Valid 181300Z – 191200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS NEAR PARTS OF
THE FLORIDA GULF COAST…
…SUMMARY…
The threat exists for a tornado or two, along with isolated
thunderstorm gusts near severe levels, tonight near parts of the
Florida Gulf Coast.
…Synopsis…
In mid/upper levels, broadly cyclonic flow is expected over much of
the central and western CONUS by the end of the period, as synoptic
ridging shifts eastward across the upper Great Lakes and Ohio
Valley. To the south of that ridging, and as heights fall across
the Plains, a shortwave trough and weak embedded low now over the
lower TX Coast will eject eastward. This perturbation should reach
the FL Panhandle by 12Z, with its central vorticity max becoming
collocated with (or absorbed into) the deep-layer cyclone evolving
from the initially separate disturbance over the central Gulf.
At the surface, 11Z analysis showed a cold front over the Atlantic
northeast of FL, becoming quasistationary across the peninsula near
Lake Okeechobee and APF, northwestward over the Gulf to around 100
nm south of the mouth of the Mississippi River, then arching
southwestward offshore from the TX coast. The boundary should move
northeastward as a warm front, faster over the northeastern Gulf
than over western/southwestern FL through 12Z. Late in the period,
the front may reach the southernmost areas of the Panhandle near
AAF, and perhaps the coastal bend. The segment from around Lake
Okeechobee eastward should remain nearly stationary for most of the
period.
…FL Gulf Coast…
An area of low pressure and scattered thunderstorms now located over
the central Gulf — Potential Tropical Cyclone 16 — is forecast by
NHC to develop into a subtropical or tropical cyclone today, while
moving northeastward toward the coastal FL Panhandle. The system
will continue to be strongly influenced by southwesterly flow and
deep shear related to the TX Coast shortwave trough. As such, its
convective distribution is highly asymmetric, with the great
majority of thunderstorms progged to remain over its eastern
semicircle, as that sector of the cyclone approaches the FL Gulf
Coast late tonight. In this regard, as well as its forecast track
and presence of a frontal zone in its eastern sector, this system
loosely resembles Earl of 1998, but with a weaker outer wind field.
As such, the tornado threat still appears lower and more conditional
than Earl, which produced several pre-landfall tornadoes near the
boundary in west-central FL.
Low-level mass response to the approaching cyclone will serve three
purposes influencing severe potential:
1. Northward shift of the baroclinic zone as a warm front.
2. Through warm advection and isentropic lift, support a large
shield of precip and embedded thunderstorms already evident to its
northeast. This precip is expected to shift northeastward in step
with and ahead of the cyclone’s translation. In turn, it will limit
inland destabilization and penetration of the frontal zone,
especially from the FL coastal bend westward.
3. Maintain backed near-surface winds across much of FL, which will
enlarge hodographs, but also, maintain relatively stable boundary-
layer trajectories through rain-cooled air in northern FL and a
large part of the Panhandle.
Given that scenario, the outlook is kept close to the coastline from
northwestern FL northwestward, where weak MLCAPE may develop late in
the period in support of the potential for a few supercells or
bowing segments to move ashore. A larger area of favorable buoyancy
is expected near and south of the front over west-central/southwest
FL. Generally weaker low-level shear is expected than farther
northwest, owing to greater distance from both the strongest winds
of the cyclone and the ambient-shear influences of the shortwave
trough. However, hodographs may be enlarged in a corridor
corresponding to the frontal zone, in support of marginal tornado
potential for any supercells that can form and interact with the
boundary. Tornado probabilities, in particular, may need to be
raised later in the period for some part of this corridor if
convective and mesoscale environmental trends better focus supercell
potential.
Refer to NHC advisories for the latest tropical or subtropical
watches, warnings, and track/intensity forecasts for this system.
..Edwards/Leitman.. 10/18/2019
$$