Hail and Heavy Rain in Florida, April 17, 2021

Area Forecast Discussion
National Weather Service Jacksonville FL
918 PM EDT Fri Apr 16 2021

...STRONG TO SEVERE THUNDERSTORMS POSSIBLE ON SATURDAY AFTERNOON
AND EVENING ALONG THE INTERSTATE 10 CORRIDOR...
...WAVES OF DOWNPOURS EXPECTED TO CONTINUE SUNDAY THROUGH TUESDAY
FOR LOCATIONS SOUTH OF I-10...

.UPDATE...Widespread area of rain continues to progress off the
east coast this evening as upper impulse shifts east. Another
wave in the WNW flow aloft will likely bring more rain to the
area late tonight and Saturday morning. Models show the frontal
boundary stalled just to our south lifting north into NE Fl to
near the I-10 corridor on Saturday as another shortwave trough
aloft and surface wave approach the area. Afternoon destabilization
with strong bulk shear present may result in strong to severe
storms near and south of the frontal boundary by late day. Storms
with hail will be possible due to strong shear and elevated
CAPE...with isolated strong winds/tornado depending on the degree
of surface based instability.

&&

.PREV DISCUSSION [741 PM EDT]...

.NEAR TERM [Through Saturday]...

Fast zonal flow will persist over our area, which will advect
shortwave energy across the northeast Gulf of Mexico this evening
and along the I-4 corridor overnight. Mostly stratiform rainfall
is expected through this evening along the I-10 corridor, but a
few strong thunderstorms will remain possible across north central
FL through the overnight hours as the wavy frontal boundary
remains nearby and instability remains confined to locations south
of a line from Gainesville to St. Augustine. Small hail and gusty
winds may be possible in Marion, southern portions of Putnam and
Flagler Counties overnight through around sunrise on Saturday
before shortwave energy departs our region. Periods of light rain
and drizzle over inland southeast GA will end shortly after sunset
for locations north of Waycross, with periods of showers
continuing closer to the I-95 corridor through around midnight.
Lows tonight will generally fall to the mid 50s across inland
southeast GA, with upper 50s along the I-10 corridor and low to
mid 60s for north central FL.

The cutoff trough currently over the southern Plains region will
be propelled eastward on Saturday by a potent shortwave trough
that will be diving southward through the northern Plains States.
This weather pattern will produce an increasingly diffluent flow
pattern over our area as winds aloft back to a more westerly
direction, with a 150-knot jet streak at 250 millibars (35,000
feet) traversing the Tennessee Valley and the Carolinas late in
the day. There should be a brief lull in convective activity
during the morning hours, followed by increasing chances for
showers and thunderstorms during the afternoon hours along the
I-10 corridor. Strong to isolated severe thunderstorms will be
possible in these areas during the late afternoon and evening
hours, with more stratiform rainfall expected from Waycross
northward. The presence of the wavy frontal boundary over north
central FL will have to be watched later tomorrow, as convection
could become surface based for locations south of I-10, with steep
mid-level lapse rates depicted by model soundings and bulk shear
values of 50-60 knots presenting a hail and damaging wind threat,
with an isolated tornado threat for these locations as well. A few
breaks in the mid and high altitude cloud cover during the late
morning and early afternoon hours will boost highs into the 70s
for most locations, except around 80 for north central FL.

.SHORT TERM [Saturday Night through Monday Night]...

Short-term, high resolution guidance suggests that convection will
continue on Saturday night, especially for locations south of I-10
as shortwave energy moves overhead in a highly diffluent zonal
flow pattern. Strong to possibly severe thunderstorms along the
I-10 corridor may sink southward overnight as the frontal
boundary nudges southward, with activity continuing through Sunday
morning for north central FL. Lows will range from the upper 50s
for locations north of Waycross to the mid 60s in north central
and coastal northeast FL.

The potent shortwave trough diving through the Plains States on
Saturday evening will pivot eastward across the Ozarks on Sunday
and across the Tennessee Valley on Sunday night. Waves of
downpours will continue to emerge from the Gulf of Mexico on
Sunday, with the heaviest rainfall expected along and south of
I-10. Rainfall amounts on Sunday could easily exceed 2 inches
across a good portion of northeast and north central FL. Highs on
Sunday will mostly remain in the 70s, except around 80 in north
central FL. The trough will cross the Appalachians on Monday
morning and will move offshore of the mid-Atlantic coast by Monday
night, possibly bringing a brief lull in downpours along the I-10
corridor beginning on Monday afternoon and evening as diffluence
briefly weakens. Waves of lighter stratiform rainfall across
southeast GA may also decrease by late Monday. Cloud cover will
remain abundant, keeping highs mostly in the 70s early next week.
Lows may fall back to the mid 50s for inland southeast GA on
Monday night, with lower 60s for coastal northeast and inland
north central FL.

.LONG TERM [Tuesday Through Friday]...

A deepening trough will dig southward from the Upper Midwest and
through the Ohio and Tennessee Valleys by midweek, sending a
strong cold front towards our region. Yet another round of
downpours and possibly a few thunderstorms is expected on Tuesday
and Tuesday night as another jet streak traverses the Deep South.
Storm total rainfall amounts of 3-5 inches, with locally higher
totals, are expected for locations south of Interstate 10 through
Wednesday morning. Localized flooding will be possible in these
areas, especially for urban and low lying locations such as
Gainesville, Ocala, St. Augustine and Palm Coast. Clouds and
rainfall will keep highs in the 70s on Tuesday, with temperatures
briefly returning to above normal levels in the upper 70s to lower
80s just ahead of the front on Wednesday. A cooler and drier air
mass will then filter into our region on Wednesday night, dropping
lows into the 40s for most inland locations. Temperatures will
only slowly moderate late this week as high pressure settles over
the Deep South. A resumption of strong zonal flow is expected by
Friday as a potentially intense storm system emerges from the
Desert Southwest into the Southern Plains states.

.AVIATION...[Through 00Z Sunday]
Lowering ceilings and vsbys with MVFR conditions expected to
develop tonight. High res models show widespread rain will
continue through around 04z with a break in coverage til later
tonight and Saturday morning. Chance of thunderstorms at NE Fl TAF
sites Saturday afternoon after 19z.

.MARINE...

A weak cold front over north central Florida late this afternoon
will stall tonight as a wave of weak low pressure develops over
the lower Mississippi Valley and moves eastward. This wave will
cross southern Georgia on Saturday afternoon and evening and will
then move offshore on Saturday night, leaving the weak frontal
boundary stalled across the northeast Florida waters through
Tuesday night as additional waves of low pressure develop over the
northeast Gulf of Mexico and move across north central Florida.
Periods of showers and thunderstorms will impact the northeast
Florida waters from this weekend through Tuesday, while activity
stays mostly south of the Georgia waters on Monday and Tuesday.
Strong to possibly severe thunderstorms may be possible for
coastal northeast FL on Saturday afternoon and evening.

A stronger cold front is then expected to sweep across our area,
bringing a brief period of west-northwesterly winds on Wednesday
and Thursday. Seas of 2-4 feet will generally prevail this
weekend and early next week both near shore and offshore.

Rip Currents: A persistent east-northeasterly ocean swell will
keep a moderate risk in place at area beaches this weekend and
early next week.

.FIRE WEATHER...

Strong to severe thunderstorms will be possible by Saturday
afternoon and evening, especially along the Interstate 10
corridor. Outside of thunderstorm activity, light surface and
transport winds will prevail this weekend through early next week,
except for locations in north central FL, where breezy west-
southwesterly winds are expected this weekend. Low daytime
dispersion values are forecast for all locations north of
Gainesville and St. Augustine during the next several days. Waves
of downpours and a few embedded thunderstorms will then focus
across locations from the I-10 corridor southward from Sunday
through late Tuesday, where widespread rainfall totals of 3-5
inches with locally higher amounts are forecast. A drier air and
progressively cooler air mass will then plunge into our region by
Wednesday and Thursday.

.HYDROLOGY...

A stalling frontal boundary and waves of low pressure moving
northeastward from the eastern Gulf of Mexico across north central
FL from Sunday through Tuesday may produce widespread heavy
rainfall, especially for locations south of Interstate 10. Current
projections are for Storm Totals of 3-5 inches through Tuesday
night for north central and portions of northeast FL, which would
likely result in at least localized flooding, especially for
urban and low lying areas such as Gainesville, Ocala, St.
Augustine and Palm Coast.

&&

.PRELIMINARY POINT TEMPS/POPS...

AMG  55  72  58  72  56 /  30  50  70  50  20
SSI  59  69  60  69  60 /  40  40  60  60  40
JAX  59  74  61  71  61 /  70  50  60  80  70
SGJ  62  72  64  72  63 /  70  40  60  80  80
GNV  60  78  64  75  62 /  70  50  60  80  80
OCF  64  79  66  80  64 /  50  40  60  70  80

&&

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