#1051 Postby weatherdude1108 » Wed Dec 12, 2018 10:20 am
Good discussion by the EWX office.
000
FXUS64 KEWX 120902
AFDEWX
Area Forecast Discussion
National Weather Service Austin/San Antonio TX
302 AM CST Wed Dec 12 2018
.SHORT TERM (Today through Thursday Night)...
Low and high level clouds are in place across much of South Central
Texas at the present time. Southerly flow has also returned and with
that an increase in surface dewpoints. With the increase in surface
moisture, some members of guidance continues to show the possibility
of some fog around or after daybreak for portions of the western CWA.
Have not seen any fog reported yet, but will continue to monitor for
any needed updates to the current area of fog we have in the
forecast.
Latest water vapor imagery shows a strong shortwave trough axis south
of California moving to the east. Ahead of it, clouds/moisture should
continue to stream east over Texas. The influences of this system
will begin to be felt later today with some weak light showers
possible in the eastern counties. Highs today will top out in the
lower 60s to near 70 degrees.
More significant upper forcing arrives this evening as the shortwave
reaches the region. Latest runs of both the synoptic and high-res
models show a decent coverage of shower activity this evening and
overnight. Will show 20 PoPs in the Edwards plateau, 30-40 PoPs for
the Hill Country and I-35/37 corridors and 50-60 PoPs for the eastern
CWA in the tonight period. There should be enough instability to
support some isolated thunderstorms as well and will mention Iso-T
with the chance of showers. Lows tonight will bottom out in the
middle 40s to near 60 with the cooler temperatures west of Highway
281.
A secondary, but stronger upper low will drop in from the northwest
on Thursday into the region. This system is now beginning to enter
the Pacific northwest and can also be seen on water vapor. This low
will be compact but dynamic with tight pressure gradients and a
strong wind field associated with it. The cold front or leading edge
to the higher winds will arrive to the western CWA in the mid-morning
hours before quickly overtaking the rest of the CWA. The main story
on Thursday will be the high winds associated with this system behind
the front. By the mid-late afternoon hours expect sustained speeds
around 25-35 mph with gusts up to 50 mph possible over a large part
of the CWA with the higher winds mainly west of the I35 corridor.
Wind speeds should be highest for the Rio Grande counties. Will
likely need a Wind Advisory for tomorrow and this may need to be
issued on the day shift, or definitely by the midnight shift tonight.
With the expected high winds and dry air there will definitely be a
fire weather threat and for more information on that see the Fire
Weather section at the end of the AFD.
The talk over the past several days has been over the eventual track
of the upper low and the influence that would have for the weather in
our northern counties Thursday afternoon through Thursday night. A
more northerly track would mean we`d get the mid-level dry slot and
not much in the way of precip chances, but a track more to the south
would equate to wrap around moisture and precipitation. As previously
mentioned, the trough axis has now moved over the NW CONUS and was
sampled by 00z upper air soundings. As a result, models have honed in
on a track that favors the southerly track the ECMWF has been showing
over the past several days. By late Thursday afternoon, clouds will
once again be on the increase as the low begins to move east of the
area and wrap around moisture moves into the area.
The focus then shifts on precip type for our northern counties. We
favor a top-down approach of the atmosphere to determine precip type.
By the evening hours, moisture quickly increases with near saturation
from the tropopause down to about 850 mb. Moisture in the dendritic
ice growth zone means snow will be produced by the lift on the
backside of the low. Then looking at the temperature profile, models
show a freezing level across the CWA in the evening hours ranging
from 3250 feet across Llano County with higher values elsewhere. This
would support all precip melting in this surface to freezing level
layer with a rain forecast. However, forecast soundings show a decent
dry layer below 850 mb. If precipitation were heavy enough, some
evaporative cooling would be expected which would cool this small
portion of the column and thus lowering the freezing level. Models
don`t really do a good job simulating this so we will have to infer
that it is possible. With these deep core systems, there can easily
be a burst in precipitation on the backsides that would support this
outcome. Therefore, some snow mixing in with the rain is possible
for portions of our CWA Thursday night as the upper low exits to the
east. For now will include a mention of rain or snow for areas north
of a line from Kerr County to northern Kendall County to Burnet
County. This area is where the GFS forecast surface wet bulb
temperatures of 34 degrees. The wet bulb temperature is the
temperature of the air if full evaporative cooling could take place.
Since we are not expecting temperatures to reach freezing Thursday
night, any impacts from possible snowfall would be minimal with brief
accumulations only possible on grassy or elevated surfaces. Rain
will be in the forecast for other areas north of a Bandera to Comal
to Fayette County line. The chances of accumulating snow will be best
well to our north, in the San Angelo and Fort Worth CWAs.
&&
.LONG TERM (Friday through Tuesday)...
The upper low is expected to push east by Friday morning and will
show a dry forecast Friday and through the early part of next week.
Shortwave ridging is expected to briefly return which should help
keep the forecast dry. Highs will then begin to moderate with highs
back into the middle to upper 60s by Monday. Models have backed up
the timing of the next system into Wednesday instead of Tuesday and
took out the low rain chances for Tuesday.
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The preceding post is NOT an official forecast, and should not be used as such. It is only the opinion of the poster and may or may not be backed by sound meteorological data. It is NOT endorsed by any professional institution including storm2k.org. For Official Information please refer to the NHC and NWS products.
The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or 
Yes, most of the models have the cold core taking a more southern route now moving the core over central Texas and southeast Texas, but I'm not sure there will be enough moisture and cold air to produce snow down here, but cold core lows can be surprising. 
