Area Forecast Discussion
Issued by NWS Houston/Galveston, TX

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FXUS64 KHGX 211727

Area Forecast Discussion
National Weather Service Houston/Galveston TX
1227 PM CDT Mon May 21 2018

Scattered SHRA and TSRA are expected to continue this afternoon.
Another round of low ceilings and fog (mainly MVFR) will be
possible again late tonight through tomorrow morning. A majority
of the area should become VFR by late tomorrow or early tomorrow
afternoon with less SHRA/TSRA coverage.  42

.PREV DISCUSSION... /ISSUED 1052 AM CDT Mon May 21 2018/


Main change to the forecast is to refine PoPs to radar and
satellite trends. Otherwise there are only tweaks to match the
immediate forecast to obs.

Related to the PoPs change is one area of mild concern - satellite
shows a mesoscale convergence axis has set up through Matagorda
and Wharton counties. So far, rain rates have largely been well
behaved, and the convergence line has been drifting slowly
eastward enough to keep rain from piling up too much. Fortunately,
soils are also quite dry in this area as it did not see much rain
yesterday. However, precipitable water values are nearing two
inches and the Midfield LCRA gauge has recorded 1.68 inches in
the last three hours, so this rainfall is not negligible. The area
will have to be monitored over the next several hours for signs of
the line stalling or rain rates increasing. Satellite appearance
implies that, if anything, the updrafts are weakening, but we`ll
be keeping an eye on it regardless.


PREV DISCUSSION... /ISSUED 349 AM CDT Mon May 21 2018/

At 300 AM, radar was rather quiet with just a few showers near
Matagorda bay. Short term guidance is bullish with redevelopment
over much of the CWA between 09-12z. Yesterday, the Hi_Res ARW
outperformed every other model but it seems overdone today as the
air mass should be worked over. 06z surface analysis shows
several boundaries lying across SE TX and these features will
likely serve as a focus for additional shra/tsra today. Fcst
soundings show convective temperatures in the mid 80`s so would
expect development beginning later this morning and continuing
into the afternoon. NAM soundings much more unstable than the GFS
with PW values approaching 2.00 inches by 21z while the GFS is
only 1.60. 850m mb temps are cooler so trimmed a few degrees from
previous forecast Max T. Will broad brush 50 PoPs today with
lower rain chances near the coast this afternoon. Rain should end
early this evening with the loss of heating. Could get some patchy
fog by early Tuesday morning as skies slowly clear.

An upper level ridge over northern Mexico will expand into Texas
through the mid week with a gradual increase in temperatures. PW
values remain below 1.50 inches and convective temperatures are in
the upper 80`s. Subsidence from the expanding ridge should limit
convective potential but fcst soundings show weak capping that
could be overcome with heating. Can`t rule out isolated showers so
will carry 20 PoPs through the period.

The end of the week is looking more interesting. Both the Canadian
and the ECMWF try develop low pressure over the southern Gulf,
both models vary in their respective placements, with the ECMWF
farther west. The GFS keeps an open wave and moves this feature
toward the FL peninsula. The Memorial Day weekend forecast will be
tied to how this system eventually evolves. FWIW, WPC currently
has a weak low over south central Louisiana and for now will lean
in that direction. Will keep rain chances in the forecast through
the holiday weekend, especially over the eastern half of the CWA.
Confidence is low and the forecast will likely change several
times by the time next weekend rolls around. It`d be a good idea
to keep up with the forecasts for next weekend. 43

Houston Hobby did not receive measurable rain yesterday so that is
29 consecutive days without measurable rain. This is the 12th
longest streak of consecutive days without rain for this location.
Bush IAH only reached 87 yesterday and this ends the 90 day streak
at 8 days. 43

Caution flags are currently flying for Matagorda Bay and western
nearshore waters as winds remain elevated this morning. These winds
should slacken off later this morning. Light onshore flow continues
through around midweek before becoming light and variable. Periods
of showers and thunderstorms are expected on and off through the
period. 11


College Station (CLL)      89  69  91  70  92 /  40  20  20  10  10
Houston (IAH)              88  71  91  73  92 /  50  20  20  10  20
Galveston (GLS)            84  76  84  78  84 /  50  10  20  10  10



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