Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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000
FXUS64 KHGX 020353
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
953 PM CST Thu Dec 1 2016

.DISCUSSION...
Evening forecast update is really just that, an update to ongoing
temperature/dewpoint trends and sky conditions. Temperatures are
falling into the 40s north of Houston but are slow to fall along
the coast. E/SE winds should bring back moisture from the Gulf
which will keep temperatures from decreasing too much with
radiational cooling.

Cloud cover should increase Friday morning with the onset of
precipitation during the afternoon. Rest of the forecast looks on
track. Previous discussion outlines latest forecast concerns and
confidence in the event. New 00z guidance will be evaluated for
any changes to the forecast and hopefully be able to highlight
where any heavy rainbands set up and when. Rainfall totals have
been increased and looking more probable for a flood watch
sometime during the weekend.

Overpeck

&&

.PREV DISCUSSION... /ISSUED 623 PM CST Thu Dec 1 2016/

..Locally Heavy Rain Possible This Weekend...

DISCUSSION...
High pressure over central Louisiana will continue to move east
tonight. This will allow east winds to continue and and maintain a
slight increase in low level moisture. Overnight low temperatures
will be warmer than last night as clouds gradually increase from
the southwest. Consider this the calm before the storm.

On Friday...moisture levels really begin to ramp up and PW values
are progged to increase to 1.50 inches by afternoon. The increase
in moisture coupled with a weak disturbance will give the area a
chance of precipitation by afternoon. This area of rain will
expand north and increase in intensity toward evening as 925 mb
winds increase. The 925-850 flow increases to 30 knots Fri night
into Saturday and forecast soundings become super saturated Friday
night into Saturday with PW values exceeding 1.70 inches. Have
added locally heavy rain to the grids for Saturday and a Flash
Flood Watch will probably be required at some point over the
weekend and possibly as early as Saturday. The Hi-Res WRF ARW and
NMM are showing some impressive rainfall totals right along the
coast Friday night into Saturday. Quite possible that a tiered or
staggered flash flood watch will be needed with the coastal
counties first and then expanding as rain falls through the
weekend. Looking at the synoptic model output, it looks like after
Friday night`s rain event more rainfall could develop inland with
isentropic lift and quite likely elevated convection. This could
set up another band of heavy rainfall between Houston and
Huntsville. This means there could be 2 bands of rainfall that
exceed 6 inches of rainfall for the event. The hard part is that
these bands could shift in location a few miles north/south which
could put one band over Houston or just north. The other band
could be just along the coast or far enough off the coast that
coastal areas only get a couple of inches of rain. Regardless
there is enough confidence that heavy rainfall could be more of a
issue for a much larger area than previously thought. And it could
get worse on Sunday as generally moderate rain continues but
falls with rather saturated grounds by this point.

Sunday night and Monday there is a growing concern for severe
weather. While WPC has a good portion of our area in a slight risk
for excessive rainfall, we do not want to neglect the severe
weather threat Sunday night into Monday morning. Deep upper low
over Mexico will begin to swing NE over Texas during this time. It
is expected to be shearing out and weakening as it does but it
will still have a negative tilt with a very strong jet. The higher
severe weather threat that SPC highlights on day 5 is east of the
area but the threat could develop sooner and back to the west more
early Monday morning. It would then quickly move east through the
day Monday. From 06Z to 12Z Monday the GFS is showing a good axis
of 1.8 inch precipitable water, 500-1000 J/kg of CAPE, 20-25 kts
of 0-1km shear, and 60-70 kts of 0-6km shear. Shear parameters are
really high and could overcome the lack of instability. This
should be a typical high shear/low CAPE type of event, but the
really strong upper level ascent may be just enough out of phase
that updrafts may struggle if there is not enough instability. By
12Z Monday the main PVA is still over the Rio Grande and does not
reach SE TX until 18Z Monday. What is troublesome is that this is
still a highly dynamic system with a strong 80-100kt jet core that
will support surface cyclogenesis along the warm front over SE
Texas. Warm sector very well could push inland giving what
instability there is a chance to be realized and yield rotating
updrafts. The threat here looks to be tornadoes and damaging
winds with a much higher threat shifting NE through the day into
Louisiana.

Finally it is looking more likely we will have a solid arctic
blast of cold air with a cold front pushing through Wednesday
afternoon next week. A 1040mb ridge settles over the Texas Big
Country by Thursday morning and 850mb temps drop to around
freezing for north of I-10. Friday morning could potentially be
one of the first strong freezes but model data not quite
supporting that yet. Certainly something else to watch in this
busy forecast cycle.

Apologies for a delayed AFD as we had to troubleshoot some system
failures.

43/39

AVIATION.../00Z TAF/
Despite some mid to high clouds working across, particularly
towards the coast, the expectation across the area for most of the
forecast period is for VFR conditions and diminishing easterly
winds tonight. Winds will begin to pick up tomorrow, with clouds
moving in from the west in the morning. Rain chances will begin to
slide in from the southwest late, but have only included a mention
of SHRA up to IAH. Of course, any changes in timing may impact
this far end of the period in future cycles.

Luchs

MARINE...
Forecast still on track for a warm front/coastal trough to setup
along the Upper Texas Coast tomorrow and over the weekend. This
will increase east winds above advisory criteria so small craft
advisories have been issued for tomorrow and will likely need to
be extended through the weekend. Strong winds and rough seas will
persist through Sunday night. A surface low may form on the
boundary Sunday night into Monday which will have winds southerly
until mid morning and then shift to the west/northwest for the
afternoon as a front pushes offshore. There will be a quick
reprieve in advisory conditions before a strong cold front pushes
off the coast next Wednesday night.

Tides...With a strong and increasing easterly fetch of winds,
above normal tides are expected over the weekend. Right now Gulf
facing beaches may experience tides 1-2 feet above normal late
Friday through Saturday morning. With astronomical tides already
reaching 2 feet above MLLW possible that tides could be 3.7-3.9
feet above MLLW or approaching 1 ft above HAT. A coastal flood
watch has been issued for these higher tides and a coastal flood
warning or advisory may be needed.

39/25

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      46  65  52  56  49 /  10  40  70  90  80
Houston (IAH)              48  66  55  61  57 /  10  50  70  90  80
Galveston (GLS)            61  69  64  68  66 /  10  70  70  90  80

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Coastal Flood Watch from Friday afternoon through late Friday
     night for the following zones: Brazoria...Chambers...
     Galveston...Harris.

GM...Small Craft Advisory from 3 PM Friday to 6 AM CST Saturday for
     the following zones: Galveston Bay...Matagorda Bay.

     SMALL CRAFT SHOULD EXERCISE CAUTION through Friday afternoon for
     the following zones: Galveston Bay...Matagorda Bay.

     Small Craft Advisory from 9 AM Friday to 6 AM CST Saturday 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.

     SMALL CRAFT SHOULD EXERCISE CAUTION until 9 AM CST Friday 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



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