Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 201553

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1053 AM CDT Fri Oct 20 2017

The surge of higher Gulf moisture certainly made its presence
known this morning with a good 3 to 7 inches of rain that fell
across portions of Galveston and Harris counties. This activity
has shifted east of I-45 this morning in line with previous days
model runs and within the axis of higher moisture with 1.8 to 2.1
inches of precipitable water. The difference is that this shift
has occurred much sooner than forecast and the same can be said
for the swath of heavy rainfall. The main changes to the forecast
for today were to have higher rain chances over the eastern most
counties of the forecast area east of I-45 with 30-40 PoP for the
rest of the area. With day time heating there still could be a few
storms that develop but think for the most part any activity this
afternoon may be more isolated than scattered. High temperature
forecast for this afternoon was also updated to have upper 70s
over the areas where rain is expected and mid 80s where cloud
cover is thin.

Early look at the 12z models show another band of convection
setting up tomorrow afternoon from Wharton up to Crockett. There
might be a weak vorticity max to swing through the area tomorrow
afternoon helping to initiate this activity. Cold front and
possible squall line of storms still on track for Sunday morning.
There are still some timing differences in the models to reconcile
with the line of storms and possibly pre-frontal trough out
running the front.



.PREV DISCUSSION... /ISSUED 642 AM CDT Fri Oct 20 2017/

The axis of showers and thunderstorms was pushing north and east
at 1130Z. Best chances will be from KIAH north and northwestward
through 14Z to 15Z. Although there is an outside chance of
thunder, expect the rest of the day will see isolated to scattered
coverage of mainly showers as the surface front/pressure trough
that help generate the earlier storms continues to push inland.
May see a brief period of MVFR conditions -- mainly ceilings --
during the late morning to midday period (15Z through 18/19Z).
Any shower activity that flares up this afternoon should dissipate
between 00Z and 03Z. MVFR conditions are expected after 09Z and
there is a slight chance that IFR or lower could develop. SREF
probabilities do show some potential for patchy fog but the main
threat will be from lowering ceilings.


PREV DISCUSSION... /ISSUED 419 AM CDT Fri Oct 20 2017/

At 3 AM, a coastal trough was located from about Port Arthur to
Galveston to Palacios. At 850 mb, high pressure was located over
the middle Tennessee valley and the flow around this feature was
bring a plume of deeper moisture into SE TX. 850 MB dew points
have increased to 15 C at CRP. At 300 mb, upper level winds were
strongly diffluent and a speed max was noted approaching West
Texas. Water vapor imagery shows two distinct disturbances with
the first approaching early this morning and the second and weaker
disturbance arriving late this morning. At 345 AM, a band of
showers and thunderstorms extended from Sugar Land to Galveston
and then offhsore. This band will continue to move north and
should be in Houston by the morning commute. The latest HRRR runs
and the Hi Res ARW both support some training of precip this
morning over Harris county and concern is growing that some
locally heavy rain could develop. Will mention locally heavy rain
in the weather grids for Braz/Galv and Harris counties for this
morning. Some of the short term guidance is producing 3 to 6
inches of rain this morning so it`s something to watch.

PW values decrease tonight to around 1.60 inches and not seeing
much forcing. Will carry 20 PoPs for now but think it`ll be dry
for most of the area. Models continue to advertise rain on
Saturday. Fcst soundings not looking impressive with a lot of dry
air between 900-700 MB. Upper level winds remain weakly diffluent
and both the GFS and ECMWF show a strong disturbance embedded in
the SW flow aloft moving across the western half of the CWA in
aftn. Will carry 30 PoPs for now but these may need to be raised.

A strong cold front will approach the area late Saturday night
into Sunday morning. Strong forcing from an upper level trough and
favorable jet dynamics favor widespread precip ahead of the front.
SE TX will lie in a 90 kt RRQ which should aid lift. Only concern
on Sunday morning is that a pre frontal trough may develop and
outrun the actual front decrease the low level convergence. Even
so, feel upper level dynamics will compensate if low level
convergence weakens. Have favored the ECMWF for Sundays precips
event over the GFS. Skies should begin to clear from west to east
Sunday afternoon with generally clear skies and much cooler
temperatures SUnday night.

Monday through Wednesday look spectacular with generally clear
skies and cool autumn temperatures. onshore winds return Thursday
and temperatures will trend warmer with afternoon high temps back
into the lower 80s. GFS and ECMWF differ significantly Friday and
beyond with the ECMWF producing more rain with next Friday`s front
and placing SE TX in a zonal flow for early next week. The GFS
shows a weaker front next Friday with less rain and it maintains
a weak upper level weakness. Have leaned a bit toward the ECMWF.

An coastal trough will help generate periods of showers and
thunderstorms today. Easterly winds over the coastal waters may
actually become southeasterly today due to the coastal trough.
Conditions are expected to reach advisory levels due mainly to wind
speeds east of Freeport with caution levels west of Freeport. That
said, winds and seas will be higher in and near thunderstorms.

Elevated tides are also a concern today. During times of high tides
later today, minor coastal flooding is possible along the Bolivar
Peninsula and near Surfside.



College Station (CLL)      82  71  87  69  78 /  30  20  30  60  60
Houston (IAH)              79  73  87  73  83 /  50  20  30  50  70
Galveston (GLS)            80  78  84  77  84 /  50  40  30  40  70


     for the following zones: Galveston...Matagorda.

     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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