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Area forecast discussion
National Weather Service Houston/Galveston Texas
826 PM CDT Thu Apr 27 2017

Discussion...
increasing clouds and strengthening southeast winds can be expected
overnight as high pressure continues moving off to the east and low
pressure deepens across northwest Texas. Some patchy fog might develop,
but think a majority of the area will just see the clouds. Not much
to change in the grids on the evening update as everything looks to
be on track. 42
&&

Previous discussion... /issued 645 PM CDT Thu Apr 27 2017/

Aviation...
deeper lower level moisture currently residing over the coastal
Bend will advect up within a regional southwesterly flow pattern
this evening. The lower 2k ft level will saturate up later this
evening...anticipating areawide MVFR ceilings by 10 or 11 PM lt.
Deepening moisture will likely have decks lowering to near...or
falling just below...IFR categories during the pre-dawn hours.
Lowering pressures over West Texas will maintain a tight enough
onshore gradient to maintain near 10 knot southerlies...strengthening
to between (sustained) 15-20 knots by tomorrow afternoon. These
winds should inhibit any significant early day fog formation.
Little change to low level moisture and a strong lower level
inversion will allow for very slow improvement to VFR tomorrow...
interior sites begin to lift and scatter out during the afternoon
hours. 31

Previous discussion... /issued 355 PM CDT Thu Apr 27 2017/

Short term...
today's nice weather with low humidity and mostly clear skies will
come to an end tonight and tomorrow. Southerly winds are already
developing across southeast Texas based on latest surface analysis. Deep
Gulf moisture is not far away and should return tonight. Short
term guidance shows temperatures falling after sunset but may
actually warm overnight as moisture return increases. Low
temperatures in the 60s will be common but even low 70s will be
possible along the coast. Patchy fog may also develop tonight with
the increase in moisture.

Long term...
main focus of the forecast update was on Saturday and Sunday with
a deep trough developing over the southern portions of the 4
corners region. Models show this upper low moving towards Texas
Panhandle and Southern Plains by 12z Sunday. GFS/NAM seem 3-6 hrs
faster with the trough than the ECMWF/CMC. A cold front associated
with the system should push into southeast Texas 00-03z Sunday, about 12
hours in advance of any strong positive vorticity advection or large scale ascent. Frontal
forcing may be enough to get a strong or severe storm to develop
late evening through the overnight. Models all maintain weak
capping if any which experience says the elevated mixed layer will
be stronger than forecast. Also very few models are actually
producing storms during the day Saturday suggesting the cap
holding. By Saturday night, instability should be decreasing with
loss of heating and without strong cooling aloft since upper
dynamics are out of phase with the surface forcing, the severe
threat may end up being more isolated. Storm Prediction Center has elevated/slight
risks for the area which if a storm forms in the environment, it would
likely become severe with hail/wind threats. Tornado threat will
be hampered by the lack of strong upper level winds which only
increase after 06z Sunday (out of phase upper trough finally
making its presence felt). This means deep layer shear may not be
strong enough to support rotating updrafts. Low level shear may
support some rotation but overall helicity values are only
borderline supportive of rotation. Storm Mode will be another
issue and expect linear convection to form along the front. There
might be a strong enough squall line to briefly support wind/hail
threats but the convection will be fighting lack of instability
and capping. There is just enough factors that support severe
weather that it deserves mention and no let our guard down.

Moisture will be increasing so there may be a locally heavy
rainfall threat. This appears to be more north of Houston where
there will better forcing. Also models show main moisture axis
shifting east Saturday night with only enough moisture convergence
along the front to increase precipitable water values. Precipitable water values
of 1.8-2 inches is nothing to sneeze at, but with storms expected
to be moving along a cold front the flood threat looks rather
small at this time for much of southeast Texas. Where storms can train
along the front as they move NE along it like in NE Texas into
Louisiana, flooding will be more likely. Overall looking at 1-2
inches of rainfall for the northern third of the forecast area and
less than an inch for much of the area.

The cold front pushes through off the coast Sunday morning which
will provide a good cool down for Sunday through Tuesday. Tuesday
southerly return flow ramps up again ahead of the next upper level
system crashing down through The Rockies into the plains. Models
show strong moisture return on Wednesday so increase rain chances
for the area. Cold front should push through Wednesday night into
Thursday. This will likely be the next time frame to monitor for
severe weather and perhaps heavy rainfall. 39

Marine...
winds over the coastal waters are continuing to shift to the east/southeast
this afternoon as the surface high moves further east. An onshore
flow is expected to be in place areawide this evening with speeds
picking up overnight. Have issued an scec for the offshore waters
for after midnight and this could be expanded to include the nearshore
waters by tomorrow morning. Small craft advisories might be needed
by tomorrow afternoon as the pressure gradient tightens in response
to deep low pressure over south Texas. The low will move NE and drag
another cold front across southeast Texas by early Sun morning. An abrupt
wind shift to the north-northwest is expected in the wake of the front and
another Small Craft Advisory will likely be required on sun. A light/moderate
onshore flow will prevail Mon/tues.

Will need to keep an eye on elevated water levels and risk of rip
currents going into the weekend. Tide levels could potentially exceed
3.5 feet around high tide early Sat morning. A coastal Flood Watch
or advisory may be required late Friday night into early Saturday.
41

&&

Preliminary point temps/pops...
College Station (cll) 68 91 75 87 57 / 10 10 20 30 80
Houston (iah) 70 90 77 85 64 / 10 10 20 30 80
Galveston (gls) 72 84 77 83 68 / 10 10 20 30 80

&&

Hgx watches/warnings/advisories...
Texas...none.
GM...small craft should exercise caution from 1 am CDT Friday through
Friday morning for the following zones: waters from
Freeport to the Matagorda Ship Channel from 20 to 60 nm...
waters from High Island to Freeport from 20 to 60 nm.

&&

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