Flood and Heavy Rain in Florida, August 16 to 17, 2019

Area Forecast Discussion
National Weather Service Jacksonville FL
409 AM EDT Fri Aug 16 2019

...FLOOD WATCH IN EFFECT THROUGH SATURDAY EVENING...

.Currently (3 AM)...

Widespread showers and a few isolated rumbles of thunder have
persisted across the Gulf Coast overnight stretching to
Gainesville and Ocala at times. High resolution guidance suggests
this moves northward into JAX metro by morning, but believe this
is overdone. Stratiform precipitation likely continues over the
next few hours to sunrise as convective cores develop in the
eastern Gulf waters.

.NEAR TERM...Today and Tonight...

With ongoing stratiform precipitation slowly fading this morning,
residual cloud cover will likely result in differential heating
between I-10/I-95 corridor and the I-75 corridor from Gainesville
to Ocala. This differential heating should lead to a surface
boundary initiating showers and thunderstorms by late morning into
early afternoon from west to east across the I-10 corridor and
from north to south from Jax metro to Daytona. Throughout the
afternoon, additional development occurs across SE Georgia from
Brunswick to Homerville. Interactions with mesoscale boundaries
during the late afternoon may lead to a few strong storms from the
FL/GA border northward into SE Georgia. This is where the best
instability (1500-2500 j/kg) will reside. The eastern activity
moves toward the coast and off the coast by mid-late afternoon due
to 15-25 knots of southwesterly storm motion flow. Throughout the
rest of the afternoon and evening, showers and thunderstorms
along the Gulf coast continue to move inland producing heavy
rainfall. Due to recent heavy rainfall over the last 7 days,
additional rainfall amounts of 3 to 6 inches favored the issuance
of a flood watch for a few select areas (Suwannee, Gilchrist,
Alachua, and portions of Marion Co.). High resolution guidance and
even some global models highlight the chance for locally heavy
rainfall associated with slow moving rain bands to begin this
evening and continuing overnight. If these do indeed develop,
these locations will have the best chance of observing flooding.
These bands of rain are forming due to the presence of vorticity
generated by thunderstorms occurring in the northeast Gulf.
Throughout the rest of the overnight, some of this activity slowly
works inland, especially if thunderstorms are able to develop.
Most of SE Georgia and NE Florida dry out overnight with isolated
showers possible.

Temperatures will be warmest across the breakfast counties of SE
Georgia where plentiful sunshine is expected with isolated
thunderstorms during the afternoon. The rest of the area struggles
to reach the lower 90s with cloud cover and precipitation
limiting the possibility.

.SHORT TERM...The Weekend...

The forecast headed into the weekend remains wet as the tropical
airmass persists over the Southeast. While the frontal boundary
gradually weakens, enhanced areas of vorticity moving into the
area from the northeast GOM, may continue to help enhance
rainfall along the I-75 corridor from Live Oak to Ocala. High-
resolution models have been persistent on slow moving rain bands
developing along the Gulf Coast through Saturday afternoon. With
PWATs of 2.3-2.5 inches expected, locally heavy rainfall will be
likely where these rain bands occur. Because of this, the flood
watch is in effect until 8 PM Saturday and it may need to be
extended into Sunday if the threat remains. For Sunday, model
guidance is less bullish on the heavy rain threat as the frontal
boundary weakens and vorticity maxima move northeast up the
Carolina coast. Abundant atmospheric moisture is still going to be
located over the region so precipitation chances remains high
despite the lack of strong synoptic/mesoscale features. Mean storm
motion flow becomes more westerly so thunderstorms will likely
develop inland and along the west coast sea-breeze before moving
eastward across the area.

Temperatures are expected to be near to below average across the
area this weekend, especially during the afternoon where showers
and thunderstorms occur. The warmest locations will be across SE
Georgia near the Altamaha River and across portions of NE Florida
toward Putnam and Flagler counties. These areas are situated on
the edge of expected rainfall/cloud cover, allowing for a few
hours of heating before thunderstorms develop. In the middle,
areas will struggle to reach the mid to upper 80s as abundant rain
showers and cloud cover limits heating starting early in the day.
While daytime highs will be below normal for many, nighttime lows
are still forecast to be near normal.

.LONG TERM...Monday through Friday...

Zonal upper level flow begins to become more amplified as a upper
level ridge builds over the Southern Plains and a trough digs
into the Southeast. Model guidance is in fairly good agreement
that remnant tropical moisture will still be located over portions
of the Southeast. An inverted trough axis associated with a
easterly wave is forecast to be moving through the Bahamas at the
start of the week pushing some of the deeper moisture west of the
area. Despite this, model guidance shows southwest flow resuming
mid- week, keeping convection chances persistent through the week.
Monday through Wednesday likely feature a strong east coast sea-
breeze, pushing convection inland during the early afternoon. Flow
becomes lighter on Thursday and Friday with merging sea-breezes
expected over inland areas.

With upper level troughing, forecast temperatures continue to be
near normal throughout the long-term (Highs: lower 90s, Lows: Low
to mid 70s).

&&

.AVIATION...

Periods of MVFR cigs and some brief IFR expected through 13z.
Mostly MVFR cigs will prevail after 13z with occasional VFR cigs
possible. Through about 15z, GNV should see much better chances of
showers and embedded thunderstorms, with rest of the TAFs seeing
convection flare up by 15z-17z and continue on and off through the
rest of the afternoon hours. Given the very moist airmass, IFR
conditions will be likely in the heavier precip areas. Should see
activity taper off in the evening with lingering precip producing
MVFR vsby conditions at times but GNV may be locked into
occasional precip for the better part of the night. Could also see
some IFR cigs return tonight for some TAFs. Southwest winds
around 5-12 knots will prevail today, with some gusty and variable
winds possible in heavier showers/t-storms.

&&

.MARINE...

Increasing winds due to a tightening pressure gradient has
resulted in SCEC conditions beginning for off shore waters.
Showers and thunderstorms will also be possible along the near
shore by late morning and early afternoon. These storms will first
develop inland before moving toward the coast. Similar
expectations for the weekend as the frontal boundary slowly
weakens and moves northward. Flow becomes more southeasterly early
next week as a weak tropical wave moves through the Bahamas.

Rip Currents: Low risk of rip currents.

&&

.HYDROLOGY...

Additional explanation for flood watch issuance...Expected
locally heavy rainfall possibilities along with collaboration from
neighboring NWS offices resulted in the issuance of a watch for
portions of the I-75 corridor. 7-day rainfall estimates from radar
data revealed several large areas of 200-400% above normal
rainfall. There have also been reports of flooding from these
areas over the last couple of days so it is clear more saturated
grounds are developing. Areas that could see up to 3 inches or
more of rain like Columbia, Union, and Bradford counties were not
included at this time due to the lack of above average rainfall.
Eastern Marion was not included because of the distance from the
Gulf coast. While broad scale rainfall is possible to verify some
minor flooding, the primary concerns will be associated with any
rain bands that form this afternoon into the overnight period. If
the lift is sufficient enough to keep thunderstorm activity
persistent into the overnight, these rainfall expectations may be
surpassed in several localized areas, especially toward the Gulf
coast. Additionally, with the forecast QPF, the Santa Fe river
could reach flood stage toward early next week. Adjustments to the
watch may be needed with time as confidence increases or
decreases.

&&

.PRELIMINARY POINT TEMPS/POPS...
AMG  91  73  89  73 /  40  30  60  30
SSI  87  76  87  75 /  50  50  60  30
JAX  87  75  87  74 /  50  50  60  30
SGJ  87  75  87  74 /  60  30  50  20
GNV  85  73  85  72 /  80  80  70  20
OCF  85  73  87  72 /  60  60  60  20

&&

.JAX WATCHES/WARNINGS/ADVISORIES...
FL...Flood Watch through Saturday evening for Central Marion-Eastern
     Alachua-Gilchrist-Suwannee-Western Alachua-Western Marion.

GA...None.
AM...None.
&&

McGinnis/Shashy