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Short Term Update

SHORT TERM [Wednesday Through Friday]...
The main emphasis for this period will be the potential for
damaging wind gusts on Wednesday. The consensus of the latest NWP
guidance (including Convection Allowing Models- or CAM) is slower
than the 12 UTC GFS, which means that the deep moist convection
ahead of the cold front won`t be moving through our forecast area
until during the daytime hours. Although there will be plenty of
clouds, we expect enough breaks in the clouds to cause some
low layer destabilization. the CAM mean SBCAPE forecast values are
generally in the 400 to 900 J/kg range (west of a line from Albany
to Tallahassee)...not bad for this time of year. Also, recent CAM
runs show less of a stable near- surface layer than was shown in
previous runs. This...coupled with unusually strong winds not far
off the surface (a 60+ KT 850 mb jet!), suggest at least a Slight
Risk of severe storms. We`ve issued a Wind Advisory for portions
of our forecast area for Wednesday just because the normal daytime
mixing could bring 40-45 MPH gusts down to the surface...even without
the aid of showers/storms.

There are, as usual, some limiting factors in terms of severe
storms. The most limiting may be the warm temperatures aloft,
with mid tropospheric lapse rates of only an anemic 5 deg/km. This
will seriously limit the updraft strength in an important part of
the atmosphere (where ice normally develops). Indeed, the maximum
updraft speeds being indicated by the 12 UTC CAM NWP guidance
don`t exceed 14 m/s, and even those values are few and far
between. As the SPC stated in their morning Convective Outlook,
there may be a lot of low- topped convection with little
lightning. Since there is no warm front nearby, the 0-6km wind
profile will be unusually "straight" for this time of year. This
indicates that the storms will tend to organize along straight bands.
Tornadoes are unlikely, but we can`t completely rule out a brief,
weak one developing if a favorable mesoscale/storm scale environment
develops. Hail is highly unlikely. The CAM consensus has the
highest probabilities for strong to severe storms over our
northwest zones, with gusts from 40 to 60 MPH. All of the NWP
guidance shows the threat diminishing later Wednesday afternoon
and evening as the cold front moves east, which is typical for our
area. Also, recent high- resolution SST data from the Gulf of
Mexico show a rather broad zone of cool shelf waters across much
of Apalachee Bay. As this stable marine layer gets drawn inland
across the FL Big Bend, it should hinder vertical mixing, and help
prevent the strongest wind gusts from reaching the surface.

The latest CAM guidance is slow to clear out the rain behind the
cold front Wednesday night, but this seems rather unlikely
(especially compared to the output from the other regional and
global model guidance, as well as MOS). Temperatures will cool
considerably behind this cold front, with highs only in the upper
50s Thursday. A light freeze is possible Thursday night, but we
have plenty of time to look at this possibility later.


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