Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 161037

Area Forecast Discussion
National Weather Service Houston/Galveston TX
437 AM CST Sat Dec 16 2017

The upper level trof currently situated over central Mexico will
kick out to the northeast and cross Texas tonight. As this occurs,
a surface coastal low is expected to develop just southwest of
Matagorda Bay and take a path parallel to the upper Texas coast.
Look for showers and embedded tstms to expand in areal coverage
from the southwest later this afternoon and evening...then exit
off to the east/northeast after 1 am.

Parameters for a good 0.75-1.5" of rainfall with localized higher
amounts will be in place (high PW`s, increased large scale lift,
llvl convergent zone, favorable upper jet position). Instability
will be lacking, so widespread severe wx is not expected - but
we`ll need to keep a close eye near the immediate coast. Significant
low level turning & shear is depicted in forecast soundings and the
northward extent of a tornadic/severe wx potential will be dependent
upon where the coastal low tracks. A track just off the coast
will keep the warm sector and potential for stronger surface based
storms offshore. A track a 10-20 miles further inland would bring
the risk onshore. Guidance is in a variety of camps...most keeping
the warm sector at the coast or just offshore. But there have
been some that take 64-68F dewpoints as far north as a Matagorda
Bay-south Harris Co-Liberty line. I went ahead and inserted the
mention of isolated severe in the forecast south of that line for a
narrow timeframe this evening.

Most areas will see a break in precip Sunday as this system departs.
But a lingering moisture axis offshore will begin moving back
inland Sunday night and Monday bringing increased chances of
precipitation back to the region (and sea fog at the coast). Most
all of the guidance has trended towards last nights ECMWF - continuing
precip Mon night, Tuesday and Tuesday night until the next upper
upper trof now digging southward thru California, eventually moves
across north Texas. Have bumped POPs up quite a bit in that time

The much talked about cold airmass is still penciled in to arrive
Thursday night and Friday. Overrunning precipitation is looking
like a decent possibility, but it`s still too early to pin down
temps, precip type, timing with any degree of confidence this far
out. 47


The generally light winds this morning will be veering/strengthening
this afternoon with the development of a strong coastal low pressure
system approaching from the middle TX coast. This system is expected
to produce scattered/numerous showers and thunderstorms later in the
afternoon through tonight, with a few of the stronger storms capable
of producing waterspouts through early Sunday morning. Have issued a
Small Craft Advisory starting this afternoon on into midnight.

A brief period of light offshore flow is expected behind this system
will then become onshore by Sunday night. The warmer/moist flow will
be moving across the cooler shelf waters...and should result in res-
ult in periods of sea fog through Tues (ahead of the next cold front
progged to move off the coast). Moderate to occasionally strong off-
shore winds will prevail through Weds.

Looking ahead, onshore winds are forecast to return to the area Thur
and strengthen to SCEC criteria that afternoon through early Fri...
all ahead of a strong cold front that is progged to barrel into the
coastal waters Fri afternoon. 41


Not planning on a lot of changes with the 12Z TAFs...with regard to
timing of the onset of the activity later this afternoon/early even-
ing. Short-range guidance appears to be on track with the main trof
approaching from the SW tonight and then sweeping to the E/NE after
midnight. One issue for the early Sun morning hours may be MVFR/IFR
visibilites and ceilings. 41


College Station (CLL)      54  45  65  53  66 /  80  90   0  20  50
Houston (IAH)              58  48  68  56  70 /  20  90  10  30  60
Galveston (GLS)            60  56  64  60  69 /  20  80  30  50  70


GM...Small Craft Advisory from noon today to midnight CST tonight 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.


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