Area Forecast Discussion
Issued by NWS Austin/San Antonio, TX

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000
FXUS64 KEWX 150507
AFDEWX

Area Forecast Discussion
National Weather Service Austin/San Antonio TX
1107 PM CST Sun Jan 14 2018

.AVIATION... /06Z TAFS/

Coastal cig/vsby obs continue to suggest delayed mvfr conditions near
I-35 through at least 09z and probably closer to daybreak. Light
decoupled evening winds may also play a role in the slow to return
clouds. CIRA-SIMWRF fog product shows an aggressive spreading of low
level saturation, so DRT may only be an hour or two in seeing the low
cigs versus the I-35 sites. Boundary layer moisture should be less
saturated at DRT at 12Z, so conditions could potentially drop to
lifr there. Once the moist layer is established will defer to the
more pessimistic NAM guidance on an afternoon return to vfr skies and
mid evening return back to mvfr.

The much anticipated arctic front enters the picture toward the end
of the TAF periods, and an earlier timing is adopted. Will show a
primary FZRAPL for AUS, but leave SAT at RAPL until the overnight
crew can further assess the timing of post-frontal cooling to
freezing.

&&

.PREV DISCUSSION... /ISSUED 931 PM CST Sun Jan 14 2018/

UPDATE...
No major changes needed to the short-term of the forecast tonight, as
the previous forecast is mostly on track. Only changes were made to
the hourly grids based on current trends. 00z guidance is beginning
to come in and a full update to the package will be done tonight
regarding the upcoming winter weather on Tuesday.

&&

PREV DISCUSSION... /ISSUED 250 PM CST Sun Jan 14 2018/

SHORT TERM (Tonight through Monday Night)...
With southerly flow in place temperatures overnight tonight into
Monday will be warmer than we have seen the last few days. Clouds
will also be on the increase overnight and will remain in place
during the day on Monday. Highs Monday will top out in the lower 60s
and will be the warmest temperatures Central Texas will see through
the work week.

Models have trended a bit earlier with the cold front passage with
the NAM, GFS, and ECMWF bringing it into the Hill Country and Austin
area by midnight, Del Rio and San Antonio around 3am, and into the
Coastal Plains by daybreak on Tuesday. Precipitation due to
isentropic lift will be ongoing ahead of the front and will continue
into Tuesday. Temperatures will plummet behind the front. To use
Austin as an example temps will go from the upper 50s Monday evening,
to around 48 at midnight, to 35 at daybreak (behind the front), and
continue dropping into the mid to upper 20s by Tuesday afternoon. See
the LONG TERM discussion below for a more in depth look at the
wintry precipitation forecast beginning Monday Night.

LONG TERM (Tuesday through Sunday)...
Bottom line up front is a wintry mix of precipitation and very cold
temperatures are likely across most of Central Texas beginning Monday
night through Tuesday.

Synoptically isentropic lift and dynamic lift from impulses embedded
in the longwave 500mb trough will help produce precipitation ahead
of the front moving through Monday night into Tuesday and continue
precipitation through the day on Tuesday. Precipitation will linger
into the afternoon hours behind the front but begin to clear from
north to south late Tuesday afternoon and evening. Atmospheric
soundings from the NAM and GFS show precipitation transitioning from
rain to freezing rain and sleet to snow across western and northern
parts of Central Texas. The GFS soundings in these northern area are
more moist in the dendritic growth zone which will favor a switch to
a snow/ice mix across these areas.

This leads to a discussion on precipitation types. Based on the
timing of the front and the strong cold air advection behind the
front the Hill Country and Austin Metro area could see the wintry mix
beginning after midnight and continuing through 6am. This transition
will happen between 6am and noon for San Antonio and the U.S. HWY
90/I-10 corridor, and during the afternoon and evening for the far
southern areas. Initially the mix will mainly be sleet and freezing
rain, but as the atmosphere continues to cool as discussed above some
snow could possibly be mixed in across the Hill Country and Austin
area during the afternoon, and across parts of the Rio Grande plains
during the afternoon and evening. As precipitation exits late in the
day into the overnight hours parts of the coastal plains could also
see some of the wintry mix before precipitation ends.

In terms of accumulations and impacts as of right now we are looking
at possible accumulations of 1/10 of an inch of freezing rain and
sleet with some isolated higher accumulations possible up to 1/4 of
an inch. This does not include any possible snow accumulations,
remaining less than 1/2 an inch, which will again be focused across
the northern and southwestern areas. With this much ice and
temperatures expected to dip down into the 20s Tuesday afternoon the
icing could impact not just the typical bridges and overpasses, but
also primary and secondary roads. This would create significant
travel impacts during the day on Tuesday into Tuesday night as
temperatures remain well below freezing.

Now that we have covered the precipitation expected we do need to
talk briefly about temperatures which will fall through the day on
Tuesday. Model guidance is dropping temperatures by Tuesday afternoon
into the upper 20s across parts of South Central Texas, including
the Austin and San Antonio metro areas. Coupled with the strong north
winds this will create windchill values in the teens for most areas.
These windchill values will continue overnight as lows drop into the
low 20s and upper teens. So after the precipitation ends the cold
will continue to be a concern. This also means that any ice/snow that
does accumulate will stick around through the night Tuesday into
Wednesday morning until temperatures rise above freezing Wednesday
afternoon. There is only a 5 to 8 hour window on Wednesday afternoon
where temperatures will be above freezing before dropping again for
Thursday night.

The GFS and ECMWF are now more in line with one another clearing out
precipitation late Tuesday evening from the Interstate 35 corridor
and out of Central Texas completely by Wednesday morning. Wednesday
will be dry and cold with temperatures slowly rising above freezing
by the afternoon.

Another strong upper level disturbance will approach and move
through South Central Texas Wednesday night and Thursday. With the
cold airmass in place and more precipitation expected, a second round
of wintry precipitation will be possible overnight Wednesday across
the Edwards Plateau and Rio Grande Plains, stretching into the Hill
Country by Thursday morning. The 12z GFS has shifted the focus of
this freezing rain and sleet event further west than previous runs.
Confidence for this second event remains low to moderate as some
differences in the models exist with temperatures and the location
of the wintry precipitation. For now have gone with low end PoPs and
a rain/sleet/freezing rain mix. Of note is the colder trend in the
GFS and ECMWF with temperatures. This would result in a larger area
being impacted by wintry precipitation. Temperatures warm above
freezing by noon Thursday transitioning all precipitation to rain as
the system exits.

Southerly flow returns Friday with dry and warmer weather Friday and
Saturday. The next trough will move across Texas late Saturday into
Sunday dragging a cold front with it. Models are showing slight rain
chances across areas east of Interstate 35 along the front as it
moves through for Saturday night and Sunday.

&&

.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry              39  62  28  29  22 /   0  10  70  60  10
Austin Bergstrom Intl Airport  37  62  29  30  23 /   0  10  70  60  20
New Braunfels Muni Airport     38  63  30  30  23 /   0  10  60  60  30
Burnet Muni Airport            34  58  24  25  18 /   0  10  70  60  10
Del Rio Intl Airport           39  61  34  34  27 /   0  -   60  50  10
Georgetown Muni Airport        37  60  25  27  19 /   0  10  70  60  10
Hondo Muni Airport             39  64  33  33  24 /   0  10  60  60  20
San Marcos Muni Airport        37  63  30  30  22 /   0  10  60  60  20
La Grange - Fayette Regional   38  64  31  31  22 /   0  10  50  50  30
San Antonio Intl Airport       40  62  32  32  25 /   0  10  60  60  30
Stinson Muni Airport           40  64  34  34  26 /   0  10  50  60  30

&&

.EWX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late Monday night through Tuesday afternoon
for the following counties: Bandera...Bastrop...Bexar...Blanco...
Burnet...Caldwell...Comal...Edwards...Fayette...Gillespie...
Gonzales...Guadalupe...Hays...Kendall...Kerr...Kinney...Lee...
Llano...Medina...Real...Travis...Uvalde...Val Verde...Williamson...
Wilson.

&&

$$

Mesoscale/Aviation...Oaks
Synoptic/Grids...Hampshire
Public Service/Data Collection...33


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