FXUS64 KHGX 131155
AFDHGX
Area Forecast Discussion
National Weather Service Houston/Galveston TX
655 AM CDT Sun Sep 13 2020
.AVIATION [12Z TAF Issuance]...
Satellite picture is looking clear this morning with all terminals
reporting VFR conditions, which should generally persist through
the current TAF period aside from temporary reductions caused by
afternoon storms. Precipitation coverage will be on the rise by
the afternoon as a disturbance in the Western Gulf has provided a
surge of low- level moisture to SE Texas. Although latest high-
resolution models have backed off on initial shower coverage,
expect thunderstorms to begin to develop around 18Z with the
greatest coverage between 21 and 00Z. with all terminals likely to
see some development. Have maintained PROB30 groups from previous
package, but have expanded to include wording at all terminals
given higher confidence in precip coverage compared to yesterday.
TEMPOs for gusty winds and reduced visibilities will likely be
needed in subsequent TAF packages.
&&
.PREV DISCUSSION... /ISSUED 404 AM CDT Sun Sep 13 2020/
DISCUSSION...
The first of the Gulf disturbances we`ve been tracking the past
several days is making its way by our area well offshore. This
will funnel in higher moisture air to the area, helping boost rain
and thunderstorm chances for a day or two. Then attention turns
to Tropical Storm Sally in the eastern Gulf.
There is increasing confidence that Sally will track far to our
east, which will limit or eliminate any direct impacts. However,
the situation remains fairly fluid as to the indirect impacts of
the storm in our area. For marine interests, we will be watching
carefully how quickly the storm strengthens in the open waters,
and how that trend will affect marine winds and seas.
For those of us on land, the track of the storm and size of its
wind field will at least modestly influence our winds. Where we
stand relative to the storm`s moisture envelope and subsident edge
will pretty strongly influence our rain chances later in the
week. The forecast is trending drier as expectations become for a
more organized storm. Still, it wouldn`t take much of a wobble or
an expanding moisture envelope to put us right back in a situation
in which we see outer fringe rains from the storm, so we may not
want to get super attached to the mid-week forecast just yet.
Finally, late in the week, we will go back to looking at the
potential of an incoming front. College Station has already gotten
its first taste of fall, while most of the Houston metro is still
waiting. I don`t know if this is going to be the front to do it,
but it looks strong enough that it is at least a possibility on
the table for now.
SHORT TERM [Through Monday Night]...
Rainfall chances will once again be on the rise as we close out the
weekend. The moisture axis associated with an upper-level inverted
trough/weak surface low in the western Gulf of Mexico continues to
advance towards SE Texas, with the 12Z suite of deterministic model
guidance in fairly good agreement showing surface PWs rising to the
vicinity of 2.5 in today near the coast and across the Houston metro
area. While increasing cloud cover may inhibit heating slightly,
convective temps in the lower 90s should nonetheless be reachable by
the afternoon. Have backed off slightly from forecast highs in the
latest national blends given the expected precipitation coverage and
cloud cover, though most locations should still see a rise into the
90s by late afternoon. Given the ample near-surface moisture, I
remain confident in widespread shower/storm coverage and as a result
have maintained likely PoPs (60-70%) for most of the area today.
Severe impacts are currently not anticipated, though any stronger
storms that do develop this afternoon may produce brief locally
heavy downpours and gusty winds. Convection should taper off this
evening with the loss of daytime heating.
Heading into the beginning of the work week, our attention turns to
what is now Tropical Storm Sally in the Central Gulf of Mexico. SE
Texas remains unlikely to receive any direct impacts with most model
guidance currently depicting a landfall near the MS/LA border,
though the approach of the system will result in continued moisture
availability near the surface as well as increased seas across the
coastal waters (see Marine section). Showers and storms look to once
again develop throughout the day on Monday, though coverage should
be a bit more confined to the coast where near-surface moisture
availability will be best. Highs near climatological normals are
expected with most locations seeing a rise into lower 90s.
LONG TERM [Tuesday Through Sunday]...
By Tuesday morning, Sally should be approaching the northern Gulf
Coast. The official forecast is for it to be a hurricane as it
does so, which seems very probable given the expected environment
and model guidance. This actually makes for a fairly important
distinction to our sensible weather, considering how far to the
west of the forecast track we are - even with the recent leftward
shift in the most recent advisory.
Essentially, our earlier forecasts worked under the assumption
that the Sally would remain a fairly broad and disorganized storm,
steered more by lower level winds which were more easterly. This
brought us well under the influence of its moisture envelope,
resulting in high rain chances.
Since then, the guidance consensus has moved strongly upwards, and
the forecast is now for a hurricane in the north central Gulf.
While still clearly feeling some northwesterly shear, Sally`s
satellite appearance is improving, and the upper levels of the
storm look favorable. A stronger, more organized storm is likely
going to have become a more consolidated structure/moisture
envelope. Instead of getting swiped by a broad, messy, slug of
tropical moisture, there`s an increasing chance we end up with
drier northeast flow and square under the storm`s subsident
fringe. As a result, I`ve continued my trend of hacking at the
PoPs in response to this. Sadly for me, I have nobody else to
point the finger at - it`s my own initial forecast and I have
nobody to curse but myself on this one.
Beyond the impact of Sally, the big feature of the forecast in the
extended is the potential for another cold front to drop into the
area - now that we`re pushing into mid-September, this will become
more frequent. The last front overperformed and managed to give
even the westernmost parts of the Houston metro a (very brief)
taste of fall. This next front - broadly speaking - also looks
pretty strong. But here`s the thing - this subtropical ridge is
also quite strong. If not for a combination of that storm, Sally,
and a shortwave getting drawn from Northwest Mexico out of the
southern stream and into the northern, I don`t think the upper
trough with this one would really dig any deeper than the Ohio
Valley.
So, while there is a pretty chilly cold pool this front is
working with, I`d expect that cold air to stay hemmed up far to
our north. Cold advection down at our end of the front will likely
be pretty minimal. Instead, we`ll look for the wind shift to
bring us drier post- frontal air, which should allow for better
overnight cooling. With the lower temperature floor and past the
solar angle maximum, that should help afternoon highs also drift
in a cooler direction late in the week. But, uh...beyond that and
some higher PoPs as the front pushes through...I`m not ready to
commit to anything stronger than that yet.
MARINE...
Light to moderate offshore flow should continue through the
remainder of the weekend and into the early part of next week. As an
upper-level inverted trough continues to advance into the W/SW Gulf,
a surge in moisture associated with this feature will result in
shower and thunderstorm coverage increasing this afternoon. Gusty
winds and brief heavy downpours will be possible with any storms
that do develop.
Although SE Texas is not expected to receive any direct impacts,
Tropical Storm Sally`s approach towards the coast this week will
bring an increase in seas to up to 5 feet across the coastal waters.
The system remains on track to make landfall near the MS/LA border.
Mariners in the Gulf should continue to monitor the latest tropical
weather information closely. The most up-to-date forecast
information can be found at
www.hurricanes.gov.
TROPICAL...
The Atlantic basin continues to have a lot of activity. Here`s the
scorecard to what`s out there and their implications for Southeast
Texas:
Weak Gulf Disturbance: Passing along the Texas coast, well
offshore. It will help provide a little moisture to help showers
and thunderstorms get going today into tomorrow, but that`s about
the extent of its impact.
Tropical Storm Sally: Now in the eastern Gulf, Sally is forecast
to gradually strengthen and head towards the north central Gulf
coast. Landfall somewhere on the Lousiana, Mississippi, or Alabama
coast is anticipated on Tuesday as a hurricane before getting
dragged to the northeast. While we should see our weather
indirectly influenced by the storm, confidence is increasing that
we will see no direct impact.
Hurricane Paulette and Tropical Depression Rene: Both in the open
Atlantic, both are likely already too far north and too likely to
be caught up by Atlantic troughs to be a concern for Southeast
Texas.
Tropical Depression Twenty and Invest 97L: TD 20 has been
designated. The NHC forecast track and model guidance continue to
provide positive indications that neither will be a concern for
Southeast Texas. Still, both storms are very, very far to the east
and at a low latitude, so it will behoove us to continue to
monitor both for any changes to those expectations.
CLIMATE...
For the second day in a row, Galveston has tied its record high of
94 degrees. Friday`s temperature matched the record last seen in
1937, and Saturday`s temperature ties a record that dates back to
1912. Today, Galveston`s record increases to 95, and with greater
cloud cover and winds with a bit more of an onshore component
expected, we should come in beneath the record today. Still,
things could become more interesting if the sky is sunnier than
anticipated, and especially if winds can switch - even temporarily
- to being offshore.
&&
.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL) 91 73 89 72 91 / 40 20 30 10 20
Houston (IAH) 91 77 93 77 94 / 60 30 30 20 20
Galveston (GLS) 90 81 91 81 92 / 70 50 30 30 30
&&
.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...NONE.
&&
$$
DISCUSSION...Luchs
SHORT TERM...Cady
LONG TERM...Luchs
AVIATION...Cady
MARINE...Cady
TROPICAL...Luchs
CLIMATE...Luchs