Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 040034
AFDHGX

Area Forecast Discussion
National Weather Service Houston/Galveston TX
734 PM CDT Fri Apr 3 2020

.AVIATION...

A cold front moving across the region overnight will generally
undercut this ongoing rain and thunderstorm activity with lower
level cooler, drier air. Thus, this TAF forecasts mainly just
light rain or spotty showers going into the early Saturday morning
hours. Moisture trapped beneath the near surface frontal inversion
will produce and maintain IFR and/or low MVFR ceilings through
mid to late Saturday morning...MVFR decks through tomorrow
afternoon. Periods of early evening light precipitation should
end through the midnight hour but return once again across more
southern hubs late tomorrow morning. A series of weak disturbances
passing across a mid level boundary will provide the needed lift
within a more moistened atmosphere (per a southern moisture surge
tomorrow morning) to initiate more persistent Saturday daytime
precipitation. So, once IFR decks lift up to low-end MVFR for
many hubs by noon Saturday then expect increasing rain probabilities
as rain fills in from the west-southwest to the east-northeast.
31

&&

.PREV DISCUSSION... /ISSUED 359 PM CDT Fri Apr 3 2020/

DISCUSSION...

The much advertised start of a stretch of unsettled weather is
finally upon us. As of this afternoon, a severe thunderstorm watch
has been issued for areas on the Edwards Plateau and in Hill
Country to the west of our area, with numerous storms erupting in
that area. Some important notes to keep at the front of your mind:

- We will be on the lookout both for potential severe weather and
  excessive rain. The focus for both will likely be to our west in
  the Hill Country and in the I-35 corridor, but could possibly
  survive into our western area of responsibility, including
  Brenham, Columbus, and Eagle Lake.
- Some potential for heavy rain may persist into our area
  Saturday, but the severe threat will wane. In general, we are
  expecting less heavy rain in short periods from intense storms,
  and more a slow piling up of rainfall totals from light to
  moderate rain over several hours. The latter is likely less
  dangerous to us than the former given drought conditions, but is
  not entirely without risk.
- Periods of showers and thunderstorms are expected to continue
  through the weekend and early next week as unsettled weather
  continues. That should give way to another brief preview of
  summer in the middle of next week.

&&

SHORT TERM [Through Saturday Night]...

Feeder shower bands setting up in the southwest counties indicative
of the strong lift approaching and feeding into the very large
clustering of storms in the Hill Country. As the upper jet arrives
late this afternoon/evening expect this area of showers and
thunderstorms to fill in and spread east and southeastward. As it
does so expect strong thunderstorms with 2+"/hr rain rates over the
western counties shifting southeast and south. Some will probably
make it into Harris county as they weaken. Some spots out west will
probably get some spotty 3-4" amounts and 1/2-2.0" elsewhere. The
areas where these heavier rains should fall will probably be over
the areas that have had the more significant drought of late so good
news there. Gusty winds of 20-30kts and maybe some small hail will
accompany the stronger storms along and south of the front as it
intrudes into the region tonight. On that note have been
undercutting the guidance since 9 am given the strong temperature
falls up north. Apparently not enough and sent a third update just a
few minutes ago. These storms should follow with a quick weakening
trend over the southern half of the region between 8-10
pm...lingering showers by around midnight and probably very light
showers at that. Cool night with temperatures falling into the upper
40s near Caldwell and 60 at IAH...upper 60s still at Galveston. The
front pushes offshore and winds start to strengthen around sunrise.
Next s/w in the series arrives late Saturday morning through the
afternoon and will likely have light rain start with the upglide
followed by freckling of showers and eventually a band of showers
and thunderstorm embedded in the afternoon waffling along the wavy
875-850mb frontal surface...or east to west generally along the I-10
corridor slowly sagging southward in the late afternoon. Put another
way...take your umbrella if you are going tomorrow. Rain chances
focus over the coastal areas in the evening then shift out into the
Gulf.

&&

LONG TERM [Sunday Through Friday]...

By Sunday morning, the upper ridge over the Yucatan should be
working to build and increase its influence over our area. But
with that said, there is increasing confidence that we`ll see
several shortwave troughs/vort maxes streaming through the
southwest flow on the backside of the ridge, keeping at least some
chance of rain in place early next week. These small ripples in
the flow aloft can be very difficult to time, so I don`t try
exceptionally hard at what will probably be a futile effort. I
keep PoPs splattered across early next week, with a slight lean
towards the afternoon hours, as daytime heating would tend to
increase potential instability.

One main exception to "not trying too hard" comes Monday
afternoon. We`ll have two features that point to this being a
particularly favored time for shower and thunderstorm development.
First, there is very strong consensus among the guidance for a
weak coastal trough to emerge (potentially the remnants of the
cold front currently incoming). This will provide onshore flow
from the Gulf for low level moisture, and surface convergence to
force upward motion. Second, there is additional consensus -
albeit not as strong - for a strong enough upper trough that it`s
able to dent the height contours in our Gulf ridging. There`s
probably more predictability in a wave of that strength compared
to the much more subtle disturbances I`ve generally described for
this time frame. (The emphasis here really is on probably, and the
word should be read with an upward inflection, so watch me pay
for this dearly on the verification) I`m selling out to try and
make this period stand out a little more from the rest of the
early week with higher PoPs.

The middle part of next week should see the upper ridge really
flex its muscle for us. And while we may not be enough into summer
yet to completely suppress all convection, I think we should
definitely be trending to hotter, less rain, but still quite
humid. I would fully expect highs on Wednesday to be in the
vicinity of 90 degrees if this upper pattern plays out the way it
does in the guidance.

For the late week, the ridge axis should move far enough east that
we start to see some upper height falls, and some modest relief
from the heat. I`ve got my eye on the bullseye of 2 inch
precipitable water progged in the GFS ahead of a cold front that
should move through at the end of the week. There`s a non-zero
chance we end up in a very similar place to today one week from
now. (It could also be a little bit sooner and be more of a
Thursday thing, but that doesn`t sound as poetic)

AVIATION [21Z TAF Update]...
Active weather tonight across the area terminals. Showers and
thunderstorms the main impacts with MVFR ceilings lowering into IFR
early Saturday morning and persisting into the mid morning hours.

MARINE...

Onshore winds continue to bring in the wind waves and elevated tide
levels continue with a Rip Current statement in effect through 6 am.
Cold front will push off and across the coastal waters and may see
strong storms with the passage. Again could see another flare up of
strong thunderstorms 9 am to 2 pm well offshore. With the increase
in winds after the fropa have hoisted an SCA for 18-21kt winds gusts
25-30kts and seas building to 5 to 8 feet. Wind should finally start
to relax Sunday morning. Warm front moving back in late
Sunday/Monday morning and southeast winds return.

HYDROLOGY...

Much needed rainfall is knocking on our door and dry soils will
help capture most of the stormwater runoff. With that said, the
Brazos/Navasota River basin continues to be the area of main
concern for new river flooding, while the Trinity River is
expected to remain in flood from any additional rainfall over the
next 24 hours.

Current forecasts take into consideration 24 hours of forecast
rainfall, but we will need to monitor streamflow conditions as
the event unfolds especially where the axis of heavy rainfall sets
up. Given the placement of the current bullseye in the rainfall
forecast, the tributaries on the Brazos River are the most
sensitive to rapid rises from intense rainfall rates.

After this initial shot of rain, soils will become increasingly
saturated which may increase our flood potential with additional
rainfall next week.

&&

.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL)      51  57  57  75  64 /  80 100  80  50  30
Houston (IAH)              60  66  62  76  67 /  60 100 100  40  30
Galveston (GLS)            69  72  69  75  71 /  50 100 100  40  20

&&

.HGX WATCHES/WARNINGS/ADVISORIES...
TX...High Rip Current Risk until 7 AM CDT Saturday for the following
     zones: Brazoria Islands...Galveston Island and Bolivar
     Peninsula...Matagorda Islands.

GM...Small Craft Advisory from 5 AM Saturday to 1 AM CDT Sunday 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...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.

&&

$$

Aviation...31


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