Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 221507
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1007 AM CDT Sun Oct 22 2017

.UPDATE...
This morning`s line of storms continues to push towards and off
the Southeast Texas coast this morning, carrying wind gusts mostly
in the 30 to 40 mph range, but also with occasional gusts to
around 50 mph in the vicinity of Galveston Bay, likely aided by
decreased friction of the water. Currently, the line intersects
the Gulf Coast near Freeport, and so the focus is gradually
turning towards the offshore waters.

One exception to this is the apparent rejuvenation/formation of a
secondary line running from roughly from Moulton to Hempstead
thanks to a kink in the boundary. GOES-16 imagery shows that a
small wedge of clearer air emerged behind the initial line, and
there`s likely enough relatively moist, unstable air there to
allow for new updrafts to form. SPC mesoanalysis shows MLCAPE
ranging from 500 j/kg at the northern point of the wedge to nearly
2000 j/kg at the edge of the front/cold pool at the southern edge
of the wedge. This range of values probably isn`t quite right,
given the small scale of the feature that likely isn`t being
resolved well by the mesoanalysis model scheme, but the updrafts
on G-16 imagery do seem a little more vigorous on the southwestern
end of this new line, so the pattern is likely good. As long as
that wedge doesn`t expand, and satellite trends indicate it is
actually getting smaller, I would not expect this secondary line
to amount to terribly much more than it already is.

Otherwise, temperatures seem generally on track. The morning`s
rain has knocked temps down a little bit more than were forecast,
and so I`ve had to adjust the first several hours of the forecast
accordingly. Also kept highs steady or nudged them downwards
slightly. Given the relatively quick motion of this front, as long
as we can get the clearing on the back edge as expected today, we
should be okay without a ton of changes to the temperatures.
However, if we start pushing too far into the early afternoon
without sun, some more substantial changes will likely have to be
made.

Luchs

&&

.PREV DISCUSSION... /ISSUED 627 AM CDT Sun Oct 22 2017/

AVIATION...
The leading edge of the thunderstorms were moving into KUTS and
KCLL at 1115Z. Winds were gusting to 34 knots at KUTS. The storms
should arrive at KCXO by 12Z, KIAH around 13Z, KHOU and KSGR
around 1330Z, and KLBX and KGLS between 14Z and 15Z. Both the
Texas Tech and the HRRR move the area of storms off the coast by
17Z to 18Z.

Impacts ahead of the storms will be patchy fog, occasionally
dense. As the storms arrive, wind gusts of 25 to 30 knots will be
possible along with ceilings IFR or lower and visibility
restricted to 2 nm or less due to heavy rain. An hour or two of
lighter rain will follow the initial period of heavier rainfall.
VFR conditions will develop during the mid to late morning period.

Later this afternoon after the cold front passes through each
site, expect northwest to north winds will gust from 20 go 25
knots at most sites. These should taper off during the early
evening. Along the coast, KGLS should see wind gusts persisting
through much of the night.

40

PREV DISCUSSION... /ISSUED 351 AM CDT Sun Oct 22 2017/

DISCUSSION...
Warm air advection producing scattered showers early this morning
in advance of a fast moving cold front over North Texas. The front
will surge south this morning reaching College Station around 12z
and the Houston area around 15z and off the coast between 17-18z.
850 mb moisture looks good with dew pts between 14-15 C. 300 MB
winds still show a broad split over SE TX and the area is progged
to lie in an 85 kt RRQ between 12-15z. All in all, it`s shaping
up to be a wet morning as the front crosses the area and jet
dynamics get tapped. Fcst soundings show PW values between 1.80
and 1.90 inches, CAPE values around 2000 J/Kg, LI values around -7
and shear values remain modest at best. A few storms will likely
be strong with some minimal wind gusts exceeding 35 mph and maybe
some pea size hail but the overall threat for severe weather is
low. The front is moving fast so heavy rain is not expected and
rainfall amounts look to average between 0.50 and 1.00 inches
today. Drier air will work into the region this afternoon and
sunshine should return by early afternoon over the west and mid to
late afternoon toward the coast.

High pressure builds into the region tonight and skies will remain
clear and temperatures will finally cool off with Monday morning
lows in the upper 40`s to mid 50`s. A secondary surge of cooler
air will surge into late SE TX Monday night and 850 temps cool
significantly and surface dew points will drop as well in response
to the dry air mass. Tuesday and Wednesday both look dry and cool
with high temps in the 70`s and low temps in the upper 40`s to mid
50`s. RH values will fall to around 25 percent both Monday and
Tuesday so will have to watch for a elevated risk for fire
weather, particularly on Tuesday as sfc winds look a bit stronger.

Onshore winds return Wednesday night and a brief warm up will
begin on Thursday and temperatures will warm into the lower 80`s.
Moisture levels will also begin to deepen as onshore winds
strengthen. Still some significant disagreements on whether the
next cold front will cross the area on Friday afternoon (GFS) or
holds off until Saturday morning (ECMWF). The ECMWF also has a
well developed upper trough that lags the sfc front setting up an
isentropic rain event for Saturday as SW upper level winds over
ride NNE sfc winds. The GFS isn`t as deep with the upper trough
and is much further north. Both models have shown good run to run
consistency so still not sure which will be correct. For now will
split the difference and continue to carry rain chances through
Saturday night. FWIW, both the MEX and ECE numerical guidance
looks to warm next weekend. 850 MB temps cool significantly and
sfc temps may struggle to reach 60 degrees next Saturday
especially if clouds hang around. 43

MARINE...
A cold front is expected to move off of the coast by early afternoon
today. Winds behind the front will likely to advisory levels by
early this evening.  The winds will diminish to below small craft
advisory levels on Monday but stay elevated into Tuesday evening. A
secondary front will then move off of the coast Tuesday night with
winds behind this front strengthening to advisory levels once again
early Tuesday morning. These offshore winds could diminish slowly
below advisory levels on Tuesday afternoon but then strengthen to
advisory levels Tuesday night. As high pressure builds into South
Texas, winds over the Upper Texas coastal waters and bays will
diminish during the day on Wednesday. Onshore winds will redevelop
Thursday morning as the high pressure area moves across the Gulf of
Mexico into the lower Atlantic Seaboard region.

40

FIRE...
A cold front will move across Southeast Texas today. Impacts behind
the front include minimum relative humidity values between 30 and 35
percent on Monday. However, wind speeds will likely on range from 5
to 10 mph at most during the afternoon. These conditions should only
lead to slightly elevated fire weather conditions.

A second cold front will then move across Southeast Texas on
Tuesday. More elevated fire weather conditions will exist behind
this front Tuesday afternoon, as relative humidity values fall to
between 25 and 30 percent and northerly winds range from 10 to 15
mph.

Slightly elevated fire weather conditions will continue into
Wednesday as dry conditions will persist. However, winds during the
day on Wednesday should only range from 5 to 10 mph.

Onshore winds will return on Thursday and alleviate any elevated
fire weather conditions of the previous few days.

40

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      78  49  80  55  75 /  70  10   0   0   0
Houston (IAH)              80  55  79  57  77 /  90  10   0   0   0
Galveston (GLS)            82  63  76  65  77 /  70  10   0   0   0

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...SMALL CRAFT SHOULD EXERCISE CAUTION from 1 PM this afternoon to
     7 PM CDT this evening 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...Galveston Bay...Matagorda Bay...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...25



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