Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 161722
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1222 PM CDT Sun Jul 16 2017

.AVIATION...
A bit calmer than 24 hrs ago with two areas of convection, the
first of which is affecting the immediate coastline and KGLS and
KLBX. This area of precip is on the periphery of a very weak low
level circulation off the middle coast. A second area of
convection is well to the north and is moving SW and could affect
KCLL and KUTS later this afternoon. Most of the short term
guidance keeps this area of storms north of the Houston terminals.
Daytime heating could still trigger some isolated convection near
both KHOU and KIAH later today. Better rain chances are expected
on Monday with deeper mstr and the area will lie on the east side
of a weak inverted trough. Winds will remain light through the TAF
cycle except in and near storms. 43

&&

.PREV DISCUSSION... /ISSUED 1120 AM CDT Sun Jul 16 2017/

UPDATE...
All of this morning`s shower and storm activity has been focused
around western Gulf lower pressure. Ample early day sun to the
surface will likely allow for the inland development of afternoon
precipitation. Highest rainfall amounts should remain along and
south of the I-10 corridor with widely-scattered latter day
activity blossoming further north. A general height weakness...or
upper shear zone...positioned across the northern Gulf coastline
may not be an actual inverted trough but just enough to initiate
diurnal showers and thunderstorms...forming over the local waters
and then transitioning inland through the daytime hours. An unstable
profile by late morning as surface temperatures achieve the middle
80s with greater than 2 inch PWATs will keep at least 30-40 daytime
POPs in place over the next several days. Diurnal temperatures
should parallel the climo curve with generally no more than a
three degree fluctuation. The only caveats will be possible early
morning patchy fog development over more open (rural) expanses
and maximum afternoon heat indices briefly touching 103 to 107 F
from around 2 through 5 PM. 31

PREV DISCUSSION... /ISSUED 410 AM CDT Sun Jul 16 2017/

DISCUSSION...
The 3AM observations across SE TX are very different from 24 hours
ago. Temperatures are a rain cooled low/mid 70s with Galveston the
warm spot at 78F. There is still no wind but the rain cooled air
has allowed for fog to develop across the area. The good thing is
that you can walk outside the office and not suffocate from the
moisture. Radar shows showers and a few thunderstorms off the
Upper Texas Coast where the airmass remains moist and unstable.

Upper level analysis at 500mb still shows upper level ridge over
the Rockies with troughing through the Great Lakes. Water vapor
imagery shows an elongated shear axis and weakness in the ridge
over the Miss River Valley stretching back towards the Texas Gulf
coast. Yesterday`s vorticity maximum that was over the Mid-South
ended up being more potent than forecast and continues to linger
over the Texas Gulf coast. This feature may still play a role in
today`s forecast but lift may not be as strong.

GOES 16 precipitable water derived imagery shows a broad area of 2
inches or more and the LCH 00Z sounding had 2.2 inches of
precipitable water. Based on the 00Z LCH sounding and model
soundings across the area, convective temperatures should again be
in the low/mid 80s with moist adiabatic lapse rates supporting
very talk and skinny CAPE. Yesterday`s vorticity maximum helped
provide stronger upper level wind so storm motions were to the SW
at a decent speed. While an easterly wind profile is expected,
wind speeds are 10-15 knots with downshear Corfidi vectors at 9
knots. Outflows may be the saving grace to keep storms moving
today and limit the flooding threat. Any storms that form today
will again produce high hourly rain rates and gusty winds. Given
these conditions, the forecast will go with 50 to 60 PoPs along
the coast and 40 PoP for the northern third of the area. Rainfall
amounts will again be in the 1 to 3 inch range but again not
everyone will observe those kinds of rain amounts.

Monday and Tuesday look for the moist airmass to linger over the
region support more thunderstorm chances. The upper level ridge
does build over the Plains over this time but think any subsidence
will remain north of the region. There may also be another piece
of vorticity to move over the region in this time frame to aid in
thunderstorm activity.

Precipitable water values finally drop below 2 inches Wednesday
through the rest of the work week but there still may be enough
moisture to support isolated thunderstorm coverage. The upper
level ridge does expand into the region for the end of the week so
the models may be over doing rain chances for the end of the week.
Still there enough confidence to keep 20-30 PoPs in the forecast.
For the end of the week Thursday into Friday there is a piece of
vorticity that shears off along the Atlantic coast and retrogrades
under the ridge across the northern Gulf through Saturday. So
higher rain chances may be needed Saturday should this system
garner a bit more moisture and if the ridge remains north or
northeast of the area.

Overpeck

MARINE...
As is pretty typical for the summertime, high pressure will be in
control over the eastern Gulf with lower pressure over West Texas
will maintain generally onshore winds for the next 5 to 7 days.
Since the pressure gradient is pretty slack and the wind field
quite light right now, outflow boundaries and land breezes may
confuse the wind field significantly. More consistent onshore flow
should become established deeper into the week as the pressure
gradient tightens up some. Periods of scattered showers and
thunderstorms may be expected across the waters much of the week.
Seas should remain around 1-2 feet through the weekend, with
locally higher wind and waves in and near showers and storms.
Later in the week, seas should build some as winds pick up mildly,
but don`t look for a significant increase.

Luchs&&

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      98  75  93  75  94 /  30  30  50  20  40
Houston (IAH)              93  75  91  76  92 /  50  30  60  20  50
Galveston (GLS)            88  81  88  81  89 /  50  40  50  40  60

&&

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

$$

Discussion...31
Aviation/Marine...43



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