Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Tampa Bay Ruskin Florida
949 PM EDT Monday may 25 2015
01z water vapor and h4 rap analysis shows the eastern Gulf of
Mexico and Florida Peninsula underneath a large ridge of high
pressure aloft. The ridge axis is just to the east of our
area...with WV imagery showing a large expanse of very dry
middle/upper level air extending from over the Florida East Coast out
into the Atlantic. This drier air will somewhat influence our
forecast for Tuesday as it makes some progress westward over our
At the surface...a large 1030+mb Atlantic high centered off the
middle- Atlantic/New England coast ridges back to the southwest into
the southeastern states. The Florida Peninsula is situated to the
south of the ridge axis...providing our state with a synoptic
easterly low level flow.
Saw quite a bit of thunderstorm activity today...with the majority
of the activity from coastal Pasco County southward along the
Suncoast. More scattered convection occurred northward through the
rest of the Nature Coast and over the inland area. Most of the
activity has now ended down toward The Fort Myers area...but
around the general Tampa Bay vicinity...the lingering effects of a
late day sea-breeze/outflow merger continue to fire off storms.
Most of this activity is now on the western side of the I-75
corridor...with storms trying to push out into the Gulf of Mexico.
May take another hour or two for things to really quite down
across parts of Manatee/Pinellas counties...but overall...will
expect a quiet/dry overnight over the land zones after 03-04z.
Storms are likely to linger longer over the marine zones...as
shown by just about all hires convection allow guidance.
For Tuesday...the ridge position shifts slightly southward...but
still remains to our north...this slight shift will bring our low
level winds out of the southeast. Now...generally...given a normal
atmospheric column for the warm season...this southeast flow
regime is one of our most active for West Coast convection. What
will complicate the forecast later Tuesday will be the slow
arrival of much drier air aloft. The GFS/ECMWF/NAM all show our
region on the western periphery of the really hostile middle/upper
level air. At the very least we can say the environment aloft will
not be as favorable as previous days. Therefore...we really have a
battle between a favorable low level convergence pattern to get
updraft going...and a hostile thermodynamic environment that the
updrafts/convective columns will be growing into.
Most of the hires guidance members are suggesting a more widely
scattered convective coverage...and mainly near the coast the second
half of Tuesday afternoon...with storms drifting west/northwest into the
Gulf. Current forecasts peak at 40-50% probability of precipitation for later
Tuesday...with no likely 60% or greater rain chances mentioned.
This seems reasonable given the mix of good and bad toward deep
convection. If the dry air makes more progress westward...then
the probability of precipitation may need to be nudged down a bit. Keep in mind that upper
level moisture is one of the poorest forecast variables by
numerical weather prediction. Either way...it will be another day
where boaters on the coastal waters need to keep a close eye on
the weather and be prepared to react to quickly developing storms
near the coast...since these storms will rapidly move offshore.
One concern for the storms that do develop...is the lower thetae
air arriving aloft that may actually set up a situation more
favorable for wet microbursts. The lower thetae air will promote
evaporative cooling through entrainment of the air into the
convective column. Was checking out the values for the wet
microburst severity index (wmsi) from the latest NAM...and they are
actually quite high for late Tuesday afternoon. Basically...storms
look to be more prone to downburst/microburst events on Tuesday
given the expected environment aloft.
Storms are likely to be late day occurrences once again. Before
the storms fire...our position under the strong ridge should again
allow many locations away from the immediate coast to reach the
lower to middle 90s for high temperatures.
convection is taking its time settling down around the Tampa Bay
area. Ksrq/kpie/ktpa/klal...will continue at least a small threat
of a shower or storm through 04z...and then VFR conditions should
generally prevail through the remainder of the overnight through
the early afternoon hour Tuesday. Late in the day...widely scattered
storms will develop...mainly after 19-20z. All terminals will run
the risk of a brief downpour or round of gusty winds...although
storm coverage is expected to be less than previous days.
east to southeast winds will remain in place across the waters
through Tuesday...other than a turn onshore during the afternoon
with the sea breeze. An easterly surge to exercise caution-level
winds is expected for portions of the Gulf waters during the late
night and early morning hours of Tuesday. Otherwise...no
headlines are expected. Late Tuesday afternoon...widely scattered
thunderstorms are expected to develop near the coast and then
rapidly push westward into the Gulf. Boaters on the coastal waters
should keep a close eye on the weather during the later afternoon
and early evening hours...and be prepared to seek safe Harbor
quickly should threatening weather approach.
Preliminary point temps/pops...
tpa 75 91 74 93 / 50 50 10 30
fmy 74 93 72 94 / 20 40 10 30
gif 74 93 72 93 / 10 20 0 20
srq 74 92 72 92 / 60 50 10 30
bkv 72 93 70 93 / 20 50 10 30
spg 77 91 77 90 / 60 50 10 30