FXUS64 KHGX 200444

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1144 PM CDT Thu Oct 19 2017

Higher moisture moving onshore from the southwest will thicken up
and lower cloud cover to periodic MVFR...more persistent through
Friday. As Matagorda Bay region moisture moves up within mid-
upper level southwesterlies and rides up and over lower level
easterlies...light precipitation will begin to develop across more
western terminals through Friday morning. An approaching upper
level trough will strengthen south(west)erly steering flow...
maintaining the moisture pump and breaks in cloud should warm
surface temperatures up to near 80F...or the temperature that may
create enough instability to initiate the development of afternoon
thunderstorms. 31


.PREV DISCUSSION... /ISSUED 1015 PM CDT Thu Oct 19 2017/

Evening radar shows increasing shower with embedded thunderstorm coverage
making its way toward the Matagorda Bay area, generally associated with
a weak frontal boundary. Expect this activity and future development
overnight and tomorrow to spread across Southeast Texas. We'll need
to monitor the possibility of locally heavy rainfall from late this
evening on through mainly Sunday morning when the next cold front sweeps
through the area and dries us out.

Current forecast already has rising rain chances for tonight and tomorrow,
and only some minor tweaks were made on the evening update. 42


PREV DISCUSSION... /ISSUED 354 PM CDT Thu Oct 19 2017/

Radar this after showing some light rain W and SW of a Bay City to
Wharton line this afternoon. This activity lines up well with a
moisture axis coming from the Gulf as forecast. Moisture should
increase tonight and especially tomorrow as a warm front moves
north. Overall the short term forecast remains on track with
moisture axis beginning to set up west of I-45 tomorrow morning
where models have been pretty consistent with the onset of
scattered showers and thunderstorms.

Instability tomorrow should be strong enough for a few
thunderstorms as a weak vorticity max moves over the area tomorrow
morning. Activity will likely persist into the early afternoon but
think activity will begin to decrease in coverage towards the late
afternoon as the weak disturbance moves NE.

Saturday forecast still looks drier compared to rain chances
tomorrow and Sunday. Upper level trough coming into the Pacific NW
will move across the Rockies Saturday with a vorticity lobe
breaking off from the main flow Saturday night. SW flow aloft will
be in place Saturday but really no good trigger for thunderstorm
activity. This means any activity will be diurnally driven and
initiated from any pre-existing boundaries on the mesoscale.
Forecast keeps 40 PoPs in place since forcing will be weak.

By Sunday morning the main upper level trough splits with the
northern upper low moving into Canada and Great Lakes. The
southern vorticity lobe begins to pinch off from the main flow and
close off by Sunday night over the Arklatex and Ozarks. By Monday
the models vary greatly in handling the evolution of this system
as it moves east. The good thing is that until Monday the models
are in good agreement. This means a cold front is on track to push
through early Sunday morning. It is quite possible a line of
storms will accompany the front. There is some question as to
whether severe weather will develop with the line of storms. The
upper trough dynamics and front are a bit out of phase but still
enough forcing to erode any capping. The NAM is a bit concerning
with its lack of QPF along the front compared to the GFS/ECMWF but
then again it is the NAM. The reason for the NAM's lack of precip
seems to be boundary layer winds veering to the SW and W which
pushes all the moisture east ahead of the front. GFS/ECMWF show
similar patterns but are much slower to shift moisture east. These
solutions have been fairly consistent. Rainfall amounts of 1 to 2
inches looks reasonable for Friday through Sunday across the
area. Storms should be moving at fast enough rates that flooding
should not be a concern but some of the stronger storms could
produce isolated higher amounts closer to 3 inches. The front
should be through the area 18z Sunday through 00z Monday with
gusty northerly winds behind the front.

High pressure builds over the area Monday into Tuesday with a
reinforcing shot of colder air behind another front on Tuesday.
Wednesday low temperatures could be in the 40s with high
temperatures in the mid 70s. Cooler and drier conditions should
persist into the end of next week and models show another front
coming down next Friday. This make sense given the upper level
pattern becoming quite amplified with a mean long wave trough over
much of the eastern U.S.


High pressure over the eastern U. S. and lower pressures over the
southern Gulf will help to produce moderate/strong easterly winds
across the coastal waters through this evening. The SCEC has been
extended into the early evening (for the coastal waters). As this
high moves further to the N/NE, the low-level flow is expected to
become more southeasterly overnight through early Fri morning. As
this moderate/strong onshore flow persists over the weekend, deeper
moisture over the Gulf will help to produce some isolated/scattered
showers mainly during this period. The next cold front will also
produce some additional activity Sunday. A strong/gusty N/NW flow
in the immediate wake of the front should prompt Caution and
Advisory Flags for Sun night/Mon morning. This offshore flow will
likely persist through the first half of next week. 41


College Station (CLL) 66 83 69 87 68 / 20 60 30 40 60
Houston (IAH) 70 80 72 86 73 / 20 60 30 40 50
Galveston (GLS) 76 81 77 84 77 / 20 60 30 40 40


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...Matagorda Bay.

Small Craft Advisory until 4 AM 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.

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.



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