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)

                            
227 
FXUS64 KBRO 280519 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1219 AM CDT Mon May 28 2018

...06z Aviation Update...

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

&&

.AVIATION...VFR conditions can be expected for all terminals
through this TAF cycle. The only entity to really focus on this
cycle will be winds. HRL and MFE may see a slight bump in the
afternoon Monday with southeasterly winds backing slightly to more
of an east southeast direction in the afternoon before veering 
back to the southeast around sunset. Otherwise, rather benign 
flying conditions into/out of Deep South Texas over the next 24 
hours. 

&&

PREV DISCUSSION... /issued 309 PM CDT Sun May 27 2018/ 

.Broken Records (literally) as Valley Heat Wave Continues to
Build...

SHORT TERM (Tonight through Monday Night): Quiet weather will prevail 
tonight with mid level ridging just upstream, with a flat surface 
pressure gradient across the area. Low temperatures tonight will be 
in the lower to mid 70s, near to slightly above normal under mostly 
clear to clear skies. The upper ridge will remain a player on Mon, 
supporting dry air aloft and stable, above avg temperature weather 
at the surface, under sunny skies. High temp records may fall Monday 
with 97 degs forecast at Brownsville and a record of 98, 100 degs 
forecast at Harlingen with a record of 98, and 101 degs forecast at 
McAllen with a record of 101. Heat index values will range from 103 
to 108 degrees for a few hours Monday during peak afternoon heating. 
Meanwhile, Sub-TS Alberto will drift north in the northeast Gulf, 
with relatively little impact to the local area. Low temperatures 
Monday night will be in the lower to mid 70s, near to slightly above 
average, under mostly clear to clear skies.

LONG TERM (Tuesday through Sunday): Little change to the going
forecast as heat wave dominates the region through the period. 500
mb ridge is locked in "La Canicula" position...very similar to a
mid July situation...and the atmospheric warming as mentioned here
yesterday with 1000-500 thicknesses exceeding 583 decameters (dm) 
each afternoon for the entire area and peaking between 587 and 589
dm roughly from US 281 west to the Rio Grande Plains Wednesday and
perhaps Thursday is identical today with the GFS, and the ECMWF
has caught up a little as well. Closing the week and starting
June, and carrying through next Sunday, there is no change to
these parameters, so no reason not to pump up afternoon highs a
couple degrees to match those of the rest of the period. 

As for winds...the same idea mentioned yesterday (lowering
pressure across the lee of the Sierra Madre Tuesday through
Thursday) will push up afternoon and evening winds to 'breezy'
levels but if anything offering a bit of relief to the heat by
mid to late afternoon spreading from east to west. Winds relax a
bit Friday through Sunday with the Canicula ridge expanding north
and northeast and increasing surface pressures a tad over these
areas. Thus more of a fresh breeze vs. breezy next weekend. 

As for temperatures: Raised readings Wednesday and Thursday to
account for the atmospheric persistence, along with the dry air
that thickens into the mixing layer which will ensure full dry
adiabatic mixing. One thing to keep a slight eye on is the veering
winds in the mixing layer toward the southeast which could hold
temperatures from getting too far up the scale especially
along/east of US 77/IH 69E Tuesday and Wednesday...but believe dry
air and mixing will overcome this. For the meteorologists who
follow this discussion tracking statistical guidance and noticing
the Day 2-Day 5 or so trends keeping mid Valley temperatures in
the upper 90s, note that we've found in this type of atmosphere
combined with seasonal equations (March-May) that could struggle
to handle strong outliers (or never seen) parameters such as
these, that the output tends to be 4 to 6 or more degrees too low.
Such is why the actual forecast is this much higher than the
statistical guidance. As for records...see the climate section
below for records and forecast highs through June 1.

MARINE: Now through Monday night...Subtropical storm Alberto 
will move north across the eastern Gulf of Mexico coast through 
Monday, but a weak pressure gradient will continue over the lower 
Texas coast. Light to moderate southeast to south winds and low to
moderate seas will dominate through Monday night. A modest swell 
generated by Alberto may add a foot to wave heights late in the 
period.

Tuesday through Friday night...Little change from earlier, with
caution conditions (15 to 20 knots) likely on Laguna Madre
Tuesday through Thursday afternoon before easing up Friday. For
the Gulf, overnight winds could sneak into the same range Tuesday
and Wednesday night before fading back below 15 knots Thursday and
Friday night. Seas will remain moderate, holding at 3 to 4 feet
for most of the period as pressure gradient across the Gulf
combined with lower 80s sea surface temperatures support these
values.

CLIMATE...Shown below are the forecasts and records (year)
through June 1 for Brownsville and McAllen for sake of interest.
We should tie or break a number of them on most days shown here.

McAllen/Miller (since 1942)

Date       Fcst     Record (year)
May 28     101      101 in 2017 
May 29     104      102 in 1975 
May 30     105      103 in 2004 
May 31     105      106 in 1967
Jun 1      104      100 in 2001

Brownsville (since 1878)

May 28      97       98 in 1919
May 29      98       98 in 1922
May 30      99      100 in 1919
May 31      99      100 in 2005
Jun 1       98       97 in 1909

&&

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

$$

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

Aviation...Frye-55