Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 230535

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1235 AM CDT Fri Sep 23 2016

Evening showers near CLL have mercifully come to an end, leaving
scattered clouds to clear skies across the area - with CLL as an
exception at the moment with lingering high clouds. Still want to
bring in MVFR visibility and some clouds below 3k feet for the
northern sites late tonight/early this morning as Gulf moisture
continues to build. With a lack of marine showers over the Gulf,
have stepped back the onset of rain chances near the coast, but
haven`t given up on them entirely, given our place between the 1.7
inch and 2 inch precipitable water observed in the evening
soundings at LCH and CRP. But, if showers do not fill in more, the
VCSH may need to be put off until there`s sufficient heating and
bring in MVFR ceilings and/or light fog earlier.



.PREV DISCUSSION... /ISSUED 1001 PM CDT Thu Sep 22 2016/

Showers finally dissipated this evening but not before dumping a
good 1-2 inches of rain from near Hempstead to south of College
Station. Deeper moisture has returned with the 00z LCH sounding
showing 1.7 inches of precipitable water and CRP showing 2 inches.
Water vapor imagery shows deepening upper level low over the Great
Basin. Upper level ridge remains over much of Texas but has not
done much to suppress convection much.

After looking at 12z/18z model guidance, here is what we think we
know and what we think we don`t know. Models are in pretty good
agreement through about 3 days which really isn`t much. Upper low
organizes over the Rockies and swings east towards the N Plains by
Sunday. Models are good with breaking off a piece of vorticity
which drops south into the S Rockies by Sunday. At the surface, a
cold front pushes into the plains as the surface low organizes
over the N plains. The front then makes slow progress south into
Texas on Sunday. Friday through Sunday the boundary layer flow
increases off the Gulf of Mexico and moisture continues to
increase. Precipitable water values reach 2-2.2 inches Saturday
into Sunday. So looks like a good chance of rain Saturday through
much of the weekend and early next week. Rainfall amounts will
probably be around 1-3 inches areawide for the next 5 days.

Considering there are large variability`s between model solutions
after Saturday, there are certain details that we don`t know. It
is unclear how the upper trough and possible cut off low
development evolves. GFS and ECMWF are quite different. This is
key because this evolution also determines when the cold front
pushes through the area or as the case is with the ECMWF...not
push through till much later next week. The front may also be
weaker. The other challenge is figuring out the details of
where/when any locally heavy rainfall will occur. There are bound
to be isolated areas of higher rainfall amounts given storms that
could produce high rain rates and strong moisture advection off
the Gulf with decent upper level support. But right now it looks
like the main axis of heavy rainfall will be more through C TX
into OK and E KS. This axis could shift depending on how the
trough and upper low evolve. The main point being that it looks
like a wet weekend and still have time to monitor forecast data.
With the weekend coming up, it would not hurt to prepare and have
back up plans in case of heavy rainfall.


PREV DISCUSSION... /ISSUED 342 PM CDT Thu Sep 22 2016/

Scattered precip will taper off by early evening with the loss of
heating. Expect it to regenerate off/near the coast during the
late night and morning hours Fri...then expand inland during the
day. Similar diurnal scenario will persist Saturday and probably
again Sunday...albeit with probably slightly more coverage each
consecutive day as moisture levels increase.

Forecast from Sunday night into the middle of next week becomes
more problematic in regards to the overall pattern which would
impact local frontal timing (if any), temps & precip. Not going
dive into the specifics regarding different model
solutions/scenarios that far out as they`ll change again in the
next set of runs.

In general, rain chances will increase late in the weekend into
early next week as an upper trof & surface front nudge closer to
the region. When and where best coverage (and potentially heavier
amounts) occur will be dependent on positioning of the trigger and
focusing mechanisms described above.

Overall forecast confidence is low beyond Sunday. Grids mainly
consist of a blend weighted with a higher dose of the slower &
warmer ECMWF soln. 47

Lee cyclogenesis will allow SE winds to persist through Friday. The
low pressure system over Colorado will then move N-NE and drag a
cold front into North Texas on Sunday night. There is still some
uncertainty with regard to the frontal position on Monday. With the
upper flow parallel to the surface front, prefer to follow the lead
of the previous fcstr and lean toward a slower fropa for early next
week. A weak surface low is expected to develop over the SW Gulf and
surface winds will probably become E-NE in response to these
systems. Tide levels will probably become slightly elevated early
next week in response to the E-NE winds. Deeper moisture over the
Gulf and disturbances rotating around abroad upper low over western
Mexico will keep showers and thunderstorms in the forecast through
early next week.

FWIW, both the GFS and the GEM bring the front through on Monday. 43


College Station (CLL)      93  75  93  76  91 /  20  30  40  10  40
Houston (IAH)              93  76  91  77  90 /  20  10  40  10  40
Galveston (GLS)            89  81  90  82  88 /  20  40  40  30  40




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