AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
419 AM EDT Mon Aug 20 2012

…Periods of heavy RAIN possible through tonight…

.SYNOPSIS
The 03 UTC regional surface analysis showed a wavy, QUASI-STATIONARY
FRONT from coastal NC, through the GA-TN border, through central MS,
and into east TX. Our forecast area was within a belt of fairly light
west to southwest winds, north of a subtropical RIDGE. Vapor
imagery and upper AIR data showed an anomalously deep, positively-tilted
TROUGH from the Great Lakes to the northwest Gulf of Mexico, with
unusually strong 500 MB winds (southwest at 35- 45 KT) over our
region. Precip Water values were slightly above CLIMATOLOGY. Area
radars showed a FEW patches of light rain across our forecast
area.

&&

.SHORT TERM…
[Today through Wednesday] The latest NWP guidance consensus is in
very good agreement in forecasting a wet day today. The
CONVECTION Allowing Models (CAM) forecast a quick recovery of the
marine boundary layer (which was temporarily stabilized by a cold
POOL from last evening`s storms) by SUNRISE, followed by the rapid
development of a large rain area over a large portion of our
forecast area sometime between MID morning and mid afternoon. The
SAME guidance indicates a near repeat of this scenario playing out
for tonight and Tuesday. AS the 500 mb trough gradually begins to
fill and drier air infiltrates our western zones on Wednesday, our
POP will decrease to near-average levels for this time of YEAR,
ranging from 50% in Cross City to 20% in Dothan.

All of the CAM forecast ISOLATED 6-hourly QPF totals of 5 to
6 inches today and tonight, but they differ on their exact timing
and location. If realized, such RAINFALL could be enough to cause
localized FLASH flooding, depending on when and where these QPF
maxima occur. (For instance, 5-6 inches of rain falling in less
than 6 hours in an urban area would cause flash flooding). To
account for this threat we issued a Flash FLOOD WATCH through
Tuesday morning. There is also a LOW-end CHANCE (5%) for strong to
severe storms today, mainly from I-10 southward, and mainly from
marginally damaging WIND gusts of 40 to 60 MPH. Unusually strong
winds aloft (at least 40 KT at 500 mb) will aid in some STORM
organization, possibly allowing for a few bowing segments to develop.
Ironically, the main limiting factor for a more WIDESPREAD severe
storm threat today will LIKELY be below-average SBCAPE (from all
the expected CLOUD cover and rain). The SBCAPE from the MEAN of
the CAM output is under 1000 J/kg (but higher offshore). Interestingly,
the CAM guidance forecasts a couple of cells with very HIGH,
persistent UPDRAFT HELICITY values over our coastal waters, indicating
a threat for a SUPERCELL. This could indicate at least a small
threat for an isolated WATERSPOUT near the coast.

.LONG TERM [Wednesday NIGHT through next Monday]…
Although the highly amplified mid-upper level trough present over
the eastern US in the short term period will dissipate and weaken by
Thursday, we are likely to remain in a weakness in the subtropical
ridge through the weekend. 500mb heights are forecast by the GFS to
be about 1 standard deviation below NORMAL for much of the extended
period. Meanwhile, a low level ridge axis along the LENGTH of the
eastern seaboard is likely to keep FLOW E-NE, with a stalled front
lingering near the southern part of our local forecast area, or just
to the south. This general scenario was depicted well by the 19.18z
suite of HPC Guidance so that was incorporated into the new extended
forecast, blended with 19.12z ECMWF and 19.00z ECMWF MOS. The end
result with slightly lower heights and thicknesses, as well as ENE
flow in the low-levels, should be temperatures slightly below
climatology with drier conditions day-to-day than what we have seen
recently. This is very consistent with the forecast from last night,
and thus there have only been minor tweaks to the previous forecast
beyond Thursday with this grid update.

&&

.AVIATION [through 06z Tuesday]…
In the next 6 hours (through 12z Monday): the best chance of lower
CIGS in the MVFR/IFR RANGE will be at DHN and ABY. Nearby OBS just
to the north indicate low CIGS near or below 1000 FT, and NARRE
probabilities of low ceilings are highest at these two terminals.

Beyond 12z Monday: Showers and thunderstorms should develop again
late tonight or early in the morning and overspread the area fairly
early in the day. MVFR/IFR expected in the +SHRA or TS, most likely
at ECP, TLH, or VLD. There is a chance for some redevelopment of TS
to the north near DHN and ABY in the afternoon.

&&

.MARINE…
Winds and SEAS will generally be near typically low summertime
values today, with west-southwest winds around 10 KT and significant
wave heights of 2 to 3 ft. Winds and seas will increase a bit on
Tuesday, increasing to 10 to 15 KT with seas of 2 to 4 FT.

&&

.FIRE WEATHER…
A wetting rain is expected over most of the area either Monday or
Monday Night. Although some slightly lower dewpoints will be moving
into the area just behind a slow-moving front later in the week, the
relative HUMIDITY is unlikely to FALL below critical thresholds.
Additionally, recent rainfall will likely keep fuel MOISTURE
relatively high.