Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 230157
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
857 PM CDT Wed Mar 22 2017

.DISCUSSION...
Still looking for increasing clouds and fog chances overnight and
a continuation of above normal overnight low temperatures. Do not
need to make much of a change to the current forecast package. 42
&&

.PREV DISCUSSION... /ISSUED 736 PM CDT Wed Mar 22 2017/

AVIATION...
Evening VFR with an areawide lowering into MVFR between 07Z
through 11Z Thursday morning...primarily low stratus per a near
5-10 knot onshore wind within a saturating lower 2k ft layer.
Where interior surface winds fall off to under 5 knots...expect
fog to be mixed in with the stratus. Similar conditions as this
past morning...patchy around many terminals with locally dense
over more rural hubs around and an hour or so past sunrise.
Improvement to MVFR through 15Z...VFR by 18Z. A tightened onshore
pressure gradient...with a strengthening low level jet...per
lowering western Texas pressures will pick up afternoon surface
southerlies across more southern and western county TAF sites. 31

PREV DISCUSSION... /ISSUED 342 PM CDT Wed Mar 22 2017/

DISCUSSION...
Temperatures this afternoon have reached the low/mid 80s for a
nice day in May, I mean March. Dewpoints range from the 50s to
low/mid 60s along the coast so it does not feel completely
horrible outside with the humidity. Winds should decrease enough
overnight to allow for more fog Thursday morning. Thursday
forecast should be a repeat of today with perhaps stronger surface
winds as pressures fall in the lee of the Rockies due to the next
approaching upper level trough.

A developing upper level trough is quite evident on water vapor
imagery and upper level analysis just off the coast of California.
Downstream of it over the S Rockies and Texas, upper level
ridging is keeping conditions warm and dry. This ridge should move
over the region tomorrow as the trough moves east into the Desert
SW.

Friday is a day to watch for severe weather. SPC day 3 outlook as
a large area of slight risk over much of SE Texas, primarily
north of I-10. Overall this looks reasonable but think 12Z
forecast data supports more of a threat over NE Texas into the
Arklatex Friday afternoon. Closer to SE Texas, think the threat
for severe weather will be north of Houston mainly from Brenham to
Cleveland northward for Friday afternoon/evening. The thinking
for a more northern severe weather threat is twofold. First this
looks to be where the best positive vorticity advection will occur
which will help erode the cap. The elevated mixed layer should
advect over the region from the SW albeit not quite as strong as
it could be. That said since surface dewpoints may only be in the
mid 60s which could limit instability. Which is the second reason
for a more northern threat area. Instability will be limited but
likely form along the dry line under the best upper level forcing.
This will be where surface based convection will have the best
chance to realize CAPE and support organized convection. Still the
lack of overall instability and upper level forcing along with
capping south of this line will be limiting factors for severe
weather.

Factors that do support severe weather will be the PVA north of
Houston and then very strong shear both 0-6km bulk shear and 0-1km
bulk shear. Model soundings support increasing shear through the
day at all levels which should support deep convection if those
updrafts can tap and realize every bit of CAPE. With the dryline
approaching from the west, there should be a line of storms,
likely a squall line or QLCS, that moves into the area during the
afternoon. The most recent 12Z Texas Tech WRF-ARW supports this
idea quite well. Peak heating will help instability but still may
not be enough if there is not quality low level moisture. As the
QLCS moves into better shear, possible to have a straight line
winds and tornado threat. Hail will be secondary and heavy
rainfall may be brief since storm motions will be high. Activity
should move east Friday evening and out of SE Texas by midnight.

Active upper level pattern looks to continue based on latest
GFS/ECMWF/Canadian solutions. The dryline/pacific front will
likely stall across the area Saturday and push back north on
Sunday. By Monday another approaching trough will move into the
southern Plains Sunday night into Monday. All 3 models have very
different solutions so not going all that high with rain chances.
Rain chances of 20/30 PoPs look reasonable for now. This again may
be a scenario for severe weather but more likely well north of
the area. Model solutions continue to be very different for the
middle of next week but next Wednesday/Thursday could be active
with thunderstorm activity. 39

AVIATION...
Only a few tweaks to the 18Z TAFS as most of the main ingredients
re- main in place through tomorrow. Did go with MVFR CIGS/VIS
tonight at most sites given the lighter winds. We should see
conditions improve by mid morning as winds begin to mix down once
again. 41

MARINE...
Not a lot of changes with the overall forecast through tomorrow with
a generally light/moderate onshore flow remaining in place. However,
we should be seeing some changes tomorrow night as the pressure
gradient begins to tighten in response to the next system deepening
over western Plains. SCA/SCEC flags may be needed at this time
across the coastal waters. As this low moves east on Sat, it will
be dragging a cold front into the state. Models are keeping this
boundary over the inland portions of SE TX and we will likely
maintain S/SE winds over the marine areas over the weekend.
Another storm system (perhaps not as strong) is progged to
develop/move off the Rockies into the Central Plains late Sun/Mon.
Increased wind speeds in response may prompt SCA/SCEC flags once
again at this time. 41

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      64  85  66  79  57 /  10   0  10  90  20
Houston (IAH)              64  84  68  80  66 /  10   0  10  70  60
Galveston (GLS)            68  79  70  79  70 /  10   0  10  50  50

&&

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

$$

Discussion...42
Aviation/Marine...31



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