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Area Forecast Discussion (AFD)

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

                            
000
FXUS64 KBRO 121811 AAB
AFDBRO

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

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

&&

.AVIATION...Isolated showers are moving north into the CWA, 
mainly reaching Hidalgo County, where there are broken multilevel 
low clouds and MVFR ceilings. 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. Enhanced southeast 
winds will, however, continue funneling moisture north into 
southern Plains storm systems. That said, current model guidance 
suggests VFR the next 24 hours, with isolated showers possible in 
the morning hours of Saturday for the lower Valley and the TAF 
sites.

&&

.PREV DISCUSSION... /issued 635 AM CDT Fri Oct 12 2018/ 

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Satellite and radar images as well as surface
observations indicate showers continue to develop across northeast
Mexico and move into extreme southern portions of the CWA this
morning. Low to mid level clouds were across western portions of
the CWA. Ceilings were near 2100ft at KAPY to near 9000ft at KEBG.
Expect VFR conditions across the Rio Grande valley this morning
through this afternoon even as a weak cold front across northeast
Mexico moves northward as a warm front today before dissipating. 
Isolated to scattered showers will develop along the warm front as
it lifts northward this morning.

PREV DISCUSSION... /issued 357 AM CDT Fri Oct 12 2018/ 
SHORT TERM (Today through Saturday):Elongated 500mb ridge across 
central Mexico and the Gulf of Mexico will provide a westerly flow
aloft across Texas today as Tropical Storm Sergio makes landfall 
across Baja California and moves into northwest Mexico this 
afternoon. Low to mid level moisture will remain high across 
northeast Mexico and northwest Texas today before the remnants of 
Sergio move into west Texas tonight. Moisture will increase across
west Texas tonight and best rain chances will remain well 
northwest of the CWA today through Saturday even as the 500mb 
shortwave associated with Sergio moves across the southern plains 
Sat afternoon. Will mention isolated showers and thunderstorms 
across the Rio Grande valley and northern ranchlands today as the 
weak cold front across northeast Mexico lifts northward as a warm 
front before dissipating. Will likely see isolated streamer 
showers overnight tonight and isolated showers and thunderstorms 
Saturday for seabreeze convection. Will go with a blend of NAM/GFS
MOS guidance for temperatures today through Saturday which is 
near normal to above normal.

LONG TERM (Saturday Night through Thursday):Models continue in
fair agreement with the development of a highly amplified mid 
level pattern over North America and the CONUS. A strong cold 
mid/upper level trough digs through the heart of the country (Sun-
Tue) as strong high pressure builds north along the West Coast 
well into British Columbia, Canada. By the middle of the week a 
winter like trough will be established over the Midwest with an 
upper low settling over the Desert Southwest while a mid level 
ridge builds over the Gulf of Mexico. All this will set up a major
temperature and weather change for the RGV as a much advertised 
cold front surges through Texas Sunday night into Monday. The 
front is still on schedule for a Deep South Texas/RGV passage 
Monday with some differences in timing between the GFS and ECMWF. 
GFS is currently lagging behind the ECMWF and temperatures 
guidance spread between the models shows some differences for days
4-6 (Mon-Wed). With plenty of time to smooth these differences 
out as we approach this event only minor changes to the 
temperature and pops were made in the latest forecast and grid 
package.

Overall, Sunday will be another warm to hot day with a possible 
afternoon shower or thunderstorms. Subsidence ahead of the front 
might be a factor with a potential heat spike. Then we can expect
the front to move through the northern and western Ranchlands 
early in the day Monday with steady falling temperatures. The Mid 
and Lower RGV might not see the front well into Monday afternoon 
and possibly as late as sunset at BRO and SPI. This will allow for
temperatures to approach 90 degrees in the Lower Valley if clouds
and rain hold off. The front should be pushing well offshore 
Monday night with Tuesday and Wednesday being a shock to the 
system with temperatures falling into the 50s and 60s for daytime 
highs and 40s and 50s for overnight lows. Rain chances will be on 
the increase with some widespread rainfall amounts of 1-3 inches
possible (especially near the coast) as isentropic flow maximizes
and mid level instability allows for some elevated convection. 
Models suggest cool temperatures and rain chances to stick around 
Thursday as the upper low over the Desert SW remains in place 
maintaining the warm moisture air overriding the cool surface air.
Not until Friday does the stubborn upper low begin to lift out 
allowing for a slow recovery back to warmer temperatures and for 
some sun to break through the thick overcast. Expect day to day 
changes in temperatures and pops as we get closer to the frontal 
passage and models get a better handle of the rapidly changing 
pattern.

MARINE:(Today through Saturday): Seas were near 5 feet with east 
winds near 18 knots at buoy020 early this morning. Light to moderate 
southeast winds will prevail across the coastal waters today with 
surface high pressure across Missouri and Illinois extending 
southward into the Gulf of Mexico. The pressure gradient will 
increase across the lower Texas coast tonight as the remnants of 
Tropical Storm Sergio move across northwest Mexico into west Texas. 
Moderate southeast winds will prevail across the western Gulf of 
Mexico tonight and increase slightly Saturday with low pressure 
across northwest Texas. SCEC conditions will develop across the 
coastal waters Saturday as a result. 

Saturday night through Tuesday...A moderate onshore flow and
moderate sea is expected Sat night through early Monday in advance
of the first strong cold front of the Fall season. The front is
expected to work its way South of Port Mansfield and Port Isabel
Monday afternoon to early Monday evening. North winds to steadily
increase and remain elevated Tuesday through Thursday. An extended
period of small craft advisories are anticipated with very rough
sea as strong high pressure builds in behind the cold front. Rain
chances increase with periods of rain and some thunderstorms
making for a nasty and hazardous conditions for the middle part of
next week. Next Weekend looks to be much improved. 

&&

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

$$

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