Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 280456
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1156 PM CDT Tue Jun 27 2017

.AVIATION...
Ongoing marine convection is well offshore, so despite insistence
of short range models, will delay onset of showers. Wind obs do
show coastal convergence should set up near shore, so will
continue to bring in potential for showers late tonight near the
coast as well as the emergence of some MVFR ceilings. However,
trend in HRRR and TTU WRF does keep IAH a bit drier, so step back
down to VCSH there.

Coastward, have to make the call on PROB30 to
TEMPO/prevailing/nothing. Don`t have the confidence in hitting
with TEMPO or prevailing at this point - not so much the existence
of rain, but in the precise timing. So for now will step back to
VCs. VFR conditions should return around mid-day. By late
afternoon or early evening, the coastal trough should draw far
enough offshore and take convection with it for most to dry out.

Luchs

&&

.PREV DISCUSSION... /ISSUED 903 PM CDT Tue Jun 27 2017/

DISCUSSION...
Compared to the last couple of nights, the radar this evening is,
well, quiet. Most of the activity is out off the Upper Texas
Coast in the Gulf. Aloft water vapor imagery shows a short wave
shearing out over the area within a mean weakness in the upper
level flow with ridging over the Baja Peninsula. At the surface,
there is a weak pressure trough or coastal trough which has been
driving the shower activity in the Gulf. HRRR/RAP trends show this
activity increasing through the overnight hours and then pushing
inland. WPC has a marginal risk for excessive rainfall for
tomorrow morning due to the possibility of bands of storms pushing
inland. CAMs show most of the activity in the Gulf and then
pushing east towards LA through the day. Forecast was updated for
ongoing trends which show little activity but then PoPs increase
through the night in response to developing showers/storms with
the coastal trough. Most of the activity will remain south of I-10
but where stronger storms do form and with some training, there
is the potential for high rainfall rates hence the excessive
rainfall outlook. The 00Z CRP/LCH soundings still show 2 inches of
precipitable water, certainly enough for heavy rainfall. Outflows
coming from the storms will likely drive convection through the
morning hours into the afternoon. As has been the case lately,
convection will also likely be able to support a funnel cloud or
two and perhaps a waterspout for the coastal areas.

Overpeck

PREV DISCUSSION... /ISSUED 645 PM CDT Tue Jun 27 2017/

AVIATION...
Forecast period opens with a fairly straightforward big picture -
slowly diminishing storms for the northern sites, while the worked
over south stays drier. However, timing the specifics for CLL,
UTS, and CXO into this evening and how quickly things wind down
will be tricky, and may (but hopefully will not) require
amendments.

Otherwise, the next challenge will be the position of a potential
coastal trough late tonight into tomorrow for development of
showers and storms tomorrow. Kept continuity with the previous
forecast and brought VCSH as far north as IAH, but latest HRRR and
RAP have been playing coy about penetration of rain that far
inland. Any further offshore, and IAH and perhaps even SGR and
HOU will be drier. Otherwise, tried to highlight a short window
for best chances of storms before this trough gets dragged east
and activity winds down for the day.

Luchs

PREV DISCUSSION... /ISSUED 346 PM CDT Tue Jun 27 2017/

DISCUSSION...
Scattered showers and thunderstorms continue to spread across the
region this afternoon, with isolated amounts of nearly 2 to 2.5
inches having fallen across parts of the Houston metro so far
today. Expect this activity to linger through the evening hours,
weakening but not entirely dissipating as a shortwave trough over
South Central Texas translates across the region. A coastal low
along the Middle and Lower Texas coast is also expected to nudge
into the northwest Gulf as a result of the passage of this
feature, but short term guidance differs somewhat on the exact
trajectory this low will take... which will have a dramatic impact
on rain chances for the region.

Speed convergence along the trough axis is expected to result in
new development along the coastal trough overnight (aided by lift
from the approaching shortwave), with model solutions ranging from
scattered to numerous thunderstorms across the bottom two tiers
of counties with a more northeast trajectory to a dry overnight
forecast with a more easterly trajectory. Have trended with a more
northeast trajectory given greater consensus from 12Z guidance
and have maintained the mention of thunderstorms for areas south
of a Columbus to Cleveland line during the overnight period in the
forecast but confidence is low in just how far inland storms will
extend. What is particularly concerning about the northeast
trajectory is a few solutions (RAP/HRRR) have shown fairly
consistent run-to-run solutions highlighting a swath of greater
than 3 inch amounts across parts of Galveston, Brazoria, Chambers,
and possibly southeastern Harris counties early to late morning
Wednesday. These kinds of amounts exceed flash flood guidance, but
because of concerns about the actual trajectory of the trough
have held off on a Flash Flood Watch for now. However, residents
are advised to check road conditions before beginning their
morning commute should the more pessimistic RAP/HRRR solution
materialize.

Elevated atmospheric moisture levels (precipitable water values in
excess of 1.8-2 inches) and daytime heating will allow
thunderstorm activity to develop again inland on Wednesday, with
daytime inland thunderstorms and nighttime marine thunderstorms
persisting on Thursday and Friday. Highs through the end of the
week are expected to gradually warm from the mid to upper 80s
Wednesday to upper 80s to low 90s by Friday. Thunderstorms may
skirt the northern counties of Southeast Texas Friday night as an
upper trough rotates across the Great Plains, but dry and hot
conditions are expected this weekend as upper ridging builds in
from the west. A passing upper level disturbance may weaken the
ridge enough to allow for a few showers (or maybe even a storm)
to develop early next week... with heat index values in the
100-105 degree range persisting this weekend into the beginning of
next week.

Huffman

MARINE...
A coastal trough will develop tonight and east winds will
strengthen. The trough will help to focus numerous showers and
thunderstorms over the coastal waters tonight and mariners in
small craft should exercise caution tonight. Winds will diminish
on Wednesday as the coastal trough moves east. The pressure
gradient will tighten Wednesday night through Friday as low
pressure deepens in the lee of the Rockies and moves across the
central plains on Thursday. Onshore winds will persist through the
weekend but speeds will decrease as the gradient relaxes.

Tides will remain elevated through Wednesday as E-SE winds
prevail. Some minor coastal flooding will be possible on the
Bolivar peninsula. Rip currents could strengthen due to the strong
and persistent onshore flow.

43

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      71  90  74  91  77 /  10  30  10  30  10
Houston (IAH)              73  88  75  89  78 /  30  50  30  60  20
Galveston (GLS)            78  85  79  87  81 /  70  70  50  70  40

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...SMALL CRAFT SHOULD EXERCISE CAUTION until 5 AM CDT Wednesday for
     the following zones: Coastal waters from Freeport to the
     Matagorda Ship Channel out 20 NM...Coastal waters from High
     Island to Freeport out 20 NM...Waters from Freeport to the
     Matagorda Ship Channel from 20 to 60 NM...Waters from High
     Island to Freeport from 20 to 60 NM.

&&

$$

Discussion...39
Aviation/Marine...25


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