Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 242355

Area Forecast Discussion
National Weather Service Houston/Galveston TX
555 PM CST Sat Feb 24 2018

.AVIATION [00Z TAF Issuance]...

KCLL/KUTS...Main concern the next 6 hours will be the cold front
and line of convection along it that is currently near near
Killeen/Temple area NW of the terminals. Capping is still strong
enough per latest AMDAR aircraft soundings, but this convection
seems to have broken what little cap there is along the front. The
key is outflow/front is out running the convection so it may not
last more than the next hour or two. For now will go with SHRA
along the front which should push through 02Z. Ceilings may drop
to MVFR ahead and just behind the front and gradually improve. VFR
ceilings continue into the day Sunday but another short wave
trough should be enough to trigger another rough of showers.
NAM/GFS highlight this second round of showers and may deserve
more attention with the 06Z TAF update.

KCXO/KIAH/KHOU/KSGR...Showers will be possible the next 6 hours at
the terminals but only passing at that. The main question from 06Z
to 12Z Sunday will be convection. NAM/GFS seem to be a bit more
aggressive with convection than the convective allowing models
like the WRF ARW/NMM and HRRR. Inclined to lean more on those
models for convective trends which don`t show much. That makes
sense due to the cap but need to be wary of the short wave trough
coming into the area by 12Z Sunday. For now will leave showers in
the forecast but will continue to evaluate convective trends.
Ceilings should drop ahead of the front. Front does reach KCXO
around 06Z and then Houston area around 08-10Z. HRRR really slows
down the front along the coast while other models push it to the
coast. Given moisture MVFR/IFR conditions likely especially for
KSGR/KHOU. Still a chance for those conditions at KIAH. Rain
chances ramp up again after 15Z Sunday and may need to be stronger
in the TAF for showers for the 06Z update.

KGLS/KLBX...Sea fog and IFR conditions will likely be the main
concerns for the next 6 to 12 hours. Front reaches the coast close
to 12Z with a around of convection. May need to added TSRA for
KLBX TAF in the morning in addition to KGLS which already has it.
This seems more likely with higher moisture along the coast and
the better lift from the short wave coming into the area. Showers
likely continue through the day Sunday, but will be a matter of
amending TAFs for TSRA. Ceilings will be an issue and hard to
time when there will be any improvement. Most likely time will be
during episodes of convection.



.PREV DISCUSSION /Issued 344 PM CST Sat Feb 24 2018/...

.SHORT TERM [Tonight Through Monday Morning]...

Will be letting the Wind Advisory expire at 4 PM based off trends
in observations over the last few hours. Winds have taken on more
of a southwesterly component along the prefrontal trough out
ahead of the main cold front this afternoon. Radar imagery has
revealed linear convection consistently redeveloping along the
prefrontal trough, oriented parallel to upper level flow with
500mb winds also out of the southwest. In north central Texas,
radar imagery is already showing a line of showers and
thunderstorms developing along the main frontal boundary, which
should arrive in our northern counties shortly after sunset. The
area of low pressure tied to the frontal boundary, is expected to
continue pushing off towards the northeast this evening. Short
term guidance is such as the HRRR and Texas Tech WRF indicate this
line of scattered thunderstorms north of the Trinity to
Washington County line possibly arriving between 00Z to 03Z as the
frontal boundary sags southward. This arrival is a little faster
than trends indicated on radar through simple extrapolation of
overall movement.

Strong gusty winds and low level rotation will also be possible
within stronger cells mainly north of I-10 this evening, as
determined through the analysis of the following severe weather
parameters. Low level wind sheer especially north of the
Washington to Trinity county line, has been pushing 30 to 35 mph
this afternoon. The RAP13 shows the region of best effective bulk
sheer pushing southward towards I-10 this evening, indicative of
possible gusty winds. As the front sags south through the evening
hours, scattered thunderstorms will be possible mainly north of
I-10 where there is ample moisture and decent instability out
ahead of the main frontal boundary. In terms of instability,
forecast soundings for IAH indicate both surface and MU CAPE
values near 1000-1500 J/kg. Lapse rates between the surface and 3
km range between 6.0 to 6.5 deg C/km, while mid-level lapse rates
appear even more favorable for instability with values closer to
7.0 deg C/km. Low level rotation will also be possible within
stronger storms, given the storm relative helicity (SRH) values
ranging between 150-250 m^2/s^2. Although the all of the
ingredients for a few strong storms are there, the lack of low
level convergence could dampen overall coverage of these showers
and thunderstorms. Therefore, will have to keep a close eye on
radar trends this evening for the possibility of these stronger
storms developing mainly before midnight as the frontal boundary
moves southward.

While showers and isolated thunderstorms will also be possible
tonight as the shallow cold front slides south of I-10, the chance
for tapping into the best instability and decent wind shear lessens
the closer you move towards the coast. Although the chance for some
gusty winds below stronger cells still remains, the concern for a
severe wind threat will transition more into the possibility for
training cells after midnight. Where this training sets up will
heavily depend upon the location at which this cold front stalls
out, transitioning to a stationary front. At 500 mb an region of
high pressure located over the northeastern Gulf of Mexico also
helps to block the cold front from pushing any further southward
off the coast Sunday. Both the 12Z runs of the NAM and GFS show
the cold front stalling out just north of the coast between 12-18Z
Sunday. As a result, this stationary front will act as a forcing
mechanism, providing lift for shower and isolated thunderstorm
development along the boundary. Upper level flow remaining
parallel to this boundary, will also act to provide good support
for training showers and thunderstorms. The concern for convection
south of this boundary will be possible beginning in the early
morning hours Sunday morning and continuing into Sunday afternoon.
Near record breaking precipitable water values exceeding 1.8
inches will be possible along the upper Texas coast. Locally heavy
rainfall cannot be ruled out, as both global and short term
guidance hint at a decent swath of 1-2 inch rainfall across the
coastal counties by Sunday morning. This area of training
convection over the coastal counties could begin shortly after
midnight. Because of the inconsistencies in vertical accelerations
amongst guidance, it remains unclear the duration from which this
rainfall will occur. The longer the duration at which these 1-2
inches of rainfall occurs, the less of a chance we will see any
localized impacts such as ponding on roadways. While if this
duration is over a shorter time scale, localized impacts will be

The stationary front will slowly slide back north Sunday morning,
increasing the chance for precipitation once again across and
slightly north of I-10. Therefore, this region including the City
of Houston will also have the possibility of receiving 1-2 inches
of rainfall accumulation by Sunday afternoon. Overhead,
southwesterly flow aloft will also help continue to advect upper
level disturbances across the region, providing dynamic forcing
and lift through late Sunday. These upper level disturbances can
be seen in water vapor imagery stretching from northeast Texas,
southwestward through northern Mexico.

A reinforcing area of surface high pressure will build in behind the
front boundary late Sunday into early Monday morning, helping to
finally slide this feature off the coast. Upper level ridging will
also help to send the frontal boundary south of the region, as 500
mb winds turn more easterly. This should make for a much drier day
Monday, with precipitable water values lowering to 0.8-1.3 inches
across southeast Texas. The chance for patchy fog over the coastal
waters will remain in the forecast until this frontal boundary is
able to push off the coast Monday morning. With water temperatures
around Galveston Bay in the upper 60s to low 70s, there will be
less of a chance for patchy fog becoming dense. More information
on fog development is provided in the marine section below.


.LONG TERM [Tuesday Through Saturday]...

Behind the cold front, Monday will shape up to be a nice day with
high temperatures in the low to mid-70s, partly cloudy skies, and
rain chances staying well offshore. The wind will veer to
southerly on Tuesday as once again, the boundary from Sunday
recedes inland as a warm front, bringing back Gulf moisture and
dew points well into the 60s. The combination of this enhanced
moisture with a series of upper-level disturbances and good
dynamic forcing will increase rain and isolated thunderstorm
chances Tuesday afternoon and evening.

Chances for showers and thunderstorms remain through Thursday as our
next cold front passes through the area mid-week. Confidence for
timing the surface front is low, so a chance for showers and
thunderstorms is kept from Wednesday morning through early Thursday.

Behind the cold front, northerly winds will knock our temperatures
back down near climatological average for this time of year (lows in
the 50s, highs around 70). Winds will slowly veer to easterly
through the weekend as a surface ridge moves into the area from the
west. This high pressure system will keep rain chances low and allow
for a slow warming trend through early next week. 22


Platforms show visibilities in the 3-4 mile range this afternoon and
with winds across the coastal waters almost due south would not
expect and resurgence in the sea fog until winds begin to back ahead
of a cold front closer to sunset this evening. Expect the cold front
to make a run for the coast near sunrise on Sunday with winds
backing to the east or possibly northeast in the vicinity of the
boundary. Periods of showers and thunderstorms are expected along
the coast tomorrow as a result of the front. Water temperatures have
warmed into the 66-71 degree range this afternoon, which in addition
to the rain will help limit the dense fog threat, but brief windows
may still exist as an easterly or northeasterly flow allows for some
of the cooler shelf waters off southwest Louisiana to be pushed
towards Southeast Texas. Expect the front to be nudged off the coast
Monday morning as surface ridging skirts Texas with the potential
for sea fog returning on Tuesday as the cold front lifts inland as a
warm front.




College Station (CLL)  53  64  50  72  53 /  30  60  20  10  10
Houston (IAH)          61  64  55  74  58 /  60  80  30  10  10
Galveston (GLS)        65  68  61  71  62 /  80  60  50  20  10



     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.



LONG TERM...Huffman
AVIATION...Overpeck is the U.S. government's official web portal to all federal, state and local government web resources and services.