Area Forecast Discussion
Issued by NWS Jacksonville, FL

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FXUS62 KJAX 131911
AFDJAX

Area Forecast Discussion
National Weather Service Jacksonville FL
211 PM EST Thu Dec 13 2018

...POTENTIAL FOR HEAVY RAIN AND A FEW SEVERE STORMS FRIDAY
AND FRIDAY NIGHT...

.NEAR TERM.../Thru Tonight/...
High pressure will remain centered well NE of forecast area, while
intense storm system develops over the western Gulf Coast region.
Clouds will continue to stream over forecast area from Gulf of
Mexico. High-res guidance suggests a few coastal showers that may
affect our easternost counties this afternoon. As lift increases
over forecast area Tonight, may see scattered showers develop,
with more numerous showers over western portion of our forecast
area. Guidance suggests low temperatures will be met around 06z
with steady or slowly rising temperatures toward dawn.

.SHORT TERM.../Friday thru Saturday/...
Storm system will move very slowly eastward Friday through Friday
night, with associated cold/occluded front moves approaches forecast
area from the West. Models seem to be in decent agreement that a
semi-organized line of intense convection will develop over the
central Gulf of Mexico late Tonight, tracking gradually eastward
across the eastern Gulf of Mexico Friday morning, then move inland
over northern FL peninsula into southern GA during the late
morning and early afternoon hours. The airmass will be too stable
for near surface-based convection, thus anticipating the line will
gradually weaken with just some elevated storms. This activity
progresses eastward during the afternoon. Very strong vertical
wind shear will be in place. The big question is the level of
airmass instability to promote intense updrafts. Global models
like the GFS are not suggesting much destabilization due to
widespread precipitation. Higher-resolution models like the NAM
and 36-hr HRRR (12z run) suggest widespread precipitation during
the late morning/early afternoon will be confined to srn GA and
the Suwannee Valley region of nrn FL, and show temperatures
increasing into the lower 70s with upper 60s dew points to support
about 400-800 J/Kg CAPE across much of ne FL. This may be
sufficient to support strong/severe storms with rotation, either
in the form of a squall line with embedded circulations, or semi-
discrete supercells. In fact, the HRRR suggested scattered
supercell storms across ne FL late in the day. The peak period of
the severe weather threat appears to be the 3pm-9pm window
particularly along and SE of a JAX/GNV line, with more isolated
threat further N.

Also looking at potential for multiple rounds of heavy rain with
this event. The first may be over our far western counties during
the morning, then across forecast area during the afternoon, with
the HRRR suggesting a third potential round Friday night
spreading across ne FL from the eastern Gulf of Mexico. At this
point, looking at 1-2 inches of rain for eastern half of ne FL,
2-3 inches with local 4-5 inches further N and W. Will maintain
existing Flood Watch, though the greater threat for flooding
appears to be across se GA and Suwannee Valley region due to
previous heavy rain event. High temperatures Friday will be in the
upper 60s across se GA, lower to mid 70s across ne FL. Cloudy
showery conditions expected later Friday night, with mild min
temperatures in the upper 50s across se GA, lower 60s across ne
FL.

Activity gradually decreases from W to E on Saturday. Given the
occluded nature of the system, expect considerable cloudiness,
with not much of a cool down. High temperatures Saturday are
expected 68-70 across se GA, lower 70s across ne FL.

.LONG TERM.../Saturday night thru Wednesday/...
Storm system shifts E of oour Forecast area Sunday, while cool
high pressure builds toward our area from the W. This will lead
to high temperatures Sunday in the lower to mid 60s, possibly
upper 60s Marion County, under partly cloudy skies.

High pressure ridge at the surface and aloft build over forecast
area Tuesday, shifting east of forecast area Wednesday.
Temperatures are expected to be near or slightly below normal
Mon-Wed.

&&

.AVIATION...VFR with mainly mid clouds between 5000-10000ft this
afternoon and evening with possibly a sprinkle at SSI. After 06z
Tonight, low level winds increase to 35-40 knots above the
surface and have included LLWS - Low Level Wind Shear in the
current TAF set. Expect CIGS to generally remain VFR in this
period with a few sprinkles possible, but main chances for rain
and lower MVFR CIGS/VSBYS will hold off until the 12-15z time
frame, or possibly later in the morning, but showed some lowering
conds at TAF sites for now. Southerly winds will start to mix down
towards the surface after 15z with 12-14G20 kts possible.

&&

.MARINE...High pressure will move Northeast of area waters Today
and Tonight. A warm front will lift North across the region
Tonight. A cold front will sweep East across the region Friday
into Friday evening, with a round of showers and storms expected.
Some of these storms could become strong to severe. Winds will
increase into caution level nearshore, and advisory level offshore
Tonight and Friday.

A secondary front will move through Saturday, as low pressure moves
to the North. High pressure will build from the West early next
week.

Rip Currents: Moderate to high risk Friday.

&&

.HYDROLOGY...
Flood Watch remains in effect for all of SE GA and interior portions
of NE FL for a widespread 2-4 inches of rainfall, with locally
higher amounts, while amounts closer to 1-2 inches expected for
points SE of GNV/JAX across the rest of NE FL. A localized 4-6
inch rainfall band possible in the Flood Watch area, but too early
to speculate exactly where this will fall. Widespread Minor to
Moderate flooding expected for most river basins in the Flood
Watch area, along with potential Major FLooding for the Alapaha
River Basin in Echols/Hamilton counties.

&&


.PRELIMINARY POINT TEMPS/POPS...
AMG  57  68  58  68 /  90 100  90  20
SSI  58  67  58  67 /  60  90  90  30
JAX  60  73  61  72 /  40  90  90  30
SGJ  61  73  62  72 /  40  80  90  40
GNV  61  74  61  72 /  40  90  90  30
OCF  61  75  62  73 /  40  80  90  40

&&

.JAX WATCHES/WARNINGS/ADVISORIES...
FL...Flood Watch from late tonight through late Friday night for
     Alachua-Baker-Bradford-Clay-Coastal Duval-Coastal Nassau-
     Columbia-Gilchrist-Hamilton-Inland Duval-Inland Nassau-
     Suwannee-Union.

GA...Flood Watch from late tonight through late Friday night for
     Appling-Atkinson-Bacon-Brantley-Charlton-Clinch-Coastal
     Camden-Coastal Glynn-Coffee-Echols-Inland Camden-Inland
     Glynn-Jeff Davis-Pierce-Ware-Wayne.

AM...Small Craft Advisory from 10 PM this evening to 3 PM EST
     Saturday for Waters from Altamaha Sound GA to Fernandina
     Beach FL from 20 to 60 NM-Waters from Fernandina Beach to
     St. Augustine FL from 20 to 60 NM-Waters from St. Augustine
     to Flagler Beach FL from 20 to 60 NM.

&&

$$

Wolf/Hess


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