Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 150539
AFDBRO

Area Forecast Discussion
National Weather Service Brownsville TX
1239 AM CDT Fri Mar 15 2024

...New AVIATION...

.UPDATE...
Issued at 1047 PM CDT Thu Mar 14 2024

Been keenly watching the incoming Convectively-Allowing Models
(CAMs) and the NAM-12 that have come in over the past few hours.
All are now advertising some degree of convection to blossom
around or after midnight, generally along the Rio Grande between
southwest Hidalgo and southern Zapata, before quickly scooting
east/northeast across much of the region. Of course, the devil is
in the details and the models differ...from the more explosive
HRRR and WRF-ARW (which imply a reasonable worst-case scenario for
hailstorms) to the weaker HRW-FV3 and incoming NAM-12.

A quick glance at the 01Z NBM QPF matches up (somewhat) with
consensus...and a quick update to the NBM rain chances is somewhat
in the ball park.

The big question? Are we unstable enough west of IH-69C to allow
quick convective development ahead of the weak (and small scale)
short wave that these models are keying on? The answer may be yes,
as point-soundings show less of a cap and plenty of CAPE to make
the case.

Bottom line? Preferred to jump on a mention of isolated to
scattered showers/thunderstorms after midnight through daybreak
(12Z...7 AM) rather than leave a general cloudy sky.

Forecast updates have been sent.

&&

.SHORT TERM...
(Tonight through Friday night)
Issued at 216 PM CDT Thu Mar 14 2024

The short term forecast period for the most part is rain free until
Friday night as a cold front moves through Deep South Texas and the
Rio Grande Valley. This front will bring enough instability and lift
to help showers and thunderstorm develop along the front. Due to the
southeasterly flow, plenty of moisture will move into the region
that will help provide fuel for these developing showers and
thunderstorms. SPC also has Deep South Texas and the Rio Grande
Valley in a general thunderstorm area for the duration of the short
term forecast period. The activity for showers and thunderstorms are
expected to start out towards the west around Zapata and Starr on
Friday and then the rain chances will spread from there during
Friday night.

Moving into the temperatures for the short term forecast period. The
low temperatures for tonight are expected to be mostly in the 70s,
while a few locations along the coast could be in the upper 60s.
Then the high temperatures for tomorrow could be mostly in the low
to mid 90s, while the highs along the coast are expected to be in
the 80s. Finally, the low temperatures for Friday night, are
expected to be in the range of upper 60s to low 70s.

On the beaches, a moderate risk of south to north longshore
currents is expected to continue through tomorrow due to
persistent south-southeast moderate swell. Surf height stays
around the range of 2 to 4 or 5 feet...even as southeast winds
remain generally around 10 to 15 mph by morning and 10 mph in the
afternoon.

&&

.LONG TERM...
(Saturday through next Thursday)
Issued at 216 PM CDT Thu Mar 14 2024

As was mentioned over the past several days, an unsettled synoptic
weather pattern is in store for much of the extended forecast
period across our CWA. A Rex Block pattern situated across most
of the Western US and portions of Western Canada will persist
through next Tue. The net result will be little movement of the
main synoptic features one of which being the mid/upper closed
low pressure area located across the Desert Southwest and Great
Basin. This closed low will maintain an unstable southwest flow
aloft across our CWA through most of the period.

The best chances for strong to possibly severe convection will
occur during the weekend. Moderate instability, a weakening cap,
an approaching cold front, and several shortwaves impacting the
CWA within the southwesterly flow aloft will be some of the main
ingredients in place this weekend. There is some indication from
the HRRR that a potential MCS may pass just to our north on Sat
morning, leaving a potential outflow boundary across our area. The
better chances for convection are expected for Sat night into Sun
as a potent shortwave interacts with the southward-sagging
frontal boundary. Another potential MCS, this time tracking
further south across portions of our CWA, may impact the area from
west to east overnight Sat into Sun. Initially, this convection
will be enhanced by upslope flow into the Sierra Madre. A
Marginal SPC risk lines up well with this potential scenario. The
main threats would be large hail and wind, mainly across the Rio
Grande Plains and Upper Valley. Although conditions are expected
to be not quite as unstable, another round of convection will be
possible Sun night into Mon as additional shortwave energy
interacts with the frontal boundary lying across the CWA.

A lull in the activity may occur by Mon afternoon into Mon night
as low level divergent flow and shortwave ridging aloft prevail.
This will help to drive the surface high pressure area behind the
front further southward across the CWA. The coolest period is
expected to be from Mon night into Tue night as a result. Most of
the guidance indicates that any lull will be short-lived with rain
generally expected to return on Tue. This will be generally due to
additional shortwave energy aiding coastal trough development by
Tue into Wed. As a result, will shift the highest POPs over the
waters and coastal counties with this coastal trough scenario.
Warmer conditions are expected on Thu as the main closed low
finally opens up and begins accelerating eastward across the
Plains. Residual moisture should maintain at least isolated POPs
on Thu.

Rainfall amounts through the period will generally by around 1-1.5
inches. However, the 95th percentile storm total QPF values
indicate that 2-3 inches will be possible, especially if short-
lived training occurs with any potential MCS.

Minor tidal run-up will be possible on Mon-Wed as persistent
moderate to strong E/NE flow prevails across the W Gulf.

&&

.AVIATION...
(06Z TAFS)
Issued at 1238 AM CDT Fri Mar 15 2024

Through 06z Saturday.....MVFR conditions with maybe some instances
of IFR conditions due to ceilings are possible through the overnight
hours. MVFR visibilities are also possible due to haze. During the
day on Friday, mainly MVFR conditions are expected to continue.

Winds will continue out of the southeast 5-15 kts through the 06z
TAF cycle. Winds could gust up to 20 kts during the afternoon hours
on Friday.

&&

.MARINE...
Issued at 459 PM CDT Thu Mar 14 2024

Updated through Tonight...
Marine layer of much cooler nearshore waters vs. the sensible air
mass above them has sharply reduced wind speeds at the beach, and
by extrapolation out to 15-20 nautical miles, there as well. Model
blends continue to struggle with wind speeds in the nearshore
waters in such cases, and were more than 20 knots above observed
as of 3-4 pm.

For Laguna Madre, cloud cover and some minor marine layer
influences nipped the winds a bit as well, with speeds in Small
Craft Exercise Caution (15 to 20 knots and gusty) rather than
full-blown advisory conditions (20 knots and gusty) by mid
afternoon.

For each reason...the Small Craft Advisory was cancelled prior to
issuing the forecasts...and gridded winds were dropped
considerably to match the current observations.

Remainder of the discussion remains as is.

Tonight through Friday Night...Favorable conditions return
tonight with pressure gradient slackening steadily. Onshore flow
is expected to be light to moderate with low to moderate seas,
which is expected to persist through Friday night.

Saturday through Thursday...Convective chances will increase
through the weekend across the coastal waters given the many
factors referenced above. The actual cold front will slowly push
through the waters by Sunday night into Monday. Small craft
advisory conditions will then persist behind the front through
Tuesday night. Rain chances will persist Monday through Thursday
as shortwave energy interacts with an adequate moisture profile
and coastal troughing. The rain chances may be ending by late
Thursday as the main low finally begins to move past the area.

&&

.PRELIMINARY POINT TEMPS/POPS...
BROWNSVILLE             86  71  84  70 /  10  10  10  20
HARLINGEN               90  69  87  67 /  10  10  10  30
MCALLEN                 92  72  89  70 /  10  20  20  40
RIO GRANDE CITY         94  71  87  68 /  20  30  30  50
SOUTH PADRE ISLAND      76  69  75  69 /  10  10  10  20
BAYVIEW/PORT ISABEL     83  68  81  67 /  10  10  10  20

&&

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

$$

SHORT TERM...52-Goldsmith
LONG TERM....80-MB
AVIATION...23-Evbuoma


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