermodynamic profiles in those areas later in the afternoon and
into early evening. This should offset modest mid/upper-level lapse
rates enough to support 1000-1500 J/kg MLCAPE from northern FL and
the eastern Panhandle across southeastern GA and parts of SC, with
areas of 500-1000 J/kg MLCAPE possible farther northeast toward the
NC/southeastern VA Tidewater areas.

The main band is expected to become better-defined and more
organized through the day as the aforementioned upper-level
processes occur, with strengthening of both low-level mass response/
convergence and deep shear. Low-level bulk shear also will
strengthen over the warm sector, with an expanding, 70-80-kt LLJ
corridor being part of the mass response. Effective SRH values in
the 250-400 J/kg range may result, even with only modest curvature
of the hodograph. Deep-shear and mean-wind vectors will be oriented
largely parallel to the main band of convective-scale forcing,
leading to a dominant quasi-linear mode, though a few discrete to
semi-discrete, warm-sector supercells are possible as well, given
the weak CINH. Bow/LEWP formations and embedded QLCS
mesocirculations will focus severe potential locally both in the
forms of damaging to severe downdrafts and sporadic tornado
potential. The “enhanced risk” area represents the most favorable
convectively active parameter space forecast for thunderstorm wind
in particular, including downward momentum transfer from a deep
layer of strong and intensifying flow above the surface.

..Edwards/Kerr.. 02/06/2020


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