AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
354 PM EST Tue Feb 21 2012
SYNOPSIS...
The long wave pattern is continuing to progress. The RIDGE that was
over the SE US yesterday has moved east over the MID-Atlantic.
Currently over our region is a SHORTWAVE TROUGH that brought some
light RAIN this morning and some CLOUDY skies. Besides a FEW light
showers this morning, today has been a CALM day in terms of weather.
IR and visible imagery show mid-upper level clouds. RADAR is QUIET
with a few leftover showers on the outskirts of the forecast area.
The trough should continue to progress eastward, but more slowly
than yesterday`s ridge AS it will begin lifting and our region
will begin to see more zonal FLOW until a second LOW approaches
Thursday NIGHT. This low could render some weather on Friday.

&&

.NEAR TERM [Today and Tonight]...
Heavy CLOUD cover and light rain from this morning have made us
lower today`s forecast HIGH today to the mid to upper 60s. Skies are
expected to clear for a little bit with a short break in the cloud
cover. With light southerly winds overnight and high MOISTURE from
RAINFALL and ADVECTION by said winds, FOG is expected across most of
the area starting early tomorrow morning and continuing into the
afternoon. Model forecast soundings and statistical guidance
indicates that the fog could be dense in spots.

&&

.SHORT TERM [Wednesday through Friday Afternoon]...
Plenty of weather concerns in the short term period, as nearly 48
hours of low-level warm advection and associated various rain
chances will be followed by the arrival of a strong cold FRONT on
Friday. We edited the GRIDS today out through 00z Saturday to ensure
a more smooth transition of weather elements into the previous
long-term forecast from last night.

First, model preferences. Differences have re-emerged from what was
a fairly good consensus amongst the 21.00z model suite. This seems
to be largely tied to two features: (1) the upper-level CUTOFF low
that is centered near 28N/117W or just west of Baja California, and
(2) an elongated PV ANOMALY / axis of SHEAR VORTICITY north of a
strong JET streak in the Gulf of Alaska. The general theme amongst
the models is that the shear vorticity will consolidate and dig
southeast into the Plains by Thursday as the jet streak develops
eastward along the NE rim of the Eastern Pacific ridge. This is
expected to lead to low-level CYCLOGENESIS in the Great Lakes or
Ohio River Valley on Thursday, and will aid in driving a deeper cold
AIR mass southeast behind a surging cold front. The new 12z GFS is
much faster with this cold front, pushing it through our area by 18z
Friday, whereas the global model consensus (and prior 00z GFS) was
much slower. In collaboration with surrounding WFOs, TAFB, and HPC,
we have decided to go with the slower cold front timing, with most
weather elements a blend of 21.00z GFS, 21.12z NAM, and 21.00z
ECMWF. There are also major differences in how the models handle the
Baja low, with potential implications for the long term period
(weekend and beyond). That will be sorted out on the overnight shift
by the long term forecaster.

For tomorrow, there is good agreement on a quick round of rain with
a fairly sharp N-S POP/QPF GRADIENT, with highest values south. This
seems to be forced largely in the low levels as isentropic ascent
kicks in as a response to a subtle shortwave trough ejects east from
the Rockies to the east coast in fast, nearly-zonal flow. Arguing
for greater rain coverage is the added benefit of being in the right
entrance region of a 120-130kt upper level jet streak. High-res
model guidance is in excellent agreement that showers will affect
mostly our marine and Florida zones, entering the western half of
our area mainly 14-18z, and then moving quickly east and diminishing
by early Wednesday evening. The various 4km WRF models all indicate
some small amounts of CAPE with some weak-moderate UPDRAFT
velocities out over the Gulf, so THUNDER was added over the water.

The earlier prospects of SCATTERED thunderstorms on Thursday seem to
have dwindled on the latest model guidance. As we are LIKELY to warm
into the upper 70s and potentially near 80 degrees, we should see at
least a weak sea-breeze CIRCULATION develop by the afternoon. 12z
model guidance does seem to support this notion with some light QPF
(0.01-0.05") amounts hugging the coastline during the day. Forecast
soundings don`t indicate much INSTABILITY, with LCL-300mb lapse
rates almost at moist ADIABATIC levels. The SREF model MEAN SBCAPE
is around 300 j/kg with a few members up around 600-800 j/kg. There
will also be quite a bit of WIND shear, with 0-6km bulk shear around
60 knots. Therefore, the threat of strong to severe thunderstorms is
non-zero. One potential flaw is strong WAA in the 900-800mb layer
just atop the boundary layer, particularly in the afternoon. This
could limit updraft strength and keep CONVECTION mostly as shallow
showers. Given the amount of shear, there are a wide RANGE of
convective possibilities on Thursday, so stay tuned!

It looks like we will see a lull for most of Thursday Night as
overnight storms would tend to fire to the northwest of us along the
cold front in the Mid-South. With the strong WAA regime and likely
increase in low cloud cover, it should be a warm night with lows in
the mid 60s. For Friday, severe weather is a possibility and we are
currently outlooked in the Day 4-8 OUTLOOK" onClick="return popup(this, 'notes')">OUTLOOK with severe probabilities
at or above 30%. More details on this in the coming days, but the
main points with this forecast update are: (1) the timing of the
greatest severe weather threat appears to have shifted into the
daytime on Friday, (2) a DIURNAL timing of the storms would probably
favor a bit more instability, (3) regardless of eventual timing the
ATMOSPHERE will be highly sheared. While there are differences in
the models regarding timing and some MESOSCALE details, they all
agree that there will be an arc of thunderstorms along the front.


.LONG TERM [Friday Night through next Tuesday]...
On a more positive note, the cold front and associated
thunderstorms will be exiting quickly to our east Friday evening
setting up a dry...although cooler weekend period. A more zonal
upper level pattern over-TOP surface high PRESSURE building in
from the west supports near zero rain chances Saturday and Sunday.
The airmass arriving is quite chilly through. The late February
sun is getting stronger, so still anticipate highs into the 60s,
however a chilly night looks to be in store for Saturday night. As
of now have gone with WIDESPREAD mid/upper 30s away from the
immediate coast, with normally colder spots down near freezing by
SUNRISE. This will also need to be monitored closely, as the
eventual position of the surface high Saturday night will
determine if a more widespread light FREEZE will be possible.
Seasonal conditions with rebounding TEMPS then expected for the
early portion of next week.

&&

.AVIATION [through 18z Wednesday]...
12Z model guidance is showing lower CIGs and VISBYs for the BR Wed
morning and conditions deteriorating earlier for ABY and DHN than
forecast at the 12Z TAF issuance. BR setup time remains similar to
the last issuance, beginning around 04Z, deteriorating near 09Z, and
improving at 16Z, earlier at ECP and VLD. Model agreement is quite
strong on low VSBY, but weak on CIGs, especially after 14Z. Expect
VFR conditions until around 04Z Wed when conditions FALL to MVFR.
At 09 conditions will fall further to IFR, then return to MVFR
around 15Z.

&&

.MARINE...
Generally quiet marine forecast until at least Wednesday. A
NOCTURNAL SURGE of southerly winds could REACH" onClick="return popup(this, 'notes')">REACH SCEC headline
levels Wednesday Night, subsiding briefly on Thursday, before
ramping up ahead of a cold front Thursday Night into Friday. We
went entirely with a SEAS forecast based on the SWAN model, as the
WW3 (based on the 12z GFS) seems to be: (1) too fast with frontal
timing, and (2) too weak with surface winds. The winds were a
blend of the ECMWF and NAM models which bring the front through
the coastal waters on Friday afternoon. Ahead of the front this
gives SSW winds around 25 KT with seas building to 7-9 feet. The
combination of the rapid ramp-up in wind-waves, plus longer period
SWELL originating from the 15-20kt SSW flow Thursday to Thursday
Night is expected to build surf heights to ADVISORY levels on
Friday (6ft or so), with dangerous rips likely.

&&

.FIRE WEATHER...
High relative HUMIDITY values keep us well above criteria level
across the region through the end of the work week. Relative
humidity is forecast to decrease to around 30 percent on Saturday
across the area for several hours, so Florida may reach red flag
criteria, but it`s too early to see ERC values, or to be certain of
what the winds will do.