Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Brownsville Texas
334 am CDT Wednesday Sep 17 2014
Short term /today through Thursday/...column moisture will remain
elevated across deep south Texas and the Rio Grande Valley through
the short term...with precipitable water hovering around two inches. Middle level
ridging overhead...supporting high pressure over the northwest
Gulf...will tend to stabilize weather conditions however...with a
situation more likely to support a sea breeze rather than widespread
precipitation. Nonetheless...a few heavier showers could develop...which
could be of concern to areas which recently received a lot of rain.
Middle level ridging will slowly succeed in developing some subsidence
and inhibition in the sounding...and feel that today might be better
for westward moving sea breeze convection than tomorrow...but another
sea breeze round will still be possible on Thursday. Should see a
mix of clouds and sun today with near normal temperatures and light to
moderate south southeast winds. A few high clouds originating with
Odile will still be in the mix. Middle 70s will be on tap tonight with
a few more low clouds due to winds backing a bit to a more southeast
onshore flow. Look for increased low clouds Thursday due to a weak
cap but still partly cloudy with moderate southeast winds. Temperatures
will again be near normal. Maintained much of the inherited forecast
package...but trended probability of precipitation down just a wee bit...10 percent here and
there...based on more stability and a lesser source of moisture from
the south. Used a blend for temperatures but bumped up high temperatures a degree
from the blend based on tuesday's performance and chance of seeing
some sun again today.
Long term /Thursday night through Wednesday/...
Thursday night into Friday we will retain rich column moisture with
850-700mb relative humidity values over 70 percent. Northwesterly 300-500mb flow
will develop as ridging develops in that layer over west/central
Texas and northern Mexico. Embedded in this flow is weak
perturbations and residual moisture from the remnants of Odile. Low
level flow will be moderate out of the southeast. Kept 20 to 30 percent
probability of precipitation in the forecast in the morning hours increasing to 40 percent valley
wide in the afternoon with 50 percent over the ranchlands where slightly
better middle level ascent is forecast to be in place enhancing
seabreeze convergence. Precipitable water values around 2.0 to 2.2
inches will position any cell that gets going to be a very heavy
rainfall producer...although storm motion and coverage should
prevent a widespread flooding risk...assuming limited convective
coverage in the days leading up to Friday.
Saturday the GFS disrupts the east to southeast 850 to 700mb
moisture tap from the Gulf of Mexico while the European model (ecmwf) keeps about 10
to 15 knot flow going. Both seem to be keying in on weak waves
developed by convection over Belize and the Yucatan. The European model (ecmwf)
appears to have a better handle based on how these systems have
behaved this season and leaned the forecast in the direction of the
European model (ecmwf)...which matches with the existing forecast well. This calls
for low probability of precipitation in the morning peaking in the afternoon at 20 to 30 percent.
Sunday and Monday the synoptic pattern remains little changed. Middle
level ridging persists over northern Mexico and central Texas. We
remain in moist 850-700mb southeasterly to easterly flow with no
real discernible waves to enhance showers/storms. Went with pop
pattern favoring overnight showers on the Gulf of Mexico working
into seabreeze initiated showers and thunderstorms in the afternoon
with 30 percent probability of precipitation in the favored zones...basically valley wide Sunday
and in a Raymondville/Hebbronville/Falfurrias Triangle on Monday
surrounded by 20 percent probability of precipitation. This is below guidance consensus but
persistent 500mb subsidence in the vicinity will likely limit middle
level lapse rates and instability and ultimately convective coverage
by making new updrafts fight a more hostile airmass.
Tuesday the European model (ecmwf) pushes a front into the area from the northwest
while the GFS holds off. The European model (ecmwf) is slower and more amplified with
ridging over the Dakotas which ultimately helps push the front
through the area. The GFS brings a jet maximum into the Pacific
northwest from nearly due west while the European model (ecmwf) is more amplified
which better agrees with the ongoing pattern over the central and
northwest Continental U.S.. GFS ensemble members also lean in a slight majority
towards the more northwestward oriented speed maximum which lends
credibility to the European model (ecmwf) solution which is less progressive and
stronger with ridging that results in the front pushing through.
Introduced 20/slight chance probability of precipitation on Tuesday given the presence of
the front. This is below guidance consensus but run to run
continuity has been poor over the last few runs.
Wednesday the GFS sends a fairly robust cold front through the area
while the European model (ecmwf) holds off. The European model (ecmwf) is slower and less amplified
with a 300mb West Texas trough while the GFS shows an
anomalously strong trough which generates a West Texas surface cyclone
that rides over a stalled front in the area producing widespread
rainfall. The European model (ecmwf) holds up better run to run and given the anomalous
strength of the 00z GFS trough went with that for the forecast just
holding a slight chance of showers in the forecast. A shift though
towards a final GFS solution will result in significant increases in
rain chances by the middle of next week. /68-jgg/
today through Thursday...broad high pressure over the north west
Gulf will support light to moderate south southeast winds through
the short term period. Two to four feet seas will persist on the
Gulf...with a light to moderate chop on the Bay. An isolated marine
shower may develop from time to time.
Thursday night through Monday...
moderate east to southeast flow and low to moderate seas are
forecast Thursday night through Monday along the Lower Texas coast.
Morning shower and thunderstorm chances will persist through the day
but widespread showers and thunderstorms are not currently forecast.
Winds and seas are expected to remain below exercise caution levels
for the period.
Preliminary point temps/pops...
Bayview/Port Isabel 90 80 90 79 / 40 20 40 20
Brownsville 91 78 91 78 / 40 20 40 20
Harlingen 92 78 93 77 / 40 20 40 20
McAllen 94 78 94 78 / 30 20 30 20
Rio Grande City 94 77 95 78 / 30 20 20 20
South Padre Island 87 81 87 80 / 40 20 30 20
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