Area Forecast Discussion
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FXUS62 KTAE 230807

Area Forecast Discussion
National Weather Service Tallahassee FL
407 AM EDT Tue May 23 2017

.NEAR TERM [Through Today]...
A very broad and anomalously deep positively tilted upper level
trough is northwest of our area, with an axis from the Great Lakes
down to the southern Rockies. This trough has embedded shortwaves,
which will help sustain convection today. At the surface, a surface
front is draped along the east coast, southeastern CONUS, and the
LA/TX coastline. Ahead of the front, our area is situated in a warm,
moist environment, with surface dewpoints in the low 70s and model
PWs for the afternoon around the 99th percentile of our
climatological records. MLCAPE will increase through the afternoon
hours up to around 1500-2000 J/kg, with the highest values in our
northwestern zones. The strong low will also increase the low level
jet, driving 0-1 km shear up to 15-20 kts and bulk shear values up
around 40 kts over our area this afternoon. SPC has placed northern
portions of our area under a slight risk of severe thunderstorms.
The main threat will be damaging wind gusts, but an isolated tornado
or two will also be possible. Heavy rain will also be possible-
around 1-1.5" accumulations are expected during the day today across
most of the area, though isolated spots could see higher totals.

.SHORT TERM [Tonight Through Thursday]...
Main forcing for Tuesday`s precip will be moving across the
Carolinas at the start of the short term period this evening and
our region will be in a brief lull between rounds of showers and
storms. Model guidance is in fairly good agreement with the
overall synoptic pattern showing a deepening upper trough
dropping from the Central Plains into the Mid South by Wednesday
morning. As this occurs strong vertical ascent will lead to
numerous showers and thunderstorms developing across the Eastern
Gulf Coast States ahead of an approaching frontal boundary. Lift
will maximize late Wednesday morning through the afternoon hours
as the frontal boundary moves through the region. Drier conditions
will arrive quickly behind the cold front on Wednesday evening.

The difficulty is in the details, which the individual models are
having trouble resolving. While we`re very confident that there
will be a large area of rain across the area on Wednesday, there
are uncertainties with respect to the severe potential and the
rainfall amounts.

23/00z NAM model soundings show 0-6km shear increasing to near 50
kt across the Florida Panhandle and into Srn GA on Wednesday
morning and early afternoon, with 0-1km shear values in the 20-25
kt range, all fairly impressive values for this part of the
country at the end of May. But previous convection on Tuesday will
limit how much destabilization occurs on Wednesday. Thus, the
overall risk is still fairly uncertain. Wind damage would be the
primary threat and this has been added to the forecast this cycle.

The rainfall forecast is also complicated by the multiple waves of
convection on Tuesday evening through Wednesday. The deep layer
forcing is quite impressive for this time of year and with
precipitable water values exceeding 1.8 inches, it`s likely that
at least some areas will see some hefty rainfall totals by
Wednesday. However, it`s difficult even at this range to pinpoint
where the heavier amounts will occur. Given the higher than usual
degree of uncertainty, will delay a flood watch issuance until
later shifts to assess how today`s convection will influence
tomorrow`s forecast. More info can be found in the hydrology
section below.

Finally, a 24-30 hr period of strong onshore winds ahead of the
frontal system will overspread the northeastern Gulf starting
Tuesday afternoon and continue through Wednesday evening. This
setup tends to favor coastal flooding for the Florida Big Bend
around the time of high tide, generally east of Saint Marks
through the Suwannee River. A coastal flood watch has been issued
for this area valid Wednesday. More details can be found in the
hydrology section below.

.LONG TERM [Thursday Night Through Tuesday]...
Little or no impact weather is expected through the long term
period of the forecast. In the wake of the departing storm system
on Thursday, much drier and somewhat cooler air will overspread
the region through Friday. Temperatures will be several degrees
below normal with low temperatures on Friday morning in the 50s. A
warming trend will commence on Friday as high pressure builds back
over the region and temperatures return to more seasonable levels.

Model guidance has a frontal zone near the forecast area on
Sunday, when rain chances (mainly in the afternoon) will begin to
increase and remain in the 20-40 percent range through the end of
the forecast period.


.AVIATION [Through 06Z Wednesday]...
Light to moderate showers are near DHN/ABY/ECP at the moment, with
development approaching TLH in the next couple of hours.
Thunderstorms will develop around sunrise, with showers and
thunderstorms on and off throughout the day across the area. Cigs
and vsbys will be mostly MVFR, though periods of IFR will be
possible. Winds will generally be from the south-southwest and will
increase through the morning, with gusts of 15-20 kts likely during


Winds will increase quickly today to advisory levels by afternoon.
Though a brief respite from these conditions is possible after
midnight, winds will rapidly increase before daybreak Wednesday
to 25 to 30 knots and persist through the afternoon when a cold
front will move through the marine area. Winds will then begin to
decrease Wednesday night and drop below advisory levels on
Thursday morning. Winds will further subside by Friday as high
pressure builds near the marine area.


Showers and thunderstorms are expected across the area through
Wednesday. With a good soaking rain for the area, there are no fire
weather concerns.


The rainfall forecast continues to be complicated. Model guidance
suggests the potential for 1-2 inches with locally heavier totals
across the region today as an initial disturbance moves through
the region. The more potent disturbance on Wednesday suggests at
least some potential for more widespread heavy rainfall,
particularly across Srn GA and the Florida Big Bend. However,
confidence in the amounts and placement of the heaviest totals is
fairly low. The local ECAM output suggests however, the heaviest
rain amounts will be confined to the Florida Big Bend and perhaps
into far South Central Georgia. At this point in time it appears
that the second round of rain on Wednesday should feature rainfall
amounts of 2-4 inches across much of the area. However, it`s
possible that some isolated heavier totals of 4-7 inches could
occur on Wednesday. The recent dry conditions and the overall
uncertainty in the forecast again argue against the issuance of a
flash flood watch for Wednesday on this cycle.

For the rivers, only the Choctawhatchee River is vulnerable at
this time to river flooding from direct rainfall in the region.
South Georgia rivers are running at very low levels, so even if
the higher amounts materialize over these basins, the river flood
risk would be minimal. Increased flows are being routed down the
Chattahoochee River given expected heavier rains in the ACF system
above Columbus GA. As a result, rises downstream of Woodruff Dam
are expected through the latter part of this week. At the current
time the Apalachicola River at Blountstown is expected to remain
below flood stage.

For the coastal flood threat - strong southwest to west
southwesterly flow across Apalachee Bay initially at 20 knots on
Tuesday afternoon will increase Tuesday night into Wednesday
morning to near 30 knots. This setup increases the likelihood of
coastal flooding for the eastern portion of Apalachee Bay,
generally from St Marks southeastward through the Suwannee River.
Output from both the ETSS and ESTOFS models maximize the highest
surge in the 14-17z time frame, which is in between low and high
tide on Wednesday. Even if the peak surge occurs just prior to
high tide, the models suggest water levels of 1 to 2 feet above
mean higher high water. The ETSS output at 00z was even a little
higher, approaching 3 feet above MHHW. Deviations of this
magnitude typically result in flooding of low lying roads along
the coast and potentially affecting non-elevated structures
immediately adjacent to the beach. Will issue a coastal flood
watch this cycle for Wednesday. Future shifts will evaluate the
need for either a coastal flood advisory or coastal flood warning
for the watch area.



Tallahassee   82  70  77  58  82 /  80  60  90  20   0
Panama City   79  73  77  64  78 /  70  60  90   0   0
Dothan        79  68  78  57  79 /  90  60  80  10   0
Albany        79  68  76  57  79 /  90  50  90  20   0
Valdosta      81  68  76  59  80 /  80  60  90  30   0
Cross City    84  71  79  64  81 /  50  60  90  60  10
Apalachicola  83  74  81  63  82 /  60  60  90  10   0



FL...Coastal Flood Watch from Wednesday morning through Wednesday
     afternoon for Coastal Dixie-Coastal Jefferson-Coastal
     Taylor-Coastal Wakulla.

     High Rip Current Risk through late tonight for Coastal Bay-
     Coastal Franklin-Coastal Gulf-South Walton.

GM...Small Craft Advisory from 5 PM this afternoon to 8 AM EDT
     Thursday for Apalachee Bay or Coastal Waters From Keaton
     Beach to Ochlockonee River Fl out to 20 Nm-Coastal Waters
     From Ochlockonee River to Apalachicola Fl out to 20 Nm-
     Coastal waters from Suwannee River to Keaton Beach out 20
     NM-Waters from Suwannee River to Apalachicola FL from 20 to
     60 NM.

     Small Craft Advisory from 11 AM this morning to 5 AM EDT
     Thursday for Coastal waters from Apalachicola to Destin FL
     out 20 NM-Waters from Apalachicola to Destin FL from 20 to
     60 NM.



LONG TERM...Godsey
HYDROLOGY...Godsey is the U.S. government's official web portal to all federal, state and local government web resources and services.