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Area Forecast Discussion (AFD)

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

FXUS64 KBRO 252108

Area Forecast Discussion
National Weather Service Brownsville TX
308 PM CST Sun Feb 25 2018

.SHORT TERM (Now through Monday Night): For parts of the Rio 
Grande Valley, a decent shot of rain with still mild temperatures 
to begin the work week will keep the slightly ahead of schedule 
"green-up" on overdrive. But not everyone will "enjoy" the rain, 
which in this form is never a bad thing here - as there will be a 
sharp edge from southeast to northwest and some areas which have 
missed some of the beneficial heavier rains this winter may well 
miss this event as well. Those rains fell along a stripe along US 
77/IH 69C between Brownsville and east of Raymondville earlier 
this month (2 to 4 inches) month and across a good part of Starr 
County most recently (1 to 2.5 inches).

The players include a weak cold front which will slide through the 
lower RGV this evening, followed by a shear axis at 500 mb which 
builds up a nice little moisture pool between 1000 and 500 mb along 
with sufficient lift to produce occasional rain, probably in the 
form of showers, most focused on the Lower Valley (mainly Cameron 
County into the Lower Texas Gulf waters) but with some westward 
extension toward McAllen. Enough lift and a still-unstable 
atmosphere should be sufficient for embedded rumbles in the better 
rain chance areas, with MUCAPE still above 1K Joules during the peak 
of the event.  

Deterministic models are in slight disagreement this afternoon with 
the ECMWF backing off a little on precip chances and QPF as well as 
slightly delayed timing, while GFS is most bullish in both camps. 
See no reason to chance a good forecast when the big 
picture/conceptual model remains the same, but decided to lean a 
little earlier with onset (pre-dawn, perhaps as early as 1-2 AM)and 
hold light rain/QPF into late afternoon Monday particularly along 
and just north of the Rio Grande near the coast, including 
Brownsville, Port Isabel, and SPI. Edited QPF (mean areal) is 
generally 0.25 to 0.5" from Harlingen to McAllen, and up to 0.75" in 
Brownsville to SPI...but with precipitable water literally off the 
climo chart (1.88" according to the GFS) it wouldn't take much 
mesoscale forcing to see locally 2" fall in the "likely" zone, which 
includes Harlingen and Weslaco as a buffer.

The rain and low clouds will keep a lid on temperatures across the 
lower and perhaps mid Valley, which current forecast had and both 
GFS/NAM guidance have come around to.  Warmest temperatures will be 
in minimal or rain-free areas that cover the Upper Valley and 
ranchlands, where atmosphere is warm enough and some sunshine is 
likely to peek through to support mid to upper 
70s once again Monday - though with more comfortable humidity.

For Monday night, 500 mb subtropical ridge nudges northwest from the 
Gulf and the shear axis exits northeast, leaving light east to 
southeast winds and low clouds that will inevitably sink toward or 
to the ground as fog or dense fog as temperatures in the rain-cooled 
area change little with 100% humidity, while temperatures and 
dewpoints farther north/west meet at some point during the night 
where winds will be near calm. Did not add "dense" fog now but 
signal is strong for another round mainly after midnight and through 
the morning commute Tuesday before the wind machine returns (see 
long term for this).

.LONG TERM (Tuesday through Sunday): Surface high pressure across
the mid-Atlantic U.S. Tuesday and surface low pressure across the
Texas panhandle will allow the pressure gradient to increase
across the lower Texas coast. Warm air advection will increase
across south TX Tuesday with breezy conditions across most of the
Rio Grande valley. A strong low level jet is progged to develop
across the coastal sections of the CWA Tues night into Wednesday
and windy conditions will be likely across most of the RGV with
wind advisories likely needed across the coastal counties
Wednesday. Unseasonably warm temperatures will return Tuesday and
continue Wednesday before a weak cold front moves through south TX
Wed night into Thurs morning. Low to mid level moisture appears
limited ahead of the front but will continue to mention a slight
chance of showers and thunderstorms across the CWA Wed night and a
slight chance of elevated convection across southeast portions of
the CWA Thurs morning before drier air filters into the area.
Temperatures will not be as unseasonably warm Thursday and Friday
as Tuesday and Wednesday. The rest of the forecast period is
dependent on moisture increasing across deep south TX late Friday
into Saturday as the surface ridge across the central U.S. Friday
moves eastward. This will provide at least a slight chance of rain
through the weekend and with the increase in cloud cover should
keep temperatures in the upper 70s to lower 80s. 

.MARINE (Now through Monday Night): Sea fog lifted a little before 
noon, but has returned to the beach and nearshore waters as of this 
writing with visibility around 1 mile.  With wind fields relaxing as 
the front sags across the waters overnight, the fog may thicken for 
up to six hours (mainly from 6 PM to midnight or so) and another 
dense (marine) fog advisory may be needed.  Thereafter, enough 
"push" to the front should thin out the fog, most likely across the 
waters north of Port Mansfield at or before midnight and south of 
Mansfield after midnight.  That said, the combination of developing 
light rain and fog, along with slow modification of the warm/humid 
air mass just above the surface, could keep some fog going into 
Monday across the nearshore waters and have kept a mention through 

Otherwise, expect seas to continue a slight subsiding overnight into 
early Monday, before a slight uptick later Monday.  Generally in the 
4 to 5 foot range and a combination of swell and wind wave. Behind 
the front, northeast winds pick up to moderate levels but nothing 
too crazy.  With the rain and fog, Lower Laguna Madre and the busier 
waters off South Padre will not be that inviting; Port Mansfield has 
a better shot with less rain and higher visibility.  Light east 
winds and moderate seas continue Monday night but expectation of 
fog, becoming dense, will make for a tricky early morning Tuesday 
boating/fishing departure.

Tuesday through Friday...Moderate southeast winds will prevail
across the coastal waters Tuesday with surface low pressure across
the Texas panhandle and surface high pressure across the mid 
Atlantic U.S. coast. Winds will increase Tues night as the
pressure gradient continues to increase across the lower Texas
coast. May need small craft advisories for the offshore waters
Tues night. Moderate to strong southeast to south winds should
prevail across the coastal waters Tues night into Wednesday and
small craft advisories will likely be needed for the Laguna Madre
and the offshore waters. A weak cold front is expected to move
offshore the lower TX coast Wed night into early Thurs morning. 
Winds will diminish and veer to the southwest Wed night ahead of 
the front and shift to the north and increase in the wake of the 
front. Moderate to strong north to northeast winds will provide 
SCEC to possible SCA conditions across the offshore waters
Thursday. Winds and seas should diminish Thurs night into Friday
as the pressure gradient diminishes across the lower TX coast with
surface high pressure across the state moving eastward.

BAYVIEW/PORT ISABEL  64  70  67  81 /  60  60  20  10 
BROWNSVILLE          65  71  67  83 /  60  60  20  10 
HARLINGEN            63  69  67  84 /  60  60  20  10 
MCALLEN              63  70  67  86 /  40  40  20  10 
RIO GRANDE CITY      61  73  65  86 /  20  20  30  10 
SOUTH PADRE ISLAND   64  68  67  76 /  60  60  20  10 



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