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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

                            
000
FXUS64 KHGX 182353
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
553 PM CST Mon Feb 18 2019

.AVIATION...
Still looking at gradually deteriorate conditions beginning this evening
and continuing overnight and Tuesday with lowering ceilings and rising
rain chances. MVFR ceilings expected to develop from south-to-north
tonight. Chances of rain increase tomorrow, and expect all sites to
transition to IFR ceilings and visibilities with expanding areas of
mainly -RA/RA developing in the morning and persisting throughout the
day. A few isolated TSRA are possible, but continue to leave mention
out of the TAFs as overall chances of an individual site being impacted
look fairly low. Looking at mainly NE/ENE winds at 10 to 15 knots 
(occasionally gusty, especially closer to the coast).  42 
&&

.PREV DISCUSSION... /ISSUED 252 PM CST Mon Feb 18 2019/ 

NEAR TERM [Through Tonight]...

Temperatures this afternoon have reached the mid 60s with a 
steady stream of thin high clouds with some breaks. High pressure 
continues to build over the Plains keeping NE winds in place over 
SE Texas with dewpoints in the 30s. Frontal boundary continues to 
push into the central Gulf slowly. Overall it has been a nice day 
to be outside and enjoy.

Upper level analysis from 12Z upper air data shows a very potent 
jet stream from west Texas NE to New England today with an upper 
low approaching the Four Corners region. Jet streak and vorticity 
maximum seen in water vapor imagery along the California coast 
should continue to round the base of the low tonight into 
tomorrow. As this happens, model guidance shows isentropic lift 
increasing overnight in response to lowering 850mb heights in the 
lee of the Rockies. Isentropic lift should be ongoing on just 
about any level, but the 295K theta surface seems to be in the 
thick of it all.

All this means is that moisture will be riding over the frontal 
surface leading to drizzle, rain and eventually fog tomorrow 
morning. Fog looks to be somewhat delayed and a function of 
drizzle. To avoid any weather type confusion, forecast will read 
showers but there will be a mix of drizzle/light rain when there 
are not showers passing.

Overpeck

SHORT TERM [Tuesday Through Wednesday Night]...

Tomorrow it is going to rain for a number of reasons. First there
is the aforementioned isentropic lift that only increases during 
the day which will support areas of showers. As the upper low 
moves into the southern Rockies, the right entrance region of the 
upper level jet becomes more oriented over SE Texas allowing for 
large scale ascent for more rain. Next in response to this jet 
streak, there is an increase in the LLJ in which by tomorrow 
afternoon there could be 850mb winds near 40 knots across the 
eastern half of the area. This will serve as a moisture axis as 
well helping to channel shower activity. After that, let's throw 
in an increase in moisture with precipitable water values around 
1.7 to 1.8 inches for the eastern half of the forecast area 
aligned with the LLJ. Looking at forecast soundings, lapse rates 
will be enough to support elevated thunderstorms. Recent SPC day 2
outlook has a marginal risk drawn closer to the forecast area 
than before. Given the set up of the LLJ this seems to make sense 
and where elevated convection may be strongest for a hail/wind 
threat. Individual storms will be moving quickly from S/SW to N/NE
so rainfall rates may not be quite as high. Rain rates may also 
be limited due to the instability being elevated and not as robust
as compared to surface based convection. A coastal low should 
develop along the front tomorrow and move along it up the Upper 
Texas Coast. This will continue to help pump moisture north and 
increase tide levels at the coast. Not only that, there is the 
small chance the warm sector could move just inland but guidance 
is consistent with keeping the warm sector airmass just inland of 
SW LA. This might be where there is a slightly higher threat for 
severe weather than SE Texas.

Where may we expect higher rainfall amounts tomorrow? First along
the LLJ and moisture axis. Hi-res models keep this mainly east of
I-45 but still a chance it could be centered to the west. Even 
then, rainfall totals look to be around a half inch with passing 
showers albeit somewhat training. The second location will likely 
be from Huntsville to Lufkin NE into the Arklatex where 850mb 
frontogenesis will be maximum. Models are really hitting areas NE 
into Arkansas with the higher rainfall amounts over 1 inch. Closer
to SE Texas along this axis, rainfall amounts look to be between 
a half inch and an inch. Again a solid half inch to an inch of 
rain across the area probably will not cause much of a flood 
problem although given very moist soils, any rainfall should run 
of pretty quickly. There could be some rapid rises on area creeks
and bayous but likely stay in banks.

A cold front should push through in wake of this upper level
trough and push into the Gulf Wednesday night. The front then
stalls as upper level flow remains from the SW and another trough
forms over the Great Basin by Thursday morning.

Overpeck

LONG TERM [Thursday Through Monday]...

We are going to quite literally write wash...rinse...repeat for 
this part of the forecast discussion for Thursday through Saturday
and then again next Monday and Tuesday. You can pretty much copy 
what was written above and insert hear for the discussion of the 
Friday/Saturday system, take a break Saturday night into Sunday 
and copy it again for Monday next week. Now there is more 
uncertainty in the details for next week but there is some model 
consistency to be had. That means we will be looking at another 
half inch to an inch of rain for the area and the sogginess 
continues (thanks El Nino).

Overpeck

MARINE...
A fairly complex/changing marine forecast this week with virtually 
all kinds of hazards in play at some point. In the short term, a 
moderate fetch of steady 20-30kt ene winds has generated 8-12 ft 
seas offshore. Not anticipating this to change much overnight so 
will keep a persistence type forecast going. Will extend the SCA 
overnight in the bays, though it'll primarily be for the southern 
portions closer to the Gulf. Tides are about 1.5' above normal and 
we'll need to keep an eye on the Bolivar area (Highway 87@124) as 
observed levels might peak around 3 ft at high tides tonight & Tue 
which may produce some minor overwash. Will hold off on a coastal 
flood advsy for now and let the next few shifts monitor trends.

Low pressure area in the Yucatan will move up the coast Tue & Tue 
night. Winds will become more se in advance and bring higher 
dewpoints over the colder waters leading to a short duration of sea 
fog. Winds will shift back to the north as the low move off to the 
ne and ends the threat Wed morning.

Another coastal trof sets up later in the week, followed by a 
probable return of sea fog Fri afternoon thru Sat night. 47

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      44  46  43  61  44 /  40  90  30  10  20 
Houston (IAH)              47  56  49  62  50 /  40  90  50  20  50 
Galveston (GLS)            53  65  54  61  56 /  70  80  50  40  50 

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...Small Craft Advisory until 9 AM CST Tuesday for the following 
     zones: Galveston Bay...Matagorda Bay.

     Small Craft Advisory until noon CST Tuesday for the following 
     zones: Coastal waters from Freeport to Matagorda Ship 
     Channel TX out 20 NM...Coastal waters from High Island to 
     Freeport TX out 20 NM...Waters from Freeport to Matagorda 
     Ship Channel TX from 20 to 60 NM...Waters from High Island 
     to Freeport TX from 20 to 60 NM.

&&

$$