Flash Flooding and Flooding in Florida, September 11 to 12, 2022

Area Forecast Discussion
National Weather Service Tallahassee FL
704 AM EDT Sun Sep 11 2022

.AVIATION...
[Through 12Z Monday]

Widespread IFR/LIFR conditions will prevail through at least 13z
this morning. Widespread showers and storms over the Gulf will
begin to move inland through the morning, possibly affecting
ECP/TLH after 13z. Conditions improve to MVFR/IFR levels after
13/14z but they'll likely stay at or around MVFR levels all day
due to low ceilings. Additionally, widespread coastal convection
will move inland through the day bringing potential lower
restrictions and TS in any of the stronger storms.

&&

.PREV DISCUSSION [407 AM EDT]...

.NEAR TERM [Through Tonight]...

One last day of widespread showers and storms, especially across our
coastal locations, is expected for today. Similar to the last few
days, the greatest coverage and rainfall amounts is anticipated to
be across the immediate coastal zones where the greatest instability
and low level convergence along a weak warm front is anticipated.
The upper level trough that's been driving these round of showers
will be in the process of weakening today and will move east out of
the region by the evening hours. As this occurs, we'll see
decreasing coverage of storms as instability decreases and the
primary upper level support begins to move away.

The main concerns through this afternoon will continue to revolve
around the threat for flooding and flash flooding. With precipitable
waters of 2 to 2.25 inches expected, and the potential for training
convection in our coastal areas, rainfall totals could again quickly
add up in locations. Widespread rainfall totals will be around 1 to
3 inches along the immediate coast and taper off to around 1 inch
across I-10. Amounts of less than an inch are anticipated across
AL/GA. The flood watch was left intact for this afternoon despite
the lower-than-forecast rainfall totals we have observed the last
few days along the I-10 corridor. While these locations have only
seen around 0.5 - 1 inches of rain west of Tallahassee, and 1 to 2
inches east of Tallahassee, the higher end potential of 2 to 4
inches in any stronger storms and training convection still remains
possible across the watch area in isolated areas. It's still quite
possible that abundant morning convection somewhat limits the
potential for these heavier amounts inland. For our coastal areas,
the upper end possibilities are a little higher due to the better
meteorological setup. Additional amounts of 3 to 6 inches can't be
ruled out in isolated areas. With much of the ground quite saturated
across our coastal zones , a flooding threat will likely develop
quickly in response to heavier rainfall and slower moving storms.
Please be sure to monitor local radar trends in the weather and
local officials through the afternoon.

.SHORT TERM [Monday Through Wednesday]...

The upper trough currently trailing south from the Upper
Mississippi Valley will continue east across the Great Lakes
region through Tuesday, exiting northeast from there on Wednesday.
A weak trailing upper trough will help push a cold front through
our Alabama and Georgia counties on Monday, limping south through
our Florida counties and into the far northern Gulf on Tuesday.

In advance of the front on Monday, the pre-frontal air mass will
be characterized by PW values of 1.6-1.8 inches, which is
sufficient for deep, moist convection. Moderate instability and
about 15-20 knots of deep layer shear in southwest flow will
exist. The southwest flow will still allow morning showers and
thunderstorms to form over the 85-86 degree Gulf waters and push
onshore into our FL counties.

As the front reaches our AL and GA counties on Monday afternoon
and encounters this air mass, inland convection will develop
closer to the front on its southward journey.

The front will limp just offshore into the Gulf on Tuesday. Well
behind the front over AL and GA, the air mass will dry out. PW
values will drop well below 1 inch, and surface dewpoints will dry
out into the very comfortably un-muggy 50s. Further south over
FL, a lesser degree of drying will take place. The front will get
mixed in with the afternoon seabreeze, so an isolated shower or
thunderstorm is still possible along the seabreeze front on
Tuesday afternoon. It should be pinned fairly close to the coast,
so low rain chances will be confined south of I-10.

The story on Wed will be similar. Along and NW of a PAM-TLH-VAD
line, the air will be pleasantly dry, and the forecast will lack
convection. As you work south from there, moisture gets
increasingly deep, and rain chances reflect that.

.LONG TERM [Wednesday Night Through Sunday]...

The front will dissipate on Wednesday night and Thursday. Surface
high pressure over the Great Lakes will bridge south, bringing a
turn to easterly flow. A return of modified maritime air will
return off the Atlantic. PW values will climb into the 1.5-1.7
inch range, and surface dewpoints will climb back into the upper
60s to near 70 from late Thursday on. Coverage of showers and
thunderstorms will rise back into scattered territory over the
course of Thu and Fri, which is fairly typical in mid-September.
This setup will continue through next weekend.

When the next 7 days are taken as a whole, we will exit the
current heavy rain pattern and see our first taste of fall. At
our latitude, this entails a brief foray with a drier air mass and
a reminder of what life is like outside of the hot, wet blanket
that we have endured all summer. The drier air will support better
radiational cooling for a few nights. There will be a few
pleasantly cool mornings in the 60s, especially from Tuesday
through Thursday mornings. Enjoy!

.MARINE...

Buoys 42039 and 42036 are observing 5- and 4-foot seas
respectively, owing to the long fetch of southerly breezes over
the eastern Gulf. Southerly breezes will become southwesterly over
the course of today and tonight, and the fetch will shorten. This
will allow seas to slowly subside.

From CWF synopsis...Low pressure over the north-central Gulf will
get absorbed on Monday into a cold front that will slip south
through Alabama and Georgia. That front will arrive over the
northern Gulf on Tuesday, bringing a shift to northerly breezes.
The front will then turn up stationary through Wednesday. High
pressure over the Great Lakes will bridge south across the waters
on Thursday, bringing a freshening easterly breeze.

.FIRE WEATHER...

Humid and wet conditions, especially for coastal locations, will
remain in place this afternoon. This will keep fire weather concerns
low. Only concerns for today are low dispersions.

Little change is expected through Monday. Then late Monday, a
cold front will slip south through our Alabama and Georgia
districts, limping into the Gulf on Tuesday. A shift to northerly
breezes is expected. A significant drop in daytime relative
humidity is forecast on Tuesday and Wednesday, with the biggest
drop over our Alabama, Georgia, and Panhandle districts.

.HYDROLOGY...

We now enter Day 3 of this ongoing heavy rain event. So far, the
heaviest rain has been confined to coastal communities from Panama
City eastward. There is little more than one more day left to
receive hydrologically significant rainfall, so the prospects for
riverine flooding are decreasing. There is still a small chance
of minor flooding on the Ochlockonee and Withlacoochee Rivers, but
it would take well over 2 inches of rain in these basins to send
the rivers out of their banks. This is now very unlikely but
cannot yet be ruled out.

Otherwise, the Flood Watch will continue as is until the end of
today. Additional heavy rain of 2 to 4 inches is possible in the
same coastal communities that have gotten such heavy rain in
recent days. Given saturated soils and the current vulnerability
to additional rainfall along the coast, continuing the Flood Watch
is wise. Pockets of heavy rainfall are possible today as far
inland as I-10. On Monday, there could still be some heavy rain
along the Nature Coast and Lower Suwanee Valley.

Beyond Monday, no hydrologically significant rain is expected.
Therefore, no additional flooding is expected through next
weekend.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not requested. However, spotters are always
encouraged to safely report significant weather conditions when they
occur (while following all local, state, and CDC guidelines) by
calling the office or tweeting us @NWSTallahassee.

&&

.PRELIMINARY POINT TEMPS/POPS...

Tallahassee   85  73  87  70  87 /  70  40  70  30  20
Panama City   86  74  87  70  87 /  80  50  30  30  10
Dothan        84  71  85  64  83 /  60  20  50  20   0
Albany        85  72  86  67  84 /  70  30  50  20   0
Valdosta      85  73  86  70  86 /  80  30  70  20  20
Cross City    88  74  88  72  89 /  70  60  80  40  40
Apalachicola  85  75  85  72  85 /  90  70  60  20  20

&&

.TAE WATCHES/WARNINGS/ADVISORIES...
FL...GA...None.
AL...None.
GM...None.
&&

NEAR TERM...Dobbs
SHORT TERM...Haner
LONG TERM...Haner
AVIATION...Dobbs
MARINE...Haner
FIRE WEATHER...Dobbs
HYDROLOGY...Haner

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE TALLAHASSEE FL
749 AM EDT SUN SEP 11 2022

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0740 AM     HEAVY RAIN       2 WNW BLUE MOUNTAIN BEA 30.35N 86.23W
09/11/2022  M4.20 INCH       WALTON             FL   MESONET

            MESONET STATION 1451W ED WALLINE BEACH
            ACCESS MEASURED 4.2 INCHES OF RAIN IN THE
            PAST 24 HOURS.

&&

JLF