Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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000
FXUS64 KHGX 171511
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1011 AM CDT Tue Oct 17 2017

.DISCUSSION...
Surface high pressure over the central Appalachians was ridging
into Texas at mid morning. Cold air advection was combining with
the surface pressure gradient between the surface ridge and a
cold front over the central Gulf of Mexico to generate breezy
northeasterly winds along and off of the immediate Upper Texas
coast. Some minor tweaks to wind. Otherwise, the current forecast
was on track.

40

&&

.PREV DISCUSSION... /ISSUED 632 AM CDT Tue Oct 17 2017/

AVIATION...
VFR conditions will prevail for the next 24 hours as high
pressure over the eastern US ridges into SE TX. 43

PREV DISCUSSION... /ISSUED 407 AM CDT Tue Oct 17 2017/

DISCUSSION...
Surface ridging extending from the Ohio Valley southwest into
Texas this morning is promoting light northeasterly winds across
the region. These light winds and clear skies have allowed for 4
AM CDT temperatures to fall into the mid 40s to mid 50s inland
this morning, with coastal temperatures in the lower 60s. After a
cool start this morning, little to no clouds and dry air in place
today will allow for temperatures to rise into the upper 70s to
low 80s. With average daily temperatures for all four climate
sites running 3-7 degrees above normal for the month of October so
far, these near to below normal high temperatures are certainly
welcome. A listing of these deviations is provided in the climate
section below.

Another cool night is expected tonight as temperatures fall into
the upper 40s to mid 50s under clear skies, but the approach of a
disturbance now 300 miles west of Baja California will result in
shortwave ridging building over the region on Wednesday and high
temperatures warming a few degrees into the low to mid 80s.
Additionally, the departure of the surface ridge on Wednesday will
allow for light southerly to southeasterly winds to resume with
moisture advection Wednesday night keeping low temperatures
almost 10 degrees warmer than tonight in the mid 50s to mid 60s.
Continued moisture advection on Thursday morning will allow for a
few showers to stream across the coastal waters and into the
southwestern counties during the day, with scattered showers and
thunderstorms possible on Friday as the disturbance near Baja
California lifts across Texas. Can`t rule out a few heavy
downpours on Friday as precipitable water values rise into the 1.5
to 1.75 inch range, but storm motions 15-20 MPH should keep that
risk for any one area brief.

Divergence begins to increase across Texas on Saturday ahead of a
strong upper trough arriving from the West Coast, and combined
with daytime heating, this will encourage another round of
scattered showers and thunderstorms across Southeast Texas during
the day. Mid-level lapse rates steepen ahead of this trough and
with increasing upper level support, what appears to be a warm
front lifting into the region and some marginal 0-6 km shear
(20-25 knots), could see a strong thunderstorm or two develop on
Saturday. As this upper trough swings across the state Saturday
night into Sunday, medium range guidance continues to advertise a
cold front surging across the state and clearing the region on
Sunday with scattered to widespread showers and thunderstorms
developing along the front. With the parent trough providing
strong upper level forcing, steep mid-level lapse rates, and
increasing shear along and ahead of the front, the threat for
strong to possibly severe thunderstorms may exist on Sunday for
Southeast Texas.

While most of the 00Z medium range deterministic guidance appears
to be converging on a solution that the trough will remain open
and progressively sweep across the region on Sunday, have concerns
that model runs have been inconsistent with their treatment of a
possible cut-off low solution (even as recent as the 16/12Z ECMWF
advertising it). As a result of this and continued GFS ensemble
support for a cut-off low to develop, have held on to low rain
chances into Sunday night should the trough move through slower or
possibly pinch off into an upper low late in the weekend. Should
current 00Z guidance verify, the front will quickly sweep off the
coast by Sunday night with dry weather and seasonable temperatures
to start next week. However, will need to monitor model solutions
(which will hopefully become more consistent once the upper
trough is sampled by the Alaska radiosonde observation network
today and the lower 48 sites by the end of the week) as the cut-
off low solution would provide for possibly a slower-moving front,
warmer temperatures, and rain chances extending into next week
longer than currently advertised by the forecast.

Huffman

MARINE...
Winds remain strong over Galveston Bay and the Gulf waters early
this morning. Daytime heating should help stabilize the thermal
gradient and the pressure gradient is also decreasing so am
expecting winds to subside quickly this morning. Will need to
extend the SCA and SCEC through part of the morning. NE winds
expected today into tonight with a ridge of high pressure to the
northeast and a weak area of low pressure near the Yucatan. The
ridge of high pressure flattens a bit and surface winds will
become east by Wednesday night and gradually increase in speed. A
moderate to possibly strong onshore flow is expected
Thursday/Friday as the gradient tightens in response to lee
cyclogenesis. Tide levels will increase on Friday in response to
the persistent onshore flow and tide levels will likely exceed 3.3
feet on Friday. 43

FIRE WEATHER...
Min RH values this afternoon will fall to between 27-35 percent
along and NNW of I-69 but winds will be considerably lighter
today. The lighter winds should limit the threat for fire weather
today. Moisture levels increase significantly on Wednesday through
the end of the week. 43

CLIMATE...
All four primary climate sites are currently reporting above
normal average daily temperatures for the month of October so
far. These anomalies through October 16 are listed below.

LOCATION               DEGREES ABOVE NORMAL
                   (AVERAGE DAILY TEMPERATURES)
Houston IAH                  4.6 F
Houston HOU                  6.9 F
College Station              5.9 F
Galveston                    3.9 F

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      79  50  84  58  84 /   0   0   0   0  10
Houston (IAH)              79  54  83  62  84 /   0   0   0  10  10
Galveston (GLS)            79  65  82  70  83 /   0   0  10  10  10

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...SMALL CRAFT SHOULD EXERCISE CAUTION until 4 PM CDT this
     afternoon for the following zones: 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...40
Aviation/Marine...47



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