Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 211749

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1249 PM CDT Sat Apr 21 2018

Showers and thunderstorms increasing in coverage as a weak
disturbance approaches SE TX. Fcst soundings look quite aggressive
with PW values increasing to 1.90 inches by 21z. Based on
instability parameters and moisture, it would appear that the best
chance for convection will be late this afternoon into this
evening. A cold front will cross the area between 06-12z with
additional shra/tsra along the boundary. A mix of MVFR/VFR
ceilings expected through tonight with clearing skies expected on
Sunday. 43


.PREV DISCUSSION... /ISSUED 1017 AM CDT Sat Apr 21 2018/

The warm front has extended into the southern counties with
scattered light showers across the region. A cluster of
thunderstorms was located south of Matagorda Bay this morning, so
POPs and weather grids were changed to be reflective of the storms
moving into the extreme southwest zones. Water vapor imagery
shows the upper level trough progressing into Texas, leading to
ample diffluent flow over eastern Texas this afternoon. Aside from
the decent upper level dynamics for lift, there remain some
phasing issues at the surface between instability and shear for
severe convection this afternoon. Forecast is now reflective of
the greatest thunder chances beginning at 21Z. However, surface
dewpoints are rising faster than models have predicted, so
isolated thunderstorms are possible beginning at 18Z.

Moving into tonight, once the warm nose around 700 mb erodes,
thunderstorms become more likely. One factor we will keep our eye
on is the track of the triple point currently over the southern
Texas Panhandle. While instability will remain a limiting factor
through the overnight period, high resolution guidance has been
increasing thunderstorm coverage for the northern counties late
this afternoon through this evening. Should this be the case, the
favorable wind profile will lead to the threat of severe wind and
perhaps an isolated tornado.

Timing the line of thunderstorms tonight still seems on track with
previous forecast, from 06Z up north to 12Z at the coast as the
cold front tracks through Southeast Texas. Flash flood threat is
still low at this time. 22

PREV DISCUSSION /Issued 444 AM CDT Sat Apr 21 2018/...

NEAR TERM [Through Today And Tonight]...

Today`s forecast really hinges on careful and thorough analysis,
but even with that, we meteorologists are hoping model boundary
layer parameterization schemes handle the situation correctly.

Water vapor imagery this morning shows an elongated upper level
low over the central and southern Rockies with a vorticity center
at the base of the low over New Mexico. Polar jet shows up nicely
with drying in all three water vapor channels from the Great Basin
down to the Desert SW and across to the southern Rockies. There is
also evidence of the subtropical jet coming across the U.S./Mexico
border into Texas with some anti-cyclonic curved cirrus over
central Texas. Subtropical jet may also be contributing to some of
the drying on water vapor imagery over the southern Rockies.
Polar jet curves north over the Plains and can see diffluent flow
over much of Texas. As the upper low moves east, this diffluence
along with the coupled jet and height falls should allow for
increasing large scale ascent. NAM/GFS/ECMWF all show strong q
vector convergence in the 500-300mb layer by this afternoon over
much of Texas from I-10 north to the Red River. South of I-10 the
lift tapers off quite a bit and more likely where the elevated
mixed layer will be stronger.

At the surface, the warm front was located fairly close to the
leading edge of 60F dewpoints. At 09Z this basically was along
I-10. Case in point, KIAH had a T/Td of 61F/54F, KHOU at 66F/62F.
Warm front should continue to push north as warm air advection
increases with cyclogenesis over NW central Texas by 18Z today.
Dewpoints in the mid 60s should spread over the area with maybe
some upper 60 dewpoints along the coast. Model soundings show
increasing boundary layer moisture but 3-6km lapse rates going
from 8C to closer to 6.5C-7C as the atmospheric column moistens.
The elevated mixed layer does erode with the lift from 18Z today
to 00Z Sunday so the warm sector should have some destabilization.
The problem will be persistent cloud cover which will limit any
diabatic heating in the boundary layer. Should there be enough
breaks in the clouds, there could be some pockets of better
heating and destabilizing the atmosphere. As it stands, warm air
advection will have to be enough to increase boundary layer
temperatures to create instability. Most models show around 500 to
maybe 1000 J/kg of CAPE with the higher amounts over central
Texas towards the Brazos Valley. This is where boundary layer
parameterization schemes in the models need to be at their best.
Should there be more heating or a lot more moisture than currently
forecast, then the threat for severe weather increases. As it
stands, the marginal at best thermodynamic environment will limit
the threat.

Should surface based updrafts form this afternoon/overnight, there
is expected to be plenty of 0-6km bulk shear (45-55kt) for
organized storms. Even 0-1km bulk shear of 20-25kts will be enough
for storm rotation and a small tornado threat. SPC does have a 2%
tornado probability along with a 5% damaging wind probability for
the area. With lapse rates decreasing during the day due to
atmospheric moistening, hail will be a minor threat. Basic timing
of any severe weather looks to be after 21Z today for today. There
should be a window from 21Z today to 03Z Sunday (4PM-10PM) for
severe weather over the Brazos Valley over to east Texas. Timing
of the threat shifts south to 03Z to 12Z (10PM to 7AM Sunday)
Sunday for areas south of College Station to Lufkin. At this time
storm evolution may go from a few individual cells to a line of
storms or a weak/broken QLCS depending upon interaction with the
elevated mixed layer.

Lastly the heavy rainfall threat which I really do not think there
is much of a threat. Nonetheless, WPC has the area in a marginal
risk for excessive rainfall exceeding flash flood guidance. Yes
precipitable water values climb to around 1.7-1.8 inches which is
above the 90-95th percentile of climo per NAEFS guidance. There is
strong moisture transport with 30 knot LLJ during the day. Overall
think there could be on average a good half inch of rain for the
area with isolated 1-3 inch amounts. Latest basin 3 hr FFG shows
most areas needing more than 4 inches of rain in 3 hrs. So other
than your typical flooding of low lying areas or rapid responses
on creeks/bayous, flood threat should be low especially with storm
motions of 30 to 40 knots.

SHORT TERM [Sunday Through Monday]...

Cold front pushes through early Sunday and off the coast 12Z to
18Z allowing for drying conditions. Sunday afternoon through
Monday should be some of the nicest spring conditions you can get
in SE Texas. High pressure will build down through the Plains
keeping northerly winds in place and some cold advection through

LONG TERM [Monday Night Through Saturday]...

Overall upper level pattern remains quite amplified with a strong
ridge over the Pacific NW which means the long wave trough
pattern should persist over the central U.S. into the Great Lakes.
A couple of short wave troughs dip south into Texas within NW
flow aloft. Forecast will keep some chances of showers and perhaps
elevated thunderstorms for Wednesday with a stronger trough
coming through Thursday night into Friday. GFS and ECMWF have
trended stronger with this second system so something to monitor
with future forecasts.



Moderate to strong winds are expected to continue through today and
into tonight, and even potentially longer in the offshore waters.
Winds will become more out of the southeast today, and with speeds
of 15-20 knots and seas of 3-5 feet, caution is urged for small
craft on the Gulf today. Some drizzle or light showers...and the
afternoon even an isolated thunderstorm will be possible as a warm
front drifts through the area. The onshore winds will also help
elevate tide levels, which may cause run up at times of high tide.

A cold front will push across the waters through Sunday morning,
veering winds to northwesterly, and remaining moderate to strong.
Currently expecting these winds to be similar in strength to the pre-
frontal winds, which would be not quite enough for a small craft
advisory, but still necessitate caution until winds begin to
diminish Sunday night. After that, generally offshore winds prevail
until Tuesday or so, at which point we`ll have a brief period of
onshore winds before the next front in the middle of next week.



College Station (CLL)      72  54  74  52  77 /  60  60   0   0   0
Houston (IAH)              74  60  76  55  79 /  50  70  10   0   0
Galveston (GLS)            74  65  76  62  76 /  30  70  10   0   0


     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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