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Area forecast discussion 
National Weather Service Tampa Bay Ruskin Florida
310 am EST sun Dec 21 2014

Synopsis...
08z water vapor and h4 rap analysis shows a currently unamplified
upper level pattern across the Continental U.S.. flow is mainly zonal along the
eastern Seaboard down to the Florida Peninsula...but watching a
broad upstream trough ejecting eastward across the Southern Plains.
Energy from the base of this trough will quickly move eastward
across the northern Gulf Coast during the next 24 hours and interact
with a surface front across the same region to bring increasing rain
chances to the central and northern Florida Peninsula. However...for
most of US...that will not be until much later today or
tonight...and will discuss in more detail within the short term
section below.

Further upstream we see strong upper jet energy arriving over the
Pacific northwest coast. It will be this energy that evolves into a
significant pattern amplification/deep trough through the central
portion of the County by Tuesday. In response to this amplifying
trough...the downstream ridge over the western Atlantic is also
prognosticated to undergo a significant amplification. Needless to
say...the upper level pattern will have little resemblance to this
morning within only a couple short days.

Lower levels...a quasi-stationary surface frontal boundary is
located from the central Gulf of Mexico northeastward to the
northern Florida Peninsula. Return flow in the lower levels from the
south is developing in response to the approach of the upper wave to
the west. This flow is beginning to undergo isentropic upglide along
the 295-305k surfaces...and we can see more and more Lower/Middle level
clouds breaking out across the eastern Gulf and Florida Peninsula this
morning. This upglide has been maximized to the north of the front
along with decent surface focus. Regional radar have been showing
areas of showers developing across the Florida Panhandle/NE Gulf as a
result. So far...much of this activity has been remaining to our
north...but will see a tendency for the showers to migrate toward
the Nature Coast as we head through the day.

&&

Short term (today through monday)...
much of the forecast area will see a dry morning as atmospheric
moistening as a result of large scale upglide will be slow to
occur...and any added surface convergence as a result of surface
warm front will be confined to our far northern zones. Will bring a
low shower chance into Levy/Citrus counties this morning...but
elsewhere probability of precipitation will be 10% or less. Skies will be mostly
cloudy...although better chances for periods of sun will exist south
of the Tampa Bay region. Approaching upper level energy will
increase its interaction with the surface boundary during the second
half of the day...and especially into the evening hours. Surface
pressure pattern will sharpen across the northeast Gulf of Mexico
increasing the low level convergence/focus...and by late in the
day...this should combine with continued upglide and a developing
favorable rrq coupled jet pattern aloft to produce increasing
showers coverage into the Nature Coast zones. Nwp ensemble members
are not in complete agreement with the placement of the lifting
features...and therefore have somewhat different quantitative precipitation forecast patterns. The
NAM continue to be furthest north with the best lift...while the
GFS...and especially the European model (ecmwf) show better shower coverage southward
into west-central Florida. The new 21/03z sref ensemble mean has
trended toward the global models with decent quantitative precipitation forecast now down to the I-4
corridor. So...as seems to often be the case...the most uncertainty
in the rain probability forecast late today into tonight will be
through the Tampa Bay/I-4 corridor...and the tightest pop gradient
will be across this region. Will have high end likely to categorical
probability of precipitation anywhere north of I-4...with likely probability of precipitation through the Tampa Bay
region...and then quickly decrease rain chances further south and
southeast. With respect to the European model (ecmwf)/GFS have gone with a 30-40%
chance near the coast down to Sarasota/Charlotte counties.

The placement of the upper jet is most favorable for significant
upper divergence over the northern half of the region tonight. The
GFS divergence fields are actually quite impressive. Many of the
models seem to be picking up on this and ramping up quantitative precipitation forecast amounts
during the overnight hours. Therefore...the potential does appear to
exist for some periods of heavier rainfall after 00z from Tampa Bay
northward through the Nature Coast. Due to the increased synoptic
lift in this region (north of I-4) and showalter indices at or below
zero have added in isolated thunder to the forecast for elevated
convection.

Monday...
early Monday morning looks to be just a continuation of the forecast
from tonight...however as the morning progresses we see not only the
upper jet position become less favorable...but also appears the low
level front will begin to move northward as a warm frontal feature
and reach at least the I-10 corridor by early afternoon. The loss of
large scale lift and a decrease in surface focus should result in
areas of showers become less organized and lighter in nature. Ramp
down rain chances through the day...and remove all mention of likely
probability of precipitation (even for the nature coast) by the afternoon hours. Rain
chances are actually looking less and less as time GOES along south
of Tampa Bay for Monday. If we can get enough sun to break
through the clouds...the atmospheric temperature potential is going to be
very high and afternoon highs for the southern half of the forecast
area in the 80s may become widespread. Temperatures are more likely to be
held in the 70s from Tampa Bay northward due to more shower and
cloud potential later into the day.

&&

Long term (tuesday through next sunday)...
models in fairly good agreement through the period. One upper
low/long wave trough initially over the middle Continental U.S. Moves east and off
the eastern Seaboard by the end of the week with a cold front
tracking into the eastern Gulf Tuesday then crossing Florida Wednesday. For Thursday
into Sat...ridging aloft and surface builds into the Gulf and shifts
over the state to Atlantic waters. During Sat a second long wave
trough...a bit less robust this time...slides from the
middle-Mississippi Valley to the east states by sun with a cold front
sagging into Florida.

The upper trough amplifies with increasing deep moisture as it moves
across the Gulf then Florida Tuesday and Wednesday. Expect areas of sea fog to
spread southward during Tuesday...and possibly become locally dense...as
large areas of convection move across from northwest to southeast.
There will be the potential for a few strong to possibly severe
storms. The bulk of the convection will exit to the east Wednesday
afternoon and evening. Cooler drier air works in during Thursday and Friday
as the ridging moves by. As noted above the next upper trough is
weaker and will have less moisture to work with so only slight
chance showers are expected...across the north Sat then spreading
into central counties sun. Temperatures start off above normal then
cool through the last part of the week before warming to around
normal by sun. Winds increase into exercise caution/advisory speeds
late Tuesday and Wednesday as they veer from south to west and north behind
the front. Winds diminish during Thursday.

&&

Aviation...
general VFR conditions early this morning under plenty of middle-level
clouds. Although guidance is hinting out some fog potential for
klal/kpgd...thinking the current clouds will prevent much in the way
of visible restrictions. General VFR conditions should prevail through
the daylight hours with a light southeast wind shifting south in the
afternoon. Probability for showers will be increasing during the
late afternoon and evening hours for mainly ksrq to ktpa and klal.
The showers and lower clouds may result in periods of MVFR
restrictions through the overnight hours into early Monday morning.

&&

Marine...
a stationary frontal boundary is located across the northern
waters...just to the south of Cedar Key early this morning. Winds
are east to southeast south of this boundary and from the northeast
on its northern side. Areas of showers/rain expected to increase in
coverage of the marine area to the north and west of Tampa Bay
today...showers reaching further south tonight. The warm front is
expected to lift north of the forecast waters for Monday with
southerly flow holding in place until a cold front crosses the
region sometime on Wednesday. Winds will shift northwest later
Wednesday and Wednesday night and possibly increase to advisory
levels for a time. Winds and seas quickly decrease Thursday and
Friday as high pressure builds over the region.

Deep southerly flow is expected to bring conditions favorable for
sea fog to develop in the next several days. Patchy fog is possible
as early as Monday...especially north of Tarpon Springs...however...
the sea fog is anticipated to become more widespread...and extend
further south Monday night into Tuesday. Once fog develops it is
likely to linger into Wednesday morning before a cold frontal
passage finally clears the area. The fog is also anticipated to
become locally dense at times...especially Tuesday and Tuesday
night.

&&

Fire weather...
no significant fire weather concerns expected through Monday.
Wetting rainfall is expected from Tampa Bay northward tonight and
Monday...with more scattered nature showers further south. Another
round of wetting rainfall for all zones is expected later Tuesday
night into Wednesday with the passage of a cold front. Drier weather
will then return for Thursday and Friday. Locations near the coast
are anticipated to see areas of sea fog move ashore...especially
Tuesday into early Wednesday. The fog may become locally dense at
times within a few miles of the coast.

&&

Preliminary point temps/pops...
tpa 77 66 76 66 / 40 70 50 20
fmy 80 65 78 66 / 20 20 10 20
gif 79 64 77 64 / 40 60 50 20
srq 77 65 76 65 / 30 50 40 20
bkv 77 64 77 64 / 50 80 70 30
spg 76 65 74 66 / 40 70 50 20

&&

Tbw watches/warnings/advisories...
Florida...none.
Gulf waters...none.

&&

$$

Synopsis/short term/aviation/marine/fire weather...mroczka
long term...rude

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