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Area forecast discussion
National Weather Service Houston/Galveston Texas
415 am CDT Fri Sep 20 2019

Short term [today through Saturday morning]...

.. Flash Flood Watch extended through Friday afternoon...

Radar imagery this morning is continuing to show some spotty showers
developing over Chambers, Jackson, and Polk counties this
morning. Matagorda Bay and the Gulf waters also show some showers
in more of a streamer shower like fashion, or bands oriented
north-south this morning. The center of the upper level low
pressure associated with the remnants of Imelda will continue to
eject to the northeast today, allowing an upper level ridge to
start building in across the region from the east. This region of
high pressure should take hold of the synoptic pattern tomorrow,
but for today, precipitable waters remain high above 2.0 inches
through much of the day into early Saturday morning, before
beginning to lower back down less than 2.0 inches. The Flash Flood
Watch has been extended through this afternoon to account for the
potential for heavy downpours resulting in locally heavy rainfall
that still remains through the afternoon hours. Showers and
isolated thunderstorms look to redevelop between 09 to 12z this
morning, mostly along and east of I-45. In general, bands of
precipitation look to be more north-south oriented compared to
the last two days where convective bands were oriented more west-
east. Development should spread inland and west of I-45 with the
help of daytime heating. The hrrr and conshort which have
performed rather well the last few days, have also initialized
well this morning, so will be leaning closer to their solutions
with this package. In general, the hrrr spreads development west
of I-45 through the late morning and into the afternoon hours
along an axis of speed convergence. These short term models look a
touch sparse in coverage in comparison to what the nam12, arw,
and tt WRF show for this afternoon. Nam12, arw, and tt WRF show a
boundary that starts off east of I-45 in the morning and then
shifts further west throughout the day, with better coverage along
this boundary. Rain rates of 1 to 2 inches look possible with the
stronger storms today. In addition, the 850mb jet still shows
some decent inflow of 25 to 30 knots across the region today and
into early Saturday, continuing to advect in Gulf moisture. Any
additional rainfall that we see today will only exacerbate ongoing
flooding over areas that are already saturated from previous
rainfall over the past few days, therefore decided to extend the
Flash Flood Watch. Where the high resolution models do agree on
the timing of when most of the convection ends for today, which
should be late this afternoon into the early evening hours, with
the loss of daytime heating.

Hathaway

&&

Long term [saturday morning through friday]...
on Saturday, a 592 dm ridge over the deep south will expand westward
and heights will begin to build over southeast Texas. The upper ridge will
become the dominant feature for the rest of the weekend. Moisture
trapped beneath the ridge will allow for mainly diurnally driven
showers and thunderstorms. High temperatures will once again warm
back into the lower 90's over most of the area. A weak inverted
upper trough over the Gulf of Mexico will move west and approach the
middle Texas coast on Tuesday. This will bring a subtle increase in
moisture and lower heights. Will trend a bit cooler with high
temperatures mid-week with a continued chance of showers. An upper
level low over California will dive into the southern rockies on Wednesday
and move toward the Southern Plains on Thursday. The upper flow will
become SW and a few disturbances embedded in the upper flow will
bring a chance of showers to the area. Some differences with the
position of the upper low with the European model (ecmwf) further north and
showing more of a sheared system with the GFS stronger and further
south with this feature. Have leaned toward the better performing
European model (ecmwf). As the mid week system pushes northeast, another upper
level ridge building over the southeast US will once again expand
westward into Texas. This feature will bring drier weather and an
increase in temperatures for the end of the week and into next
weekend. 43

&&

Marine...
small craft should continue to exercise caution over both
Matagorda and Galveston bays and the Gulf waters this morning
through at least the early afternoon hours. Moderate onshore flow
with the occasional gust around 20 knots have been observed over
the past few hours across the Gulf waters and bays, and seas
remain at four feet across much of the Gulf waters. Onshore flow
slacken early this weekend as the upper level ridge builds in.
Additionally, tide levels remain elevated through portions of the
weekend. A coastal flood advisory may be needed for early
Saturday. Tide levels at Galveston Bay entrance rise to 3.5 feet
above MLLW during high tide Saturday. A beach hazard statement is
in effect through Saturday to account for elevated tide levels and
risk for strong rip currents.

Hathaway

&&

Hydrology...
area rivers and bayous responded rapidly to yesterdays heavy
rainfall. Most rivers have crested and have begun to fall. Only the
San Jacinto river at Sheldon has yet to crest but it will later this
morning and then begin to fall. As of 4 am, Peach Creek at
Splendora, East Fork of the san jac at New Caney, West Fork of the san jac
at Humble and Greens Bayou at 59 remain in major flood. Most of the
Harris County bayous are falling and the break in the rainfall
overnight should allow a further drop through the morning. The
Trinity River at Liberty and Moss Bluff may briefly reach flood
stage later this morning. 43

&&

Climate...
after that very wet day yesterday needless to say records were set
for rainfall.

Bush intercontinental recorded 9.21 inches of rain which surpassed
the previous record of 6.92 inches from 1979. This also puts the
rain recorded at iah in first place for the wettest day in September
which moves ahead of the old record Holder of 7.73 inches associated
with Hurricane Ike on September 13th.

Galveston set a record rainfall yesterday as well with 6.81 inches
exceeding the previous 1979 record of 5.11 inches. This will place
yesterday's September 1st through the 19th total at Galveston in the
3rd place for the wettest early September.

Galveston sept 1st-19th rain totals
1885 21.74
2018 18.74
2019 17.83

Amongst the ASOS sites cxo was the unfortunate winners in the
rainfall totals yesterday with 11.18 inches though iah gave it a run
with 9.21.

This begs the question what happened back in 1979 on this date that
held the records...surprise it was another tropical system. Tropical
depression 20 which formed off of Brownsville and moved inland
through southeast Texas producing severe flooding in Brazoria,
Galveston, Harris counties...at the time it was responsible
for two drownings and flooded 1950 homes in Harris County.
45

&&

Preliminary point temps/pops...

College Station (cll) 90 74 91 74 91 / 30 10 40 10 20
Houston (iah) 85 75 90 74 90 / 60 10 40 20 30
Galveston (gls) 84 81 89 79 90 / 60 10 40 20 30

&&

Hgx watches/warnings/advisories...

Texas...Flash Flood Watch through this evening for the following zones:
Brazoria islands...Chambers...coastal Brazoria...coastal
Galveston...coastal Harris...Fort Bend...Galveston Island
and Bolivar Peninsula...Grimes...inland Brazoria...inland
Galveston...inland Harris...Montgomery...northern
Liberty...Polk...San Jacinto...southern
Liberty...Trinity...Walker...Waller.

GM...small craft should exercise caution until 1 PM CDT this
afternoon for the following zones: coastal waters from
Freeport to Matagorda Ship Channel Texas out 20 nm...coastal
waters from High Island to Freeport Texas out 20
nm...Galveston Bay...Matagorda Bay...waters from Freeport
to Matagorda Ship Channel Texas from 20 to 60 nm...waters from
High Island to Freeport Texas from 20 to 60 nm.



&&

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