Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 211822

Area Forecast Discussion
National Weather Service Brownsville TX
1222 PM CST Wed Feb 21 2018

.DISCUSSION...With cold front already barreling into south-central
TX this afternoon (with temps in the 40s in its wake), updated
temps/dewpoint/RH/winds through 18Z Thursday with NAM-heavy
short-term blend to better reflect observations and match up with
neighboring office. May have to trend even colder for the
afternoon package. Updated text products have been issued.


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


.AVIATION...Challenging aviation forecast over the next 24 hours.
For the afternoon, modest southerly breezes and MVFR/VFR ceilings
will be in place. However, a cold front is approaching Deep South
Texas this afternoon with seasonally cold air behind it. This
evening, ahead of the front, expect IFR/low-end MVFR CIG`s to re-
develop quickly after sunset.  Still some uncertainty as to how
fast this front will ultimately drive toward the RGV. Shorter-
term models (mainly the NAM), which have handled these fronts much
better this winter season, bring the wind-shift into MFE as early
as 03Z this evening. Current TAF set goes with NAM trends but
slows its FROPA by about 3 hours for the TAF sites. Front expected
to slow down as it reaches BRO, but should move through by around
12Z. Some light is possible in the few hours ahead of the front.
Some convection is possible right along the front, so have
included a PROB30 group for MFE where models are placing the
better potential. Behind the front, expect CIG`s around 010 with
some light rain/drizzle, without much improvement by 18Z Thursday.


.PREV DISCUSSION... /issued 539 AM CST Wed Feb 21 2018/

DISCUSSION...Updated for latest aviation discussion below.

AVIATION...Lowest cigs around 700 to 800 feet continue to
fluctuate between sct and bkn this morning, with secondary layer
around 1500 feet. Expect clouds to slowly lift above 200 feet
later this morning, and hover between there and 3000 through the
day. Southeast winds expected to rise to 15 to 20kts again this
afternoon. The cold front will begin to move into the region later
this evening. It will first bring scattered showers and a few
thunderstorms to the area, especially KMFE. The front will slow
down as it moves into the region, and it`s final stalling spot is
still not settled, so some areas may see briefly northerly winds
late tonight and early tomorrow, before shifting back from the
southeast during the day Thursday.

PREV DISCUSSION... /issued 405 AM CST Wed Feb 21 2018/

SHORT TERM (Now through Thursday):Upper level flow remains
highly amplified with large trough along the Rockies and ridge off
the east coast. Impulse embedded in the southwesterly flow
between these brought the showers to the area yesterday and is
moving quickly to the northeast. A few remain a possibility today,
mainly in the northern ranchlands away from the river. The well-
advertised cold front is sweeping southward through the Hill
Country, and should be arriving in the ranchlands after sunset.
Models have been more consistent recently in bringing the front at
least partially into Deep South Texas, but are still hesitant in
fully clearing the river. So temperatures today ahead of the front
will again reach the 80s, with modest southerly flow through the
day. The front will bring a good late-season blast of colder
temperatures, with temps falling into the 50s and 60s by sunrise
Thursday. The main question, again, is how far the cold air drops
into the region. Right now, the models are evenly split on this,
which hinders confidence in the temperature forecast for the lower
valley for Thursday. For now, have kept highs in the 50s for the
northern ranchlands with modest north winds, and have held light
southerly flow for Cameron County, keeping highs steady in the mid
60s. Again, if the north winds are able to reach further south,
the lower valley will not reach 60 degrees Thursday. The front
will also bring a good chance for showers and a slight chance for
thunderstorms tonight into Thursday.

LONG TERM (Thursday Night through Tuesday):Large scale mid/upper
level pattern to remain persistent with a large broad trough in
the West and an anomalous ridge along the east coast. Models
trend to break down the ridge with the center steadily moving west
across the Gulf of Mexico as we move ahead into next week and
next week. Shortwaves are shown to traverse across the conus with
the stronger one possibly pushing a cold front through the region
next Sunday into Monday.

The Western trough is the deepest Thursday/Thu night with low
pressure deepening over the Plains through Saturday. This will
allow for a strengthening pressure gradient with the RGV wind
machine turning on Friday and reaching the highest setting
Saturday with the potential for another widespread wind advisory
day. Temperatures respond with well above normal temperatures
likely for the end of the week and the first half of the weekend.
Moisture remains limited and at this time so not expecting much in
the way of rainfall.

Sunday and Monday..mid level shortwave trough moves out of the Plains
dragging the next potential cold front into the region. However,
with the Subtropical ridge building over the Gulf deepening
southwest winds to strengthen around this time with the cold front
encountering the parallel flow making it difficult for a
significant passage. WPC shows the front pulling up stationary
over or close to the CWA with the GFS the strongest of the
deterministic models pushing the front just south of the river. At
this time the WPC depiction looks within reason and it compliments
the lower temperature and higher pop guidance.

Tuesday looks to end up as a recovery day as mid level flow
trends more westerly and surface high pressure tracks east.
Temperatures should warm up and rain chances tapper off.

MARINE:(Now through Thursday): Relaxing winds and seas will
continue briefly today ahead of the approaching cold front. Seas
will  drop to 4 to 5 feet, with southeast flow of 10 to 15 knots.
Confidence in the frontal passage tonight into tomorrow is modest,
as  the models differ considerably on final stopping location.
Current forecast keeps the front just to the north, with southerly
flow continuing for most marine areas tonight into Thursday. The
proximity of the front will allow some showers and thunderstorms to
develop mainly tomorrow, with some gusty winds possible. The general
onshore flow will keep seas around 5 feet through sunset tomorrow.

Thursday night through Sunday...Southeast to South winds will be
on the increase Friday through Saturday as deepening low pressure
over the Southern plains combines with broad surface high over the
Gulf. The gradient should become strong enough Friday for possible
small craft advisories on the Laguna Madre. Stronger winds are
possible Saturday with the Gulf waters seeing winds approach 20
knots and seas steadily building with the long duration of the
moderately strong southerly flow. Small Craft advisories may be
needed for all or portions of the Coastal waters Saturday. The
next cold front approaches Sunday but it is uncertain on if this
front moves offshore and how strong.




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