Area Forecast Discussion
Issued by NWS -- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KTAE 280738
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
338 AM EDT Sun Sep 28 2014
...FLASH FLOOD WATCH EXPANDED TO INCLUDE ENTIRE FORECAST AREA AND
EXTENDED THROUGH MONDAY AFTERNOON...
.Near Term [Through Today]...
The very weak middle and upper level pressure pattern will remain
in place for one more day before a shortwave moving through the
Ohio and Tennessee Valleys links up with the cluster of convection
in the northern Gulf tonight.
Forcing for showers and thunderstorms will likely come from two
different features this afternoon. First, convection ongoing over
the Gulf this morning will become more organized this afternoon as
a weak area of low pressure develops under a weak MCV spawned
from yesterdays convection. As the wave of low pressure
strengthens, so will southerly flow an isentropic ascent, forcing
showers and thunderstorms inland over the Panhandle primarily west
of the Apalachicola River. Later in the afternoon, a surface
trough will develop over land where temperatures will have climbed
into the middle to upper 80s. This feature will bend from the
southeast Big Bend, around the central Big Bend and across extreme
southern Georgia. Expect a second round of convection to form
along this trough in the late afternoon. Eventually, both forcing
mechanisms will blend together and a rather solid line of showers,
thunderstorms, and heavy rain will spread north through our
Alabama and Georgia counties.
Satellite derived PWAT values this morning match very well with
area soundings and depict that a swath of 2+" has essentially
overspread the entire Tri-State region. This evenings Tallahassee
sounding showed PWAT values around 130% of normal. With these
values expected to only become more anomalous through the day, the
threat for heavy rain remains very high. On average, areas
receiving rain today should expect to see about an inch, with
isolated higher amounts as high as 4 inches. Keep in mind that
these amounts will accumulate very quickly and for that reason,
the Flash Flood Watch will remain in effect for today and has also
been expanded to cover our entire region.
.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.
[Through 06Z Monday] Generally, a patchy deck of MVFR ceilings
will plague the region through the morning hours. This afternoon,
we`ll likely remain socked in under a VFR, low to mid- level cloud
deck. As showers and thunderstorms spread inland expect IFR to
LIFR conditions in very heavy rain.
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 85 71 83 72 87 / 60 80 80 30 20
Panama City 80 74 83 70 84 / 80 90 80 30 20
Dothan 81 69 81 66 84 / 80 90 70 20 10
Albany 82 69 81 68 85 / 70 90 80 30 10
Valdosta 87 70 83 70 85 / 80 70 80 40 20
Cross City 87 70 85 70 86 / 70 60 80 40 40
Apalachicola 79 75 82 72 84 / 80 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-