Severe Thunderstorm Warning 121, NWS Jacksonville, FL

Area Forecast Discussion
National Weather Service Jacksonville FL
329 PM EDT Thu Jun 28 2018

...SLIGHT RISK OF SEVERE STORMS TODAY...

.NEAR TERM...This afternoon and Tonight...

Active west coast sea breeze convection aided by a weak weak mid
level impulse were firing storms over a very unstable and moist
airmass impacting much of NE FL this afternoon. There was a weak
east coast sea breeze, still just east of the St. Johns River this
hour, with enhancement of convection expected as the sea breezes
and outflows converge over NE FL over the next few hours. Locally
heavy rainfall, frequent lightning and wet downbursts of 40-60
mph are the main convective concerns. Upstream, still generally
north of the Altamaha River basin, was a more robust line of
strong to severe thunderstorms sagging steadily south toward SE
GA, which was continuing to destabilize this hour with only
discrete convective development. Through early evening, expect
this line of strong to severe storms to track southward impacting
SE GA through 00Z. The main convective hazards will be damaging
wet downburst winds of 50-70 mph and the Storm Predicition Center
continued to highlight this area as under a "Slight Risk" of
severe thunderstorms. Into the early evening, this line of storms
will settle farther southward over NE FL toward the I-10 corridor,
but with a gradual decrease in convective coverage and intensity
as the airmass will become less conducive for maintaining strong
convective growth due to earlier sea breeze convection. Overnight,
will maintain a low chance of showers and isolated tstorms after
midnight generally focused over SE GA and the Suwannee River
Valley under a meandering mid level trough and moist westerly
flow. Temperatures tonight will trend above normal in the low to
mid 70s.

.SHORT TERM...Fri through Sun...

Unsettled weather pattern with elevated rain chances will
continue each day through the weekend with above normal
atmospheric moisture content, sea breeze forcing, high diurnal
instability and a meandering mid level trough axis all phased
across the local area. Subtle changes in steering motion,
lingering outflow boundaries and also lingering debris clouds will
dictate where rainfall initiates each day.

Fri looks like an early start to convection with ongoing showers
and potentially a few morning tstorms across under a developing
mid level cut-off low within the broader trough axis sliding
southward across the eastern Gulf Coast region and northern FL.
Friday afternoon, a weak but dominant west coast sea breeze will
further enhance convective development as it moves inland across
the Suwannee River Valley and toward the Atlantic coast through
the evening, where it will rendezvous with a slowly inland
progressing east coast sea breeze near the St Johns River basin.
Although storm motion will be WNW 5-10 kts, it will be so light
and actually likely decrease into the afternoon and evening under
the developing cut off low, that localized heavy, potentially
flooding rainfall will be possible given very high PWAT values of
2-2.6 inches. In addition to the heavy rainfall threat, cool
temperatures aloft of -7 to -8 deg C near 500 mb under the mid
level low will enhance the potential for strong winds gusts in
storms with wet downburst potential of 40-60 mph especially during
afternoon/evening sea breeze and outflow mergers. Rainfall will
tapper off in coverage and intensity Fri evening through midnight,
but will maintain low chances through Fri night given lingering
surface through mid level trough axis across the local area and
high moisture content.

Sat and Sun the broad upper level low and trough axis ever so
slowly begins to retrograde west as it becomes bridged by high
pressure building offshore of mid-Atlantic coast. This subtle
shift of the mid level low farther west will transition steering
flow to SW across northeast FL Saturday, with light and erratic
storm motion over much of SE GA under the trough axis. By Sunday,
the mid level low will retrograde farther west, thus increasing SW
steering flow across the entire forecast area to near 10 kts.
Both days expect a prevailing west coast sea breeze regime with
rainfall coverage the most across our western zones morning
through early afternoon, then increasing in coverage and intensity
into the afternoon and early evening hours as it approaches the
east coast and a weak east coast sea breeze. Rainfall will
decrease in coverage and intensity each night, with the best
chances of a late night/early morning shower focused across the
Suwannee River Valley under the WSW steering flow and also across
SE GA under the meandering trough axis.

Temperatures will trend near to slightly above normal with highs
near 90 to lower 90s. Low temperatures will also range near to
slightly above normal with values in the low to mid 70s.

.LONG TERM...Mon through Thu...

Mon & Tue...A decrease in overall rain chances as high pressure
strengthens north and east of the area as the pesky mid level low
shifts farther west and across the central Gulf Coast region,
taking with it the swath of very high PWAT air. Expect a decrease
rainfall coverage these two days as less forcing and less moisture
will be in place, with a return to more scattered afternoon
showers and tstorms. Steering flow Mon will be more southerly
which will enable both west/east coast sea breezes to propagate
inland and trigger convection. By Tue, steering flow transitions
to more SE which will bring a stronger east coast sea breeze
across the local area and favor the better chances of
afternoon/evening convection toward the interstate 75 corridor.
Showers and tstorms will fade inland each evening with loss of
forcing and instability.

Wed & Thu...A return to higher rain chances as a TUTT (tropical
upper tropospheric trough) moves west across the western Atlantic
with a surface wave/trough axis pivoting toward the SE Atlantic
coast. As this system approaches, expect an increase in deep layer
moisture from east to west which will increase local coverage of
diurnally driven convection Wed. By Thu this TUTT feature is
expected to be over the local region, which will enhance both
tstorm coverage due to high moisture content but also intensity
with the GFS40 indicating fairly cool 500 mb temps of -9 to -10
aloft.

Temperatures will trend near to slightly above normal with highs
near 90 to lower 90s. Low temperatures will also range near to
slightly above normal with values in the low to mid 70s.

.AVIATION...18Z TAF period...

Showers and storms are expected to impact area TAF sites this
afternoon and evening. Restrictions along with gusty winds will be
expected with any storms which develop. Much of the activity will
dissipate toward midnight, with prevailing VFR conditions
anticipated for the Overnight period. Convection is expected to
develop once again Friday morning.

.MARINE...

Headlines not expected. A trough of low pressure will linger over
southern Georgia through the weekend with an unsettled weather
pattern in place. South to southwest winds will transition to
south to southeast in the afternoon with the east coast sea
breeze. Waves of showers and thunderstorms are expected. Winds
will transition to more onshore early next week as high pressure
builds northeast of the waters.

Rip Currents: Moderate risk today and Friday due to elevated SE
winds in the afternoon, small swells and numerous sandbars.

&&

.PRELIMINARY POINT TEMPS/POPS...
AMG  74  90  74  92 /  60  60  40  60
SSI  77  89  78  89 /  70  60  40  50
JAX  75  91  75  92 /  70  70  40  70
SGJ  76  92  75  89 /  60  60  40  60
GNV  75  92  75  90 /  60  60  40  70
OCF  75  91  75  90 /  50  60  40  70

&&

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

Enyedi

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