SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0654 AM CST Sun Dec 20 2020

Valid 201300Z - 211200Z


Isolated severe thunderstorms may affect parts of the coastal
western Florida Peninsula late this afternoon and evening.

In mid/upper levels, broadly cyclonic flow will cover much of the
central and eastern CONUS this period, with several embedded
shortwave troughs.  Among them, a leading perturbation -- initially
located over the southern Appalachians to western FL Panhandle, will
weaken and eject east-northeastward away from the ongoing convective
regime extending southwest/northeast across the Gulf.  A stronger
shortwave trough -- apparent in moisture-channel imagery from
central KS to west TX -- is forecast to dig southeastward through
the period, reaching portions of the Mid-South and LA around 00Z,
and extending from the Carolinas to northern FL around 12Z tomorrow.

At the surface, an elongated area of low pressure was analyzed from
near the mouth of the Mississippi River eastward across shelf waters
south of PNS.  A warm front was drawn southeastward across the
eastern Gulf and Keys, and a cold front southwestward to the west-
central Gulf Coast of MX.  The warm front will move eastward
diffusely across the northeastern Gulf and into parts of west-
central FL this evening, while the cold front crosses the central/
northeastern Gulf.  This front may catch up to a currently primary/
prefrontal band of broken convection extending northeast-southwest
across much of the southwestern/central Gulf.  The front should
cross northwestern FL this evening and tonight, reaching east-
central/southwestern FL by 12Z.

...FL Gulf Coast...
Thunderstorms from the Gulf may cross parts of the FL coastline from
midday through this evening.  Marginal threat for a tornado and/or
damaging gusts is apparent for a short-lived, late-afternoon/evening
episode over parts of coastal west-central FL.

The north and east sides of a marginally thermodynamically favorable
warm sector will brush across the southward-extending AAF area of
the southeastern FL Panhandle today, and just onshore the west-
central FL Peninsular coastline late afternoon into evening.  A
strong storm cannot be ruled out in the small notch of warm sector
briefly passing over the AAF area; however, flow is progged to veer
in the warm sector as the low passes and before cold frontal
passage, while destabilization is limited and uncertain due to
ongoing/foregoing antecedent precip.  Potential in between the two
areas -- around northern Apalachee Bay and northern parts of the FL
coastal bend, still appears lower and too conditional for an outlook

Farther south, a window of opportunity onshore may develop near the
warm-frontal zone, where flow veers enough to advect better-modified
marine air inland, but before too much westerly near-surface
component reduces lift, shear and storm-relative flow in the
boundary layer.  Upper-60s F surface dew points will represent
enough boundary-layer moisture to offset modest midlevel lapse
rates, for supporting a deep buoyant profile (but with low CAPE
density).  Time series of forecast soundings suggest potential for
500-1000 J/kg MLCAPE along the immediate coastline area and 40-50 kt
effective-shear magnitudes, with hodographs supporting 150-300 J/kg
effective SRH during the early part of the most-buoyant period,
prior to the further veering.  The main uncertainty is how much
mature prefrontal convection can cross this environment while it
lasts.  As convection reaches the coast farther southward past about
the FMY area, both deep shear and low-level hodographs will be
shrunken relative to near Tampa Bay.

..Edwards/Smith.. 12/20/2020

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