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000
FXUS62 KTBW 140012
AFDTBW

Area Forecast Discussion
National Weather Service Tampa Bay Ruskin FL
712 PM EST Thu Dec 13 2018

.UPDATE (Overnight and Friday)...
00Z water vapor and H4 RAP analysis is showing a vigorous
upper level low closing off over the southern Plains this
evening. It will be this system and associated surface
low/cold front that evolve eastward through the day on
Friday and bring increasing rain/storm chances to west-
central Florida. The system to impact us on Friday/Friday
night is a rather complex one with an atypical frontal
structure and evolution. The front is not going to be very
progressive the next 24-30 hours, especially as it reaches
the eastern Gulf of Mexico/Florida west coast. Several
mid/upper level disturbances moving over this lower level
boundary in conjunction with a highly diffluent upper level
southern stream jet pattern suggest several rounds of storms
may form and pivot eastward into Florida. This setup makes
detailed timing for highest rain chances difficult. Other
than a few showers after midnight for the northern Nature
Coast, most areas should remain dry for the overnight hours.
It appears likely the first round of significant
showers/storm coverage will impact areas north of the I-4
corridor during the morning hours, followed by another round
or two impacting the Tampa Bay Area during the mid/late
afternoon or early evening hours. Yet another round, or the
lingering earlier round from Tampa Bay, will then settle
south and east into our southern and interior southern zones
later in the evening and overnight. These initial rounds of
storm (especially in the afternoon/evening) look to be the
ones with the greatest potential for embedded severe and
rotating storm structures. The initial round into the Nature
Coast may very well have organized convection, but we will
have to see if any of this activity is rooted at the surface
or if its an entirely elevated convective situation.
Forecasters will be closely monitoring the lower level
environment to determine the potential for concerning
elevated radar signals to actually impact at ground level.
Greater potential for surface based convection exists later
in the day across the region due to a combination of
moisture/thermal advection from the south and diurnal
heating.

Even after the severe threat diminishes, the slow frontal
movement means we are unlikely to see a defined clearing of
the rain Friday night, but rather lingering elevated chances
for showers/weaker storms for all areas.

Of note: Many of the higher resolution NWP guidance members
are showing a squall-line coming into the I-4 corridor/Tampa
Bay Area region during the evening hours Friday, even late
evening in a few outlier solutions. While the slow progress
of the cold front could certainly make this timing a
reality, the overwhelming bias associated with these model
solutions is to bring MCS/squall-lines into our region late,
sometimes by several hours, compared to what actually ends
up occurring. The reasoning for this lies in the difficulty
all NWP models have dealing with cold pool
generation/evolution and propagation under and ahead of the
convection. This is especially true over the open waters of
the Gulf of Mexico. These cold pools of air very often move
quicker in reality compared to the model simulations, and
therefore we see the rain/storms arrive and exit quicker
than the model ensembles will forecast. this bias has been
applied to the current forecast related to timing of higher
rain chances / severe weather threat.

&&

.AVIATION (14/00Z through 15/00Z)...
VFR conditions as BKN-OVC mid deck slowly lowers through the
night. E and ESE winds under 10 knots overnight begin to
increase between 12-15knots with higher gusts and become SE
to SSE AFT 14-15Z. Approaching cold front bring increasing
chances for showers/storms to KTPA/KPIE/KLAL/KSRQ by early
to mid afternoon and then continue into the evening hours. A
few storms later afternoon/evening may be strong to severe.
Rain/storm chances look to wait until after 21Z for
KPGD/KFMY/KRSW but then linger through the evening and
overnight hours.

&&

.Prev Discussion... /issued 246 PM EST Thu Dec 13 2018/

..SLIGHT RISK OF SEVERE THUNDERSTORMS FRIDAY...
..HAZARDOUS CONDITIONS ARE EXPECTED TO IMPACT MARINE WATERS AND
BEACHES...

DISCUSSION...
Robust mid level low over TX this afternoon takes an easterly
track...as ridging east of FL moves away...along the western Gulf
coast Fri then over the southeastern states during Sat. At the
surface - ridging over FL and the east Gulf slides to the east as
the surface reflection of the TX mid level low drags a cold front
across the Gulf. The front reaches the west-central and southwest
FL coastal waters Fri then slips across the state to the Atlantic
Sat.

Moisture and wind fields begin to increase over the Gulf later
tonight ahead of the approaching system...shift into the coastal
waters early Fri...then cross west-central FL Fri morning into the
afternoon and southwest FL afternoon and evening. These times are
first guess and will be adjusted over the next couple of forecast
cycle. A squall line or two is expected to form ahead of the
front with strong to severe thunderstorms that can produce
damaging wind gusts...tornadoes or waterspouts...and locally heavy
rainfall with flooding. The most active weather will be moving
east across the state toward the Atlantic late Fri night. Large
waves and swells will be generated in the western Gulf that
translate onto area breaches for the weekend with a high risk of
rip currents expected.

For Sun - Thu: The mid level low exits the southeastern coast early
in the period as ridging builds into the nation/s mid-section from
Mexico then slides east through Wed. Surface high pressure settles
into the western Gulf Sun then tracks to the eastern seaboard by
midweek. On Thu a mid level short wave trough/low moves into the
southern plains with a surface low/cold front forming on the western
Gulf coast. The upper and surface ridging dominate for much of the
period with dry seasonable temperatures. Moisture increases enough
by Thu to support some showers and a few thunderstorms.

MARINE...
Increasing winds ahead of an approaching storm system will require
small craft advisories on some of the waters tonight...and spread to
the rest of the waters as the system moves into the coastal waters
early Fri with thunderstorms and possible waterspouts. The
front crosses the state Sat then moves out over the
Atlantic. This system is generating large waves and swells
in the western Gulf that will translate into the local
waters for perhaps much of the weekend. High pressure begins
to build into the western Gulf Sun then slides to the
eastern seaboard by midweek with winds veering and
diminishing.

FIRE WEATHER...
Atmosphere continues very moist with no low humidity concerns. Storm
system coming in from the west will bring showers...thunderstorms...
and locally heavy rainfall mainly Fri with some showers and a few
storms into Sat.

&&

.Preliminary Point Temps/PoPs...
TPA  65  76  65  72 /  10  90  90  30
FMY  66  81  69  74 /   0  40  70  40
GIF  64  80  65  74 /  10  80  70  40
SRQ  66  76  67  72 /  10  90  90  30
BKV  63  78  65  72 /  30 100  90  30
SPG  66  76  65  72 /  10  90  90  30

&&

.TBW WATCHES/WARNINGS/ADVISORIES...
FL...None.
Gulf waters...Small Craft Advisory until 7 PM EST Friday for Waters from
     Englewood to Tarpon Springs FL out 20 to 60 NM-Waters
     from Tarpon Springs to Suwannee River FL out 20 to 60
     NM.

     Small Craft Advisory from 1 AM to 7 PM EST Friday for
     Waters from Bonita Beach to Englewood FL out 20 to 60
     NM.

     Small Craft Advisory from 4 AM to 7 PM EST Friday for
     Coastal waters from Bonita Beach to Englewood FL out
     20 NM-Coastal waters from Englewood to Tarpon Springs
     FL out 20 NM-Coastal waters from Tarpon Springs to
     Suwannee River FL out 20 NM.

&&

$$

UPDATE/AVIATION...Mroczka
Previous Discussion...Rude