Day 1 Severe Weather Outlook

Valid 311300Z – 011200Z

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
SOUTH FLORIDA AND THE KEYS…

…SUMMARY…
A few severe storms may occur across parts of south Florida and the
Florida Keys tonight, with isolated damaging wind gusts and perhaps
a couple of tornadoes.

…Synopsis…
In mid/upper levels, a positively tilted trough will shift slowly
eastward across the central CONUS, mid/upper Mississippi Valley and
northern MX through the period. The associated broad field of
cyclonic flow will be traversed by numerous small shortwaves and
vorticity maxima at various latitudes, as the synoptic-scale trough
moves to a 12Z line from the upper Great Lakes across the Mid-South
region, deep south TX, and north-central MX. A leading/southern-
stream shortwave perturbation — now apparent in moisture-channel
imagery over the TX coast and adjacent waters — will deamplify
while ejecting east-northeastward across AL/GA around 00Z, then off
the southern Atlantic Coast overnight. More directly upstream from
the outlook area, assorted models vary in timing/strength of weak
vorticity lobes/shortwaves, initially placed over the southwestern
Gulf or central MX, that extrapolate across south FL and the Keys
overnight. This reinforces considerable subjective uncertainty in
their character, based on lack of clear presentation of any such
features in moisture-channel imagery at this time.

At the surface, a broad area of low pressure was analyzed over the
north-central/northeastern Gulf, with a center roughly south of PNS
and west of SRQ at 11Z. This low should shift slowly eastward and
weaken through the period, while another develops over the Atlantic
off the GA coast around 00Z. The newer/eastern low is forecast to
deepen and move northeastward while remaining offshore from the
Carolinas. By the end of the period, a cold front should extend
from this low southwestward across central FL, through the weak/
western low west of FMY, and southwestward toward the Yucatan
Peninsula. A quasistationary front — initially drawn from the Gulf
low southeastward through the Straits of Florida — will become a
slow-moving warm front and drift northeastward over the Keys and
parts of south FL through the period.

…South FL, Keys…
Scattered thunderstorms are expected to develop today into this
evening over the eastern Gulf, southeast of the Gulf low and on
either side of the warm front. This activity should shift/develop
eastward toward south Fl and the Keys tonight. As projected
inflow-layer parcels destabilize with approach/passage of the warm
frontal zone across the outlook area, the severe threat will
develop, including the potential for damaging gusts and a tornado or
two. A mixture of convective modes may develop in a rather messy
regime, with multicells, small bows and/or a few supercells
involved.

Rich low-level moisture is present south of the stationary/warm
front, with surface dew points in the low 70s F common in available
METAR/ship/buoy obs from the northern Yucatan Peninsula across the
channel to waters around western Cuba, and southward over the
northwestern Caribbean. Associated high theta-e in the boundary
layer will offset modest low/middle-level lapse rates enough to
yield warm-sector MLCAPE in the 800-1500 J/kg range. Meanwhile,
favorable deep shear is expected across the area, especially in the
warm-frontal zone with some enhancement from backed near-surface
winds. Effective-shear magnitudes mostly are forecast in the
35-45-kt range. The largest hodographs will be near the warm front
as well, especially the last 3-6 hours of the overnight period when
mass response to Atlantic cyclogenesis strengthens 850-mb flow.

..Edwards/Kerr.. 01/31/2020

$$

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