Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 241510

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1010 AM CDT Wed Aug 24 2016

A couple of showers have managed to spring up over the offshore
waters this morning. Isolated showers and thunderstorms are
possible today, although strong capping will probably prevent
convection from developing inland until later in the afternoon.
Heat will be a concern today with highs in the low/mid 90s and
heat indices in the 100-107 degree range. No significant changes
were made to the previous forecast.

As for the tropics, nothing substantial has changed with Invest
99L since the last forecast package. There is an Air Force
Reserve Hurricane Hunter aircraft currently conducting a flight
into 99L, and we will have to wait and see if they are able to
find a closed circulation. For now, the National Hurricane Center
has given 99L an 80% chance of development over the next 5 days.




Low clouds have stayed mainly to the west of area TAF sites this
morning. Could still get a brief window of MVFR cigs but conds
will remain mainly VFR. Daytime heating will trigger scattered
shra/tsra later today but 00z CRP/LCH soundings show a decrease
in moisture from yesterday morning so would expect slightly less
coverage compared to yesterday. Latest RAP/HRRR show some
potential for shra between 18-22z. Some cirrus overnight with
clouds increasing on Thursday as moisture levels deepen
significantly. Showers possible along the coast by the end of the
TAF cycle. 43


Southeast Texas remains under the influence of an expansive upper
ridge centered over Louisiana/Mississippi early this morning, with
00z radiosonde observations reporting very little change in the
overall strength of this feature as mid-level heights have (597
decameters) remained relatively static since the 12Z observations
yesterday. Early morning water vapor imagery also shows a compact
shortwave trough lifting out of the Four Corner region and across
New Mexico/Colorado. At the surface, temperatures as of 3 AM CDT
ranged from the upper 70s inland to mid 80s along the coast with
patchy fog beginning to develop in a few sheltered locations. A
few showers were observed on area radars over the coastal waters,
with little activity occurring farther inland.

For today, expect patchy fog to persist through mid-morning
before mixing out with daytime heating. The shortwave trough
lifting across the Rockies this morning will continue to across
the Central Plains today, nudging the Gulf Coast ridge farther
east and north. Locally, this will result in a slightly weaker
ridge by the end of the day (mid level heights decreasing 1-2
decameters) but upper ridging over the region today will maintain
enough influence to result in another hot day. With surface
moisture from recent rainfall and afternoon temperatures in the
upper 80s to mid 90s, elevated heat index values in the 100 to 107
range will be possible across the region. Similar to yesterday,
isolated showers and thunderstorms will be possible across most of
the region primarily during the afternoon and early evening hours.
Marine areas may see isolated showers throughout the day along an
axis of deeper moisture extending from the Upper Texas coastal
waters east towards Louisiana.

Any showers or thunderstorms that are able to develop will quickly
dissipate with loss of heating this evening, with another dry
night expected for Southeast Texas. As the upper ridge lifts
northeast of the region today and tonight, an associated surface
high over the southeastern US will also slide away from the region
resulting in the establishment of easterly surface winds. This
will allow for tropical moisture located over the northeastern
Gulf to be advected towards the region, and this returning
moisture will first be noticed by an increase in nocturnal showers
across the coastal waters Thursday morning.

Rain chances will increase for the region during the day on
Thursday and Friday as the displacement of the upper ridge will
allow for a series of small disturbances to rotate west underneath
it. Forecast precipitable water values surge into the 2-2.2 inch
range during this time and scattered showers and thunderstorms are
expected across the region as these disturbances translate across
the area. While forecast rain totals are not terribly high for
this time period at 1 inch or less, the very high atmospheric
moisture content coupled with relatively slow forecast storm
motions (850-300 MB winds around 5-10 knots) will create a concern
for locally heavy rain. Otherwise, increasing cloud cover and
rain chances will keep temperatures on Thursday and Friday a few
degrees cooler than today.

By this weekend, a stronger disturbance moves under the upper
ridge as it stretches from the Southern Plains to the Mid Atlantic
coast and nearly stalls over the region from the influence of the
broad, flat ridge to its north. This will result in a continuation
of rain chances heading into the weekend, with periods of showers
and thunderstorms across the region.

Have low confidence with the far extended portion of the forecast
(Monday/Tuesday) with considerable divergence amongst medium
range model guidance. The ECMWF shears out the disturbance over
the region, while the GFS hangs onto it before pushing it into
Mexico. Given this spread, have highlighted low rain chances with
temperatures near climatology for this portion of the forecast.


High pressure over the northern Gulf and lower pressures over the
central plains will maintain a light to occasionally moderate
onshore flow. The high elongates E-W and retreats to the east and
surface winds will become more easterly. An east wind will prevail
through the weekend but speeds will generally remain between 10 and
15 knots. The prolonged easterly fetch should produce slightly
elevated tides over the weekend.

An area of low pressure might enter the Gulf of Mexico early next
week. Although there is a great deal of uncertainty with regard to
the movement and intensity of this feature, mariners should be
prepared for possibly stronger winds and higher seas beginning as
early as next Tuesday. Mariners are urged to keep up the latest
forecast. 43

A broad area of low pressure and associated tropical wave in the
Caribbean (Invest 99L) continues to be monitored by the National
Hurricane Center for potential tropical cyclone development early
this morning. This system is expected to continue to move west-
northwest over the next few days, with conditions becoming
increasingly favorable for development as it approaches the
Bahamas later this week. Model guidance continues to offer a wide
range on solutions for the intensity and track of any cyclone that
develops from Invest 99L. If it does move into the Gulf, the
strength and placement of the aforementioned upper ridge over the
southeastern US as well as the strength, speed, and track any
potential upper troughs moving across the CONUS from the west will
play big roles in steering where Invest 99L ultimately goes.
Characteristics of these upper features can be difficult to
resolve 7-10 days out and it is important to not fixate on any one
model solution as a guarantee of what will happen to Invest 99L.

The bottom line is that late August through early October is the
climatological peak of the hurricane season and Southeast Texans
should make sure they keep up with the forecast and hurricane
preparedness regardless of whether or not a system is in or
approaching the Gulf of Mexico.



College Station (CLL)      93  75  91  74  89 /  10  10  30  20  50
Houston (IAH)              93  76  91  75  89 /  20  10  50  30  50
Galveston (GLS)            91  81  87  78  86 /  10  10  40  50  50




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