Area Forecast Discussion
Issued by NWS Brownsville, TX

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FXUS64 KBRO 140528 AAB
AFDBRO

Area Forecast Discussion...UPDATED
National Weather Service Brownsville TX
1128 PM CST Mon Nov 13 2017

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

&&

.AVIATION...Midnight forecast package is pretty much an update of
the previous package, with the same degree of difficulty during the
09Z to 15Z period for evolution of potential IFR visibility and
associated ceilings (fog).  One change was to clear out clouds to
start the period which adds a bit more confidence for fog to develop
around 3 AM or so.  While it remains possible for LIFR visibility
and ceilings...favoring Harlingen and McAllen as discussed
previously...confidence is not there to forecast full on fog (i.e.
1/2SM FG VV002) mainly based on *just* enough wind to mix some
slightly drier air into the equation. Still undercutting guidance
which is not handling the situation where it rained fairly heavy the
past couple of days including parts of the populate RGV.  The
possibility exists for denser fog and aviators should keep close
tabs for early morning flights.

Low MVFR ceilings MVFR visibility could hold fairly late into the
morning with light east winds and mid November sun struggling just a
bit to burn everything off.  Model soundings continue to show a
potential low VFR ceiling toward noon, before skies should begin
clearing out for the afternoon.  Latest guidance suggesting a bit
longer period of ceilings so have added another group at 21Z to
break things out.  Generally light east winds into evening with
mainly clear skies to for the first half of Tuesday night.

By mid to late afternoon, the forcing begins to nudge southward but
have maintained mention of precipitation without visibility
restrictions and a low VFR ceiling per forecast mixing heights.
After 00Z the better forcing shifts into Mexico and drier air begins
filtering south, but still enough moisture to argue for mixed layer
clouds.  Beyond this forecast, question turns to late night fog
development and lower visibility but should be after 06Z (midnight)
on Tuesday.

&&

.PREV DISCUSSION... /issued 630 PM CST Mon Nov 13 2017/

DISCUSSION...Updated for latest /00z/ aviation discussion below.

AVIATION...Forecast is fairly simple at the start and end, but
tricky in the middle. With radar much quieter to begin the evening
and low level atmospheric forcing and tropical moisture shield
dissipating or shifting south into Mexico, there will be no
localized surprises as we saw between 1 and 4 AM at McAllen/Miller
early this morning. VFR mid level deck will persist through
midnight before scattering out if the GFS/ECMWF moisture trends
are correct. At this point, the trickiness begins. The first
question is whether MVFR/Low MVFR decks can develop as we`ve seen
often in warm humid conditions. GFS soundings favor the MVFR deck
for all three airports, but at the same time GFS and models
overall may not be handling the moisture in the ground from recent
rains and lack of evaporation due to plenty of cloud cover and
lengthening nights. Interestingly, the NAM guidance does pop in a
daybreak LIFR ceiling category which could be a response to the
record-breaking soaking there in the wee hours this morning.

For the forecast, then: Maintained a MVFR visibility with
prevailing MVFR ceilings (low MVFR in the mid Valley) toward
daybreak, but added TEMPO groups for IFR visibility and some IFR
ceilings as well especially at Harlingen and McAllen where this is
probably a better bet based on the rainfall (McAllen) and
climatology at Harlingen which is adjacent to farmland and known
to sock in. Note that we`ll be watching closely trends of
scattering out of the mid deck and any early ground-type (BCFG)
development in the area toward midnight which would increase
confidence for some landing/takeoff issues early Tuesday in more
widespread LIFR and possibly VLIFR conditions. Just something to
keep in the back of one`s mind.

If IFR conditions develop, it may take a little while to fully
break; possibly not until 10 AM or so in the mid Valley. But by
noon or so ceilings should lift and/or scatter in low VFR range -
lifting higher at McAllen/Miller based on mixing arguments (5K
feet) by mid afternoon under a generally light easterly wind
pattern.
52/BSG

PREV DISCUSSION... /issued 244 PM CST Mon Nov 13 2017/

SHORT TERM (Tonight through Tuesday Night): The present moisture
slug supporting the intermittent afternoon showers will push
inland tonight and conditions will improve. Drier air from high
pressure over the southeast U.S. will be drawn out over the Gulf
and toward the CWA. Satellite total PWAT indicates drier air
working this way from the northeast. Pops have thus essentially
dropped out of the forecast for the next three periods. Light east
northeast winds will veer to east on Tuesday and then to
southeast Tuesday night. Patchy fog will be possible tonight and
is included in the overnight to near dawn timeframe. Warmer than
average temperatures will continue, with mid to upper 60s at night
and lower 80s during the day. Look for mostly clear skies at
night with partly cloudy to mostly sunny skies during the day.

LONG TERM (Wednesday through Monday): The latter half of the week
continue with the H5 ridge off to the southwest, continuing to
provide dry NWly flow aloft. At the surface, southeast flow
continues to pump Gulf moisture in, allowing much above normal
temperatures each afternoon. The main weather feature in the long
term continues to be the next cold front sweeping into the region
sometime Saturday. GFS and Canadian models continue to show
strong forward motion with the front along with a stronger surface
high, bringing the front through before noon. The EC is
persistent indicating a weaker surface high, thus a slower frontal
passage later Saturday afternoon/evening. With a very cold
airmass already primed in western Canada, would lean more toward a
stronger surface high breaking off and sweeping southward, thus
will lean more toward the quicker passage scenario. This will
bring minimal daytime heating for Saturday, keeping highs in the
low 80s before frontal passage, while Sunday will see good CAA and
highs struggling into the 70s.

MARINE:
Now through Tuesday night: Initial moderate east northeast winds
will weaken tonight as high pressure spreads over the Gulf, though a
long Gulf fetch will support slightly higher seas. Wave heights
offshore will run 4 to 6 feet tonight and Tuesday. Light east winds
on Tuesday will veer to southeast Tuesday night, as moderate seas
continue.

Wednesday through Saturday: Tranquil marine weather remains for
the Gulf waters for the latter half of the week, with high
pressure along the northern Gulf Coast. This will keep winds at or
below 15 knots, with seas around 3 feet. The next front is
forecast for sometime Saturday, with a rush of northerly winds
reaching 20 knots at passage. Advisories look likely at this
juncture.

&&

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

$$

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