Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 150450 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1050 PM CST Wed Feb 14 2018

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

&&

.AVIATION...As expected, fog/mist and VLIFR (VV002) ceilings have
settled in across the Valley terminals this evening, and have
amended TAFs to adjust for bouncing visibility trends between LIFR
fog (1/2 mile) and IFR mist (1 mile) for the overnight. Eastern
terminals are on the western edge of the dense (1/4 mile) fog zone,
and expect this to continue overnight as the sea breeze "push" of
fog dissipated soon after sunset, and there should be just enough
southeast flow to keep visibilities from dropping much lower than
where they are now. Thereafter, forecast was left unchanged:  Expect
the VLIFR ceilings and LIFR fog periods to continue through 15Z
before visibility lifts into a light mist/haze and ceilings climb
back to IFR or low MVFR as southeast winds begin to crank. By noon,
mixing will be in full swing with gusts possibly reaching 30 knots
at Harlingen.  Ceilings will scatter out and lift to MVFR/low VFR,
possibly going completely clear by 22Z. For the evening, gusty
southeast winds will diminish but still be fresh at all three sites;
while sea fog will lurk along the coast, expect too much southeast
flow and dry air mix-down to pin the fog from Port Isabel/Port
Mansfield to the beaches and into the nearshore Gulf, leaving all
three terminals clear through 06Z/Friday.
52/BSG

&&

.PREV DISCUSSION... /issued 636 PM CST Wed Feb 14 2018/

DISCUSSION...Low clouds and fog bank getting a nice assist from
sea breeze circulation late this afternoon, courtesy of the
overachieving afternoon highs in the lower 80s along US 77 with
the chilled down beach in the upper 50s to lowre 60s against the
chilly nearshore waters. Bayview (KPIL) quickly crashed to 1/4
mile visibility and beach cams showed 1/4 to 1/8 mile visibility
(estimated) as early as 5 PM.

Based on the current conditions as well as satellite loops showing
a steady westward push, have issued a Dense Fog Advisory for the
coastal communities as well as inland communities, primarily along
and east of US 77 which should see at least a six hour period of
dense fog this evening.

Beyond midnight, the question is how much fog will remain as the
land/seas thermal gradient shrinks sharply and surface winds veer
a bit more to the southeast. Situation could evolve to a ceiling
vs. visibility situation for the inland portion of
Cameron/Willacy/Kenedy County and the thicker fog may recede back
to the beaches. Will cross that bridge later, but for now have the
Dense Fog Advisory into the higher populations of Brownsville,
Harlingen, and Raymondville.
52/BSG

AVIATION...Both difficult and easy forecast at the same time
overnight. Easy is the idea of low ceilings in all areas, as well
as a period of fog, locally dense, for Brownsville and Harlingen
this evening in the aforementioned fog bank moving across both
terminals. Have issued Airport Weather Warnings for expectation of
a period of dense (1/4 mile visibility) fog this evening through
midnight. Thereafter, visibility may (should?) rise to about 1
mile as winds veer to the southeast, though can`t rule out periods
of 1/2 mile visibility similar to the inherited forecast. For
McAllen/Miller, IFR ceilings will likely develop below the
inversion later tonight, and visibility should also dip into IFR
range (2 miles or so) through mid morning as winds are just not
quite strong enough - yet - to mix out the lighter fog.

By late morning, the Valley Wind Machine will begin to crank up as
south/southeast flow gets going between the eastern Gulf ridge and
low pressure troughing in the lee of the Sierra Madre. ECMWF/GFS
in good agreement for a period of 20+ knot sustained winds in the
afternoon, and could see some gusts to 30 knots (mainly
Harlingen). Ceilings will lift and scatter out to MVFR near the
coast and low VFR inland.
52/BSG

MARINE...(Now through Thursday Night)...Made a few adjustments
to fog and wind through the short term period. Marine layer will
be classicly stout given upper 60s dewpoints over upper 50s water
temperatures in the near shore. Less of a marine layer beyond 20
nm out (TABS Buoy K carrying 70 degree SST) but wind field is a
bit removed from the inland "Valley Wind Machine". Lowered daytime
speeds to 10-11 knots in the nearshore, more from the due
southeast, Thursday and just a brief uptick Thursday evening
before leveling off overnight. Added patchy fog through the day
Thursday from Laguna Madre through the nearshore waters where
surface winds will sharply drop, connecting the fog mention with
the Thursday night and early Friday period.

Dense fog advisory for <1 nautical mile visibility will continue
overnight and likely be extended into mid morning Thursday based
on current conditions and analogs for similar events in the past.
How much mixing of drier air to the surface will determine if
advisories need to be extended or let expire later Thursday.

No significant changes to wave height, though did nudge down per
<3 foot values at Tabs K buoy and held at 3 feet or less for much
of Thursday before some increase, mainly offshore, Thursday night.
52/BSG

PREV DISCUSSION... /issued 356 PM CST Wed Feb 14 2018/

SHORT TERM (Tonight through Thursday Night): Main forecast issue
in the short-term will be fog. Earlier had made some adjustments
to weather/sky/temp grids to account for the persistence of low
cloud cover. Surface trough/remnant warm front has been slower to
lift north/inland than previously thought, but finally seems to be
making its move. Little change was made to forecast grids after
00Z tonight, apart from weather. Through Thursday night, CWA will
still be dominated by the western periphery of an H5 high over the
western Gulf. Very dry air and subsidence at mid-levels will
continue to be present, so have maintained PoP`s at silent levels.
Some short-term guidance indicating a slight chance of precip
over the northern Ranchlands this evening as the warm front/trough
continues lifting NW, but don`t believe this would be any more
than sprinkles, if it actually occurs. Temps overnight won`t vary
too much over the NW one-third or so of the CWA, where daytime
heating was inhibited today. Morning lows for Thursday are
expected to be in the low-mid 60s area-wide.

With dewpoints also rising into the lower 60s overnight on the
return flow, fog formation is once again expected.  Not confident
enough that it will become dense over land areas, though, with
surface winds actually progged to increase a bit through the night.
MOS guidance and HRRR/RAP visibility progs suggesting the fog will
form fairly quickly after sundown, so have added patchy fog
beginning at 03Z in the Lower Valley and areas extending north and
west, where winds are expected to be a little lighter.  Bumped those
same areas up to areas fog coverage from 09-15Z, with patchy fog
elsewhere.  Dense marine fog is expected to continue on the Laguna
Madre and 0-20nm offshore Gulf waters; see Marine section below for
more details.

Still not seeing any reason to go against a significant warmup
tomorrow with the strengthening southerly flow and much more
sunshine that we have seen lately (once morning mist/low clouds burn
off).  Inherited forecast maybe just a bit warmer than latest MOS,
but have leaned toward the inherited values, which are mainly in the
80s (mid-70s along the coast).  Winds will also become breezy by mid-
day, especially for the Lower Valley, reaching up to 20G25 mph.
Another night with lows in the 60s is in store for Thursday night.
Can`t rule out some more patchy fog, but with winds expected to stay
up in the 8-mph range and MOS guidance sending mixed signals, will
leave out of the grids for now.

LONG TERM (Friday through Wednesday): Shallow 500 mb ridging will
prevail over the Gulf of Mex and the U.S./Mexico Gulf Coast region
throughout the weekend into the first half of next week. A steady
progression of fast moving short waves will be moving through the
broad troffing over the northern two thirds of the nation into the
weekend. One of these short waves will push southwards towards
South TX this weekend which will in turn push a fairly weak cold
front down near south TX which could spark off a bit of conv
locally. However both the GFS and ECMWF keep the better moisture
values pooled to the north of the RGV. So for now will only
mention some 20-30 % chcs for the Fri-Sun period. The strong 500
mb PVA and deeper moisture values then lift out to the north away
from the RGV for Mon which will lower the conv potential a bit.
Further west a deeper 500 mb trough will dig down over the western
states which in turn builds the ridge axis over the western
Atlantic and the SE states. At the surface a series of stronger
surface lows will move out across the central Plains states ahead
of the digging western trough axis. This will interact with the
surface low to our east to tighten up the PGF over the RGV
producing breezier and windier conditions for early next week.
This western 500 mb trough will start to push eastwards around
midweek next week which may push through a milder cold front
through the RGV on Wed.

The persistent surface and 500 mb ridging over the region will
maintain overall temps well above average for Feb. CAA in the wake
of the weaker front on Wed may pull the temps down to near normal
late in the forecast period.

Will go close to a model blend of the GFS and the ECMWF for temps
and pops through next Wed. Both of these models are in good
agreement through Tues for both the overall synoptic pattern and
for temps/pops. Both begin to diverge a bit for the solution for
Wed with the anticipated fropa with the GFS showing a stronger
passage versus the ECMWF. Overall confidence in the longer term
period is generally above average through Tues and near average
for Wed.

MARINE (Now through Thursday night): After considering webcams on
South Padre Island, satellite trends, and collaborating with other
Texas coastal offices, Marine Dense Fog Advisory for the Laguna
Madre and nearshore Gulf waters out to 20nm has been extended until
3am Thursday.  Some slight improvement may be noted over the next
several hours, but the advection fog should again quickly become
dense this evening.  Some hint in the model guidance that flow may
increase enough to improve visibilities later tonight, but will
defer to later shifts to make the ultimate call on any advisory
reconfiguration.  At least patchy fog is expected once again
Thursday night in the same areas.

Otherwise, moderate SE winds and moderate seas of 3-5 ft. on the
Gulf will prevail.  May have to watch the Laguna for Small Craft
Exercise Caution wind conditions Thursday afternoon, but if it stays
socked in with fog, winds would be held down to 10-15mph.

Friday through Monday Night: The PGF will be somewhat weak for
Fri and Sat as the surface ridge moves further eastward. This will
hold down the winds and seas for the weekend. The PGF will then
start to strengthen on Sun and Mon/Mon Night as a strong surface
low pressure system develops over the central Plains states
resulting in stronger S-SE low level marine flow. The one limiting
factor for the stronger surface marine winds is that both the Bay
and Gulf water temps remain very cold in the mid 50s. This will
tend to limit the downward mixing of the stronger southerly flow.
So conditions may remain marginal for SCA conditions in the longer
range period despite the strengthening gradient.

&&

.BRO WATCHES/WARNINGS/ADVISORIES...
TX...Dense Fog Advisory until 6 AM CST Thursday for TXZ251-254>257.

GM...Dense Fog Advisory until 9 AM CST Thursday for GMZ130-132-135-
     150-155.

&&

$$

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