Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Austin/San Antonio Texas
1136 PM CST Thursday Dec 5 2013
precipitation is advancing very slowly from the northwest. Little change
to the cloud cover forecast is expected. Looking for dz to begin
next few hours and eventually turn to rain overnight. Aus may see
freezing rain mix in after about 10z for a few hours. With the mix of rain
and freezing rain not expecting significant impact on operations. Precipitation
should end by late morning Friday. Winds will be from the north at
10 to 15 kts for most of the period.
Previous discussion... /issued 932 PM CST Thursday Dec 5 2013/
we have updated the forecast to include Val Verde County in the
Winter Weather Advisory.
temperatures continue to slowly trend downward this evening as
cold air filters into the region. Given the latest temperature
trends across the southern Edwards Plateau and increasing radar
returns out west...we have opted to include Val Verde County in
the Winter Weather Advisory. The main area of concern will be the
northern half of Val Verde County as this area is most likely to
see temperatures at or below freezing. While we did lower the
overnight low at del Rio a few degrees...suspect most of the
freezing precipitation will remain north of the city.
Otherwise...no other significant changes have been made to the
forecast at this time. We did opt to lower overnight lows along
the Rio Grande plains.
Previous discussion... /issued 339 PM CST Thursday Dec 5 2013/
Short term (tonight through Friday night)...
the early stages of these south central Texas winter weather events
can be tough to pin down when the expected overrunning
precipitation is delayed. Radar trends upstream over west central
Texas depict rain and freezing rain behavior more reflective of
the drier NAM or European model (ecmwf) quantitative precipitation forecast forecast...and this is the main reason
to hold off issuing a Winter Storm Warning. However...the delay of
precipitation onset has allowed for a broader area of south
central Texas to cool down to the middle 30s to near 40...which
should result in a wider impact in both time and space for
freezing rain and freezing drizzle to develop tonight into Friday.
Afternoon temperature trends have leveled off for the most
part...but with nightfall and the increased potential for
evaporative cooling effects from the onset of rain into the dry
air mass...thus will increase the speed of cooling this evening
and expand the Winter Weather Advisory forward in time to 8 PM to
account for the colder areas of the Northwest Hill country.
By daybreak...the freezing temperature line could sag south to a
Rocksprings to Boerne to Lockhart to Giddings line by daybreak
Friday. Higher rainfall rates has previously been expected to
overwhelm the surface inversion near and to the east of Austin
metropolitan...but the drier initial conditions could enable a few
pockets of 32 degree weather and efficient icing from the higher
Areas generally along and north of a Fredericksburg to Giddings
line will need to be watched closely tonight for a possible Winter
Storm Warning upgrade...but the current thinking is that areas of
moderate sleet showers would be needed to enhance the icing
potential to create this kind of impact. Precipitation is expected
to diminish fairly quick from west to east after daybreak
Friday...but the cold air at daybreak and cooler trends in the
forecast highs for the day may make icing concerns last beyond
noon. Subsidence Friday night is expected to continue lowering
temperatures to well below freezing with a morning freeze expected
across nearly the entire forecast area.
Long term (saturday through thursday)...
the cold conditions Saturday morning could wind up resulting in a
higher concern for icing from freezing drizzle over the weekend.
Currently precipitation amounts are expected to be very light. The
areas of concern would likely fall east of Highway 281 from midday
Saturday through midday Sunday...with the GFS model again showing
higher potential than the NAM/ECMWF. The departing shortwave
passing to the north Sunday will leave subsidence and increasing
depth of the cold and dry layer...which should bring an end to the
icing potential for the weekend. However...cold days remain Monday
into much of next week...and another weak disturbance is expected
to bring a chance of light precipitation on Wednesday. With a
deeper layer of dry air expected to remain over the area at this
time...will continue with silent 10 probability of precipitation for now. With the models
continuing to show an active subtropical jet will continue partly
to mostly cloudy skies with below normal temperatures for the
remainder of the extended forecast.
Preliminary point temps/pops...
Austin Camp Mabry 33 37 26 33 29 / 80 60 10 20 30
Austin Bergstrom international Airport 32 37 25 34 28 / 80 60 10 30 40
New Braunfels Muni Airport 34 39 26 34 30 / 80 50 10 20 30
Burnet Muni Airport 29 34 22 30 26 / 90 40 10 20 30
del Rio International Airport 37 45 32 40 31 / 40 10 0 - 10
Georgetown Muni Airport 30 34 22 32 27 / 80 60 10 20 30
Hondo Muni Airport 37 43 30 37 30 / 70 20 10 10 20
San Marcos Muni Airport 32 38 26 34 30 / 80 60 10 20 30
La Grange - Fayette regional 35 38 29 36 32 / 60 80 20 40 40
San Antonio International Airport 35 40 29 35 30 / 70 40 10 20 30
Stinson Muni Airport 37 41 29 36 31 / 70 40 20 20 30
Winter Weather Advisory until 2 PM CST Friday for the following