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Area Forecast Discussion (AFD)

Issued by Houston/Galveston, TX (HGX)

                            
000
FXUS64 KHGX 161718
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1218 PM CDT Sun Jun 16 2019

.AVIATION...
Shower and storm activity filling in from the southwest to
northeast this afternoon. TAF calls for areawide showers and
thunderstorms with periods of heavier thunder that could put down
winds in excess of 25-30 knots and lower visibilites to under 3SM
within heavy rain. After this first round of afternoon activity,
there should be a 3 to 6 hour VFR break early this evening. Then,
there could be an overnight (late evening through early Monday
morning) return to thunder. Confidence is high with this afternoon
convective activity and the moderate for a brief respite this
evening. Confidence is moderate at best for an early Monday 
morning evolution and passage of a Central Texas squall line 
clipping our forecast area in the 04Z to 09Z time frame 
tonight...it really depends on this next 6 hour window of 
convection behavior. 31

&&

.PREV DISCUSSION... /ISSUED 1036 AM CDT Sun Jun 16 2019/ 

UPDATE...
Scattered showers and thunderstorms will begin to fill in across 
the northern half of the forecast area late this morning and into 
the early afternoon hours. A highly moist (1.8 inch pwats) and 
unstable air mass is over us in the downstream of activity that 
is expected to develop just south of the Red River in northeastern 
Texas. Steep mid-level lapse rates equating to gt 3.5 or 4.0k 
J/kg CAPE and -11 L.I.s with the somewhat negatively tilted mid to
upper low/trough over the TX Panhandle creating broad jet level 
diffulence across eastern Texas today. With an upstream jet streak
passing across El Paso this morning, expect this feature to 
possibly enhance a south to southeastern-tracking MCS or QLCS 
feature later today into the evening hours. It appears that the 
window for the threat of severe weather will be late this 
afternoon through the early Monday morning hours. The main threat 
will be downburst winds with the secondary threat, at least early 
in the event, being that of isolated tornadoes. 31

&&

PREV DISCUSSION /Issued 353 AM CDT Sun Jun 16 2019/...

DISCUSSION...

An extremely tricky forecast in the short term as mesoscale
details will wreak havoc on the larger scale setup for at least
the next 36 hours. In general, the environment is not very
conducive for severe and heavy rain threats, but a better
environment farther north may birth clusters of strong to severe
storms that will bring us at least a marginal threat for damaging
winds primarily in multiple rounds.

In the extended period, rain chances look to diminish towards the
middle and late week ahead, as focus for higher temperatures and
humidity emerge towards the middle and later portions of the week
ahead.

SHORT TERM [Through Monday]...

Let's start with the one place there is some actual certainty in
the short term forecast...what's happening now. Radar and
satellite show an organized, but slowly weakening and finally
beginning a broad turn to the right as suggested by CAM guidance.
Meanwhile, at the coast, streamer showers are beginning over the
Gulf and pushing a county or two onshore before dissipating.

GOES-East TPW product shows a plume of high moisture air with
precipitable water pushing two inches, though cloud cover makes
determining the precise extent of this plume impossible.
Regardless, this will support the continued development of
streamer showers on the onshore flow, with a preference for that
deepest moisture channel.

Though the MCS is slowly weakening, it is beginning to finally
make a turn right towards SE Texas. Forecast soundings for
Huntsville show that the low level environment is not terribly
stable, but nor is it terribly conducive for sustaining the MCS.
So, without selling out hard, I gradually paint, and connect the
highest PoPs from the onshore flow and from remnants of the MCS
coming down from the north around 14 or 15Z. By now, uncertainty
is already beginning to climb.

In general, think it's reasonable that we'll see multiple rounds 
of showers and storms possible through tonight and into tomorrow 
morning - the TTU-WRF may not have the worst idea in developing 
and swinging through two to three different MCSs through the area 
while the upper shortwave trough works across Oklahoma. Shear will
be lousy, and so I'm very much in agreement with SPC that the 
severe threat will be marginal for most, with a small area of 
higher potential at our northern edge. I don't think the synoptic 
environment is very supportive at all, but if an MCS riding a 
healthy/balanced cold pool can come out of the better environment 
farther north, it will carry some decreasing severe potential 
before it can weaken in the lesser environment in our area. DCAPE 
values are largely around or above 1000 J/Kg with small inverted-V
profiles, so damaging straight-line winds appear to be the 
primary threat as the updrafts collapse in the strongest storms. 
Hail and tornado threat will be smaller threats relative to the 
damaging wind potential.

I try to carry out a multiple round scenario, with brief lulls in
PoPs followed by another spike as the next round comes in. I also
follow a similar trend with cloud cover, but in general the 
clouds and rain should keep temperatures for today and tomorrow 
suppressed a handful of degrees from what we saw yesterday.

LONG TERM [Monday Night Through Sunday]...

South to southeast winds will persist through the period as
surface high pressure remains centered in/around the eastern
Gulf of Mexico and Florida Peninsula area. The best chance 
of rain looks to be on Tuesday as another possible impulse/
disturbance moves across the area. Cannot totally rule out
possible mainly daytime shower/thunderstorm development for
the remainder of the week as much of the area remains on the
northern fringes of upper level ridging, but think that a
majority of the area should stay on the dry side. Warming
temperatures still look like a good bet with mid to upper 
90s expected for afternoon highs, and low temperatures might
be struggling to fall under 80 degrees at some spots. Heat 
index values could be peaking at or above 105 degrees for
several days, and precautions should be taken for heat 
safety. Currently showing a slight increase in rain chances 
next weekend, and this would shave a couple degrees off the
afternoon high temperatures. 42

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      91  75  91  75  92 /  60  40  60  20  30 
Houston (IAH)              92  76  91  77  92 /  60  50  70  20  30 
Galveston (GLS)            88  80  88  81  89 /  40  50  70  20  40 

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...High Rip Current Risk through Monday morning for the following 
     zones: Brazoria Islands...Galveston Island and Bolivar 
     Peninsula...Matagorda Islands.

GM...SMALL CRAFT SHOULD EXERCISE CAUTION through Monday morning for 
     the following zones: Coastal waters from Freeport to 
     Matagorda Ship Channel TX out 20 NM...Coastal waters from 
     High Island to Freeport TX out 20 NM...Galveston Bay...
     Matagorda Bay...Waters from Freeport to Matagorda Ship 
     Channel TX from 20 to 60 NM...Waters from High Island to 
     Freeport TX from 20 to 60 NM.

&&

$$

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