Area forecast discussion
National Weather Service Houston/Galveston Texas
354 PM CDT Tuesday may 21 2013
Discussion...
a strong upper level trough moving across the Southern Plains was
combining with a cold front to generate thunderstorms across North
Texas this afternoon. A fairly large cap was inhibiting any
convection over the hgx County Warning Area through 330 PM.
However...do expect the cap to erode during the latter part of the
afternoon especially over the northern half of the area. The high
res models were pushing the line of thunderstorms into the far
northern set of counties between 5 and 7 PM...and then into the
central set of counties between 7 PM and midnight. The model
forecast soundings and a special 18z sounding taken near cll all
forecast the cape to be around 2800 to 3000 and a downdraft cape
around 1450. Feel that the main severe threat will be winds. There
is a lessor threat for large hail and cannot rule out an isoalted
tornado due to helicity values of about 150. Agree with the
moderate risk generally along and north of a line from near Kurten
in northern Brazos County to Madisonville to Lovelady to
Centralia. A slight risk was mainly north of a line from Eagle
Lake to Houston to Liberty. The main time frame will likely be
during the evening between 6 PM and midnight.
The models bring another shortwave trough across the area on
Wednesday. Combining this fact with a seabreeze and the onshore
flow means rain chances should linger through the day on
Wednesday. An upper level ridge will then build over southeast Texas through
the weekend. The European model (ecmwf) and GFS differ on the strength of the ridge
but the main effect will be a drying trend with high temperatures
reaching to near 90 or into the lower 90s. Both models then
develop a northwesterly flow Monday through Wednesday.
40
&&
Marine...
a moderate to occasionally strong onshore winds will persist through
this evening in response to the approaching upper level disturbance/
storms associated with it. Will let the scec expire as scheduled. As
high pressure builds into the area the gradient is expected to relax
and winds should begin to diminish overnight into Wednesday. However this
onshore pattern is expected to continue through the rest of the week
and into the weekend (and mayhap the rest of the summer). 41
&&
Aviation...
Did not make a lot of changes with the 18z tafs...just mainly tweak-
ing the timing of the onset of the storms approaching from the north
and west. Models have not terribly consistent with the timing of the
precipitation...but based on current trends/extrapolations it looks like thunderstorms and rain
times may have to be re-tweaked again. 41
&&
Preliminary point temps/pops...
College Station (cll) 73 91 72 91 71 / 60 20 10 10 10
Houston (iah) 73 90 72 90 72 / 50 30 10 20 10
Galveston (gls) 75 84 75 81 74 / 40 30 10 20 10
&&
Hgx watches/warnings/advisories...
Texas...none.
GM...small craft should exercise caution until 4 PM CDT this
afternoon for the following zones: waters from Freeport to
the Matagorda Ship Channel 20 nm to 60 nm...waters from
Freeport to the Matagorda Ship Channel out 20 nm...waters
from High Island to Freeport 20 to 60 nm.
&&
$$
Discussion...40
aviation/marine...41
