AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
511 AM EST Tue Jan 22 2013

…UPDATED FOR AVIATION DISCUSSION…

.NEAR TERM [Today]…

The large scale longwave pattern during the predawn hours is
highlighted by a ridge along the Pacific Coast and a trough
dominating much of the rest of the continent, particularly east of
the Rockies. The western ridge continue to create a split flow
pattern with the southern branch of robust jet at base of trough
coming northeastward across the Gulf of Mexico and Southeast
transporting a fair amount of cirrus across our area. At the
surface, low off mid-Atlc coast with reinforcing cold front SWD down
WRN Atlc then SWWD across N FL coast. Front noted by temp/dew points
/subtle wind comparison. i.e. just along front around 50/42/near
calm winds while over SE AL, mid to upper 40s/low to mid 30s/NW wind
5 to 8 mph. Dew points across Cntrl AL/GA running around 10 degrees
drier than same time yesterday and this will translate SWD into this
morning. In wake of front, as Alberta Clipper low exits EWD across
ERN Canada, 1030mb Canadian high centered over IA/MO. Locally
this translates to a reinforcement of cold arctic air riding on dry
NLY flow moving into SE region. This reflected in area model
soundings. i.e. 12z Tues GFS TAE with 0.28 PWAT and NLY flow to H7.

During today, Alberta low will move into Canadian Maritimes with
amplified troughing remaining east of Rockies and into WRN Atlc this
morning. Trough/low begins to lift this afternoon with local flow
becoming more zonal by tonight. Moisture will continue to be absent
from area, in fact PWAT remains below one-half inch thru 12z Thurs.
Front will push south of coast soon after sunrise. In its wake,
surface high will expand SEWD from MS Valley to across TN Valley and
then into SE region ushering in a colder and much drier airmass.
Inland dew points will drop to the upper teens to low 20s. Expect
decreasing high clouds with gradual clearing from N-S. Still even
with clouds filtering ample sunshine, cold air advection mixing
means that max temps will only reach from mid 50s north to mid 60s
SE Big Bend.

&&

.SHORT TERM [Tonight Through Thursday]…

The local area will reside on the backside of the mid/upper
longwave trough. At the surface, high pressure will gradually
slide south across the Tri-State region, into the northern Gulf by
Thursday. The main forecast challenge through the short term will
be pegging overnight low temperatures and afternoon highs.

Tonight will be the coldest night of the week, with sub freezing
temperatures expected for a large portion of the area. Thus, a
freeze watch has been issued for areas likely to see temperatures
fall below freezing for more than 2 hours. Although the boundary
layer appears to remain fairly dry overnight, patchy frost will be
possible closer to dawn in locations across SE Alabama, north
Florida, and extreme southwest GA, where RH values are most
favorable.

Strong subsidence on the backside of the aforementioned trough
will is well agreed upon by most of the guidance, with a solid
inversion expected at or below 900 mb on Wednesday afternoon. This
will inhibit afternoon mixing, and thus keep temperatures
moderated. Expect temperatures around 60 degrees across our
Alabama and Georgia counties, with lower 60s expected across north
Florida.

Southwest flow will advect higher dewpoint air into the region
overnight Wednesday. This will limit how low temperatures can
fall, even with light winds and few clouds expected. Expect an
east/west gradient divided by the Apalachicola and Chattahoochee
rivers, with upper 30s east, and lower to mid 40s to the west.

By Thursday, above average afternoon temperatures will return and
more efficient mixing is expected. Warming 850 mb temperatures and
a more zonal upper flow regime will translate to nearly all
locations across the Tri-State area reaching the 70 degree mark.

No rain is expected through Thursday.

&&

.LONG TERM [Thursday Night through Monday]…

The extended forecast period will be marked by a more amplified flow
pattern as a shortwave currently south of the Aleutian Islands in
the north Pacific digs southeast out of the Canadian prairies on
Thursday and Friday. Surface cyclogenesis initially occurs ahead of
the digging wave over the Mid-South and then develops more rapidly
along the east coast later Friday and Saturday – pushing a cold
front through the Gulf coast region. Generally the global models are
within 12 hours of each other on the timing of the front (centered
around 00z Saturday). However, they disagree on the strength of the
low-level cold air advection and how far south the surface high
pressure center gets. The 21.12z GFS is an outlier in this regard,
keeping the high center mostly north of the Ohio River and never
cooling 850mb temperatures below 5-6C in our area. However, the
21.12z runs of the UKMET, CMC, and ECMWF all show the high getting
much closer to the Gulf coast with 850mb temperatures dipping below
0C. More confidence lies in the latter solution at this time, which
would mean a chilly weekend.

&&

.AVIATION…[through 12z Wednesday]

VFR conditions will continue through the 24 hours. Expect only high
level cloudiness this morning becoming mostly clear during the day.
Winds will be light from the north to northeast then calm after
sunset.

&&

.MARINE…

Cautionary level winds will subside through the day as a cold
front moves further from the region and high pressure begins to
take over. Relatively light winds and calm seas are expected until
Friday when they will begin to respond to an approaching front.
This front could bring Advisory conditions to the northeast Gulf
by this weekend.

&&

.FIRE WEATHER…

In the wake of a dry cold front…cooler and much drier air with
PWAT`s AOB 0.35 inches will overspread the region today and linger
into Wed. This combined with temps rising into the mid 50s to low
60s will enhance vertical mixing thru the day. This will induce
long durations of low RH everywhere across N FL today while
similar conditions favor low RH across all but the WRN portions of
FL on Wed. However, neither dispersion nor wind criteria will be
realized. Critical ERC values are expected both days across Leon and
Wakulla counties thus a red flag warning and fire wx watch are in
effect here for today and Wed respectively. Several other counties,
especially Washington and Bay have ERC`s approaching critical levels
and need to be monitored in case a warning or watch is necessary.
The air mass will begin to moisten Thurs and Friday as a front
approaches. In its wake, the air mass will begin to dry out during
the weekend.

&&

.HYDROLOGY…

Significant rainfall is not expected through the weekend. Thus,
river levels will remain steady as well.