Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 152340
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
540 PM CST Mon Jan 15 2018

.AVIATION...
Scattered showers have developed to areas southwest of the Houston
terminals and are moving NE and could brief clip IAH this
evening. There should be a gap in the precip this evening. Precip
will redevelop in the wake of a strong cold front that will reach
KCLL around 06z and Houston around 09z and the coast around 11z.
Light rain will develop in the wake of the front and transition to
a mix of light rain/freezing rain and sleet. The transition will
occur first at KCLL, KUTS and KCXO as early as 12z but more likely
between 12-15z. The transition at KIAH will occur between 15-17z
with a mix of rain/freezing rain and sleet. Fcst soundings do
support a change over to snow during the afternoon. A mix of rain
sleet will develop at KSGR and KHOU by 18z with the frozen precip
lingering during the afternoon. Mainly rain is expected at KGLS
with some sleet mixing in by afternoon. Most of the precipitation
will end by 01z. Strong north winds will develop in the wake of
the front and will begin to subside toward Tuesday evening.
Generally MVFR ceilings are expected behind the front with VFR
conditions develop by late afternoon over the northern TAF sites
and after 00z over the south. 43

&&

.PREV DISCUSSION... /ISSUED 357 PM CST Mon Jan 15 2018/

DISCUSSION...
It is so nice to see temperatures in the mid/upper 60s today. So
wish I could be outside to enjoy instead of bringing the bad news.
What`s that saying? If you don`t like the weather in Texas just
wait 5 minutes? That`s the forecast for tonight/tomorrow. So let`s
get to it.

Currently the arctic cold front is pushing into Red River region
and north Texas. Latest surface obs at 12Z show a 1048-1050mb high
sitting over NE Montana/NW North Dakota. Water vapor imagery shows
that nice closed low over the Great Lakes and broad vorticity
maximum over the Plains swinging southward. Upper air analysis
shows highly amplified pattern with ridge over the western U.S.
driving the polar jet due south over the Plains.

Tuesday...This is probably one of my top 5 toughest most
challenging forecasts to make and one of the toughest forecast
decisions to make. Big thanks to our forecast team collaborating
on this but 12Z models all show very similar trends. Ensembles
like SREF/GEFS seem to be showing something similar.

First thing we saw was temperatures in the boundary layer are
trending colder so we had to trend surface temperatures colder and
sooner than previous forecast. This calls for freezing
temperatures for most of the day on Tuesday with precipitation
falling.

Second the transition of precipitation types looked on track with
a transition Tuesday morning from rain - freezing rain - sleet -
snow. Model soundings from NAM/GFS show this trend. The ECMWF also
showed colder 850MB temps by 18Z Tuesday indicating a similar
trend. There is a fight between the moisture over the frontal
surface and precip falling versus the drier boundary layer air
coming into from the north. This could allow for precip to end
sooner than expected but it will have to overcome quite a bit of
thermodynamic wet bulbing to do it.

Third increase in frontogenetic lift at several levels. GFS/NAM
all show frontogenesis at 925-850mb and 850-700mb layers. There is
also jet stream configuration with the right entrance region over
much of SE Texas and quite a bit of 500-300mb Q vector
convergence supporting large scale lift. These seem to come
together a little bit late but still lift over the area 18Z
Tuesday to 00Z Wednesday. This all suggests possible banded
precipitation and you even see hints of that in the QPF output in
the models. I`m not going to get into a discussion of CSI and
lapse rates but cross sections do show pockets of CSI and steeper
lapse rates in these areas. Frontogenesis will be driving most of
the banding anyway.

Fourth we need to emphasize freezing rain/sleet as the primary
threat for most of the area. Most ice accumulations will be from a
few hundredths to a tenth of an inch. Banding could lead to
higher amounts of ice leading to need a winter storm warning. With
this potential in mind, we decided on a winter storm warning for
areas along and north of Interstate 10. Farther north there could
be a band of snow from Brenham/Conroe/Livingston north with
generally 1 inch of snow possible which would fall on already icy
conditions from freezing rain/sleet prior to snow. There very
well could be some sleet/snow late afternoon/evening in the
Houston area but freezing rain will be the main issues south of
Conroe.

Precipitation should come to an end Tuesday night with low
temperatures Wednesday morning requiring a hard freeze warning. In
the extended forecast there looks to be a chance of rain Friday
with warm air advection. This continues Saturday into Sunday with
another front pushing through Sunday. This time it looks like a
line of showers and a few thunderstorms Sunday. High pressure
builds in with more cold conditions.

Overpeck

MARINE...
A Gale Warning is now in effect for the coastal waters tomorrow
afternoon through early Weds morning. A Small Craft Advisory is
also in effect for the bays during this same time frame.

Light onshore winds the rest of this afternoon and tonight will be
giving way to strong/gusty offshore winds tomorrow morning on into
the afternoon and overnight hours. This cold arctic airmass is ex-
pected to bring frequent gusts to 34 knots, mainly during the late
afternoon/evening across the coastal waters, with a sustained N/NE
wind of 20 to 30 knots persisting into early Weds morning. Seas to
build into 6 to 8 feet nearshore and around 10 or 11 feet offshore.
Precipitation should be rain/showers...but isolated locales in the
upper parts of the bays could experience some freezing rain during
the late morning/early afternoon time frame on Tues. Low water is-
sues could also present themselves Tues/Weds given the strong off-
shore flow combined with the predicted low astronomical tides.

Winds/seas should be decreasing to below SCA/Gale Warning criteria
Weds morning/afternoon...but SCEC conditions could linger over the
offshore waters as the surface high re-centers itself over the S/E
U.S. and maintains a tight east-to-west gradient over the northern
Gulf. Models keeping with the idea of a coastal low developing at/
near the middle TX coast Thurs/Fri. Along with a brief shifting of
winds (around this system), unsettled weather to return before the
low moves east/washes out Sat. Light/moderate onshore winds should
prevail the rest of the weekend , with the next cold front progged
to move offshore late Sun. 41

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      27  29  20  37  24 /  70  60  10  10  10
Houston (IAH)              33  35  23  39  27 /  60  70  20  10  10
Galveston (GLS)            42  44  28  40  34 /  50  70  30  10  10

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Winter Storm Warning from 3 AM Tuesday to midnight CST Tuesday
     night for the following zones: Austin...Brazos...Burleson...
     Colorado...Grimes...Harris...Houston...Liberty...Madison...
     Montgomery...Polk...San Jacinto...Trinity...Walker...
     Waller...Washington.

     Winter Weather Advisory from 3 AM Tuesday to midnight CST
     Tuesday night for the following zones: Brazoria...
     Chambers...Fort Bend...Galveston...Jackson...Matagorda...
     Wharton.

GM...Small Craft Advisory from noon Tuesday to 9 AM CST Wednesday for
     the following zones: Galveston Bay...Matagorda Bay.

     Gale Warning from noon Tuesday to 9 AM CST Wednesday for the
     following zones: Coastal waters from Freeport to the
     Matagorda Ship Channel out 20 NM...Coastal waters from High
     Island to Freeport out 20 NM...Waters from Freeport to the
     Matagorda Ship Channel from 20 to 60 NM...Waters from High
     Island to Freeport from 20 to 60 NM.

&&

$$

Discussion...14
Aviation/Marine...43



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