Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 210002
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
702 PM CDT Sun May 20 2018

.AVIATION...
Showers and thunderstorms are building southward this evening while
overall motion of precipitation is toward the east. CLL is
beginning to see clearing while UTS, CXO, IAH are continuing to be
impacted by thunderstorms. An outflow boundary is currently
racing out ahead of the main line of thunderstorms and pushing
through SGR and HOU. Winds will be variable with gusty conditions
beneath stronger storms. Anticipating most of the thunderstorm
development to weaken by 03Z, with showers lingering into the
early morning hours. The precipitation should clear from the
northwest to southeast. Some patchy fog will also be possible
between 09-15Z across our northern TAF sites.

Should see a small break in the way of precipitation for the
coastal sites tomorrow morning. The possibility of shower
development will return at LBX and GLS around sunrise. MVFR
ceilings will begin to rise to VFR in the late morning and into
the early afternoon hours. E-SE winds will return by the afternoon
between 5-10kts.

Hathaway

&&

.PREV DISCUSSION... /ISSUED 404 PM CDT Sun May 20 2018/

DISCUSSION...

Showers and storms will largely be welcome rainfall for much of
the area today. However, there is enough instability that an
isolated strong to marginally severe storm is still possible, and
these strongest storms could create a very localized minor
flooding problem is it dumps enough rainfall quickly enough over a
vulnerable location. Beyond that, we settle into a rather
summer-like pattern for most of the week.

NEAR TERM [Through Tonight]...

For the first time in a while, the radar is lit up with a number
of showers and thunderstorms near and to the northwest of Houston.
IR satellite shows the coldest cloud tops right around -70C,
though most of the area shows warmer tops. As one might expect
given this, rainfall totals have varied wildly from near nothing,
to a maximum gauge total of 2.36 inches north of IAH, though
radar estimates imply a cell has dropped roughly three inches in
extreme southwestern Montgomery County as well.

The streamer showers and storms that had made up the bulk of the
morning`s rainfall has largely merged with a broken line of
thunderstorms following its cold pool`s outflow boundary slowly to
the southeast. Mesoanalysis shows 1500 or so j/kg of CAPE in the
environment ahead of the line, indicating we still have ample
instability for continued storms. DCAPE west of Houston is now
over 800 j/kg, which implies that our main severe threat is
damaging straight-line winds, though even that is a fairly
marginal threat. In addition, high precipitable water values and
a CAPE profile that this evening will resemble more a heavy rain
look, also means that the strongest cells could generate some
briefly torrential downpours, creating localized flooding issues
in areas of poor drainage and near flashy small streams.

However, bulk shear is pretty weak, and this will inhibit updraft
organization and the ultimate severe and rain threats. The threat
will largely be confined to the most robust updrafts before they
collapse and where cells pass over a site repeatedly.
Additionally, since these storms are largely sustained by
instability alone, we`ll see them very slowly wind down into the
nighttime hours once the sun sets. This will further limit
rainfall potential near the coast, where the forecast doesn`t
really have PoPs above 30 percent. Rainfall here may be fairly low
unless the line can push coastward more effectively in the next
several hours.

SHORT TERM [Monday Through Tuesday Night]...

In many ways, tomorrow looks very similar to today. However,
without the upstream spark of a large convective system, I would
expect rainfall potential to be much more subdued despite similar
midlevel heights and what appears to be another shortwave moving
through. Ditto, really, for Tuesday. We`ll definitely have some
things that will help support showers or storms that do develop,
but finding triggers for storms looks to be more difficult. Though
places that have seen plenty of rain today will obviously be
warmer with more isolated convection, temperatures as a whole look
to be pretty stable the next couple of days.

LONG TERM [Wednesday Through Sunday]...

We spend the latter half of the week pinned between a building
ridge over western Mexico and an upper low over the northern Gulf
Coast, which keeps us in a situation with potential for isolated
to scattered showers and storms, but lacking in obvious triggers,
keeping convective potential in check. Heights do look to rise,
and so introduce a slow rise in temperatures into the weekend.

Now, the potential wrench in the whole works: it appears the 12Z
Euro has got a bug in it to bring the upper low lifting out of the
Caribbean Sea towards Louisiana this weekend. This could put an
end to any warming trend, and open us up to higher rain chances,
though I`ve undercut the guidance as I`ve leaned a little more to
the GFS, which was a little more like yesterday`s forecast.

Now, as for what the Euro does next week? I`m not gonna talk about
it. It`s beyond the forecast period, I refuse to discuss it. It`s
probably going to change before it gets into the forecast period
anyway. And if it doesn`t change, then I will discuss it then.

AVIATION [18Z TAF Issuance]...

Impressively large area of showers and thunderstorms from Del Rio
fanning out to near Fort Worth to Houston. Storms have been
getting stronger with the continued destabilization over the area.
Can`t rule out strong thunderstorms with winds over 40 knots with
these probably near the CLL/UTS/CXO terminals this afternoon.
Expect the deeper cold pool with the large cluster of storms near
TPL-T35 to help steer the development southeastward. Have gone
more pessimistic in area TAFs with VCTS this afternoon
interspersed with TEMPO SHRA/TSRA through mid evening. Expect the
area of storms to shift southeastward late afternoon evening and
many of the HRRR/WRF support this movement/development as upper
short wave moves into the area aiding the ventilation. Between
03-08z expect that most precip will wane but a boundary is likely
to be present near the coast around sunrise and anticipate at
least some scattered showers in proximity to the boundary so will
reinstate VCSH near 12z for the coastal sites. After the storms
wane late evening some patchy fog possible in rain cooled areas.
45

MARINE...

Southeast winds 10-15 knots probably getting a little stronger
and gustier tonight as inflow to deep convection shifts close to
the coast with the storms. Storm may spread out into the nearshore
waters this evening before weakening. Boundary remains in the
morning and could be focus for additional showers. Expect lighter
wind regime Monday through Thursday. Will be keeping an eye on the
Eastern Gulf this week with most models indicating lowering
pressures as upper low meanders about.

45

HYDROLOGY...

Despite a good amount of rain upstream and some more rains
expected through tonight, widespread hydrology concerns are nil.
Rivers are well below bankfull, and given the recent dry weather,
the rain will largely be beneficial. The potential for isolated
stronger storms and high available atmospheric moisture does mean
some briefly torrential rainfall is possible with the strongest
cells. This could potentially cause very localized minor flooding
issues on flashy small streams.

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      68  90  70  91  70 /  60  20  10  20  10
Houston (IAH)              71  91  72  91  73 /  90  20  10  20  10
Galveston (GLS)            78  85  78  85  78 /  20  20  10  20   0

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...NONE.
GM...NONE.
&&

$$

Discussion...31
Aviation/Marine...08



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