Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS64 KHGX 201219
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
619 AM CST Fri Jan 20 2017

.AVIATION...
Variable conditions across the area, as much of the area struggles
to improve from dense fog and low ceilings overnight producing
LIFR and VLIFR conditions. By mid-morning, expect the process to
progress more uniformly, and bring nearly all sites through MVFR
to VFR conditions this afternoon. However, with pre-frontal
onshore flow continuing, we should expect conditions to degrade
again tonight. For now, have chosen to focus on the near-term
conditions, and only attempted to highlight when conditions slip
back to MVFR across the area. However, expect future forecast
cycles will more explicitly sketch out how low visibilities
and/or ceilings are expected to go. A fair ballpark estimate at
this time would be to see similar conditions.

Luchs

&&

.PREV DISCUSSION... /ISSUED 455 AM CST Fri Jan 20 2017/

DISCUSSION...
Latest water vapor imagery and 00z upper air analysis shows a
short wave trough with a increasing jet streak over west Texas.
Upstream a much more vigorous upper level low was moving towards
the California coast with a very strong jet. This will be the
next system to affect SE Texas over the weekend, specifically
Sunday.

At the surface the boundary layer is fairly stable with near calm
winds and widespread areas of fog. Dense fog advisory is in effect
for the southern half of the forecast area along with Galveston
Bay and some of the adjoining Gulf waters. Surface analysis also
shows that deep moisture is not all that far away. Dewpoints in
the low 70s are just off the Upper Texas Coast. It will be quite
possible to go from a stable boundary layer to unstable in the
next 12-15 hours.

Latest short range guidance such as recent HRRR, WRF ARW/NMM, NAM,
RAP and synoptic guidance like GFS show quick recovery of the
boundary layer. Precipitable water values increase to 1.4 to 1.6
inches basically Houston southward from 21Z today to 03Z Saturday.
Surface dewpoints could increase into the upper 60s during this
same time and possible that there will be a warm frontal boundary
stretched across the area. Warm front could possibly reach as far
north as Houston. This is all in advance of the strong jet streak
coming from the west so that deep layer shear will be increasing
to more than 50 knots over the area. Low level shear will also
increase to around 20 knots. Question will be instability but with
a strong jet streak capping may be quickly overcome. There will
certainly be plenty of Q-G forcing and omega for large scale
ascent over the area. HRRR model soundings at KHOU show STP
parametersincreasing from less than 1 to about 1.5-2.5 from 23z
Friday to 02Z Saturday. So even with only 500-1000 J/kg (HRRR
model soundings for KHOU shows CAPES as high as 1300 J/kg), the
environment for the southern half of the forecast area could be
supportive of severe weather, specifically damaging winds and
tornadoes with any rotating storm. SPC has a marginal risk for
part of the area today and agree the primary threat should be east
of the area tonight but really do not want to let our guard down
for the potential this evening along the coast.

Last but not least, let`s not forget the rainfall potential given
recent events. Latest HRRR runs have shown isolated areas of 1-2
inches of rain this afternoon. Storms will be fast moving but
could very easily achieve high rain rates yet again. Stronger more
organized storms could put down some swaths of 2 inches or more.
Over certain areas that have seen recent rainfall, flooding would
become a problem. Unfortunately even 12 hours away from an event
it is still hard to pinpoint exact locations for heavy rainfall
but certainly the potential for it Houston southward to the coast.
There is always the possibility the highest rainfall amounts occur
over the Gulf but given the recovery of the boundary layer along
the coast rather not let our guard down.

For the weekend a rather impressive jet stream dives into Texas
Saturday into Sunday. Upper level low develops over the southern
Plains Saturday night with a strong Pacific cold front pushing
through the area. Surface low tightens up Sunday allowing for a
tight pressure gradient resulting in strong and gusty winds. Gale
watch is already out for the marine waters and likely need a wind
advisory Sunday for winds in the 25 to 35 mph range. This system
does eventually drag down some colder Canadian air Sunday night
into Monday allowing for high temperatures in the 60s. Yes that
means that temperatures will actually be near normal. Gasp! But
don`t worry above normal temperatures return Tuesday/Wednesday
next week with a cold front pushing through later on Wednesday so
that near normal temperatures will be possible for the end of next
week. Forecast did keep some slight chance of rain going Wednesday
into Thursday. GFS shows some slight isentropic lift as the
surface front pushes through and there is some lag in time until
the 850mb front pushes through with the onset of isentropic down
glide. Basically expect some dreary couple of days with the mid
week front pushing through. ECMWF on the other hand is more
progressive with the front but think the GFS might get this right
given the recent shallow fronts that have pushed into the area.

Overpeck

MARINE...
Winds tonight have become southerly to southwesterly. Though not the
most conducive to foggy conditions, webcams and observations have
shown rapidly deteriorating conditions on Galveston Bay and very
near shore on the Gulf of Mexico, so it`s obvious that there is
still potential despite not having the most favorable winds. Patchy
fog will remain possible until a strong cold front moves through the
waters Saturday night.

In the wake of this front, look for very strong and gusty northwest
winds to pick up very late Saturday night and Sunday. A gale watch
has been issued tonight given the confidence in these conditions.
Barring a significant change in the forecast, gale warnings and low
water advisories are likely to be needed. Look for winds and seas to
gradually subside Sunday night. Winds may come up again in the
Monday night-Tuesday timeframe, and caution flags may be needed,
particularly offshore. Additionally, a front in the middle week may
also result in the need for caution or a small craft advisory.

Luchs

Strong and gusty winds on Sunday will likely be high enough for
elevated fire weather conditions, and may be cause for some concern
for the suppression of any wildfires that may begin. But for now,
observed ERCs across the area are less than the 50th percentile, and
minimum relative humidities look to be in the 30-40 percent range,
which should mitigate drops in fine fuels to some extent. Continued
low humidity is expected Monday before rebounding for the mid-week,
but also expect winds to be less than 15 mph, decreasing the concern
in spread potential.

Luchs

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      74  56  75  49  65 /  10  20  20  30  10
Houston (IAH)              75  60  77  54  67 /  40  60  30  20  10
Galveston (GLS)            71  64  72  58  66 /  50  60  30  20  10

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Dense Fog Advisory until 9 AM CST this morning for the following
     zones: Austin...Brazoria...Chambers...Colorado...Fort
     Bend...Galveston...Harris...Jackson...Liberty...Matagorda...
     Waller...Washington...Wharton.

GM...Gale Watch from late Saturday night through Sunday evening 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...Galveston Bay...Matagorda
     Bay...Waters from Freeport to the Matagorda Ship Channel
     from 20 to 60 NM...Waters from High Island to Freeport from
     20 to 60 NM.

     Dense Fog Advisory until 9 AM CST this morning for the following
     zones: Coastal waters from High Island to Freeport out 20
     NM...Galveston Bay.

&&

$$

Discussion...39
Aviation/Marine...25



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.