Area Forecast Discussion
Issued by NWS
FXUS62 KTAE 281404
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
1004 AM EDT Sun Sep 28 2014
...Heavy rain possible through Monday...
.Near Term [Through Today]...
The 8 am EDT regional surface analysis showed a quasi-stationary
front roughly 20 to 60 NM offshore the central Gulf Coast, with
progressively higher pressure to the northeast. This tight
pressure gradient has caused winds to be a bit higher over the
coastal waters than the earlier NWP guidance had forecast. Vapor
imagery and upper air data confirm what the NWP guidance has been
forecasting for several days- that precip water values have
increased to more than 1.5 times normal values across much of our
forecast area. There was a weak 500 mb short wave trough along the
Gulf Coast (nearly coincident with the aforementioned surface
front), which was helping to generate large scale ascent inland.
All of these ingredients were helping to generate numerous showers
and isolated thunderstorms near the FL Panhandle coast. We expect
that it`s just a matter of time before rain develop inland.
The 8 am EDT sounding from Tallahassee shows a large area of
nearly- saturated conditions below the -10 C level, indicating
that warm rain processes (i.e. lack of ice) are likely, which
usually means high rain rates. Just looking out are window we can
tell this was occurring by the small droplet size and poor
visibility. All of this points to the threat of flash flooding.
Based on NWP guidance and what we`ve observed already (offshore
and outside of our forecast area), isolated storm total rain
amounts could be in the 6 to 9 inch range. If that were to happen
in a city (or in any low- lying, poor drainage area), flash
flooding would occur.
[Through 12Z Monday] Low cigs (high-end IFR to low-end MVFR) will
slowly lift to mid-range MVFR levels this afternoon. Periods of
rain (and isolated TSRA) are likely at all terminals, spreading
from KTLH and KECP this morning to KDHN, KVLD, and KABY this
afternoon. This will continue into overnight. Because of the
tropical nature of the rain, vis will be at IFR levels at times.
.Prev Discussion [338 AM EDT]...
.Short Term [Tonight Through Tuesday]...
Tough forecast coming up in terms of trying to pin down exactly
where the heaviest rainfall will occur in the forecast area and
just how much it will be. It is easy to get lost wading through
the dozen or so CAMs that we have available to look at now, each
with a slightly different (but valuable) take on the situation.
Thus, it is best to step back and take a look at the big picture
first. The synoptic scale contains two important features. The
first is an upper trough currently progressing eastward through
Texas, and the second is a very impressive and large plume of
tropical moisture with precipitable water values well over 2
inches across most of the Gulf and the Florida peninsula. As the
trough continues to progress eastward, a weak area of surface low
pressure is expected to develop along the central Gulf coast
states and lift northeastward through Monday. The forecast area
will also be in the favorable right entrance region of the upper
level jet as it lifts north of the area. Checking the forecast
soundings from the NAM and GFS for different points in the
forecast area, the profiles certainly look tropical with tall,
skinny CAPE, high mean RH, and a warm cloud layer. There is also
the potential for some training echoes embedded within a larger
areas of rain. In fact, we have already seen evidence of training
echoes on radar offshore earlier tonight. Thus, many of the large
scale ingredients needed for heavy rain will be in place across
the entire forecast area as we move into this afternoon through
Monday. There is high confidence in the large scale pattern
evolution, and in fact the GFS ensemble mean continues to show a
bullseye of precipitable water values in the 99th percentile for
this time of year squarely in the middle of the forecast area by
late tonight, which is signaling a rare event on the large scale.
Now we can return to all of those CAM runs with that viewpoint in
mind. Looking at the mean of those runs, the highest QPF values
through Monday are concentrated right along the panhandle coast
and just offshore. This fits with the thinking that a weak area
of low pressure will develop along the Gulf coast, which would
tend to enhance low level convergence in this area near the coast.
Therefore, it seems like the greatest probability of seeing the
heaviest rainfall amounts is still along the panhandle coast.
However, as the weak surface low lifts northeastward later tonight
into Monday, the current belief is that the rest of the forecast
area also has a threat of isolated excessive rainfall amounts,
especially given the tropical large scale environment that will be
in place everywhere in the forecast area. Because of this, we
opted to expand the flash flood watch to the remainder of the
forecast area and extend it through Monday afternoon.
By the time we get into Tuesday, things should be improving as the
upper trough is expected to weaken and lift out of the area.
.Long Term [Tuesday Night Through Sunday]...
The 12z GFS and 00z EURO are in fairly good agreement on the timing
of the exiting upper trough and surface low Monday night through
Tuesday. However, the GFS is quicker to dry us with deep northwest
flow. Will keep a slight PoP in our easternmost zones on Wednesday
to account for this. Otherwise, deep moisture begins to return to
the region as early Thursday ahead of the next upper trough and
associated cold front. The front is forecast to push into our CWA
on Saturday. Expect temperatures to run near normal.
Although winds outside of convection are expected to be in the 10
to 15 knot range today, rather stormy conditions are also likely
with numerous showers and thunderstorms. Winds and seas will be
locally higher near thunderstorms. Stormy conditions are expected
to last through Monday with a decrease in winds and seas expected
by mid-week before another front approaches by the end of the week
and into next weekend.
Hazardous fire weather conditions are not expected over the next
General rainfall amounts of 1 to 3 inches are likely across the
forecast area through Monday, except 2 to 4 inches along the
panhandle coast. However, localized amounts around 6 inches in a
short period of time appear possible anywhere in the forecast area
through Monday given the tropical airmass in place, thus the flash
flood watch has been expanded to include the entire forecast area
through Monday afternoon. Main stem river flooding is still not
expected due to low flows from the recent dry summer and given that
any excessive rainfall amounts are expected to be isolated in
.Preliminary Point Temps/PoPs...
Tallahassee 80 71 83 72 87 / 60 80 80 30 20
Panama City 78 74 83 70 84 / 80 90 80 30 20
Dothan 78 69 81 66 84 / 60 90 70 20 10
Albany 82 69 81 68 85 / 50 90 80 30 10
Valdosta 83 70 83 70 85 / 60 70 80 40 20
Cross City 83 70 85 70 86 / 60 60 80 40 40
Apalachicola 79 75 82 72 84 / 90 80 80 40 30
FL...Flash Flood Watch through Monday evening for Calhoun-Central
Walton-Coastal Bay-Coastal Dixie-Coastal Franklin-Coastal
Gulf-Coastal Jefferson-Coastal Taylor-Coastal Wakulla-
Gadsden-Holmes-Inland Bay-Inland Dixie-Inland Franklin-
Inland Gulf-Inland Jefferson-Inland Taylor-Inland Wakulla-
GA...Flash Flood Watch through Monday evening for Baker-Ben Hill-
AL...Flash Flood Watch through Monday evening for Coffee-Dale-Geneva-