Area Forecast Discussion
Issued by NWS Jacksonville, FL

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FXUS62 KJAX 171329
AFDJAX

Area Forecast Discussion
National Weather Service Jacksonville FL
929 AM EDT Thu Aug 17 2017

...HEAT INDEX VALUES NEAR 105 TODAY WITH HEAVY RAINFALL IN
STORMS...

.UPDATE...A very saturated airmass was across the local forecast area
with the 12Z JAX RAOB measuring 2.28" of precipitable water content.
A few early morning showers near the JAX metro area and a lone storm
near St. Augustine dissipated shortly after 9 am, yielding just some
debris clouds with mostly sunny to partly cloudy skies across much
of the local forecast area with very muggy conditions as dew pts
ranged in the mid/upper 70s.

The two main weather stories today will be (1) heat and high heat
index values nearing 105 deg this afternoon and (2) the potential
for locally heavy, flooding rainfall due to a lot of moisture and
slow and erratic storm motion.

Heat: Mid and upper level high pressure across the Fl panhandle
and SW GA will slowly retrograde and weaken today while
maintaining a ridge axis across SE GA through tonight. The
stronger subsidence will bring lower convective development and
favor the hottest temps across inland SE GA where mid 90s are
likely by mid to late afternoon, and persisting into the early
evening hours.

Rainfall: Farther south across NE FL, the influence of mid/upper
level ridging was weaker, with a weak low level trough axis
extending southward from a broad low pressure center near SC along
the local coastline. The combination of a weak pressure pattern
will create very light steering flow across the local area, and
combined with the high atmospheric moisture content and diurnal
heating, above normal sea breeze and outflow induced rain chances
will continue to be advertised today. As the day progresses, the
weak trough axis near the Atlantic coast will be drawn inland as
the typical thermal low forms. This trough combined with the
Atlantic coast sea breeze and St Johns River breeze will bring
scattered midday showers/isolated tstorms to areas generally east
of Highway 301. Farther west, low level westerly flow will push
convection already firing across Apalachee Bay early this morning
farther inland along the Gulf Coast sea breeze through early
afternoon. As the convective temp of 91 is reached, airmass showers
and tstorms will begin to pop across inland areas as well.

As convection grows deeper and sea breezes and outflow boundaries
begin to converge toward each other through the late afternoon
generally between I-75 and Highway 301, storm motion will remain
light and thus storm development and propagation erratic. The
sfc-6 km mean steering flow from the JAX RAOB this morning was 3
kt. Thus, expect very heavy, locally flooding rainfall into the
afternoon hours with the potential for wet downburst winds of
30-60 mph in the stronger and deeper storms.

Convection will linger into the evening hours while gradually
decreasing and dissipating with the loss of daytime heating.

&&

.AVIATION...Prevailing VFR until convection develops this
afternoon when restrictions are expected to due TS.

&&

.MARINE...SSW winds 10 kts or less this morning will back SE near
the coast with the sea breeze this afternoon and near 10-15 kts.
Combined seas of 3 ft or less.

Rip Currents: Low risk.

&&

.PRELIMINARY POINT TEMPS/POPS...
AMG  95  76  94  75 /  40  20  30  20
SSI  91  79  90  77 /  40  20  40  40
JAX  94  76  92  75 /  60  30  60  20
SGJ  92  77  90  75 /  50  20  50  20
GNV  92  75  91  73 /  60  20  60  20
OCF  92  75  91  74 /  60  20  50  10

&&

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

$$

Enyedi/Cordero/Corless


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