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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

FXUS64 KHGX 180254

Area Forecast Discussion
National Weather Service Houston/Galveston TX
954 PM CDT Wed Apr 17 2019

At 9 PM, high pressure was located over the SE US and low 
pressure was located over SW OK with a dry line extending south 
from the low across the western TX. Some light showers continue 
to stream into area from the Gulf but precip has remain widely 
scattered and generally light. All that is about to change as a 
line of storms approaches the region from the west. Betweem 
06-09z, 300 mb winds split as an upper low over West Texas shifts 
east and SE TX will lie in a well defined jet couplet. The PW 
values surge to 1.90 inches over the western half of SE TX and 
there is some weak surface convergence along what looks like a 
weak outflow. On water vapor imagery there appears to be two short
waves lined up and the first of these will approach the region 
around midnight and help maintain the convection over Central 
Texas as it approaches SE TX. Instability parameters are 
impressive with LI values near -10, CAPE values 2800 J/Kg and 
Helicity around 220. SPC has expanded the Enhanced Risk a bit east
and this seems like the right call based on the instability and 
the approaching the short wave. 

SPC has also expanded the Slight Risk further east and this now
includes the Houston area. AMDAR soundings still show a strong
capping inversion in place at 700 mb and the million dollar
question is, will the cap hold or will it break. Short term
guidance has been trnding a bit more aggressively with the HRRR
and TT WRF all showing convection approaching Houston around
sunrise as the second short wave approaches the region from the
southwest. Feel the combination of the short wave and strong upper
level diffluence will help to erode the cap and allow a second
area of storms to redevelop over the SW zones and move NE toward
Houston and points northeastward. Instability isn't quite as
impressive as points to our west but there could still be some strong
storms with gusty winds. 

Speaking of winds, the actual front will mot reach the coast until
about 18z. The winds will become N-NW in the wake of the front and
will become strong. A Wind Advisory will likely be required for a
large part of SE TX Thursday afternoon. 43


.PREV DISCUSSION... /ISSUED 704 PM CDT Wed Apr 17 2019/ 

A mix of MVFR/IFR ceilings across SE TX this evening, should 
continue to lower in height to IFR through the overnight hours and
into tomorrow morning. Scattered light showers are continuing to 
develop out ahead of tonight's cold front. E/SE winds remain gusty
but should slacken ahead of the frontal passage tonight, and 
increase in speed out of the W/NW once again tomorrow morning. 
Strong to severe storms with gusty winds (primary hazard) and 
hail will be possible as the front passes through the region. The 
front should arrive at CLL shortly after midnight and reach the 
coastal terminals before sunrise. Ceilings will be slow to improve
tomorrow morning, but anticipating VFR criteria by early 
afternoon area wide.


PREV DISCUSSION... /ISSUED 340 PM CDT Wed Apr 17 2019/ 

NEAR TERM [Through Tonight]...

It has been a pretty soupy day today with overcast stratocu and 
continued moisture advection over SE Texas. Cloud cover has 
limited heating today which means there is not quite as much 
instability over the area as there could be and the capping 
inversion remains strong. AMDAR sounding from 20Z still shows a 
strong cap near 700mb at 12C with very steep lapse rates above 
that EML inversion. Surface Theta-E axis continues to run through 
central Texas and seeing some showers form on the eastern edge of 
this axis near the Brazos Valley. Right now there does not appear 
to be enough large scale lift to help erode the cap or a jet 
streak for divergence. If anything, water vapor imagery shows a 
jet streak departing the area so look for neutral/subsident 
vertical motion to continue the next few hours. The upper level 
low has moved into New Mexico with one part of the polar jet 
coming across the Big Bend and West Texas. Southern part of this 
jet should spread over south Texas overnight and into parts of SE 
Texas by 12Z Thursday. LLJ will be strong but veer around to the 
SW or WSW ahead of the front and strong jet forcing. There is 
still a very good chance of showers and thunderstorms mainly 06Z 
to 12Z Thursday. The WRF/HRRR continue to show a squall line 
moving into the area around 06-09Z with it becoming weaker 09-12Z 
Thursday. This is just in time for Houston morning commute from 
4AM to 8AM Thursday. We still think SPC outlook is on track 
especially concerning the capping limiting the severe threat 
closer to Houston and the coast. There could be some locally 
heavy rainfall as well but that may depend upon available 
instability and lift with the higher moisture. Again capping will 
be the main limiting factor. Upper level lift may just be enough 
out of phase to keep the cap in place. These are finer details 
that need to be monitored overnight as convection develops and 
adjust based on further analysis/guidance.


SHORT TERM [Thursday Through Friday Night]...

Deep upper level trough should develop east of the area over the 
Mississippi River Valley by Friday morning. Line of storms should 
be exiting the region Thursday morning as the initial trough axis 
moves past the area into Louisiana. A cold front at the surface 
should usher in cooler/drier air into the region for the holiday 
weekend. We will need to keep an eye on fire weather conditions as
winds will be pretty gusty from the north with after RH values 
getting down around 30-35 percent.


LONG TERM [Saturday Through Wednesday]...

Upper level ridging should continue over the area for the weekend
with one short wave trough moving across the southern Plains 
Sunday as another trough develops over the Rockies Sunday night. 
GFS/ECMWF/Canadian all have fairly different solutions for the 
upper level pattern for early next week from Monday into 
Wednesday. GFS has the trough splitting with still some troughing 
back to the west while the ECMWF/Canadian have a slower evolution 
of the trough. At this point lean more on a slower moving trough 
which the models in some shape/form somewhat agree with on 
Wednesday next week. However the surface details are all very 
different and rather not focus on those details since we are 
looking at the day 6-8 part of the forecast. Forecast will show 
rain chances slowly ramping up Monday through Wednesday.



Moderate to strong onshore winds and building seas are expected
to develop this afternoon and tonight as the gradient tightens
ahead of the incoming storm system. Showers and thunderstorms 
will be pushing west to east across the waters tomorrow morning
with a westerly wind shift. The storm system's cold front is
still expected to push through the area in the afternoon with
northwest winds increasing in its wake. There could be a period
of lower winds and seas (below caution/advisory levels) just 
ahead of the storms and again just behind the storms, but winds
and seas should come back up to those levels after the front
moves through the area. Could see some minor overwash at Highway 
87/124 on the Bolivar Peninsula tonight around high tide time 
(~4am), and then we'll need to keep an eye on possible low water 
conditions late Thursday night and on into Friday morning at low 
tide time across northern bay locations. Winds will weaken late 
Friday an on into Friday night as high pressure builds into the 
area. Onshore winds come back to the area on Saturday and persist 
for the remainder of the weekend and on into at least the first 
half of next week.  42


College Station (CLL)      80  63  76  52  74 /  40 100  40   0   0 
Houston (IAH)              78  66  80  54  75 /  40  90  80   0   0 
Galveston (GLS)            76  70  78  60  74 /  40  60  80   0   0 


     the following zones: Galveston Bay...Matagorda Bay.

     Small Craft Advisory until 6 AM CDT Thursday 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.