Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Tampa Bay Ruskin Florida
530 am EST Sat Feb 28 2015
..updated to discuss increased rain chances today...
09z water vapor and h4 rap analysis shows a currently unamplified
upper level pattern across the Continental U.S.. a general zonal flow pattern
is in place over much of the Continental U.S....although energy dipping south
along the Pacific coast is in the process of evolving a deeper
trough across the far west. This trough will have a positive
influence on our forecast as it eventually helps to build downstream
ridging over the Gulf of Mexico and Florida Peninsula by the early
portion of the upcoming week. In the short term however...our upper
level flow consists of deep layer southwesterly flow overtop a
shallow northeast to easterly regime in the very lowest levels. The
easterly flow in the lowest levels is the result of the gradient
around strong 1040+mb high pressure ridging down the Atlantic
Seaboard into our region. This pattern will persist for the
remainder of today...and help develop/expand a decent isentropic
upglide region northward over the Florida Peninsula.
This upglide regime is just staring to get its act together with a
widespread lower clouds deck and a few sprinkles/light showers
showing up on radar across the region. This cloud deck is focused
beneath a pronounced inversion where winds veer from northeast to
southeast between 950-900mb...and another inversion around 850mb as
seen on the 28/00z ktbw sounding profile. With the low level ridge
continuing to ridge southward into the daylight hours...this pattern
will ensure a mostly cloudy day with scattered light showers for
Further to the north we find narrow by focused zone of rain
extending from south of Apalachicola to the Nature Coast of
Levy/Dixie counties. While there is some weak upglide beginning up
over these areas as well...this band of rain is mainly being forced
by a approach of a subtle shortwave...and a well defined coupled
upper level jet structure. The rer (right entrance region) of
northern stream jet energy exiting up the East Coast is coincident
with the ler (left exit region) of a southern stream jet streak
crossing into the northeast Gulf of Mexico. Pronounced upper level
divergence fields are shown by the guidance suite and line up
very well with the ongoing band of rain. This band of rain will be
flirting with northern Levy County (chiefland) area into the morning
hours...before the shortwave dynamics and upper divergence shift
eastward out of our region for the afternoon.
Short term (today through sunday)...
today...welcome to the final day of meteorological winter. While
astronomical Spring does not arrive until March 20th...
meteorological winter is defined as December through February...
which means tomorrow is the start of meteorological Spring! It will
certainly be feeling like Spring around these parts by the early
part of next week.
As mentioned in the synopsis...will be watching the evolution of the
narrow but focused band of rain forced by the coupled upper jet
structure moving across Apalachee Bay into the Nature Coast.
Generally speaking the majority of this rain should stay just to the
north of Levy County...but it is a very close call...and shower
along the southern periphery of the band will periodically reach far
enough south to impact areas around Chiefland. Have likely probability of precipitation for
northern Levy County this morning to account for this. Elsewhere the
expanding upglide regime and continued moistening of the low level
column should result in widely scattered sprinkles/patchy drizzle giving
way to scattered/numerous light showers across the region. Certainly
some area will end up with conditions warranting likely rain
chances...but it is tough to predict within such a broad upglide
regime just where the best quantitative precipitation forecast fields will develop. Overall the best
upglide fields coincident with speed convergence along the East
Coast suggest the highest coverage of showers will be over our
interior zones. However...given the radar presentation at this early
hours...and the latest ecs/mav (ecmwf and GFS mos) showing
categorical probability of precipitation all the way over to the I-75 corridor...have
decided to not fight guidance and go with at least likely probability of precipitation for
all areas later this morning...and especially into the afternoon.
Will slowly ramp up the probability of precipitation across the region through the first
half of the day. This is a situation of high rain chances...but low
quantitative precipitation forecast (rainfall amounts). The showers should be on the lighter side
with quantitative precipitation forecast generally under a quarter of an inch...and in many cases
under a tenth of an inch. The upglide regime...and defined
inversions will keep our skies mostly cloudy and discourage rapid
diurnal temperature rises. The best chances at any breaks of sun in the
afternoon will be down south of Sarasota...and will allow temperatures to
break into the Lower/Middle 70s for these southern zones. Temperatures further
north will struggle more as the upglide only increases through the
day...and the ridge continues to supply drier air to the boundary
layer. Wet bulb effects from the scattered showers falling through the
boundary layer will further slow the temperature rise...and think
locations up toward Tampa will struggle to reach 70...and up toward
Levy County may struggle to rise into the lower 60s today.
upglide regime above 5000 feet will begin to pivot off to the
northeast with time leaving just the very lowest levels with any
lift. Guidance is in agreement that shower production will begin to
dissipate quickly as this occurs and also pivot northeast during the
evening hours. Will taper rain chances this evening and generally
end the showers for all areas after midnight. However...the lower
upglide regime will likely continue to support lower stratus and
will keep the forecast for mostly cloudy to clouds skies. Overnight
the ridge also begins to slowly weaken to our north and allow the
gradient to relax over our region. This weaker gradient is shown by
many of the guidance members to allow a well defined inverted
surface trough to develop just east of Florida over the warmer
waters of the Gulf Stream. This surface trough will also help to
focus the majority of the shower activity to the east and northeast
of our zones.
the ridge looses its grip on our area allowing a much weaker
gradient and weaker easterly flow to develop. Although the easterly
flow will be weaker...it is still show to be enough to migrate the
surface trough mentioned above westward over the Florida Peninsula during
the day. This will come into play during the afternoon. Sunday will
see the first signs of ridging building north from the Carribean
with our upper flow becoming more westerly. The loss of the
influence from the surface ridge will quickly dissipate the
inversion and allow any morning lower stratus to burn off...and
skies to become partly cloudy. The atmospheric temperature potential is
quite high on Sunday and the sunny breaks should help boost temperatures
into the the 70s north and lower 80s south. The strong terrestrial
heating should allow for winds to turn onshore for a time during the
afternoon with sea-breeze development. This is where things get
interesting. We look to already have a weak surface trough in place
nearby to focus some meager low level convergence...now add
convergence as the sea-breeze interacts with the surface trough and
we can not rule out the potential for a few afternoon convective
showers...or even a thunderstorm to the south of I-4. It is
important to note that Sunday is not going to be a wet day...and
outdoor plans should not be discouraged. However...have gone with a
20-30% pop Sunday afternoon for the potential widely scattered convection.
Any rainfall will be short-lived...but have to at least mention the
potential in the forecast.
Long term (monday through next friday)...
through wed; a building upper level ridge centered over the
Yucatan...and dominating the Gulf of Mexico...slowly slides
east and stretches from the western Caribbean to northeast of
the Bahamas. This is in response to a long wave trough deepening
over the western Continental U.S.. at the surface high pressure extending
across Florida from the Atlantic will move east as high pressure over the
northern and Central Plains tracks east...across the eastern sea
board to the open Atlantic. The upper level ridging will provide a
stable and generally dry and warm airmass with temperatures on the
warm side of normal and no significant rainfall. The departing high
pressure over the Atlantic and the plains high pressure moving by to
the north will result in a relaxed easterly low level flow that
gradually becomes southeast and southerly and slightly more robust.
The southerly flow will advect enough warm and humid air across the
cooler shelf waters of the Gulf to possibly form some sea fog around
Thursday-fri; by Thursday the western long wave trough has moved east...
from the upper Great Lakes to the southern High Plains...and
continues to the eastern Seaboard by Friday as it Delaware-amplifies some.
The Caribbean-Bahamas ridge flattens and shift to the southeast. A
cold front accompanying the upper trough moves into the western
Gulf-southeast U.S. Areas Thursday then the eastern Gulf and roughly the
northern half of Florida by Friday. While the models had been rather
consistent they begin to diverge Thursday and Friday. The GFS is slightly
faster and more robust with the rainfall while the European model (ecmwf) lags behind
and is drier. For now will maintain the current forecast of slight
chance to low end chance showers. Temperatures initially above
normal will cool to normal or slightly below.
widespread MVFR to IFR conditions across west-central and southwest
Florida early this morning...and conditions are likely to transition
to more and more IFR ceilings with time. These restrictions are expected
to linger into the later morning hours...with ceilings slowly raising
back into the MVFR category for all terminals before midday. The
afternoon should feature continued MVFR ceilings for the terminals along
the I-4 corridor...while more southern terminals will have a chance
to break out into VFR ceilings for a while during the later afternoon
and early evening hours. Look for scattered showers through the
day...however rainfall is generally expected to be light with
limited visibility restrictions. Ceiling heights should again lower
after sunset this evening with widespread MVFR conditions into the
strong high pressure ridging down the eastern Seaboard will keep the
local gradient tight into this evening across the eastern Gulf of
Mexico. Advisory level winds and seas over the offshore waters early
this morning will end by the afternoon hours...but may once again
briefly occur this evening before the gradient begins to relax early
Sunday morning. Winds and seas Sunday and Monday are expected to
remain below 15 knots...with winds locally turning onshore each
afternoon due to seabreeze development. High pressure will shift
eastward into the Atlantic Tuesday into Wednesday allowing winds
over the eastern Gulf to shift southeast and eventually south with
time. As winds become more southerly later Tuesday into Wednesday...
the risk looks to increase for the development of sea fog over the
near shore waters and bays.
a cloudy day on tap with occasional showers and sprinkles
region-wide. Mixing heights will be shallow keeping ldsi values on
the low side...despite steady winds in the boundary layer. Relative
humidity values will remain well above critical levels. No
significant fire weather concerns continue into Sunday and
Monday...with relative humidity values well above critical levels.
Temperatures will begin to moderate on Sunday and generally be above
average for the early portion of next week.
Fog potential...patchy fog is expected Saturday night into early
Sunday morning...with more numerous areas of fog possible Sunday
night into Monday morning...especially along and to the north of the
I-4 corridor. The fog may become locally dense early Monday morning
over these northern areas.
Preliminary point temps/pops...
tpa 70 62 77 64 / 50 30 30 20
fmy 78 66 83 65 / 50 20 30 10
gif 72 63 80 64 / 60 40 30 20
srq 72 63 78 63 / 50 20 20 20
bkv 69 60 78 59 / 50 40 30 10
spg 69 64 75 65 / 50 30 30 20
Gulf waters...Small Craft Advisory until 9 am EST this morning
for Englewood to Tarpon Springs out 20 to 60 nm-Tarpon
Springs to Suwannee River out 20 to 60 nm.
Synopsis/short term/marine/aviation/fire weather...mroczka
long term/decision support services...rude