Flooding in Florida, September 9, 2022

Area Forecast Discussion
National Weather Service Jacksonville FL
1059 AM EDT Fri Sep 9 2022

...New UPDATE...

.UPDATE...
Issued at 1023 AM EDT Fri Sep 9 2022

Overall forecast is on track this morning as a near stationary
wavy front lies across the SE GA coast where a band of heavy
showers persist just north of the mouth of the Altamaha river
with new showers now forming just offshore around a weak low
forming along the east to west frontal boundary. There's more
clearing over portions of NE FL from Ocala northeast to St
Augustine while earlier fog and low stratus over inland areas is
now lifting into a stratocumulus field underneath mid and high
level clouds moving in from the SSW as new convection is
developing over the NE Gulf coastal waters.

Many ingredients are in place for another afternoon of numerous
to widespread showers and thunderstorms with locally heavy
rainfall possible. A Flood Watch is in effect through late tonight
for coastal SE GA areas including the I-95 corridor from Brunswick
and St Simons Island south to Kingsland and Saint Marys where 2-4
inches of rainfall is possible through tonight with locally heavy
amounts possible there. The Weather Prediction Center has a
slight risk of excessive rainfall covering inland NE FL and most
of SE GA for the potential for locally heavy rain and localized
flash flooding potential as high moisture ahead of a vigorous mid
to upper low to the west overrides the frontal boundary over the
area. Thunderstorms will form along the Atlantic seabreeze and
also along the front with activity through inland SE GA and lift
northeast eventually moving offshore into the Atlantic waters
before winding down just after sunset as some slightly direr air
works in from the south along the FL peninsula.

Highs will be in the mid 80s for most of the area with upper 80s
over the Saint Johns river basin and lower 80s over inland SE GA
where more cloud cover is present. Lows will be in the low to
middle 70s.

&&

.SYNOPSIS...
Issued at 500 AM EDT Fri Sep 9 2022

Early morning surface analysis depicts a wavy stationary frontal
boundary situated across the Deep South, with weakening low
pressure (1007 millibars) positioned along the front over the
lower Mississippi Valley. Meanwhile, high pressure (1016
millibars) was building over the eastern Great Lakes and the Mid-
Atlantic region in the wake of this front. Aloft...cutoff
troughing was only slowly filling as it remains stationary over
coastal Louisiana, while ridging takes shape off the southeast
U.S. coast in between this cutoff trough and Hurricane Earl, which
has made its closest approach to Bermuda as it accelerates north-
northeastward over the open waters of the North Atlantic.
Otherwise, a deepening trough was progressing southeastward from
the northern Rockies and across the Northern Plains states. A
round of thunderstorms and heavy downpours on Thursday evening
that resulted in 2-4 inches of rainfall in portions of Glynn and
Duval Counties has lifted northward towards the Savannah and
Charleston metropolitan areas, with convection during the predawn
hours confined to our adjacent coastal waters, while another area
of convection was located over Apalachee Bay and the FL Big Bend
coast. Multi-layered cloudiness remains in place area-wide, with
low stratus ceilings expanding at inland locations. Temperatures
and dewpoints at 09Z were mostly in the 70-75 degree range.

&&

.NEAR TERM...
(Today and tonight)
Issued at 500 AM EDT Fri Sep 9 2022

The cutoff trough will only slowly fill through tonight and will
begin to nudge northwestward across the lower Mississippi Valley
tonight as ridging aloft continues to build off the southeastern
seaboard. This weather pattern will keep a deep southerly flow
pattern in place locally, with a 60-knot jet streak at 250
millibars (around 35,000 feet) located in between these features
to the south of the FL panhandle coast. Meanwhile, strengthening
high pressure over the Mid-Atlantic region will wedge down the
southeastern seaboard through tonight, resulting in a tightening
local pressure gradient near the position of the wavy, stalled
surface boundary over southeastern GA. Breezy onshore winds should
develop over coastal southeast GA shortly after sunrise. This
tightening pressure gradient will maximize surface convergence
over coastal southeast GA, where some of the short-term, high
resolution guidance indicates that a few bands of slow moving
downpours will set up over Glynn and possibly portions of Camden
Counties later this morning and into the afternoon hours.
Additional rainfall amounts could exceed 2 inches over these
coastal counties, which will increase the threat for flooding,
especially at urban and normally flood prone locations such as
Brunswick, which experienced flooding on Thursday evening due to
2-4 inch rainfall amounts. Additionally, strengthening southerly
flow will result in increasing speed shear across our region this
afternoon, and the presence of the frontal boundary and
strengthening surface convergence could result in a few strong or
even an isolated severe thunderstorm this afternoon, especially
for locations from Waycross northward. An isolated tornado cannot
be ruled out near the stalled boundary through this evening.

We expect that another round of morning convection will occur
over portions of north central FL and the Suwannee Valley, as
convection developing over the far northeast Gulf of Mexico
advects north-northeastward. This convection will likely shroud a
good portion of our area in thick cirrostratus, which will again
delay a majority of the downpours until the afternoon hours. PWATs
will remain elevated (above 2 inches), and relatively mild mid-
level temperatures will result in efficient rainfall production
within convection that develops this afternoon. A band of heavy
downpours will again likely traverse much of our area through the
early evening hours. Rainfall amounts of 0.5 to 1.5 inches is
expected at most locations through this evening, with locally
higher amounts possible. Localized flooding will be possible,
especially across southeast GA and portions of the Suwannee Valley
and north central FL, where the Weather Prediction Center has
indicated a slight risk of excessive rainfall through tonight.
Plenty of cloud cover and afternoon convection should keep highs
in the low to mid 80s across our area today.

Any downpours that linger into the evening hours at coastal
locations could result in an enhanced flood threat due to
astronomically high tides, which will peak later this evening.
Otherwise, a pocket of slightly drier air advecting northward
across the FL peninsula this afternoon will overspread our area
overnight. Additionally, the strengthening ridge off the
southeastern seaboard will likely lift the wavy frontal boundary
north of the Altamaha River basin tonight, which will result in
the best surface convergence shifting north of our region. Low
stratus ceilings will likely develop again towards the predawn
hours, with lows generally falling to the 70-75 degree range
throughout our area.

&&

.SHORT TERM...
(Saturday through Sunday)
Issued at 500 AM EDT Fri Sep 9 2022

Prevailing flow will continue to be out of the south this weekend
as the upper level low off to the west is gradually absorbed by
the prominent trough over the central US. Heavy rains capable of
producing localized flooding are expected during this period with
PWAT values measuring at times as high as 2 to 2.3 inches. Diurnal
convective developments will initially build in over the
westernmost portions of the forecast area and then expand eastward
as the afternoon progresses. Daily high temperatures during this
period will reach up into the upper 80s and lower 90s for areas to
the east of the I-75 corridor and in the mid 80s for areas to the
west of I-75 that are expected to receive convective developments
earlier in the day. Overnight low temperatures will drop down into
the lower 70s over inland areas and in the mid to upper 70s along
the coastline.

&&

.LONG TERM...
(Monday through Wednesday)
Issued at 500 AM EDT Fri Sep 9 2022

The upper level low will become fully absorbed by the trough over
the central US as it moves eastward with the prevailing flow
shifting about to become more out of the southwest-west. Chances
for heavy rainfall will continue on into Monday ahead of the
approaching frontal boundary from out of the west-northwest. Drier
air will settle in over the region with reduced chances for
precipitation expected through midweek. Temperatures for next
week will be in line with the seasonal average.

&&

.AVIATION...
(12Z TAFS)
Issued at 821 AM EDT Fri Sep 9 2022

The 12Z TAF period will begin with a range of conditions from
LIFR at KJAX, IFR at KSSI due to onshore moving heavy showers
to VFR conditions at the remaining terminals as scattered low
clouds linger around 1.0 to 2.5 kft with a lull in showers for
NE FL TAF sites. Fog and low stratus should begin to lift and
clear by 1330Z at KJAX moving from brief IFR visibility around
1/4 to 1/2 mile and broken ceilings around 200ft lifting to
VFR levels as low clouds and fog scatters out as southeast winds
begin to pick up to around 5 knots. The southeast winds will
range generally 5-10 knots at the TAF sites with some higher
gusts possible as high pressure builds to the north and
easterly onshore winds develop near the wavy near stationary
front draped east to west across coastal SE Georgia. Expecting
a round of showers to develop by 13Z-15Z across the area
terminals as energy aloft moves in from the Gulf of Mexico. This
activity will translate into increasing coverage of thunderstorms
after 17Z this afternoon with heavy downpours reducing visibility
levels into MVFR and IFR levels. This activity should begin to end
by around 00-01Z with some lingering showers exiting north and
east into the Atlantic waters by 02-03Z with broken to overcast
mid and high level clouds overnight with another round of showers
entering from the Gulf late overnight with VCSH noted at KGNV
after 06Z and generally VFR conditions and near calm winds at the
regional TAF sites to end the 12Z period.

&&

.MARINE...
Issued at 515 AM EDT Fri Sep 9 2022

A frontal boundary will remain stalled over the southeast Georgia
waters today and will briefly lift north of the Altamaha sound
tonight and Saturday. Meanwhile, high pressure will gradually
strengthen over the Mid-Atlantic region and New England through
Saturday. This weather pattern will create strengthening onshore
winds across the southeast Georgia waters this morning, with
speeds approaching Caution levels of 15-20 knots by late morning.
Combined seas will also build throughout our local waters due to
long period swells originating from distant Hurricane Earl. Seas
should remain just below Small Craft Advisory levels through
Saturday, with 4-6 foot seas forecast throughout the Georgia
waters, as well as the offshore waters adjacent to northeast FL.
Seas of 3-5 feet will prevail over the nearshore waters adjacent
to northeast FL through Saturday evening. Periods of showers and
thunderstorms will continue across our waters into early next
week, as the frontal boundary sinks slowly southward late this
weekend. Winds and seas will then subside by Tuesday ahead of
another frontal boundary that will likely stall over the northeast
Florida waters late next week.

&&

.HYDROLOGY...
Issued at 500 AM EDT Fri Sep 9 2022

Additional rainfall amounts of 2-4 inches are possible today
across coastal southeast GA, possibly extending westward along the
Altamaha River basin, where bands of slow moving downpours will
likely develop along a nearly stationary frontal boundary. Locally
higher totals will be possible, and a Flood Watch has been issued
for Glynn and Camden Counties. Rainfall amounts elsewhere of 0.5
to 1.5 inches are expected through this evening, with locally
higher amounts possible, especially across inland southeast GA,
the Suwannee Valley, and north central FL, where multiple rounds
of downpours will be possible. Localized flooding is a concern at
these locations, especially for urban and normally flood prone
locations. Additional afternoon and evening downpours this weekend
will likely result in additional rises on area rivers and
tributaries, with "flashier" basins such as Black Creek in Clay
County being monitored, as basins become increasingly saturated.

&&

.PRELIMINARY POINT TEMPS/POPS...
AMG  82  70  88  73 /  90  60  90  40
SSI  84  75  89  77 /  90  40  80  50
JAX  85  72  90  74 /  80  40  80  40
SGJ  85  75  89  76 /  80  40  70  40
GNV  85  72  90  72 /  80  40  90  30
OCF  85  71  90  73 /  80  40  90  30

&&

.JAX WATCHES/WARNINGS/ADVISORIES...
FL...High Rip Current Risk through late Saturday night for FLZ124-125-
     133-138.

GA...High Rip Current Risk through late Saturday night for GAZ154-166.

     Flood Watch through late tonight for GAZ153-154-165-166.

&&