Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 181737
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1237 PM CDT Mon Sep 18 2017

.UPDATE...
Showers and a few thunderstorms are erupting across the area as
moisture surges into the area. Water vapor imagery shows a very
subtle shortwave trough over Southeast Texas has been digging a
bit more space for itself through the morning. Additionally,
though yet non-operational, GOES-16`s TPW product shows
precipitable water near or exceeding two inches across the entire
area, including locations that earlier this morning were closer to
1.5 inches. As a result, the main change to the forecast this
morning has been to increase rain chances across the area to
reflect current trends. Secondarily, have blunted high
temperatures slightly in most areas with a greater expectation of
clouds and rain. However, this has been made trickier as obs
indicate that where clouds aren`t providing a significant
obscuration, we`re actually running a bit warmer than forecast.
The high forecast for this afternoon attempts to stitch these two
disparate ideas together. Finally, PoPs are brought to an end
rather quickly, as subsidence behind this trough along with the
loss of heating should cut convection off pretty rapidly.

Luchs


&&

.PREV DISCUSSION... /ISSUED 1219 PM CDT Mon Sep 18 2017/

AVIATION...
Inland TAF sites will be dealing with SHRA/TSRA and associated MVFR
ceilings and visibilities this afternoon as activity continues to
develop and work its way generally off to the N and NE. Should see
a decrease in coverage later this afternoon through early this evening.
Some MVFR/IFR ceilings and visibilities are possible again overnight
at inland sites, and areas near the coast could see possible SHRA
toward morning. Activity should spread inland again tomorrow afternoon,
but current thinking is that we might see less coverage than what we
are seeing today.  42

PREV DISCUSSION... /ISSUED 431 AM CDT Mon Sep 18 2017/

DISCUSSION...
The main issue for this morning is patchy dense fog. May need to
issue a dense fog advisory -- if so it will mainly north and west
of Houston. Model sounding forecasts indicate that any fog that
does form should lift between 14 and 15Z at the latest.

Otherwise, expect chances for showers and thunderstorms to
increase this morning. The models show that the 500 mb high
pressure area will weaken and move southward today. Model sounding
forecasts also show increasing CAPE and a diminishing cap. SPC Rap
analysis also shows that PWs are on the increase and at 3:45 AM
ranged from 1.7 to 1.9 along and off of the coast. Some of the
high resolution models seemed to be a bit overdone, especially
early this morning over the inland areas. Do expect isolated
coverage before sunrise over the Gulf waters spreading inland.
Scattered coverage should develop during the mid and late morning
period at the latest and continue through the afternoon.

Expect scattered daily coverage of showers and thunderstorms to
persist through at least Thursday. Both the GFS and ECMWF show
upper level shortwave troughs moving across the area each day. By
Thursday and Friday the upper pattern morphs into a trough from
the Mid Atlantic into SE Texas with an upper level high pressure
ridge developing west and north of our area. Earlier model runs
had the ridge overhead of the forecast area for the weekend.
Latest runs now have more of a trough overhead and thus better
chances for rain. Now have better confidence that the daily
isolated to scattered showers and thunderstorms will persist
through the weekend.

40

MARINE...
Southeasterly 5-10 knots and occasionally increasing to 10-15 knots
especially Wednesday and beyond. Seas 2-3 feet should be the
dominant sea state this week. Moisture levels increasing today and
Tuesday so expect coverage of showers and even a few thunderstorms
to increase. By Thursday a more typical diurnal pattern will be
established with showers after midnight tapering off by noon over
the water.
45

AVIATION...
Widespread IFR/VLIFR deck outside of IAH/HOU/CLL so far but the deck
is expanding toward CLL and could intrude into IAH...decks should
lift and mix out by 14-15Z. Guidance is very bullish on SHRA/TSRA
coverage and will continue to carry VCSH/VCTS and will probably need
to add some tempos after it develops or if HRRR gets more consistent
on timing/placement into the Metro Hubs.

Overnight return of fog and MVFR-IFR CIGS and IFR fog in the more
rural sites.
45

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      88  74  92  75  92 /  30  10  30  10  30
Houston (IAH)              89  75  90  76  90 /  50  10  40  20  50
Galveston (GLS)            87  79  87  79  85 /  30  10  20  20  30

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...NONE.
&&

$$

Discussion...25


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