AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
320 AM EST Tue Jan 8 2013
.NEAR TERM [Today]…
The 03 UTC regional surface analysis showed a ridge along the
Piedmont and a quasi-stationary, diffuse frontal system from off the
TX coast through South FL. The latest NWP guidance indicates the
western portion of this front will intensify and lift northward as a
warm front on Wednesday. Vapor imagery and upper air data showed a
split flow pattern over the CONUS, with a closed low in northwest
Mexico in the southern stream, and building 500 mb heights over the
Southeast. The 00 UTC KTAE sounding showed ample moisture between
850 and 600 mb.
Although the latest global models don`t show any Q-G forcing in
our region until tonight (and even then it`s rather weak), the
consensus of Convection Allowing Models (CAM) indicate a chance
of light rain today. The MOS from the large scale models have a
PoP of 10% or less. It`s not entirely clear what forcing will
trigger the shallow moist convection shown in the CAM, but it could
be the advancing (but weak) warm front to our south. Given the
fact that there was enough deep layer moisture to support some
light showers Monday evening (despite relatively weak forcing), we
took and average of the CAM PoP and MOS PoP, giving our forecast
area a PoP ranging from 20 to 30%. QPF values are likely to be
less than 0.10 inches.
The GFS/NAM/ECMWF MOS consensus appears quite warm for today,
especially in the FL Big Bend (with highs in the mid to upper 70s).
Monday was a good example of how MOS tends to have a warm bias
during these “cool wedge” scenarios, and we think with all the
moisture aloft (i.e. cloud cover) temperatures will still be a
little cooler than MOS. Our high temperatures forecast is a
compromise between a cloudy-and-cool scenario like Monday, and a day
where there is enough insolation for strong mixing and highs
approaching 80 degrees.
.SHORT TERM [Tonight Through Thursday]…
There is good agreement among the latest models in the continuation
of building 500 mb heights over the Southeast. This will effectively
block any major storm systems in the southern stream from directly
impacting our region- leaving us with just fringe effects. The
PoP will range from 10 to 30% as a weak warm front lifts north of
our region Wednesday and Thursday. Despite the building 500 mb heights,
the models are showing plenty of mid to upper layer moisture, so
there will be periods of considerable cloudiness. As is usually
the case during the winter, when it`s unusually warm here, there
is a chance of dense fog. The expectation of morning fog and
daytime clouds lead us to undercut the MOS consensus high
temperatures by a few degrees. Despite this, temperatures will be
well above average, even during the overnight hours.
.LONG TERM [Thursday Night through Monday]…
Not much change to the previous thinking this model cycle. The big
story remains the strong ridge in place across Southern Florida that
will prevent a series of weather systems from moving into much of
our forecast area. A decaying frontal system will be across the
western portion of the area at the start of the period. This front
will dissipate by Saturday as the ridge strengthens. The next
frontal system on Sunday night into Monday may make a little more
progress eastward but is still anticipated to stall by Tuesday. Rain
chances will be on the low side but greatest in the northwestern
zones as these areas will be closest to the series of stalling
Unseasonably warm temperatures will be the primary story through the
long term period. Max temperatures could possibly flirt with 80
degrees on Friday and Saturday with lows in the mid and upper 50s
being common throughout the period. This is effectively 10 to 15
degrees above climatology for Mid January. Even though this will be
quite warm, this won`t be record warmth as record highs for mid
January at Tallahassee range from 81 to 83 degrees.
.AVIATION [through 06z Wednesday]…
MVFR ceilings will advance northward early this morning out of the
Gulf of Mexico and impact the terminals starting around 08z.
Expect a period of MVFR ceilings to continue through mid morning
at all sites before some improvement at TLH/VLD/ABY in the
afternoon hours. With a warm frontal zone being near the area on
Tuesday night, expect more widespread restrictions, possibly into
the IFR category.
Winds will be at exercise caution levels early this morning,
followed by a brief weakening of the winds this afternoon, before
they pick up again to exercise caution levels tonight. In fact,
winds and seas are likely to remain at moderate levels through
Thursday, especially across our western marine zones.
The moistening trend will begin today with relative humidity values
remaining well above critical levels. No fire weather concerns are
anticipated through the next several days.
Given the low PoP over the next few days, rivers are expected to
remain below action stage.