Area Forecast Discussion
Issued by NWS Jacksonville, FL

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FXUS62 KJAX 182113
AFDJAX

Area Forecast Discussion
National Weather Service Jacksonville FL
513 PM EDT Sun Apr 18 2021

...ISOLATED STRONG TO SEVERE STORMS POSSIBLE AGAIN SOUTH OF
INTERSTATE 10 THROUGH EARLY THIS EVENING...

...LOCALLY HEAVY RAINFALL THREAT CONTINUES ACROSS NORTHEAST
FLORIDA MAINLY SOUTH OF THE I-10 CORRIDOR THROUGH TONIGHT...

.NEAR TERM [Through Monday]...

Tonight, the frontal boundary has nudged into NE FL this
afternoon with earlier rounds of showers and a few intense storms
near Flagler Beach having moved well offshore into the Atlantic
waters. The front`s position is partly due to a meso high that
formed behind the initial round of storms over NE FL areas late
this morning with light cyclonic flow observed from surface
observations with light SE winds near Ocala and from the west at
Gainesville. This meso high has helped deter stronger storm
development until later this afternoon over much of NE FL with
light to moderate showers now streaming east along I-10 into
jacksonville. But, strong mid level flow (effective bulk shear of
45 knots) over NE FL downstream of a short wave trough pivoting
east from the southern plains into the lower MS Valley along with
upper level jet stream diffluence in the vicinity of the Right
rear quadrant will position over the NE Gulf of Mexico into NE FL
and enhance lift over northern FL through this evening. These
features will allow additional waves of convection to stream ENE
from the Gulf of Mexico along and just north of the frontal
boundary as it sinks southward back into central FL through
tonight.

For storm potential, ML CAPE levels will increase to at least
1000 J/kg over north central FL areas generally along and south
of a line from Gainesville to St Augustine. The rest of NE FL
will See less of a threat with some potential for strong storms
ML CAPE values under 500 J/kg and elevated MU CAPE values under
300 J/kg yielding mainly a small hail and heavy rain threat with
strong damaging winds more likely the main hazard farther south
over north central FL areas. Given strong effective shear that
could allow persistent updrafts and the proximity of the frontal
boundary, an isolated tornado cannot be ruled out over north
central FL where higher dewpoints in the upper 60s to low 70s
will be.

Also, given the persistent moist airmass near the front with
PWAT values well above the 90th percentile at over 1.6 inches
from the 12Z JAX morning sounding and RH values in the low and
mid levels near 90 percent, convection will again carry highly
efficient rainfall rates late today into tonight similar to
yesterday. This will allow an additional swath of widespread
1-2 inch totals over areas south of I-10 for NE FL with locally
heavier amounts on top of areas that have received up to 3-5
inches over the past 48 hours from Ocala to Palm Coast while at
least 1-3 inches have fallen over all of NE FL in that same
window. Therefore, isolated flash flooding will remain a concern
in these areas that have seen the highest recent rainfall totals
this weekend in addition to urban centers with poor runoff.

Wet conditions and cloudy overcast skies will persist overnight
with light westerly winds and near seasonal overnight temps in
the low 60s over NE FL and the upper 50s over SE GA will be
common overnight.

Monday, the frontal boundary over central FLorida will slide
farther southward into south FL. Scattered to numerous showers
and a few storms are expected with drier conditions over SE GA
under mostly sunny skies. Winds will shift to the NW around 10-15
mph as weak high pressure attempts to build west of the region.

.SHORT TERM [Monday Through Tuesday Night]...

Monday night, the frontal boundary will be stalling near lake
Okeechobee as the shortwave trough exits east into the Atlantic.
This feature will be replaced by an upstream southern stream
shortwave trough moving east from TX that will ride through the
base of a northern stream trough swinging from the northern
plains into the Mid MS Valley on Tuesday. These features aloft
will help develop another wave of low pressure over the Gulf
of Mexico that will travel NE along the frontal boundary over
north central FL and portions of NE FL south of Jacksonville
yielding another round of showers and a few storms before the
front slides east into the Atlantic waters as another cold front
pushes into the southeast US. Another inch of rain could fall
over mostly north central FL areas. The secondary cold front
will press east through the area on Wednesday with strong high
pressure building from the northwest. This will usher in cooler
and much drier conditions with breezy NW winds through Wednesday
night.

Temperatures will be slightly below normal with highs in the 70s
with near normal for lows in the upper 50s to lower 60s through
Tuesday night, then warm to near normal temperatures on Wednesday
around 80 degrees as skies become sunny.

.LONG TERM [Wednesday Through Sunday]...

Strong high pressure will build in closer to the region from the
NW on Thursday and then move north of the region on Friday with
continued dry conditions. The high will move NE of the region on
Saturday with winds becoming southerly as a warm front lifts north
into the Gulf coast with showers pushing into the area from the
west. Sunday, timing differences between models on how quickly
a developing low pressure and trailing cold front moves through
the region yield lower confidence in any shower coverage for the
end of next weekend, so have kept scattered pops for now.

Initially cooler temperatures late this week will be warming to
above seasonal values next weekend.

&&

.AVIATION...
[Through 18Z Monday]

Bouts of scattered showers will move through the region until
later this afternoon and evening, where another bout of
thunderstorms and heavier rains is expected to move through across
NE Florida, south of the frontal boundary. Ceilings can be
expected to drop to IFR levels during this period with cloud
heights remaining low for Florida sites into Monday morning.

&&

.MARINE...

Light offshore flow will remain over the water with no headlines
expected through Monday. Winds will become onshore on Tuesday as
low pressure tracks along frontal boundary south of the waters
with main impacts still the showers and embedded storms with heavy
rainfall at times. A secondary frontal passage on Wednesday will
bring a surge of northerly winds around 20 knots Wednesday Night
and early Thursday that may lead to Small Craft Advisory
conditions.

Rip Currents: Moderate risk today for NE FL and SE GA Beaches
today. Moderate risk on Monday for NE FL beaches with a low risk
for SE GA beaches.

&&

.FIRE WEATHER...

Low daytime dispersion values will continue due to light surface
and transport winds through Tuesday for Northeast Florida with
higher dispersions likely over southeast Georgia areas on Monday.

&&

.HYDROLOGY...

Around 3-6 inch rainfall totals are noted across Marion, Putnam,
and Flagler counties with only minimal impacts due to antecedent
drier conditions. Another 1-2 inches could fall through late
tonight, but a Flood watch is not anticipated as there`s been a a
break in rainfall south of I-10 since late this morning. Not as
much rainfall expected on Monday as the axis of best rainfall
slips southward into central FLorida with more rain possible on
Tuesday as the frontal boundary works north back towards north
central FL as a new low tracks NE along the boundary from the Gulf
of Mexico. Minor River flooding may still be possible along the
upper Santa Fe Basin, but greatest flooding impacts will continue
to be the metro areas mainly south of Jacksonville through
Tuesday.

&&

.PRELIMINARY POINT TEMPS/POPS...

AMG  56  77  52  78  54 /  20   0  10  20  10
SSI  61  78  61  75  61 /  70  10  20  30  10
JAX  61  79  58  74  59 / 100  30  30  30  20
SGJ  63  75  61  72  62 / 100  60  50  50  20
GNV  61  76  56  76  58 / 100  50  50  50  20
OCF  64  77  60  77  61 / 100  80  60  70  20

&&

.JAX WATCHES/WARNINGS/ADVISORIES...
FL...None.
GA...None.
AM...None.
&&


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