Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 190014 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
614 PM CST Thu Jan 18 2018

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


.AVIATION...Low ceilings and reduced visibilities will continue
through the next 24 hours with IFR to even LIFR conditions
possible. Light rain and drizzle will also affect the local
aerodromes as an upper level disturbance moves across South Texas
and surface coastal troughing strengthens. Winds will shift from
the north to the northwest during the day tomorrow with winds
ranging from 10 to 15 knots along the coast. Model guidance
indicates the possibility of an isolated thunderstorm as the upper
level disturbance moves overhead, but confidence is low.


.PREV DISCUSSION... /issued 254 PM CST Thu Jan 18 2018/

SHORT TERM (Now through Friday Night): Weather conditions
continue to deteriorate with widespread drizzle/light rain
developing while temperatures have lowered a few degrees this
afternoon (mid-upper 30s) due to the onset of the precipitation.
These cold and damp conditions will persist along with fog (2-4
miles vsby) through the rest of the afternoon and the evening

Latest water vapor loop shows well developed mid-upper level low
over the Big Bend of the Texas. The approach of the low
tonight/Friday will provide large scale ascent and allow for a
moderately strong low level jet to develop. The sufficient lift,
modest elevated instability (300-500j/kg of cape), 500-700mb lapse
rates around 7.5c/km will combine increasing the chance of
thunderstorms late tonight and Friday morning. Mid and Lower
Valley and coastal strip are most favored areas to hear some
thunder. Best chances again our around sunrise and up until Noon
when the axis of the upper low is just over SC Texas then
diminishing later in the day with rain chance slowly ending west
to east as the low tracks east of the coast Friday night. Besides
the small chance of thunderstorms rain chances will be rather high
with increasing convergence along a coastal trough over the
coastal waters. Model guidance is pushing categorical rain
chances near the coast for tonight and Friday morning.

Temperatures tonight should not exhibit any significant lower and
should remain steady much of the night and would not be surprised
to see a few degrees of warming near the coast. Friday`s highs
will be tricky with the very high rainfall probabilities
continuing through morning and possibly mid day in the east.
Surface winds also remain northwest (west of the front) which will
not allow for a any significant warming. NAM seems to be best
model of choice for cooler temperatures and will trend in this
direction. Friday night low will have same trend as tonight`s lows
with no significant cooling and more of a steady isothermal
regime with the no more cold air advection indicated.

LONG TERM (Saturday through Thursday): A brief return to warmer
weather will occur Saturday and Sunday as mid level ridging moves
overhead and southeast return flow strengthens. Saturday night and
Sunday temperatures will be slightly above normal, near 60, and 75
to 80, respectively. Mid level ridging will shift east late in the
weekend as a southern plains storm system develops. Winds will
weaken over the CWA Sunday, even shifting to northwest over the
brush country and ranchlands late in the day. The new air mass
arriving from the northwest Sunday night will be from a milder
source, basically maritime polar air from the West Coast, and will
therefore have a weaker impact than recent arctic blasts. Winds will
shift to north Monday with temperatures right around normal, about
70 during the day. Temperatures will in fact hover close to normal
much of the remainder of the week. A brief chance of rain will be
possible Monday morning with the arrival of the cold front near
dawn, but conditions will then improve through the day. The front
will push offshore but will later move back north as a trough, with
some support from an upper short wave trough, enhancing winds and
rain chances along the coast on Wednesday. A follow-on low latitude
short wave ridge moving from west to east will help nudge the mid
week system out of the way, resulting in a fairly common setup
Thursday and Friday with ridging over the Mississippi Valley and
high pressure controlling the northwest Gulf.

Now through Friday Night: Pressure gradient has weaken as main
center of high pressure moves east while a coastal trough persist
over the coastal waters. Moderate NW winds exist along the coast
with moderate northeast to east winds east of the trough. The
approach of a strong upper low early Friday will strengthen the
trough with a possible low pressure area to develop by Friday
night. This should increase the easterly flow out over the outer
waters while near shore lighter offshore flow may continue.
Challenging is if the trough temporarily moves farther west ahead
of the upper trough. All guidance continue to indicate the trough
remaining offshore and moving northeast Friday night. Winds will
likely remain moderate with some periods of stronger winds in the
20-60 miles area with seas gradually building in all coastal
waters with time with the potential for seas to reach small craft
advisory heights later Friday.

Saturday through Tuesday night: Initially light south winds on
Saturday will strengthen from the southeast to south Saturday into
Sunday as a plains storm system deepens and tightens the gradient
with high pressure which will be slowly retreating over the
southeast United States. South winds of 15 to 20 knots will be
possible on the Gulf on Sunday, with elevated seas around 7 feet,
though similar wave heights due to swell will remain in place
through the weekend. Winds will weaken on Sunday ahead of a cold
front due Sunday night. The front will result in north winds of 15
to 20 knots on the Gulf Monday and Monday night with moderate to
high seas. Conditions will improve Tuesday and Tuesday night as high
pressure spreads over the area, with light to moderate east winds.
The best chance for small craft should exercise caution conditions
on the Laguna Madre will be monday behind the front, with 15 to 20
knot north winds. Periods of exercise caution conditions will be
possible across the near shore waters through the entire period, due
to a combination of winds and wave heights. The far shore waters
will see 4 to 6 or 5 to 7 feet waves much of the period, resulting
in small craft should exercise conditons to low end small craft
advisory conditions.




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