Severe Thunderstorm Warning 171, NWS Tallahassee, FL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
456 PM EDT Mon Aug 12 2013

...UPDATED FOR AFTERNOON AND EVENING WEATHER...

.NEAR TERM [Through Rest of Tonight]...
The large scale pattern remains highlighted in the Nrn stream by a
blocking pattern with trough from extreme Ern Pacific, ridge from
Canadian NW territories SEWD to Wrn TX and trough over NE quarter of
U.S. into Canadian Maritimes. Srn stream dominated by flat stacked
subtropical ridge gradually weakening from E-W. At surface, high
centered over Ern Panhandle. This reflected in 00z Tues RAP13 model
soundings PWATS with 1.74 inch at TAE vs 2.09 inches at DHN and in
location of Monday's aftn/early eve convection across our Wrn most
counties assocd with Gulf seabreeze and outflow clashes. Several
were strong to severe. Forecast updated for sct-nmrs convection
Wrn counties thru 023z. Also...increased QPF and SKY and reduced
temps there.

Locally, large scale pattern translates to a hot and stagnant
airmass with ample subsidence beneath ridge especially east of
Apalachicola River. As a result, afternoon temperatures rose to
mid 90s west to near 100 east counties, highest where convection
was absent. Coupled with dew points around 70, heat indices soared
to near 100 west to 105 to 107 east.

Absent upper support, evening convection should end gradually
then more rapidly with loss diurnal heating. This reflected in
HRRR and WRF. May need to update 00Z-02Z grids for lingering Wrn
POPs. Otherwise...high debris clouds from dying convection should
dissipate by midnight with skies mostly clear for much of
overnight for good viewing of the annual Perseid meteor shower.
However, Glamp, SREF again imply patchy light fog towards sunrise,
and given high PWATS will go with persistence and may add some
fog towards sunrise. Another warm and muggy night with lows in the
lower to mid 70s.

&&

.SHORT TERM [Tuesday Through Wednesday Night]...
A broad upper level trough will continue to deepen across the
eastern CONUS with an axis from the lower Great Lakes to the central
Gulf coast. This will push a weak front down to southern AL/GA and
advect deep moisture currently just to our west into our area
tonight into Tuesday. This points to increasing rain Tuesday focused
in the northwest 1/2 of our area... SE AL and the FL panhandle,
spreading across the entire region Wednesday and Wednesday night,
except portions of the FL Big Bend.

Localized heavy rainfall is likely due to the deep moisture and weak
flow aloft resulting in very slow storm motion. High temperatures
will moderate under cloudy skies from the recent hot readings, with
a NW-SE gradient and the warmest temps over the FL Big Bend. Low
temperatures will be warm due to increased overnight cloud cover.

&&

.LONG TERM [Thursday through Monday]...
A long wave trough will remain in place from the Great Lakes to the
central Gulf Coast early in the period as a reinforcing short wave
in the northern plains drops into the trough, resulting in a closed
low over the Lower Mississippi Valley by Saturday. This low
gradually fills and weakens, leaving behind a weak trough for early
next week with an axis still to our west.

The end result is a prolonged period of rain for the southeast with
periods of heavy rain likely. Total rainfall from the GFS indicates
areas of rain up to 10 inches across the FL panhandle through
Monday, due in part to its development of a tropical cyclone in the
gulf which it brings into the FL panhandle Saturday. The Canadian
also generates a tropical system similar to the GFS, however the
ECMWF does not generate a tropical system and therefore depicts
rainfall totals through Monday of about 5 inches.

Confidence of a period of heavy rain is high, and if the
GFS-advertised tropical system does develop then local rainfall
amounts could be double or triple the current WPC 5.5" totals.

&&

.AVIATION [Beginning 18Z Monday]...
VFR conditions will prevail through the period with only isolated
showers and thunderstorms this evening. Scattered storms will
develop in the late morning and early afternoon Tuesday possibly
impacting the DHN, ECP and ABY terminals before the end of the TAF
cycle. Went with Prob30 groups for those sites. Winds will be light
from the west to southwest.

&&

.MARINE...
A weak pressure pattern will persist keeping winds around 10 knots
or less, with seas 2 feet or less. Thunderstorm activity over the
waters will increase by Wednesday.

&&

.FIRE WEATHER...
No fire weather concerns.

&&

.HYDROLOGY...
Area river levels continue to slowly decrease or remain steady and
this. A wet pattern will develop by mid-week, which will begin to
increase flows on several rivers later into the week and over the
weekend depending on which basins receive the heaviest amounts. As
of now the highest model rainfall amounts for the period from
Wednesday through the weekend are focused on the FL panhandle into
southwest GA, but that is a preliminary outlook.

&&

.PRELIMINARY POINT TEMPS/POPS...
Tallahassee   74  95  75  92  73 / 10  40  20  60  30
Panama City   78  90  78  89  76 / 20  40  20  50  30
Dothan        75  91  75  89  73 / 20  70  30  70  40
Albany        75  93  76  91  74 / 20  60  30  70  40
Valdosta      72  96  75  94  73 / 10  30  20  60  40
Cross City    73  94  75  94  73 / 10  20  10  30  30
Apalachicola  78  90  79  89  77 / 10  20  20  50  30

&&

.TAE WATCHES/WARNINGS/ADVISORIES...
FL...None.
GA...None.
AL...None.
GM...None.

&&

NEAR TERM/Marine...Block/Navarro
SHORT TERM...Barry
LONG TERM...Hollingsworth
AVIATION...Hollingsworth/Harrigan
FIRE WEATHER...Barry
HYDROLOGY...Hollingsworth