Area Forecast Discussion
Issued by NWS Tampa Bay Area, FL

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FXUS62 KTBW 241318
AFDTBW

Area Forecast Discussion
National Weather Service Tampa Bay Ruskin FL
918 AM EDT TUE MAY 24 2016

.SYNOPSIS...
13Z water vapor and H4 RAP analysis continues to show the Florida
peninsula at the base of a longwave eastcoast upper trough that
this anchored by a large closed low spinning over the mid-atlantic
and northeastern states. Broad upper ridging is found upstream
from Mexico/western Gulf northward through the lower/middle MS
valley. The deep layer flow between these two features over our
region is from the west and northwest as sampled by the 12z KTBW
RAOB. Also within this NW flow we find a weak impulse/shortwave up
toward the Florida Panhandle. This feature will drop southeastward
through the day...helping to enhance the overall troughing over
the peninsula...and spreading a preceding swath of broad but weak
synoptic forcing/QVector convergence across the region this
afternoon/evening. This feature is a positive toward convection
later today. However...similar to Monday...this NW flow has also
delivered a deep layer of dry air above our heads. This dry air
is a general negative toward late day convection. 12Z sounding
shows large dewpoint depressions from just above the surface to
well above 400mb. The depth of this dry air is resulting in a very
low PW value for the end of May. The measured value this morning
was only 0.58". This is just about as dry as it gets around these
parts this time of year. If fact...it is the lowest value ever
recorded on the May 24th 12Z sounding...breaking the previous
daily min of 0.6". The 30 day moving average of the 10% percentile
PW value is a low 0.98"...so our current value shows we are sitting
under an exceptionally dry trop column for late May.

On the other hand...if one heads south...from Ruskin/Tampa Bay
area...there is a fairly tight gradient in the PW. The 12Z KMFL
RAOB sampled a PW of nearly 1.5". This gradient will define the
forecast story for today. From the Tampa Bay area
northward...despite the broad synoptic support/troughing aloft and
eventual low level seabreeze focus...it will be very difficult to
support much in the way of convection due to the overwhelming
degree of dry air for convective updrafts to overcome. The further
south one goes from the I-4 corridor...the more favorable the
atmosphere will become toward supporting or maintaining a few deep
convective updrafts.

At the surface...High pressure is centered off the SE conus coast
and ridges back to the west/SW over the northern half of the
Florida Peninsula and NE Gulf of Mexico. The gradient is fairly
light currently, but as the high better aligns itself through the
day...a more defined easterly flow in the lower levels is progged
by NWP guidance to develop. The easterly flow will be stronger
than previous days...and therefore the westcoast seabreeze will
have a more difficult time propagating inland. The better focus
staying closer to the coast should eventually align or propagate
any late day storms toward the I-75 corridor along the suncoast.
As mentioned above...despite the focus along the Nature
coast...little in the way of convection is expected...and barely
any of the hires convection allowing guidance are currently
showing storms north of Tampa Bay. An isolated pop up and short-
lived shower is not impossible, however coverage should be low
enough to exclude mention from the current forecast.

Forecast will show pops increasing from around 20% I-4 corridor to
the 30-50% range as one heads southward. Highest rain chances...as
mentioned...should align along the I-75 corridor from roughly
Sarasota southward to Fort Myers after 19-20Z. Despite still
having dry air aloft...the dry air will not be as pronounced as
further north...and combined with the other positive factors for
convection mentioned above...feel these rain coverages are
appropriate.

Temperatures today should max out within a degree or two either
side of 90 away from the beaches...and reach the middle 80s at the
immediate coast.

&&

.SHORT TERM (Tonight and Wednesday)...
Sct convective cells south of I-4 slowly dissipate between 01-03Z
with the loss of diurnal heating and sea-breeze focus. As during
any warm season day...residual outflow boundaries can focus
isolated cells for a bit longer...but these are tough to forecast
more than a couple hours in advance. With this in mind will end
pops after 03z...which is consistent with climatology. Low
temperatures by dawn will again be a bit below climo for late
May...with many inland area dropping down into the middle 60s.
Normally warmer locations with land breeze flow off the harbors
and Bays...such as southern Pinellas/coastal Charlotte/Lee
counties will likely stay in the lower 70s.

Wednesday currently looks as through it will be an inactive day
convectively for almost all zones. Shortwave impulse that will
help support some storms today will have sheared out and shifted
south of our region by tomorrow...with instead the enhanced
drying/subsidence in its wake arriving overhead. In
addition...defined easterly flow in the 1000-850mb range at times
exceeding 15kts should be enough to disrupt the organization of
the sea-breeze...or even keep it parked just offshore in some
spots. This lack of surface focus to initiate updrafts will
further decrease the potential for deep convection. Have excluded
mention of any rain chances for just about all zones. Only
exception is Lee county were a 20% chance for an isolated late day
shower has been painted. Lee county is in closest proximity to the
deeper moisture which will be down over far south Florida.
Sometimes a few storms are forced along these moisture gradients
leading to this small rain chance around Fort Myers.
However...anything in this area should be very isolated and short-
lived...and should have limited impact to any outdoor
plans/activities. Temperatures Wednesday remain seasonable with
upper 80s to lower 90s common.

&&

.MID/LONG TERM (Wednesday night through Monday)...
At the beginning of the medium range period, a large trough will
reside over the western half of the CONUS, bringing episodes of
thunderstorm activity from the Canadian border southward into the
Southern Plains. Further east, as a cutoff low lifts into the
northern Atlantic, broad upper ridging will become established from
the Gulf Coast states up into the Mid-Atlantic.

From Wednesday night through the end of the week, while weak upper
ridging looks to prevail, models continue to depict a weakness
developing across the western Atlantic near the Bahamas. Long range
solutions show this feature eventually evolving into a weak cutoff
low meandering northwestward through late week. With surface ridge
axis extending across northern Florida, a prolonged period of
easterly flow should promote increasing moisture from the Atlantic.
This increased moisture combined with weak lift in the presence of
the upper low should promote a daily increase in chances for
afternoon showers and thunderstorms across the peninsula.

From the weekend into early next week, the Atlantic upper low looks
to continue northwestward, opening up over the southeastern U.S. as
it is slowly absorbed into the larger upper trough moving eastward
early next week. Given the proximity of this feature and presence of
abundant moisture...expect scattered afternoon showers and storms
will remain possible through the weekend. Should the system track
well to the east of the Florida west coast, conditions may be drier
than currently forecast, as weak subsidence overspreads the
region.

Temperatures throughout the long range period look to remain in the
mid to upper 80s each afternoon, with lows in the upper 60s and
lower 70s most nights.

&&

.AVIATION...
24/12Z-25/12Z. VFR with FEW-SCT LCL BKN cumulus developing by
afternoon. Expect ISOLD late afternoon TSRA/CB with best odds for
LAL/PGD/FMY/RSW which have VCNTY TSRA. Light mostly easterly
winds...but shifting to onshore at coastal terminals in the
afternoon at 10kts or less. VCNTY TSRA ends by 03Z followed by
prevailing VFR and light east or variable winds at 6kts or less
through the overnight.

&&

.MARINE...
High pressure will ridge back over the northern Florida peninsula
and northeast Gulf of Mexico from the Atlantic through the
majority of the week. The position of the ridge will favor
stronger easterly surges each night and early morning. Overnight
winds are forecast to approach cautionary levels at
times...especially Wednesday night into early Thursday morning.

&&

.Preliminary Point Temps/PoPs...
TPA  88  71  89  69 /  10  10  10  10
FMY  89  71  88  69 /  40  30  20  20
GIF  89  69  87  67 /  10   0   0   0
SRQ  85  70  87  69 /  10  20  10  10
BKV  89  65  88  62 /  10   0  10   0
SPG  87  73  88  72 /  10  20  10  10

&&

.TBW Watches/Warnings/Advisories...
FL...None.
Gulf waters...None.

&&

$$

SYNOPSIS/SHORT TERM/AVIATION/MARINE...Mroczka
MID TERM/LONG TERM...Austin


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