Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 300114
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
814 PM CDT SUN MAY 29 2016

.UPDATE...
Quick forecast update to extend mention of isolated showers and
thunderstorms through the early evening hours based on latest
radar trends. Shallow convection continues to develop this evening
as a result of diurnal heating, but with sunset rapidly
approaching expect limited additional development with ongoing
activity to begin dissipating. While the atmospheric column is
relatively dry compared to the past few days (around 1.5" per GPS
MET data), this activity may be capable of producing a quick 0.5"
of rainfall as it drifts north.

Also continuing to monitor convection developing over portions of
Northwest Texas this evening along what appears to be an outflow
boundary. HRRR/TTU-WRF (which seem to have initialized the best
with this activity) show these storms growing upscale into a
thunderstorm complex that may clip far western portions of the
region as forward propagation vectors advertise continued movement
towards the southeast. The main question will be how long this
complex is able to hold together with loss of daytime heating, but
with temperatures behind the line in the mid 50s over the
Panhandle think the cold pool is strong enough to sustain the
complex into West Central Texas (possibly into Central Texas).
Similar to last night, have added low (15 PoPs) to the western and
northwestern counties to account for this possibility of some
lighter rain associated with this system reaching the area.
Otherwise, retrended temperatures and dew points based on
observations with overnight lows expected to fall into the upper
60s to mid 70s.

Huffman


&&

.PREV DISCUSSION... /ISSUED /

AVIATION...
VFR conditions will prevail across area TAF sites through 08-09z
before some MVFR ceilings try to develop. There is quite a bit of
cirrus approaching from the west and still not sure if this will
impede the development of low clouds/fog later tonight. SREF
ensemble guidance is very supportive of fog development over the
northern TAF sites after 09z but the SREF was way too aggressive
last night. Also removed the mention of vcsh for tomorrow as
probability of occurrence looks too low for a mention at this
time. That said, PW values reach 1.60 inches between 18-21z so an
isolated shower not out of the question. VFR conditions expected
Monday afternoon with a SE wind increasing to near 10 kts. 43

PREV DISCUSSION... /ISSUED /

DISCUSSION...
Visible satellite imagery this afternoon shows cloud cover
thinning across SE Texas with some cumulus/stratocumulus
development. Temperatures have reached mid/upper 80s once again
early this afternoon. There may be enough cloud cover to limit the
extent of heating today. An isolated shower or storm may still
form if bay/sea breeze can develop. Ridging aloft and cap may just
be enough to keep convection from forming. ACARS soundings show
weak capping around 800-700mb but has dry air over the moist
boundary layer. This has allowed for precipitable water values
around 1.4 to 1.5 inches across the area. Deep convection will
have a tough time forming just due to day time heating and without
any low level forcing. The same type of conditions are expected
Monday. Forecast will keep thunderstorm chances in the 10-20
percent range this afternoon through tomorrow.

Tuesday will be more of a transition day from more of a dry
pattern back to a wet pattern. Will SE Texas catch a break for
a while from heavy rainfall and flooding? It is not looking good.
Tuesday rain chances range from 20-30 percent with precipitable
water increasing above 1.5 inches and the upper level ridge
breaking down further. Best chances will be over the Brazos Valley
on Tuesday if the cap can weaken. GFS/NAM soundings seem to
suggest a weak cap but with lack of a low level boundary it may be
difficult for storms to develop. More likely scenario will be if
storms forming in west central Texas can move into the area. Even
that is a stretch for Tuesday/Wednesday.

Wednesday into Wednesday night an upper level trough over the S
Rockies moves towards the Big Bend of Texas. This helps increase
the jet over Texas and certainly provides a more favorable
environment for thunderstorm activity. Instability will be
greatest over the area Wednesday into Thursday with CAPE 2000-3000
J/kg. Deep layer shear will be increasing but remaining around 30
knots. Precipitable water increases to around 1.7 to 2.0 inches
Wednesday and more widespread 1.8-2.0 inches of precipitable water
over the area Thursday into Friday. Upper level trough slowly
moves east through the end of the week. GFS/ECMWF/Canadian all
have a broad upper level low over Texas Thursday/Friday which will
be slow moving. Given high moisture expected, upper level low and
for good measure a weak surface trough boundary in the area
Friday, there will be the potential for more heavy rainfall. Any
severe weather looks to be more on Wednesday into Thursday and
most likely a marginal threat since shear will support
multicellular convection. Hail and downburst winds will be the
main threats in addition to the heavy rainfall threat. At this
point it is still hard to pinpoint how much rain will fall over
the area but possibly another 1 to 2 inches for the Wednesday to
Friday time frame. Any additional rainfall will prolong river
flooding. River levels will likely not go up with additional
rainfall except for fast responding basins like Harris County
bayous. Rivers will just take longer to recede with additional
rainfall. Upper level trough axis will remain over SE Texas
through Mississippi River valley so likely hold onto thunderstorm
chances through the weekend.

Overpeck

MARINE...
A fairly quiet forecast for the upper TX coastal waters these next
few days as high pressure holds over the region. Light/moderate on-
shore winds will prevail with sea and tides remaining low. Changes
possible starting mid week on into the end of the week with as the
next storm system begins to move into the state. A slight increase
with wind speeds possible Weds/Thur with the approach of disturban-
ces from the west which will help to tighten the pressure gradient
over the near/offshore waters. Increased storm coverage could also
make for elevated winds/seas in/near these storms. Extended models
still hinting at the possibility of a cold front by the end of the
week...but have been a little less enthusiastic about it with this
latest run. Will be interesting to see how later runs resolve this
(but thinking FROPA less likely given the time of year). 41

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      70  87  70  85  71 /  20  20  10  30  30
Houston (IAH)              70  87  71  88  72 /  20  20  10  20  10
Galveston (GLS)            76  83  76  85  77 /  10  10  10  10  10

&&

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

$$

Discussion...14


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