Day 1 Severe Weather Outlook

Valid 011300Z – 021200Z

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

…SUMMARY…
A few severe thunderstorms will be possible across south Florida and
the Keys, with strong to damaging wind gusts as the main threat.

…Synopsis…
In mid/upper levels, a positively tilted, synoptic-scale trough
begins the period from the upper Great Lakes across the Mid-South,
deep south TX, and north-central MX. This trough will shift
eastward through the period, reaching VA, the western Carolinas, GA,
western FL Panhandle, and the north-central/west-central Gulf by
00Z. This trough should move offshore all of the Atlantic Coast
except south FL by 12Z.

At the surface, 11Z analysis showed a strong surface cyclone
offshore from NC, with cold front southwestward across north-central
FL to a weaker, frontal-wave low near PIE. The front was drawn from
there southwestward across the east-central/south-central Gulf. An
outflow boundary — surrounding the residual cold pool from last
night’s South FL/Keys MCS — extended from Grand Bahama
southwestward over the Straits of FL between the Keys and Cuba, but
moving closer to the Keys amidst low-level return flow. The western
part of this boundary extends into an area of initially poorly
organized convection located north through west-southwest of the Dry
Tortugas. The outflow boundary should shift north-northeastward
over the Keys and portions of south FL through the remainder of the
morning. As the main low off NC deepens further and ejects
northeastward, the secondary/frontal-wave low will weaken, and the
cold front should clear the peninsula before 00Z.

…South FL and Keys…
Scattered thunderstorms are possible today, mainly ahead of the cold
front from midday into early/mid afternoon, offering strong-severe
gusts. A tornado cannot be ruled out as well.

Thunderstorms moving eastward off the Gulf across the Keys/
Everglades, or developing over the southern peninsula, will
encounter a favorable environment. Boundary-layer airmass recovery
is expected into at least midday across most of the outlook area,
from south to north, as two predominant processes occur:
1. The outflow boundary moves northward and weakens, amidst
low-level warm/moist advection. This already has contributed to
some low-level recovery evident in the 12Z KEY sounding, compared to
that from MFL still showing the well-defined effects of the MCS on
the boundary layer.
2. Diurnal/diabatic surface heating, through breaks between
departing morning cloud cover and thickening anvil cirrus ahead of
Gulf convection, boosts instability inland over the southern
peninsula.

These processes should offset modest mid/upper-level lapse rates
enough to boost peak MLCAPE mainly into the 500-1000 J/kg range.
Patchy/localized higher values are possible just inland from the
southeastern coast, near the south FL metro corridor, where the
preconvective air mass over land will have the most time to
destabilize. A tightening mid/upper height gradient ahead of the
progressive synoptic trough will contribute to strengthening deep-
layer speed shear, at least before prefrontal flow veers in response
to the departing Atlantic cyclone. Forecast soundings show
effective-shear magnitudes of 40-50 kt across the region from late
morning into afternoon before convective processes modulate the wind
and thermodynamic profiles. The same veering with time will reduce
veering with height (and thus hodograph size), though some low-level
vorticity enhancement may linger around the northward-moving,
residual outflow boundary. Severe potential should end from west to
east across the area by around 00Z with continued low-level veering
ahead of the front, and cold advection behind the frontal passage
itself.

..Edwards/Kerr.. 02/01/2020

$$

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