Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Tallahassee Florida
935 PM EST Thu Dec 12 2013
..a brief light freeze is expected across many inland areas
..heavy rainfall likely Saturday and Saturday night...
near term, marine and aviation sections.
Near term [tonight]...
winds have gone calm at many inland locations already this evening.
After monitoring evening temperature trends, we have decided to
expand our freeze warning southward to include a few Florida Big Bend and
Panhandle counties. We are still not getting sufficient durations to
expand east of I-75 into south central Georgia or into the Suwannee
Valley. These latter two areas will however see the best chance for
frost formation and we have indicated that in the grids.
Short term [friday morning through Saturday night]...
after a cold and tranquil start on Friday, things will quickly
change as a storm system spreads rapidly eastward out of The
Rockies. Model guidance is in relatively good agreement with the
progression of this system developing across eastern Texas on
Friday evening and moving quickly eastward into the mid south by
Saturday morning. The initial shortwave with this system will
gradually deamplify and move into the Ohio Valley Saturday
afternoon. A trailing higher amplitude impulse will move into the
southeast late Saturday afternoon and likely lead to the
development of a wave of low pressure along the surface frontal
boundary just west of our region. As this development occurs, a
considerable increase in the coverage and intensity of the showers
and storms should occur. The primary concern from late Saturday
morning through Saturday evening is for heavy rainfall. Sref probs
for rainfall amounts in excess of 2 inches are relatively high for
an event still beyond 48 hours. The Euro and NAM are both
indicating rainfall amounts in the 2 to 3 inch range as well with
this system across North Florida and southern Georgia - which
further increases confidence in the upcoming heavy rainfall event.
While these are widespread totals, isolated maximum totals of 4 to
6 inches are possible, especially with the stronger storms.
The severe weather threat is much less certain. While deep layer
shear will likely be sufficient for organized severe storms,
surface based instability is anticipated to be relatively weak.
The 12z NAM did bring a corridor of +65f dew points well into
south Georgia, but this seems overdone given that this system is
not especially strong. As a result, think the severe threat should
be confined to areas mainly in North Florida near the coast, and
even then should only be isolated.
By Saturday evening, the secondary impulse will start moving away
from the region, the available forcing for heavy rainfall will
continue to decrease as the area of showers and storms moves into
the Southeast Florida Big Bend.
Long term [sunday through thursday]...
at the start of the long term period, the frontal boundary will be
exiting the region slowly to the south with drier air slowly
moving in. High pressure eventually takes control of the pattern
across the region Monday through Wednesday. Afternoon temperatures
should be around normal values Tuesday and Wednesday, though
overnight lows look to be a few degrees below normal as the rather
dry airmass and nearby high pressure allow for a couple of good
radiational cooling nights. The high pressure will move east of
the region on Thursday with onshore flow developing ahead of the
next storm system.
[through 00z saturday] VFR conditions will prevail through the
period. Light and variable winds tonight will shift to east and
southeast winds tomorrow morning at less than 10 knots. At the end
of the period, ensemble guidance is suggesting that MVFR cigs may
begin to settle in near ecp ahead of a developing system, but
confidence was too low for now on to put in this taf package.
borderline advisory winds will continue over the offshore waters for
several hours tonight. We did lower seas based on buoy obs and also
ended the advisory a few hours earlier. Winds will continue to
decrease with the approach of the frontal system on Friday and
Friday night. Winds will shift to offshore behind the front on
Sunday and increase to cautionary levels through Monday morning
before decreasing Monday afternoon. Light winds and low seas are
expected by Tuesday.
low level moisture will begin to increase on Friday ahead of an
approaching cold front. This will keep minimum humidity values in
upper 30s and 40s for Friday afternoon. By Saturday, showers and
thunderstorms will move into the region, with no fire weather
widespread rainfall is expected over the weekend. Total quantitative precipitation forecast amounts
differ between the models. The GFS has quantitative precipitation forecast values in the 1-2 inch
range, while the European model (ecmwf) has several areas over four inches. Average
rainfall totals of 2 to 3 inches with localized amounts of four to
six inches are expected, which will result in rises on area
rivers. A Flood Watch may be necessary for a portion of the
region on Friday.
Preliminary point temps/pops...
Tallahassee 31 65 54 71 61 / 0 0 10 90 90
Panama City 39 64 57 71 60 / 0 0 20 90 80
Dothan 31 62 51 69 55 / 0 0 30 90 70
Albany 31 62 52 69 56 / 0 0 20 90 80
Valdosta 32 65 51 72 61 / 0 0 10 80 90
Cross City 32 70 52 74 65 / 0 0 10 50 80
Apalachicola 39 64 57 71 64 / 0 0 10 90 80
Florida...freeze warning from 2 am EST /1 am CST/ to 8 am EST /7 am CST/
Friday for Gadsden-Holmes-inland Jefferson-inland Walton-
Georgia...freeze warning from 2 am to 8 am EST Friday for Baker-Brooks-
Alabama...freeze warning from 1 am to 7 am CST Friday for all zones.
GM...Small Craft Advisory until 4 am EST Friday for offshore waters
from 20 to 60 nm.
short term/long term...Godsey