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Area forecast discussion 
National Weather Service Houston/Galveston Texas
1050 am CDT Friday Jul 31 2015

Update...
one note...after checking through the temperatures decided to up
the highs for today a bit to be more in line with the mav
guidance. Drier air filtering in from the northeast will likely
aid in a bit higher numbers.

40

&&

Previous discussion... /issued 1036 am CDT Friday Jul 31 2015/

Discussion...
a cold front was entering into the northeastern counties of the
forecast area at middle morning...with a prefrontal trough across
Galveston Bay through metropolitan Houston and into the cll area. Another
trough was out in the open waters of the Gulf. Isolated showers
have endeavored to develop over the inland areas south of the
prefrontal trough while scattered larger showers were over the
open waters of the Gulf east of that surface trough.

The high res short range models looked fairly similar for the
outlook today. Liked the hrrr which has isolated convection along
and southwest of a line from about cll to Houston to Galveston
late this morning through the early afternoon. As the front
continues moving southwestward...the rain chances will push more
over the southwestern counties during the late afternoon and early
evening.

The other issue of note will be the heat index. Values are still
forecast to reach advisory criteria over the coast during the late
afternoon period. The drier air behind the front will likely have
an impact over Chambers and maybe Galveston County. Since the
timing is uncertain at this time...will keep the heat advisory up
for the immediate coastal counties but will drop counties later
today if needed.

Tweaked the probability of precipitation and weather a bit to account for the latest radar
and model trends.

40

Previous discussion... /issued 414 am CDT Friday Jul 31 2015/

Discussion...
overnight convection over the Brazos Valley continues to diminish
as it exits the forecast area...with very warm temperatures
persisting across southeast Texas early this morning. Surface
analysis at 05z showed a plume of 75+ degree dew points draped
along the Texas coast and extending into northeast Louisiana.
This... combined with leftover clouds from last night/S storms...
was contributing to heat indices in the low to middle 90s as of 3 am
CDT at a few sites. A weak frontal boundary was also analyzed on
this map stretching North Texas and on into northern Louisiana.
This front will play a major role in how the weather evolves today
for the region... both in terms of thunderstorm chances as well as
concerns about heat.

The 00z Lake Charles sounding reported a precipitable water value
of 2.26 inches... with overnight GPS-met measurements across the
region showing a similarly moist airmass /2.1 to 2.3 inches around
the Houston metropolitan and along the coast/. High resolution guidance
is fairly consistent with developing scattered showers and
thunderstorms along the front as it slowly drops southwestward
today... but agreement is poor in the actual timing of this
feature and consequently the location of the greatest areal
coverage.

The GFS appears to be by far the most aggressive and sweeps
moisture away from the Interstate 45 corridor and points east by
18z. Comparison of surface Theta-E fields however /which represent
moisture and instability present/ reveal stabilization for the
eastern half of the region closer to 00z... with relatively good
agreement between both higher resolution and synoptic models on
this solution. Based on this... highlighting highest rain chances
west of the Interstate 45 corridor today along and ahead of the
anticipated frontal position. Additionally... a dry sub-cloud
layer and dcape values 1100-1300 j/kg this afternoon raises
concerns for at least an isolated gusty wind threat from stronger
thunderstorm activity.

A later frontal passage also means that stifling conditions will
persist ahead of the front today and have gone ahead and issued a
heat advisory for the coastal counties of Matagorda... Brazoria...
Galveston... and Chambers. Heat indices in these areas reached 109
to 111 degrees yesterday. With the anticipated slow progression
of the front today combined with naefs anomalies highlighting
anomalous temperatures again along the coast /a return interval
of approximately once every 10 years/... wanted to highlight the
increased potential for health impacts. If the front moves through
faster than anticipated however... drier air filtering in behind
the front will actually help lower heat index values and advisory
criteria may not be met.

For the remainder of the short term... the front is expected to
push off the coast tonight. Drier air filtering in behind the
front will help limit rain chances over all but the coastal waters
on Saturday. This slightly drier air may also allow for some
overnight lows in the upper 60s in the Piney Woods region... with
low 70s to near 80 possible elsewhere. High temperatures are not
expected to differ much from today on Saturday but a drier airmass
in place will help mitigate against higher heat index values.

The region will remain on the eastern periphery of the upper ridge
on Sunday and through the rest of the forecast period... with the
center of this feature shifting a bit around the Southern Plains
during this time. Low thunderstorm chances will continue in the
vicinity of the stalled frontal boundary with dry conditions and
above normal temperatures inland persisting into the middle of
next week.

Huffman

Marine...

Storms have departed the waters but more could coming late this
morning through evening. Winds will generally be variable northerly
to easterly across the Upper Texas coastal waters with an afternoon
seabreeze forming. Eventually a diffuse cold front sags out into the
Gulf waters late tonight and winds increase slightly. 850mb front
appears to stall well offshore and should become a focus for
redevelopment of showers and thunderstorms but probably just southeast of the
Upper Texas coastal waters. Will need to keep an eye on the remains
of the boundary Saturday afternoon-Sunday as the Gulf is plenty warm
enough and shear relaxes. Moisture surges back into the area late
Sunday and south to southeasterly flow returns and continues through
Friday.
45

Aviation...

VFR for now but may get a few spots with mifg around sunrise mainly
the more rural sites cxo northward. Challenging forecast for today.
Dry air should intrude into the region from the northeast but a few
of the models appear too quick to dry things out. L/v winds will be
the north until the diffuse frontal passage. Am thinking that cll-hou-sgr-lbx
may see some thunder close by this afternoon and will likely add
this in the 12z package if not sooner. Iah may be on the northern
fringe of where pooled moisture and sagging boundary interact so for
now will probably leave thunderstorms in the vicinity out but it will be close. Deep mixing
again today with light winds in the profile so storms should be slow
moving. Drying northeasterly to easterly flow should overspread
iah/hou by late afternoon/early evening leading to clear skies.
45

&&

Preliminary point temps/pops...
College Station (cll) 99 73 98 74 99 / 30 10 10 10 10
Houston (iah) 100 74 98 75 99 / 20 10 10 10 10
Galveston (gls) 95 80 94 81 92 / 30 10 10 10 10

&&

Hgx watches/warnings/advisories...
Texas...heat advisory until 7 PM CDT this evening for the following
zones: Brazoria...Chambers...Galveston...Matagorda.

GM...none.
&&

$$

Discussion...40
aviation/marine...33

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