Day 1 Severe Weather Outlook

Valid 261300Z – 271200Z

…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE MID-ATLANTIC REGION AND FLORIDA PENINSULA…

…SUMMARY…
Lines of storms may pose a risk for strong to damaging winds during
the late afternoon/evening across the Mid-Atlantic States and parts
of the Florida Peninsula.

…Synopsis…
In mid/upper levels, a high-amplitude pattern covers the CONUS,
dominated by a substantial synoptic-scale trough extending from a
cyclone over far northwestern ON southwestward through a vorticity
max near UIN, then southwestward across the Arklatex region and
southwest TX near DRT. As the Canadian cyclone elongates southward
through shortwave amplification, the current UIN-DRT segment of
troughing will pivot northeastward and deamplify. This section
should reach western OH, eastern TN, and southern MS by 00Z, with
primary vorticity lobe ejecting across the central Appalachians
overnight. The southern branch of troughing should reach western
NY, eastern PA and the Carolinas by 12Z tomorrow.

At the surface, 11Z analysis showed an elongated area of low
pressure along an occluded front from southern OH to central PA,
with the associated cold front from a triple point near a line from
IAD-GSO-ABY-PAM to the central Gulf. A dominant low should
consolidate/deepen over western/central PA through 00Z, with post
cold-frontal trough (and some accompanying baroclinicity) across
western VA southward to central SC, catching up to the initial cold
front over eastern GA/northern FL, with the front south-
southwestward to the east-central Gulf. By 12Z, the low should be
deeply occluded and move to near eastern Lake Ontario, with a triple
point over the Atlantic near ACK, and a strong cold front well
offshore from there past FL.

…Mid-Atlantic region…
A band of scattered showers and thunderstorms is expected to develop
by around mid-afternoon across parts of western VA and WV, shifting
eastward and northeastward across portions of the Blue Ridge/
Shenandoah region and southern PA, with additional convection
potentially forming over central VA. Given the intense ambient wind
fields and strong deep shear, damaging wind — including isolated
severe gusts — may penetrate to the surface. Some of the
convection responsible for such gusts may be too shallow to produce
lightning.

As the trough aloft approaches, and the low-level cyclone
tightens/deepens, deep-layer forcing and low-level mass response
will intensify, supporting convective organization even in very
weakly unstable conditions. Low/middle-level lapse rates should
steepen just enough — from a combination of subtle heating and warm
advection in the low level with the leading part of trough-related
cooling aloft — to produce neutrally to convectively unstable
conditions. MUCAPE in the 100-500 J/kg range is possible over the
outlook area — generally weaker with northwestward extent as
low-level theta-e lessens. Still, given the intense winds in
mid/upper levels and fast convective motion, sporadic strong to
locally severe gusts may occur, and even gusts below defined (50-kt)
severe limits will be capable of damaging trees. If 12Z or later
guidance and mesoscale trends increase confidence in a particular
channel or swath of convective-wind potential within this area, a
15%/slight-risk upgrade could be needed in one of the day-shift
updates.

…FL…
Scattered thunderstorms should cross the outlook area from Gulf to
Atlantic Coasts late this afternoon and evening, offering strong to
isolated severe gusts. The potential will be maximized locally with
any bow or LEWP formations that may develop within the convective
band.

An outflow boundary from earlier convection/precip over central FL
has settled southward and stalled across south-central FL, and was
analyzed between SUA-PBI across the Lake Okeechobee region to just
south of FMY. This feature should weaken gradually through the day
amidst diabatic heating on both sides, and airmass recovery on the
north side due to low-level theta-e advection from the Gulf. Any
convection forming to its north through midday, as progged by
several models, may delay that process. Still, sufficient
destabilization is forecast to support surface-based effective-
inflow parcels for later convection moving off the Gulf late this
afternoon into evening, just ahead of the cold front.

As part of the return-flow/destabilization process north of the
dissipating outflow boundary, surface dew points should recover into
the mid-60s to near 70 F prior to the main thunderstorm band,
supporting MLCAPE in the 500-1200 J/kg range. While buoyancy will
be on the modest side for FL convection, along with generally small
low-level hodographs related to veered near-surface winds, deep
shear will be favorable for organized convection. Forecast
soundings show effective-shear magnitudes around 45-55 kt, locally
higher.

..Edwards/Dial.. 02/26/2020

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