Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 122331 AAA

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
531 PM CST Mon Feb 12 2018

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

.AVIATION...Satellite images and surface observations indicate
mostly low clouds across the CWA early this evening. Ceilings were
near 900ft at KBRO and KPIL to near 3800ft at KAPY. Visibilities
were near 6SM with mist at KPIL. Expect IFR conditions to continue
to develop across the coastal sections of deep south Texas tonight
and spread inland as a coastal trough continues to develop
offshore the lower TX coast. This will will provide lower ceilings
and reduced visibilities as light drizzle and mist develops
late tonight into early Tues morning.

.PREV DISCUSSION... /issued 254 PM CST Mon Feb 12 2018/
SHORT TERM (Now through Tuesday Night): Cloudy and chilly
conditions dominate this afternoon, with temperatures right in
line with forecast highs (around 50 to the lower 50s) but at least
it`s rain-free and the somewhat low dew points (30s) make it feel
a bit drier than some events have felt this winter.
Unfortunately, all of that ends this evening as warm air
overrunning the cold dome helps "sink" the inversion, and the
overcast will drop accordingly and be just several hundred feet
above the surface by late evening in most areas. Soon after, areas
of drizzle with embedded light rain (enough to wet the ground)
should arrive with weak shear energy riding the southwest flow
between retrograding southwest U.S. 500 mb trough and west-
building subtropical ridge. All models continue to strengthen the
ridge (rising heights, over 591 decameters by late Tuesday night)
which will bring large scale subsidence and push any light rain
makers into east Texas by Tuesday afternoon and especially Tuesday

The challenge remains how stubborn will the chill be, and for how
long.  The combination of time of year (still winter), nature of the
surface ridge which tapped Canadian air and brought upper 30s to
lower 40s temperatures area-wide this morning, and lack of synoptic
forcing - i.e. a building upper ridge from east to west vs. an
approaching upper level trough - make a strong case for a sloooow
recovery back to warmth. In such cases, the NAM-12, mentioned in
prior discussions, is the model of choice. One issue it does
have, however, is the handling of near-coast modification even
with offshore low level flow. We saw this last Friday when the
lower - mid Valley was able to punch into the low to mid 60s while
the ranchlands were locked down in the 55 to 60 range, even
though each area was under solid low ceilings. With a cooler start
today and tonight than we saw last Thursday, the combination of
low clouds/drizzle and light fog will keep Tuesday`s temperatures
equivalently low. While we can`t rule out "near 50" for the late
afternoon highs across the ranches and lower to mid 50s for most
of the Valley, expect a hint of moderation to push highs in the
lower 50s across the ranches west of US 281/IH 69C, and 55 to 60
for the remainder of the RGV. These temperatures are a degree or
so lower than the prior forecast for the Valley, and 4 degrees
lower for the northwest ranchlands.

For Tuesday night, nearly all signs point to fog - probably becoming
dense in many areas. The fog would be mainly a "sinking ceiling to
the ground" variety as steady temperatures are matched closely with
dewpoints (100 percent humidity) over a cooled ground which may
"sweat" a little by daybreak Wednesday toward the coast - a ground
factor that can enhance surface dense fog. The NAM-12 enhances the
offshore flow due to contrast between stubborn surface ridge and
coastal trough nudging toward/into the nearshore waters, which would
tend to reduce the possibility of fog.  However, the NAM-12
soundings just inland from these areas are saturated nearly all the
way up (actually showing a 100 to 200 foot ceiling) with light wind;
the GFS, which tends to become a better solution as the residual
cold air erodes away, is saturated below the inversion to the
ground. Because of uncertainty 36 to 48 hours out, elected to go
patchy to areas of fog and drizzle with no mentioned visibility
restrictions, though have included in current hazardous weather
outlook and will continue to do so.  Dense fog advisories look to be
a good bet at some point Tuesday night into early Wednesday.

LONG TERM (Wednesday through Monday): Generally warm (near-climo
to ~10 degrees above) and occasionally breezy through the long-
term, with a notable one-day cooldown on Saturday (though even
this is brought into question with the latest model runs).

Wednesday morning, we should finally be transitioning out of the
cloudy, gray "gloom" described above as the old coastal
trough/warm front lifts northward through the CWA. Could see some
residual fog/light rain/drizzle hanging around in the morning
before this occurs. Aloft, the 500-mb high center will be nearly
overhead, or just offshore. Southerly flow at 850-mb and gradually
decreasing cloud cover through the afternoon should allow for a
nice warmup, into the mid-upper 70s away from the coast. Trend
continues into Thursday, as lower-tropospheric flow shifts to more
S/SW, allowing 850-mb temps to climb ~3C from Wednesday, putting
high temps in the mid- to even upper 80s inland, with lower 70s to
near 80 along the coast. RH is confined to H85 or below, so not
mentioning any precip.  Will be breezy in the afternoon as well.

Models continue to play games with the strength and timing of the
cold front that was previously progged to move through the area on
Friday afternoon. At any rate, front should not arrive across the
northern tier of the CWA until around 00Z Sat or later, so still
expecting a more-or-less diurnal temp cycle. Southerly flow
weakens a bit, and moisture begins to pool ahead of the front, so
probably not quite as warm as Thursday, but still should hit the
lower 80s most locations.

Regarding the front: 12Z GFS places it just north of the CWA at
00Z/Saturday. 00Z run of ECMWF had it into the northern tier of
counties at that time, but a quick glance at the 12Z solution
shows the front stalling out, with much less cooling behind it.
Because of the Euro`s somewhat wild swings in handling this front,
have gone with the National Blend for temps/winds during this
timeframe to keep it out of the mix. GFS has now flipped to become
the cooler model for Saturday, so confidence isn`t especially
high. Chances of light rain and drizzle are expected to once again
accompany the front and persist into at least part of Saturday,
with low-level southerly flow gliding up over the cooler airmass
(wherever that boundary happens to be).

Even if the GFS is correct, the cooldown appears to be short-
lived, as SE return flow kicks in for Sunday. Model consensus
brings afternoon temps back up to near-normal. Warmer and breezy
trend looks to continue into Monday as southerly flow begins to
increase ahead of the next cold front moving down the Plains
sometime in the middle of next week.

MARINE (Now through Tuesday Night):  Stiff north/northwest winds
continue this afternoon but generally 15 knots or less; seas have
fallen to 3 to 5 feet based on buoys inside and outside of the
marine forecast area.  Expect this trend to continue overnight, with
seas likely dipping a bit more toward the coast (more of a wind wave
event).  Tuesday/Tuesday night`s wind forecast is a bit more
challenging.  Went with a modified NAM solution bringing the coastal
trough to around 30 nm offshore, where winds become east/southeast
by afternoon while north/northwest winds ~10 knots or so continue
closer to shore and across Laguna Madre. Enough fetch between the
primary surface ridge (1032+ mb) moving into the southeast should
keep 3 to 5 foot seas going over the warmer waters, slightly less
over the cooler waters nearshore.

As for fog: Lowering ceilings over the cooler nearshore waters
Tuesday and especially Tuesday night are likely to bring mentionable
fog to these waters as well. How dense the fog becomes, and when, is
a bit more uncertain but possibility is increasing for less than 1
nm visibility as early as Tuesday evening and most likely overnight
and into the pre-dawn hours of Wednesday.

Wednesday through Sunday: Might see some areas of fog over the
Laguna Madre and 0-20nm Gulf waters both early Wednesday and
Thursday mornings. Light SE flow Wednesday afternoon increases to
near Small Craft Exercise Caution (SCEC) levels on Thursday. Seas
generally moderate through this period. Flow begins to back on
Friday ahead of an approaching cold front. There is still some
uncertainty regarding the strength of this front; hence the wave
fields as well. Latest runs have come back weaker, moving the
front into the waters Friday night, but then stalling out. Should
at least see a period of SCEC to low-end Small Craft Advisory
winds Friday night into Saturday. Went above latest wave guidance
but capped off at 6 ft. for now, even offshore. Winds expected to
veer back to SE and freshen by Sunday afternoon, possibly once
again to SCEC levels.



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