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Area Forecast Discussion (AFD)

Issued by Brownsville/Rio Grande Valley, TX (BRO)

FXUS64 KBRO 242354 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
554 PM CST Sat Feb 24 2018

.DISCUSSION...Updated for latest aviation discussion below.


.AVIATION...Satellite images and surface observations across deep
south Texas early this evening indicate some mid to high level 
clouds over the area. Expect VFR conditions to prevail across 
the area this evening with breezy southeast to south winds
gradually diminishing after sunset. A weak cold front will move 
into the northern ranchlands tonight and low to mid level moisture
will increase ahead of the boundary. Expect MVFR to near IFR 
conditions to develop with low clouds/fog late tonight into early 
Sunday morning. In addition...showers and thunderstorms will 
develop across portions of the northern ranchlands tonight and 
possibly move into the Rio Grande valley early Sunday morning. 
Light to moderate winds and MVFR ceilings are expected to 
linger into early Sunday afternoon. 


.PREV DISCUSSION... /issued 307 PM CST Sat Feb 24 2018/ 

SHORT TERM (Tonight through Sunday Night): Winds continue to gust 
between 40 and 45 mph across portions of Cameron county and 
between 30 and 35 mph across portions of Hidalgo county this 
afternoon. Will continue the wind advisory for the coastal 
counties of deep south Texas through the rest of the afternoon. 
The winds will diminish early this evening as the strong low level
jet across the lower TX coast lifts northward and the pressure 
gradient begins to relax as a weak cold front moves into south 
Texas. Main concern tonight is the convective potential across the
northern ranchlands and upper Rio Grande valley as the front 
moves into the northern and western portions of the CWA late 
tonight. This morning the HRRR was progging thunderstorms 
developing across portions of Jim Hogg and Brooks counties tonight
but the latest HRRR is indicating southern portions of Hidalgo 
county before midnight. With lack of mid level moisture and a 
strong low level thermal inversion across the lower RGV...will 
keep best rain chances across the northern ranchlands and upper 
RGV tonight where showers or thunderstorms are more favorable for 
development. With fairly decent storm relative helicity across the
area...will mention gusty winds... small hail...and heavy 
rainfall with strong thunderstorms across the northern and western
portions of the CWA tonight. 

The front will likely stall somewhere over deep south TX late 
tonight into Sunday. Will likely see scattered showers and 
isolated thunderstorms across eastern portions of the CWA Sun 
afternoon where surface instability will likely be higher with 
light southerly winds across the lower RGV. The front tries to 
move across the lower TX coast Sun night and offshore early Mon 
morning with a convective boundary across southeast portions of 
the CWA Sun night. Will keep higher POPS across southern and 
eastern portions of the CWA Sun night as drier air filters into 
the northern and western portions. 

LONG TERM (Monday through Saturday): A changeable spring-like
pattern fittingly dominates the coming work week, including just 
about everything from welcome rain early to a heat spike and wind 
machine in the middle, and perhaps thunderstorms separating the 
very warm/hot day from a warm breezy and much drier day to follow 
as March arrives, then back to some clouds and perhaps a little 
rain to start the next weekend. 

The changeable weather comes courtesy of a subtle change in the
steering pattern t through the week. Though southwest flow still 
prevails at 500 mb into mid week, the subtropical ridge that has 
dominated the Gulf of Mexico and kept Florida beautiful and south 
Texas breezy to windy and generally warm/humid will flatten a bit 
and begin to shift west, and by late week this ridge axis is 
centered over Veracruz. At the same time, troughing develops along
the mid Atlantic coast, with northwest steering flow reaching 
Florida and bringing a long-awaited front through most of the 
peninsula. Why does this matter? It *could* be the start of a 
trend toward drier fronts and continued warmer than normal 
temperatures heading deeper into March, which is part of the 
spring pattern expected to dry out and eventually heat up (in a 
more notable way) the Rio Grande Valley. Stay tuned.

Monday begins with a weak 500 mb short wave/shear axis which helps
to develop a weak but important coastal trough just east of the
Lower Texas coast. The GFS was initially the most bullish on this
solution, but the 12Z deterministic ECMWF came around completely
today and given that the potential for light but notable rains
especially across the populated RGV makes sense given the big
picture, especially since Sunday's front has minimal southward
push of drier air after reaching the ranchlands. With leftover
subtropical moisture in the mix have pushed up QPF a few notches,
with potential for area-wide .25 to .5 inches across the populated
RGV. Temperatures will be a bit tricky; low clouds/light rain and
northeast flow should hold readings in the soupy mid 60s to around
70 for the Valley, but with drier air and filtered sunshine
farther north and west the mid 70s advertised by the ECMWF is in
line as well. 

Winds continue to veer to the east Monday night but light (5 knots
or less) and eventually to light southeast by daybreak Tuesday.
With only slight drops in temperature but more importantly
dewpoints catching up/remaining for 100 percent humidity, fog,
which has been a dominant weather element this winter on the edge
of transitions from cool/cold to warm, looks likely to
develop. Followed WFO Corpus' lead and added "Areas" mention to
entire region except for outer coastal waters, with slow
dissipation Tuesday along the immediate coast where marine layer
will keep winds suppressed some.

As for those winds on land, quick return to south/southeast flow
with a pickup to near 30 knots at 850 will get the Valley Wind
Machine back in gear - though not as crazy as today's version
which has pumped long durations of 30 to 34 mph gust 40 to 50 mph
across the lower Valley. As deeper moisture quickly scoots north,
expect a mix of afternoon sun and clouds in most area, and with
rapid low level warming low to mid 80s seem like a sure bet,
perhaps a few upper 80s sneaking in across parts of Hidalgo and
Starr County.

Breezy warm weather continues overnight Tuesday into early
Wednesday as region is sandwiched between what's left of
subtropical ridge and approaching (but lifting) shortwave trough
moving through the southwest U.S. Lows should be in the lower 70s
(upper 60s ranchlands) but could be a tad higher if low clouds
scoot in, especially across the RGV. This sets us up for a
"classic" wind machine/heat spike day on Wednesday. Expect 40-45
mph gusts across the US 77/IH 69E corridor by late morning through
afternoon, and with atmosphere quite warm for late February,
temperatures should achieve upper 80s to perhaps 90 in a few spots
given ample sunshine. Farther west, west to southwest flow at 850
brings the downslope, and surface winds go light. Guidance fairly
consistent on 90 to 95 along/west of US 281-IH 69C with highest
temperatures near Falcon Dam. Would not rule out the year's first
century mark readings either; guidance often late to catch on and
has to deal with late winter climatology where outliers are very
difficult to nail down. For now, went 92 to 95 from McAllen
through Zapata, but would not be surprise to see this end up in
the 96 to 100 range.

The aforementioned shortwave trough rips through Texas Wednesday
night, with a quick moving wind shift/front surging through Deep
S. Texas at some point. GFS/ECMWF in excellent agreement with a
mid evening to midnight passage for the ranches, and a midnight to
3 AM or so passage for the Valley. While this is likely to change
a bit, the timing of any precipitation (including thunder) will 
ultimately be an hour or two (not a full six hours as currently 
forecast) before refreshingly dry air sweeps in. Thursday looks 
mainly sunny with passing cirrus; the ECMWF is a bit drier but the
GFS came toward this solution with the 12Z run. Temperatures will
still be warm but dewpoints will be much lower, and we may need 
to watch combo of afternoon RH and wind for potential wildfire 
spread issues as we get closer to the event.

Clouds gradually increase Friday but still should be enough
sunshine and still comfortable humidity with lighter
east/northeast winds to make a great end to the work week.
Saturday remains uncertain for now; retained the idea of some
light rains for the Rio Grande Valley with fairly deep low level
moisture (to 800 mb) as weak short wave slides through the flat
ridge, which will be centered over Veracruz. 

MARINE: Seas were generally 5 to 7 feet with south to southeast 
winds 15 to 20 knots offshore the lower Texas coast this 
afternoon. Winds will diminish across the coastal waters tonight 
as a weak cold front moves into south Texas late tonight and the 
pressure gradient weakens across the lower TX coast. Seas will 
remain high offshore so will extend the small craft advisory for 
the 20-60NM waters through midnight but the nearshore waters 
should be below SCA criteria so will replace with SCA with SCEC 
after 6 pm. Light to moderate southeast winds will prevail across 
the lower TX coast after midnight tonight. Winds should remain 
light from the south across the coastal waters Sunday as the front
likely stalls near the coastal bend Sun afternoon. The frontal 
boundary tries to move over the lower TX coast Sun night and move 
offshore early Mon morning as a convective boundary. Winds are 
expected to shift to the north and northeast across the northern 
coastal waters early Mon morning as a result.

Monday through Thursday: A rather yucky boating day Monday with
occasional rain/showers and a nuisance stiff northeast wind, along
with mixed seas (wind chop and some swell) up to 5 feet expected.
A bit better for early morning fishing Tuesday except for
potential dense fog (radation/sea fog combo) that could delay
trips for those concerned with low visibility, with winds light
and seas moderate to start the day. Laguna winds will pick up some
Tuesday afternoon, and caution/Advisory conditions for winds
possible but will be determined by strength of the marine layer.

Winds will pick up Tuesday night in all areas (a bit less over the
nearshore waters due to remaining marine layer) and with 850 mb
flow possibly reaching 40 to 45 knots advisory conditions (wind
and perhaps seas) may develop offshore, continuing Wednesday with
20 knot sustained wind likely in both areas. Even the nearshore
could see 20 knot winds for a time, especially mid morning through
mid afternoon.  

A brief lull in winds before shifting northwest/north late
Wednesday night and continuing Thursday. With minimal cold air
advection over a marine layer, expecting sustained winds over the
nearshore waters Thursday to be around the edge of caution (15 to
20 knots) with a better opportunity over Laguna Madre. Seas will
hold at moderate to caution (6 foot) levels though northwest flow
should drop values a bit nearshore for Thursday.

CLIMATE: The heat has dominated February, and after today all
primary locations (Brownsville, Harlingen, McAllen) will be
somewhere between 5 and 5.5 degrees above the monthly average.
With above to slightly above average temperatures through Monday,
then a last hurrah of heat Tuesday and especially Wednesday, 6
degrees above average for the month is not out of the realm of
possibility. That *could* tip the scales to a winter (December-
February) just *above* average - something that seemed unheard of
when January ended. We'll do the numbers in a few days.


GM...Small Craft Advisory for winds until 6 PM CST this evening for 

     Small Craft Advisory until midnight CST tonight for GMZ170-175.

     Small Craft Advisory until 6 PM CST this evening for GMZ150-155.



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