Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

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

                            
000
FXUS64 KBRO 161548 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1048 AM CDT Mon Sep 16 2019

.DISCUSSION...Have reduced pops and adjusted cloud cover for today
with our region on the western side of the upper low. Latest radar
trends and satellite loops show more concentration of convection
over the NW Gulf with the latest CAMS favoring highest
probabilities over areas to our north. We will still see showers
and thunderstorms increase today but coverage should be less than
previous forecast suggest. Temperatures look ok at this this time. 

&&

.PREV DISCUSSION... /issued 550 AM CDT Mon Sep 16 2019/ 

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...VFR conditions expected to mostly prevail through the
TAF period. Upper level low pressure combines with weak surface 
coastal low pressure and increasing tropical moisture to continue 
the chance of showers and thunderstorms this afternoon. While most
activity is expected along the coast, any thunderstorm is capable
of occasional lightning, gusty winds, and locally heavy rainfall.
Brief MVFR to IFR CIGs are possible near any heavier showers or 
thunderstorms late this morning into mid afternoon. Light onshore 
flow will otherwise shift from the northeast to southeast through 
tonight. 

PREV DISCUSSION... /issued 405 AM CDT Mon Sep 16 2019/ 

SHORT TERM (Today through Tuesday): Upper level low pressure 
continues to work westward across the Gulf of Mexico as weak 
surface coastal low pressure tries to develop along the lower 
Texas coast. Tropical moisture continues to increase across Deep 
South Texas, especially along the coastal counties. The 0Z 
sounding showed PWAT values of 2.17 inches and model sounding PWAT
values over the next 36 hours climb above 2.25 inches today and 
up to potentially 2.50 inches into Tuesday afternoon before 
moisture heads northeast. 

Locally heavy to torrential rainfall will continue to be possible 
where heavier showers and thunderstorms persist, mainly during 
afternoon hours along outflow boundaries and the sea breeze. 
Nuisance flooding of low lying and poorly draining areas remains a 
possibility both today and tomorrow. WPC has continued to keep 
marginal excessive rain near the coast on Tuesday. 

Temperatures both afternoons will continue to depend on rainfall, 
with highs generally near normal today and near to slightly above 
normal on Tuesday, especially across western and mid to upper RGV 
locations with better rainfall chances edging further east and 
north. 

LONG TERM (Tuesday Night through Sunday): The 500 mb closed low
that will be bringing tropical rains to the RGV throughout this
week will continue meandering around the mid TX coastline Tues
Night into Wed. The 500 mb low will then shift steadily northward
across the west side of the ridge axis situated over the
southeastern States as a large and deep 500 mb trough digs into
the western States Thurs and Fri. As the 500 mb closed low is
picked up by the deep western trough axis, 500 mb ridging will
start to rebuild over Deep South TX and the RGV. This will in turn
start decreasing the conv potential over the region as the deeper
tropical moisture is shunted more to the north and east. Some
residual moisture will linger over the area mainly near the 
coastline and out over the lower TX Gulf waters. So will not
eliminate pops totally from the latter forecast periods, but will
lower them down pretty significantly as next weekend approaches. 

The latest Excessive Rainfall outlooks from WPC are placing the
best chcs for the heavier rainfall further up the TX coastline
closer to the middle and upper TX coastline which will likely be
more under the influence of the 500 mb low. This is in line with
where both the GFS and the ECMWF place the 500 mb low. This may
yield up a somewhat drier solution for the RGV by comparison. 

As the atms dries out somewhat over the RGV, expect that afternoon
high temps will creep up steadily later this week and into the
weekend. The model blends seem to remain on the cool side for
highs through Day 7/8. Will nudge up the blends a few degrees for
highs especially for the mid Valley. Heat index values may become
more of an issue later in the forecast period as the deeper
tropical moisture levels start diminishing. They will probably
won't reach into Heat Advisory levels at this time. But they could
climb into the 104-108 degree range by next weekend. 

MARINE (Now through Tuesday): Coastal low pressure tries to 
develop into Tuesday as an upper level low pressure system works 
into the western Gulf of Mexico. Onshore flow remains light 
through the period, however, observed easterly swell has increased
tonight to near 4 feet. Marine conditions are expected to begin 
to improve today into tomorrow with subsiding easterly swell. The 
chance of thunderstorms with occasional to frequent cloud to water
lightning increases again this afternoon and continues through 
Tuesday. Heavy to torrential downpours are possible across open 
waters with increased winds and seas near any convection. Will 
continue to mention waterspouts as a possibility due to continued 
very light winds and abundant tropical moisture. 

Tuesday Night through Friday Night: Surface ridging over the SE
U.S. will likely rebuild over the Gulf Coast and the Gulf of Mex
resulting in a steady light to moderate SE low level flow for much
of the extended range marine forecast period. This may build the
swells over the western Gulf of Mex a bit throughout the upcoming
work week, but do not expect any sustained SCA conditions at this
time. 

&&

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

$$

This product is also available on the web at:
HTTP://WEATHER.GOV/RGV

59/99