Area Forecast Discussion
Issued by NWS Houston/Galveston, TX
FXUS64 KHGX 230301
Area Forecast Discussion
National Weather Service Houston/Galveston TX
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.
Still a few pesky showers around north and west, as well as some
winds up around 10 kts to start off the forecast period, but both
should wrap up pretty early in the evening. After that, clearish
skies and lighter winds should prevail for much of the night.
Towards morning, sites north look to see light fog and/or some
lower clouds. For now, chose to go with some MVFR visibilities,
and some clouds, but no ceilings at MVFR levels. Closer to the
coast, nocturnal marine showers should push onshore and gradually
stream inland into the afternoon. Angled towards VCSH, and kept
VFR conditions at SGR and LBX, but there are some second thoughts
about that with some potential for similar fog/low clouds.
Generally soft-pedaling the guidance here, and banking on a
transition from last night being largely VFR through the night
towards better potential this weekend.
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
.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL) 75 93 76 91 75 / 30 40 10 40 20
Houston (IAH) 76 91 77 90 76 / 10 40 10 40 30
Galveston (GLS) 81 90 82 88 81 / 40 40 30 40 40