Day 1 Severe Weather Outlook

Valid 131300Z – 141200Z

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER NORTHERN
FLORIDA INTO SOUTHEAST GEORGIA…

…SUMMARY…
Thunderstorms capable of isolated damaging wind gusts and a couple
of tornadoes are possible over northern Florida into southeast
Georgia, mainly later this evening into tonight.

…Synopsis…
In mid/upper levels, a series of shortwaves in broader-scale
cyclonic flow will lead to an eastward shift of the mean troughing
across the central/eastern CONUS. An elongated, weak, positively
tilted trough — now apparent in moisture-channel imagery from WV-AL
and across the north-central/west-central Gulf — will shift east-
northeastward to northeastward while further weakening. This
feature should move mostly or entirely offshore from the Carolinas
to FL Atlantic coastline by around 00-03Z.

Meanwhile, an upstream perturbation — presently over the southern
Great Plains — will dig east-southeastward and strengthen, reaching
the Mid-South region and LA by 00Z. This feature should pivot
northeastward overnight to eastern TN and GA as yet another upstream
perturbation reaches the Ozarks. A pronounced upper-level jet max
— now analyzed over the southern Pacific Northwest and northern
Great Basin — will spread/expand further inland, reaching the
central Rockies and southern/central High Plains by the end of the
period, following these shortwaves.

At the surface, 11Z analysis showed a low near rig station KIKT
east-southeast of the Mississippi River mouth, with cold front
arching southward and westward across the northwestern Gulf to a
weaker/trailing low offshore from Padre Island. A warm front was
drawn from the main low southeastward across the northeastern Gulf
to southernmost FL, becoming diffuse northeastward over the northern
Bahamas. Stationary frontogenesis was evident over northern FL near
OCF and SGJ, northeastward just offshore from SC. This will become
the main synoptic warm/stationary front through the rest of the
period as the air mass over the peninsula slowly modifies via
advection from marine sources.

The current main low is forecast to ripple northeastward and remain
relatively weak, moving inland across the west-central FL Panhandle
this afternoon while secondary surface cyclogenesis occurs near the
SC coast. As the initial low moves inland into more-stable
boundary-layer air and weakens, the second low should move northward
to north-northeastward across eastern NC and the VA/Delmarva
Tidewater region overnight, while deepening. The associated cold
front should move offshore from GA by 12Z, reaching eastern NC,
northern FL, and the northeastern Gulf. The frontal segment east of
the Gulf low may shift northward temporarily across northern FL into
southern GA.

…FL/GA…
Cool temperatures and stable conditions remain widespread north of
the warm front at this time, along with broken areas of reinforcing
convection/precip, across the FL Panhandle, northern FL and southern
GA. Elevated convection over land and over this air mass — related
to the initial shortwave perturbation, should shift eastward to
northeastward with time, while offering no organized severe threat.

Additional development is possible this afternoon over the northern
peninsula, as diurnal heating destabilizes boundary-layer lapse
rates and renders surface-based effective-inflow parcels. The
strongest moisture, low-level shear and deep shear won’t arrive
until the next convective episode; however, the combination of
around 1000 J/kg MLCAPE with 30-35-kt effective-shear magnitudes may
support a few organized multicells offering a marginal/localized
severe threat.

The main severe potential still appears to be ahead of the trailing
shortwave trough this evening and overnight, as the initial low
moves inland and the warm front/sector shifts somewhat northward
across northern FL and the south-central/eastern Panhandle tonight.
Scattered thunderstorms should form on both sides of the warm front,
as well as both ahead of and behind the surface low. Activity near
and south of the boundary — including convection moving off the
Gulf — will have the best-organized severe potential.

Increasing deep-layer flow is expected as the shortwave trough
approaches, leading to strengthening vertical shear, coinciding with
favorable boundary-layer theta-e advection across the area.
Accordingly, time series of forecast soundings show effective-shear
magnitudes increasing to 50-60 kt over northern FL tonight prior to
cold frontal passage, while effective SRH strengthens to 150-300
J/kg. Despite rich low-level moisture advecting inland from the
Gulf, modest lapse rates aloft should keep MLCAPE around 1000 J/kg,
locally near 1500 J/kg. This parameter space will support a modal
blend of multicells and supercells, with damaging gusts and a couple
tornadoes possible. Veering prefrontal surface flow — coinciding
with the deepening and moving away of the second low — will reduce
low-level shear near the end of the period, though strong-severe
convection may still be ongoing over parts of western/northern FL
and southeastern GA as a frontal or prefrontal QLCS.

..Edwards/Broyles.. 12/13/2019

$$

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