Scientific Forecaster Discussion
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Area forecast discussion...updated
National Weather Service Brownsville Texas
556 am CDT Tue may 23 2017
Discussion...watervapor satellite loops indicate increasing
divergence over south Texas between a mid level shortwave trough
over northeast Texas and an upper ridge over northern Mexico.
Moisture convergence along the stationary front over deep south
Texas and the enhanced lift aloft is allowing rapidly developing
convection along the boundary. The invigorated thunderstorms are
moving very slowly with a few decent updrafts creating intense
rainfall and possible small hail. The slow movement of the storms
may be some concern for localized flooding this morning if the
storms persist. Have increased the pops for this morning.
Discussion...updated for latest aviation discussion below.
Aviation...some patchy MVFR ceilings will impact the air
terminals this morning but the main concern will be a line of
showers and thunderstorms develop north of regional airports. The
line of storms may persist for a few hours this morning drifting
slowly south. VFR conditions are forecast to prevail this
afternoon with scattered showers and thunderstorm in the area.
Another round of convection, possibly severe, is expected this
evening associated with a cold front. The cold front moves south
of the taf site before sunrise Wednesday.
Previous discussion... /issued 418 am CDT Tue may 23 2017/
Short term (today through wednesday): the quasi-stationary front that
has been meandering along and north of the northern ranchlands the
last 24 hours is currently extending east to west from the western
Gulf into central Kenedy County and into Hidalgo and Starr counties.
Convection has been developing along the boundary mainly over the
Laguna Madre between 1-3am with some isolated convection starting up
over central Starr and northern Hidalgo. Expecting convection to
increase in coverage this morning/today as warm moist low level flow
flows over this boundary with large scale ascent increasing in
advance of southeast moving upper trough over the Central Plains.
Several disturbances within the cyclonic flow to help initial
possible several clusters of thunderstorm through the day and
especially tonight with the main impulse and a cold front moving
through the County Warning Area. Storm Prediction Center maintains a slight risk of severe weather for
all deep south Texas today and tonight but believe the best
timing for severe weather in our region will be between 6pm and 3
am. Main threat with the potential storms will be large hail and
damaging winds but can not rule out a brief spin up tornado with
shear values increasing overnight as the cold front and upper
trough approaches. The strongest disturbance and lift will
coincide with the moderately strong cold front moving through the
County Warning Area between 9 PM and 3 am. Instability, cape values 2500-3000j/kg,
lapses rates at least 8.0 c/km are forecast this evening with the
approach of upper disturbance and the cold pool aloft associated
with the upper trough. Most models suggest convection to initiate
over the Sierra Madre around 6 PM and move southeast near or into
Zapata and Starr County through the evening hours...while
additional convection to form along the cold front and sweep
through the region between 9pm and 3 am. Uncertainty creeps into
the forecast if the Sierra Madre convection, which could be the
most intense, does move into the forecast area or stays just south
of the river.
Once the front moves through convection should end within a few
hours with relatively cooler and drier air filtering across deep
south Texas by sunrise. Wednesday will be much less humid as dew
points fall into the 40s and 50s as the large scale upper trough
Ushers in a brief shot of modified Canadian air. Highs will be
around 10 degrees lower then the last few days.
Long term (wednesday night through monday): northwest flow aloft
and surface high pressure across the western Gulf of Mexico will
continue to bring drier air and slightly cooler temperatures
across the area Wednesday night. Low temperatures Wednesday night
expected to be a few degrees below normal with temperatures
falling into the 60s in most locations. Mid level ridging over
the southern and Central Plains Thursday will continue to provide
subsidence across deep south Texas even as low level moisture
begins to increase due to the return of onshore flow. Temperatures
will return to above normal Thursday with high temperatures mainly
in the 90s. A subtropical ridge develops over northern Mexico
Friday and into the weekend resulting in temperatures well above
normal. Moisture will continue to increase across deep south Texas
and the Rio Grande Valley through the weekend and into early next
week as a weak cold front moves southward across Texas. This will
provide a chance of showers and thunderstorms across the region
Sunday night into Monday.
Marine (now through wednesday): a modest swell has developed over
the coastal waters with the persistent east-southeast flow over the
Gulf. This swell to persist today in advance of a cold front that
will sweep through tonight. Showers and thunderstorms to increase in
areal coverage and intensity today and especially tonight before
ending as the cold front passing through. Northeast winds and sea to
increase approaching Small Craft Advisory levels by midday Wednesday
before slowly lowering by sunset.
Wednesday night through Saturday night...light northeast winds
initially will gradually become southeast by late Wednesday night
as surface high pressure moves eastward. Southeast winds will
steadily increase on Thursday as the pressure gradient tightens
across the western Gulf of Mexico due to low pressure across the
West Texas interacts with high pressure across the northern Gulf.
Moderate to strong southeast winds will support scec to low-end
Small Craft Advisory conditions across portions of the Lower Texas coastal waters
Thursday through Saturday night.
Preliminary point temps/pops...
Bayview/Port Isabel 92 70 84 71 / 30 40 10 0
Brownsville 94 71 87 71 / 30 40 10 0
Harlingen 94 67 87 68 / 40 40 0 0
McAllen 97 70 90 69 / 30 60 0 0
Rio Grande City 96 67 90 68 / 30 60 0 0
South Padre Island 84 75 81 75 / 30 50 10 0
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