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Area Forecast Discussion (AFD)

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

                            
000
FXUS64 KBRO 130331 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1031 PM CDT Fri Oct 12 2018

.DISCUSSION...Made a few tweaks to the overnight forecast. First,
through midnight and perhaps toward 4 AM, expect little if any
precipitation anywhere with dry air dominant well above the
surface...mean layer humidity below 20 percent from 600 to 250 mb
or so. Toward morning, low level streamer showers from the Gulf 
can't be ruled out but with dry air still in place well above the
surface I'm not sure where the 18Z GFS was generating a decent
plume of moisture along/just off the northeast Mexican coast and
helping boost rain chances to near 40 percent in southern Cameron
County. Prefer the drier ECMWF solution and kept the streamers as
"isolated" as expect most areas will remain dry.

Trimmed down rain chances for Saturday as well. Fairly strong
south/southeast flow combined with the aforementioned fairly deep
dry layer aloft should keep any rain to the "passing"
variety...and not too sold on thunder for the same reason though
kept in for consistency's sake. Raised temperatures a degree to be
more in line with guidance and very warm atmospheric profile. 
52/BSG

&&

.MARINE...Winds at Buoy 42020 had reached 15 to 17 knots with
higher gusts...and with similar values just inshore at Brazos
Santiago decided to add caution wording to the nearshore zones as
well overnight.
52/BSG

&&

.PREV DISCUSSION... /issued 729 PM CDT Fri Oct 12 2018/ 

DISCUSSION...Updated for latest /00z/ aviation discussion below.

AVIATION...With south/southeast flow picking up in the increasing
gradient between eastern Gulf of Mexico ridge and general
troughing over west Texas through the Sierra Madre, along with a
boost from the remnant surface low from Pacific Tropical Storm
Sergio, the main forecast concern is gusty winds on Saturday.
Today was a preview of the stronger winds to come, and guidance
consensus near 20 knots sustained with a few gusts to near 30
knots looks realistic and have gone with this from late morning
through sunset. Otherwise, with a decent plume of low level
moisture - similar to today - expect periods of broken VFR skies
from mid morning through mid afternoon. Models not showing much
moisture above 700 mb - in fact, layer relative humidity is below
15 percent from 600 to 250 mb or thereabouts - so no need to add
any mention of showers since any that occur will move quickly and
may not even reduce visibility sufficient for mention at this
time.
52/BSG

PREV DISCUSSION... /issued 247 PM CDT Fri Oct 12 2018/ 

SHORT TERM (Tonight through Saturday Night): Isolated showers are 
moving north into mainly eastern sections of the CWA. A weak surface 
trough along the northeast Mexican coast is providing some omega 
(lift), where the showers are developing. A stalled frontal boundary 
is analyzed to the inland and southwest of the CWA. High pressure 
over the northwest Gulf and east Texas extends south over the CWA, 
with drier air struggling to push inland from the east over the 
Gulf. The models show that this drier air will not make much headway 
in the short term, as high pressure shifts east, and forecast 
precipitable water remains around 2 (two) inches. Enhanced southeast 
winds will, continue funneling moisture north toward southern Plains 
storm systems. That said, showers may diminish tonight as diurnal 
heating decreases, but isolated showers will be possible again in 
the morning hours of Saturday for the lower Valley. Look for 
moderate to breezy southeast winds through the short term with a mix 
of clouds and sun. Overnight lows will range from the mid 70s to 
near 80, while daytime highs will creep into the lower 90s on 
Saturday, with both day and night temps being well above normal.

LONG TERM (Sunday through Friday): As a strong positively tilted
upper level (H5) trough begins to slide toward the central US 
early next week, the upper level high will begin to retreat across
the Gulf waters and eventually set up over the Florida 
Peninsula. This will shift a more zonal flow to a southwesterly 
flow (more active flow) aloft. At the same time, models show a 
strong cold front and upper level parent trough from Canada moving
southward over or just east of the Rockies. This cold front will 
continue to surge southward and through the region by late 
Monday.

As the frontal boundary passes, rainfall chances will dramatically
increase during the day on Monday and into Monday night. As the
upper level trough continues to gradually shift eastward with
time, a surface coastal trough will develop allowing for continue
isentropic upglide to occur across Deep South Texas through a 
good chunk of the long term period. Models ping QPF off and on 
through next weekend, however, given the uncertainty of how long 
the coastal trough will actually last, tapered POPs a bit here and
there toward the end of the period. The only exception is Friday 
where an upper level impulse shown in the globals begins to push 
through the region. Did show this by a subtle uptick in the POPs 
for that time period. 

Finally, temperatures on Monday are a bit tricky given the
uncertainty in timing of the front. Expect the front to be through
the entire region by sunset, however, temperatures in the lower
valley still may approach the 90 degree mark where locations in
the northern Ranchlands will be falling into the 60s during the
afternoon hours behind the front. Beyond that, still went a little
warmer than guidance but kept the trend of nudging temperatures
down from previous forecasts. Tuesday and Wednesday will 
certainly feel like a cool season day due to gloomy, damp, and 
cool conditions...especially after so many months of hot weather.

MARINE (Tonight through Saturday Night): High pressure and stronger
winds will be the headline the through 36 hours (Saturday night).
High pressure over the northwest Gulf will interact with lower 
pressure upstream to support moderate to fresh southeast winds and
moderate seas. Winds Saturday and Saturday night will be in the 
15 to 20 knot range, meeting small craft should exercise caution 
criteria. The average height of the highest 1/3 of the waves will 
build to between 4 and 6 feet Saturday night offshore, also 
meeting small craft should exercise caution criteria.

(Sunday through Friday): Southerly flow out ahead of an advancing
cold front will keep winds and sea moderate with perhaps
cautionary wording needed. That said, as a strong cold front
surges southward into the area on Monday, winds will shirt to the
northeast and increase to 20-25kts. With the strong northeast
fetch, expect seas to ramp up into the 8 to 11 feet range with 
higher seas not out of the question. Model guidance suggest a 
coastal trough developing right along the coastline, which should 
keep winds in the SCA thresholds, which should also keep wave 
heights elevated. Confidence is rather high that an extended 
period of SCAs will likely be needed through much of the long term
marine period. 

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...None.
&&

$$

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