Area Forecast Discussion
Issued by NWS Jacksonville, FL

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FXUS62 KJAX 122335

Area Forecast Discussion
National Weather Service Jacksonville FL
735 PM EDT Wed Aug 12 2020


Slow moving convection will decrease in coverage this evening,
and should end by midnight. With the slow movement, a few storms
with heavy rainfall and localized flooding still a possibility
this evening. Partially clearing skies and dry weather
anticipated overnight.


[Through 00Z Friday]

Convection will decrease in coverage this evening, and is
expected to dissipate by midnight. Prevailing VFR conditions
expected overnight into Thursday morning. Convection is expected
to develop again Thursday afternoon, with restrictions in and near
storms anticipated.



.NEAR TERM [Through Thursday]...

A few strong thunderstorms will be possible late this afternoon
through the early evening hours for locations from Waycross
northward, where latest LAPS/RAP analyses indicated downdraft CAPE
values around 800 j/kg, indicating that a few storms may pulse
and produce gusty winds of 40-50 mph as convection increases in
coverage as outflow boundaries drift southward. Slightly warmer
temperatures aloft today should limit coverage of strong
thunderstorms along the U.S. Highway 301 and I-95 corridors in
northeast FL, where mesoscale boundaries will collide and trigger
slow moving thunderstorms that will zipper northward up the St.
Johns River Basin. Steering flow is almost non-existent, so
localized flooding will be possible through the early evening
hours, particularly in low lying and metro areas. Outflows from
convection moving northward into southeast GA and from convection
currently near the Altamaha River may trigger additional
thunderstorms over the Okefenokee Swamp and points eastward to the
U.S. Highway 301 corridor in southeast GA this evening, but
activity should dissipate at these locations by midnight.
Otherwise, debris cloudiness will gradually thin out during the
predawn hours, hopefully enough to adequately view the Perseid
meteor shower. Convection developing along the FL Nature coast
during the predawn hours may build northward along the Suwannee
River, with isolated coverage possible towards sunrise in southern
portions of the Suwannee River Valley. Lows tonight will only
fall to the low to mid 70s inland and the mid to upper 70s at
coastal locations.

Troughing will begin to sharpen over the lower Mississippi Valley
on Thursday, resulting in weak ridging aloft over our region,
keeping weak steering flow in place locally. A weak pressure
pattern will continue to prevail over our area, with southwest
flow gradually deepening in the low and mid levels. Tropical
moisture will remain in place with relatively mild temperatures
aloft, setting the stage for another round of slow moving
convection during the late afternoon and evening hours. Higher
coverage is anticipated across inland southeast GA and the
southern St. Johns River basin, where mesoscale boundary
collisions may be more numerous. Highs will again generally reach
the lower 90s inland and around 90 at the coast, with heat index
values climbing to around 100.

.SHORT TERM [Thursday Night Through Saturday Night]...

Convection will remain diurnal in nature and will gradually
dissipate during the evening hours, with slowly strengthening
southwesterly low and mid level flow potentially bringing activity
back towards the I-95 corridor during the early evening hours and
then pushing offshore before midnight. Predawn convection may
increase in coverage over the northeast Gulf of Mexico and along
the Nature Coast on Friday morning, with scattered coverage
possibly moving into southern portions of the Suwannee Valley
towards sunrise. Lows will generally fall to the mid 70s inland
and the upper 70s at area beaches.

Mid-level troughing will continue to sharpen over the
Ohio/Tennessee/Lower Mississippi Valleys on Friday, with this
feature developing a weak surface low pressure center over the
Southern Appalachians this weekend as this weather system
progresses slowly eastward. Atlantic ridging will attempt to lift
northward slightly on Friday, advecting drier air into the mid-
levels per model soundings and increasing the risk for pulse
strong and isolated severe thunderstorms, with the dominant Gulf
coast sea breeze acting as the trigger and slightly faster east-
northeastward storm motion anticipated. Stronger activity will be
capable of producing downburst winds of 40-60 mph. Strengthening
southwesterly flow will pin the Atlantic sea breeze boundary to
the east of I-95 through late afternoon, with highs expected to
reach the lower 90s all the way to the coast before convection
overspreads our region from west to east during the afternoon
hours. Convection should continue to remain mostly diurnal in
nature, with activity pushing offshore during the early evening
hours and isolated to widely scattered activity then moving into
the southern portions of the Suwannee Valley towards sunrise on

A similar weather pattern will prevail on Saturday as
deep southwesterly flow continues and tropical moisture pools over
our area as the base of the mid and upper trough moves slowly
eastward across the FL panhandle. Numerous mainly late morning and
afternoon thunderstorms will develop along the dominant Gulf coast
sea breeze, with storms again pulsing along the I-95 corridor as
they encounter the pinned Atlantic sea breeze boundary. Highs will
again reach the lower 90s before convection develops, with lows
falling to the 70s on Saturday night.

.LONG TERM [Sunday Through Wednesday]...

Troughing will flatten somewhat as the base of this feature
crosses our region on Sunday, with a much drier air mass
attempting to cross the Altamaha River in southeast GA late in the
day as low and mid level flow attempts to shift to northwesterly
in the wake of the trough. Deep southwesterly flow will keep
abundant moisture in place for most of our area, with the
presence of the trough likely developing convection over the
Suwannee Valley during the morning hours on Sunday and Monday,
with activity then moving eastward across the rest of our region
during the early to mid afternoon. A more significant shortwave is
then progged to dive southeastward from the Missouri Valley into
the lower Mississippi and Tennessee Valleys by midweek, which will
sharpen troughing to the west of our region and produce another
episode of deep southwesterly flow. Strengthening mid-level flow
will likely contribute to pulsing strong or isolated severe
convection each afternoon, with a pinned Atlantic sea breeze
boundary being the focus for stronger activity. Flood concerns may
increase by midweek locally in this pattern, as training of
thunderstorms over saturated grounds becomes a daily occurrence.
Temperatures will still climb to around 90 or the lower 90s early
in the week, but could decrease somewhat by midweek as convection
becomes numerous and earlier in the day.


A trough will remain situated just west of our GA waters through
the weekend, keeping the axis of weak Atlantic ridging positioned
across south FL, resulting in prevailing south-southwesterly
winds during the next several days. Showers and thunderstorms will
develop each afternoon over land and may impact the near shore
waters during the late afternoon and evening hours, with scattered
activity possible offshore during the overnight hours. The
Atlantic sea breeze will develop each afternoon, with winds
becoming onshore over the near shore waters, followed by southerly
evening wind surges. Seas of 2-4 feet will prevail both near shore
and offshore through early next week. Coverage of showers and
thunderstorms may increase early next week as a frontal boundary
stalls just to the west of the GA waters.

Rip Currents: A small but persistent long period east-
southeasterly ocean swell will keep a low-end moderate risk in
place at the northeast FL beaches each afternoon during the next
few days.


Thunderstorm activity will be slow moving this afternoon and
again on Thursday, creating the risk for localized flooding and
numerous cloud to ground lightning strikes. Outside of
thunderstorm activity, light west-southwesterly transport winds
today will shift to southwesterly on Thursday, with marginally low
daytime dispersion values expected area-wide. Onshore surface
winds will develop each afternoon at coastal locations, with
sustained speeds increasing to 10-15 mph for locations east of
I-95. Southwesterly transport winds will increase to 10-15 mph on
Friday for locations along and north of I-10, where good
dispersion values are forecast on Friday afternoon.


The St. Marys River north of Macclenny and at the Trader`s Hill
gauge downstream of Macclenny are currently in minor flood, with
levels north of Macclenny forecast to crest by late Thursday.
Additional heavy downpours may keep minor flooding going through
the upcoming weekend.



AMG  74  91  73  91  73 /  50  70  40  70  20
SSI  77  89  78  91  78 /  40  30  30  50  40
JAX  74  93  74  93  74 /  40  50  30  60  20
SGJ  75  90  76  93  77 /  40  50  40  50  30
GNV  73  92  74  92  74 /  40  40  20  60  10
OCF  73  91  74  92  74 /  30  40  20  50  20


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