Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Tampa Bay Ruskin Florida
518 am EDT Thursday Aug 21 2014
Short term (today-friday)...
the middle/upper level ridge will drift northwest into the lower
Mississippi Valley through Friday with the subsidence that has
been in place gradually weakening as it moves further away from
the region. Meanwhile at the surface the ridge will hold across
the Gulf of Mexico and Florida keeping a rather light flow in
place with the sea breezes developing each afternoon. Guidance
does indicate an increase in moisture during the period and as the
middle/upper level ridge moves further away to the northwest the
general steering flow will become more east to northeast albeit
rather weak. Therefore...with less subsidence...an increase in
moisture and the shifting steering flow we should see more
convective activity over the region compared to the last few
days. For today the highest rain chances will be over southwest
Florida...but inland areas further north should at least see low
end scattered probability of precipitation. Tonight any convection will drift west out
into the Gulf during the evening with mostly clear to partly
cloudy skies prevailing. On Friday we should once again see
scattered convection develop during the afternoon and evening with
best chances across the far north and far south where the
deepest moisture is expected. Temperatures will remain near to a
few degrees above normal thanks to the middle/upper level ridge with
daytime highs mostly in the lower to middle 90s and overnight lows
in the 70s to around 80 degrees.
Long term (friday night-wednesday)...
the extended portion of the forecast features two systems having
the potential to influence our weather. The first is the tropical
disturbance approaching the Lesser Antilles. The 00z suite of
guidance is in pretty good agreement in taking the disturbance
west-northwestward toward Hispaniola during Saturday...then
turning more northward toward the eastern Bahamas after that. By
no means is this a guarantee but the forecast will follow this
scenario for now.
The second feature is a weak back-door cold front forecast to move
southward into North Florida by Sunday...then into central or
South Florida by Monday. Saturday may be our last drier than
normal day before the front moves in by Sunday. The European model (ecmwf) does
bring drier air in behind the boundary which would keep our rain
chances below normal. The European model (ecmwf) also has a rather weak reflection
of the tropical disturbance moving northward well east of the
Carolinas on Tuesday while the other global models including the
Canadian and GFS have a more robust system near or east of the
Bahamas. A tropical cyclone moving northward near the Bahamas
would typically put is in the dry/subsident region of the system.
With so much uncertainty...did not want to put too much stock in
any one of the global models so stayed pretty close to climatology
with highest rain chances over our northern zones on Sunday with
the advancing cold front then over our southern zones Monday
through Thursday more typical of climatology.
We will also finally get into more of an easterly flow regime
which will bring more afternoon storms toward the Gulf Coast
rather than moving quickly inland as we have seen for much of
some MVFR/local IFR conditions will be possible in isolated to
scattered showers and thunderstorms this afternoon and
evening...otherwise VFR conditions should prevail.
the general west to northwest flow will persist through the
weekend as high pressure remains over the Gulf of Mexico. Early
next week winds should become northeasterly as a back door cold
front moves into the area and high pressure bridges the boundary.
Winds are expected to remain 15 knots or less and seas less than
4 feet through early next week with no headlines anticipated.
no fire weather concerns expected through the weekend as relative
humidity values should remain above critical thresholds. Could see
some areas of ground fog late each night...especially in locations
that receive some rain during the previous evening.
after further review of the tpa ASOS data it was determined that
the previously reported high temperature of 99 degrees for
August 20th was unrepresentative. The high temperature for the
date at tpa was 93 degrees.
Preliminary point temps/pops...
tpa 94 80 94 80 / 20 20 30 10
fmy 95 76 95 78 / 50 20 40 10
gif 96 77 96 77 / 30 20 30 10
srq 94 77 94 78 / 20 20 30 10
bkv 95 74 95 75 / 30 20 30 10
spg 93 82 93 81 / 20 20 20 10
Short term/aviation/marine/fire weather...69/close
long term/decision support...63/jillson