AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE TALLAHASSEE FL
345 AM EDT Fri Jul 20 2012

MESOSCALE DISCUSSION…
The large scale persistent blocking longwave pattern is highlighted
by TROUGH over West Coast, deep ANTICYCLONE/RIDGE over Rockies and
Plains, trough over ERN states and another strong ridge in WRN Atlc
east of SE U.S. Of particular local interest is trough across NE US
with axis SWWD to remnants of TUTT over Ern GA to weak 580dm LOW
near Pensacola. Satellite shows several Gulf vort MAX projected to
rotate into base of trough. At the surface, main features are HIGH
PRESSURE system over the Midwest, Bermuda high pressure to the SE
extending WSW across the CNTRL Gulf, and a surface trough across SRN
GA/AL to weak low just south of MS/AL coast. All this places local
area just to the east of upper trough axis within low pressure
weakness, and between the two deep layer ridges, AS well as just
South of surface trough. This will provide continued high PWs and
deep SW to W FLOW and ample MID-upper CLOUD blowoff from low. .NEAR TERM [Through This Morning]… With local area in weakness, 00Z soundings across the Gulf region show very moist tropical profiles. As in previous nights, ACTIVE
land breeze aided by proximity of upper low, allowed marine
CONVECTION to began early increasing overnight and moving across
adjacent coastlines during the predawn hours. Will keep sharp 80-20%
SW-NE POP GRADIENT in for morning and once again, the threat of
waterspouts is possible.

&&

.SHORT TERM [This afternoon Through Sunday]…

TODAY, models show that both of the above deep layer ridges will
continue to amplify while the Atlc subtropical ridge  continues to
expand WSW. The weakness in between will again serve as the
convection focus while low level SET up places local area in ideal
Gulf sea breeze scenario. With area between upper trough and ridge
and with Gulf ridge lifting NWD and subtropical ridge expanding wwd,
surface trough just to our N, local gradients will tighten today.

However this is a tricky forecast because much of what happens later
today is dependent on early morning convection. Recent days in this
persistent pattern have demonstrated that when marine and coastal
convection began early and intensified overnight than assocd cloud
debris hindered heating and convective development farther inland
during the AFTN and evening. This appears to be occurring at press
time, albeit less than 24 HRS ago. The more robust GFS, WRF and
ARW opt for active marine convection thus show more limited
convection later today especially well inland. Conversely, the NAM
shows more limited marine convection which translates to less clouds
and more heating favoring convective outflows generated from the
decaying pre-DAWN convection. All this is further complicated
by proximity of Pensacola low and several mid level disturbances
moving EWD over the Gulf coast. One is PROGGED to sweep under local
H5 trough/low later this aftn that could ignite convection. Also,
MCS moving SWD over TN could kick out OUTFLOW boundary that may
REACH” onClick=”return popup(this, ‘notes’)”>REACH our NRN CWA towards EVE. For all these reasons will opt for a
little more convection than yesterday. So 20-60% SE-NW POP gradient
this aftn. Local confidence tool, CAM with NIL chances of severe WX.
SPC with 5% convective WIND OUTLOOK” onClick=”return popup(this, ‘notes’)”>OUTLOOK for our area. Again, locally
heavy RAIN could lead to localized flooding. Inserted heavy rain and
gusty winds in wrn counties in GRIDS for this aftn. Highs mid 80s
coast to around 90 inland and tighter local gradients will provide
for some gusty winds.

FOR TONIGHT INTO SUNDAY, The Rockies/Plains upper anticyclone will
finally shift EWD and in response, the ERN H5 trough begins to
weaken and lift nwd allowing the Gulf ridge to amplify NWD on SAT.
This allows the surface ridge to expand further WWD. All this will
progressively bring SUBSIDENCE and drier AIR from S-N and is already
evident looking at 00z regional soundings. For example TAE with 2.28
inch PW compared to 1.31 inch PW at Tampa. By Sun ridge approaching
our SRN CWA. Under decreasing clouds, TEMPS will rise to at or above
NORMAL by SUN.

FOR TONIGHT, will go with 10-50% SE-NW POP gradient. Lows around 70
inland to mid-upper 70S coast. FOR SATURDAY, 20-60% daytime, 0-30%
at NIGHT SE-NW POP gradient. Highs upper 80s coast to low 90s
inland. Lows low-mid 70s inland to near 80 coast. FOR SUNDAY, 30-40%
POPS. Highs near 90 coast to mid 90s inland.

&&

.LONG TERM (Sunday night through next Friday)…
Weak mid/upper level ridging will be in place through the period.
The ridge will flatten somewhat by mid-week as a TROF digs off the
eastern seaboard. A backdoor cold FRONT should make it as far south
as the Carolina`s by Thursday. Otherwise, a surface ridge will
extend from the Western Atlantic across the Gulf with the axis
mostly over or south of our coastal waters. Both the EURO and GFS
show tropical MOISTURE lifting from the Bahama`s across the Florida
peninsula and eastern Gulf early next week. This appears to be from
a wave currently located near Puerto Rico. Time HEIGHT cross
sections continue to show ample deep layer moisture through the
extended period. Will keep PoPs near seasonal levels (40-50%) each
day. Max temps will generally be a FEW degrees above normal.

&&

.AVIATION…
ISOLATED showers were moving inland along the Florida coast early
this morning. Expect VFR conditions to persist through SUNRISE with
just brief periods of MVFR vsbys/cigs. Then, once again we will see
NUMEROUS showers and thunderstorms later this morning and afternoon.
Thus, convective tempo groups were added to all TAF sites.

&&

.MARINE…With trough of low pressure to our west, convection is
expected to increase across the western marine areas into this
morning. Synoptic flow is anticipated to remain in the 10 to 15 KNOT
RANGE except above 15 knots wrn waters with borderline SCEC with
slightly lower values nearer to the coast. Similar conditions are
expected on Saturday. With trough weakening ,winds should subside on
Sun. Recent runs show SWAN to be too low on winds and SEAS. So will
adjust up accordingly.

&&

.FIRE WEATHER…
Low level onshore flow will continue to feed abundant moisture into
the region. Minimum afternoon relative HUMIDITY levels are forecast
to stay well above critical levels along with rain chances each day
through the upcoming weekend and beyond.