Area Forecast Discussion
Issued by NWS
FXUS62 KTAE 290807
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
407 AM EDT SAT AUG 29 2015
.Near Term [Through Today]...
Water vapor satellite loops prominently display a mid-upper level
low centered near the mouth of the Mississippi River, with a plume
of deeper moisture situated along the eastern periphery of the
circulation aloft. Within this moisture plume, fairly widespread
convective activity was ongoing as of 07Z over the adjacent coastal
waters of the Gulf of Mexico. Other than a few light showers over
land, it was mostly dry. Fairly dense cirrus appeared to be
streaming north from the Gulf thunderstorm activity per IR
satellite, covering much of our forecast area.
With the deep moisture in place across the area, and an unusually
vigorous mid-upper level low for August just to our west, the
ingredients are there for a day with relatively high coverage of
rain across the area. Models show the strongest QG convergence
centered right over our forecast area. A potentially complicating
factor, though, is the amount of cloud cover that lingers into the
day. Persistent cloud cover could hinder destabilization. While some
rain showers would still be likely, the intensity and coverage of
rain might be lower than it would be otherwise. Additionally, some
high-resolution models show stratiform rain spreading north into the
area from Gulf convection this morning. Given how far south in the
Gulf the strongest convection is currently located, widespread rain
- at least early in the morning - seems somewhat unlikely. For now
we indicated likely PoPs (60-70%) over the southern half of the area
in the morning, with rain showers the dominant weather type. In the
afternoon, the likely PoPs were shifted into the northern part of
the area, with thunderstorms becoming increasingly likely as some
instability may be able to develop.
Regardless of the specific convective evolution, there should be
cloud cover or rain through much of the day which is expected to
hold high temperatures generally in the mid 80s. This is slightly
below normal. If Tallahassee`s high stays in the 80s, it would mark
the first back-to-back days with highs below 90 degrees since June
28th and 29th.
.Short Term [Tonight Through Monday]...
By this evening, the +PV anomaly over the north-central Gulf will
have moved very little. The associated pressure trough will become
a bit more broad as another wave slides through the center of the
country, from the Great Lakes through the Southern Plains. This
will eventually increase the mid/upper levels southwesterly flow
and advect the northern Gulf anomaly through the Southeast on
Sunday, and off the Mid-Atlantic coast on Monday. Through the same
period, the secondary wave will stall over the Lower Mississippi
Valley. At the surface a trough out ahead of the northern Gulf
anomaly will disturb the broad ridge covering the eastern CONUS.
We`ll likely have a fair amount of convection lingering into the
evening tonight. On Sunday, the western half of the Tri-State
region will be under a slightly drier airmass directly below the
+PV anomaly. To the east, the surface trough and upper forcing
contribution will initiate widespread convection. A weak surface
trough will remain along the western Atlantic coast on Monday,
forced by a ribbon of trailing PV behind the main anomaly. This
will be a focus for convective initiation, with storms spreading
west into the eastern Tri-State region in the afternoon. On
Monday, storms may have the potential to produce higher than
normal rain rates as PWATs over 2" combine with warm cloud depths
between 13-14kft coincide. This could provide an unfortunate
amount of antecedent rain leading up to the approach of Erika.
.Long Term [Monday Night Through Saturday]...
There still remains an incredible amount of uncertainty with
respect to the eventual track of Erika. The general idea is that
Erika will weaken into a depression as it crosses over Cuba, and
that`s where models continue to diverge. From the 00z suite of
guidance, once Erika emerges into the Gulf the ECMWF lifts a weak
surface trough through the northeast Gulf, the GFS redevelops
Erika into a tropical storm and moves it north toward the Tri-
State region, while the HWRF completely dissipates the system. So
at this time, the range of possibilities lies between total
dissipation and a tropical storm at our doorstep sometime Tuesday
or Wednesday. Keeping in mind that the "range of possibilities"
has really been changing with every model run. The one thing that
there remains some confidence about is that we will likely receive
a couple days of heavy rainfall as Erika treks through the region.
Highest average rainfall amounts between 5-7 inches still seem
reasonable, and will most likely be across or east of the eastern
half of the region. Stay tuned...
[Through 06Z Sunday]
VFR conditions prevailed across the area at TAF issuance time,
although satellite revealed dense high cloud cover across most of
the area. Some periods of MVFR CIGS will be possible particularly
at TLH, VLD, and ABY today, although in general VFR conditions are
expected to dominate outside of thunderstorm activity. Periods of
showers and thunderstorms are also expected across the area,
mainly after 15Z and prior to 03Z. IFR visibilities would be
possible in any thunderstorm.
Relatively light winds and low seas will prevail through the
weekend before increasing in advance of Erika early next week.
Showers and thunderstorms are likely over the next several days.
Widespread rain and elevated relative humidity is expected today,
with no red flag conditions expected in the near future.
With around an inch of rain (on average) expected through the
weekend, no impacts are expected. As mentioned earlier, Erika has
the potential to bring 5 to 7 inches in some locations, provided
it remains a somewhat progressive system. Most basins should be
able to handle these amounts considering their low flows. However,
the target area (at this time) for the highest rainfall amounts is
across the southeast Big Bend of Florida where the Steinhatchee
River remains above normal flows. Should these amounts fall in
this region, there could be some significant impacts, even outside
of river basins in rural or urbanized areas. All of this is
highly dependent on the uncertain forecast track of Erika.
.Preliminary Point Temps/PoPs...
Tallahassee 86 72 92 75 91 / 70 30 40 20 60
Panama City 84 74 87 76 88 / 60 30 20 20 40
Dothan 85 70 91 72 92 / 60 40 30 20 40
Albany 85 71 89 72 91 / 70 50 40 20 40
Valdosta 86 72 90 73 89 / 70 40 50 30 60
Cross City 87 74 89 75 89 / 50 30 60 30 60
Apalachicola 85 75 90 78 89 / 70 30 30 30 50