Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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000
FXUS64 KHGX 232103
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
403 PM CDT Thu Mar 23 2017

.DISCUSSION...
Temperatures this afternoon are back in the mid 80s or about 10
degrees above normal for this time of year. Dewpoints have mixed
out heavily from the 60s to the 40s in a few areas with gusty
southerly winds. Upper level analysis has upper level ridging over
much of Texas as expected with the strong upper level trough over
the southern Rockies. Water vapor imagery and visible for that
matter show cirrus moving over the area with anticyclonic
curvature due to the ridge.

Tonight southerly winds will remain around 10-15 mph so do not
expect any fog to develop. LLJ of 50kts at 850mb should allow for
at least some mixing and sustain higher winds overnight. Winds of
40-50kts at 850mb will continue tomorrow and veer to the SW during
the day. Look for surface winds of 15 to 25 mph and gusts over 30
mph at times.

By 18Z Friday a fairly stacked low will move into the TX/OK
Panhandles with a surface dryline extending from central Oklahoma
into central Texas. Dryline will act more like a Pacific front
which by Saturday may reach the Upper Texas Coast before stalling.
The boundary however will be the main focus for linear convection
in the form of a broken squall line or QLCS. All high resolution
model guidance have fairly consistent timing with the line of
storms pushing into SE Texas after 17-18Z Friday and push across
through 03-05Z Saturday (12PM Fri to 12AM Sat). Line of storms
could affect Houston during the evening hours from 21Z to 01Z
(4PM to 8PM Friday). Main hazards will be damaging straight line
winds and lightning, brief tornado along the line and then hail.

Overall there is good confidence in some type of line of storms
moving through the area, but low confidence in the
intensity/severity. Synoptic models still show a good trailing
vorticity max moving across the northern half of the area from 21Z
Friday to 00Z Saturday. The vorticity does become channelized
minimizing lift from any PVA farther south of I-10. Greatest
height falls and cooling aloft should be sticks to north of the
area but eventually pushes south by 06Z Saturday. By this time all
strong PVA shifts over Arkansas/Louisiana. What this means is
that the cap may be slow to erode over SE Texas and by the time it
does, there may not be any forcing for convection. Instability is
another question as models try to generate 1000-2000 J/kg of CAPE
but there is some concern that the quality of moisture may not
support surface dewpoints in the mid/upper 60s or enough to
support the instability. Low level moisture mixed out quite a bit
today and even dewpoints in the Gulf are in the mid 60s. The
severe weather threat comes from strong deep layer shear, 50-60kts
of 0-6km shear and low level shear of 20 knots. This will be more
than enough to organize severe convection like a QLCS with a
possible mesovortex along the line. Overall thinking is that the
cap will hold along the coast but remain weaker north of Houston.
Most likely areas for severe weather will be north of a Brenham
to Cleveland line. This should be where severe threat will be
greatest. Farther north instability will be weaker since moisture
return may not be quite as strong as progged. Regardless, there
are still several issues that make forecasting the threat for
severe weather challenging.

Another trough is expected to push through north Texas late
Sunday into Monday. There may be a few storms for extreme northern
areas of the forecast area but think the main threat for any
severe weather will be northeast of the area towards the Arklatex
and Louisiana. Lastly the active pattern continues for the middle
of next week. Models differ on timing but looks like
Wednesday/Thursday will be another time frame to monitor for
thunderstorm activity.

Overpeck

&&

.MARINE...
Not a lot of change with the previous thinking with regard to the up-
coming forecast. Light/moderate onshore winds prevailing so far this
afternoon, but will be expecting increased winds (and then seas) for
later tonight in response to the tightening gradient (via the deepen-
ing low moving into the western Central Plains). An SCEC has already
been issued for offshore waters for this afternoon...and have gone a-
head and posted Caution/Advisory flags for tonight/early tomorrow.

Still keeping with a slight W/NW wind shift for early Sat morning in
the wake of the weak front for mainly the bays/nearshore waters, but
this will be short-lived as onshore winds return areawide later that
afternoon. These winds should pick up once again late Sun as another
system moving off the Rockies begins to develop over the Plains. Not
sure we will see as much gradient tightening with this system compar-
ed to the one tomorrow, but cannot rule out SCEC for this time frame
at this time (Sun/Sun night). The continuation of this v progressive
pattern into next week could bring more active weather/elevated wind
and seas next Tues night-Wed night. 41

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      66  79  57  81  58 /  10  80  30  10  10
Houston (IAH)              68  81  65  83  60 /  10  50  60  10  10
Galveston (GLS)            70  79  71  80  68 /  10  20  50  20  10

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...SMALL CRAFT SHOULD EXERCISE CAUTION from 10 PM CDT this evening
     through Friday afternoon 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.

     Small Craft Advisory from 10 PM this evening to 1 PM CDT Friday
     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.

     SMALL CRAFT SHOULD EXERCISE CAUTION until 10 PM CDT this evening
     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.

&&

$$

Discussion...39
Aviation/Marine...41



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