Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Austin/San Antonio Texas
1107 PM CST Friday Nov 27 2015

not expecting much improvement in flying conditions during this
taf period. Ceilings will generally control category. Ceilings and visible will
vary between MVFR and IFR depending on rain showers. Behind the
cold front winds will continue to be from the north at 1o to 15
kts with gusts up to 25 kts. Winds should decrease late morning


Previous discussion... /issued 919 PM CST Friday Nov 27 2015/

quick update to maximum probability of precipitation at 100 percent for the next several hours
from Bexar and Medina counties...up through Llano County. This
area on radar has been fairly persistent with the northward trend
of rain and isolated thunder. Kept the isolated thunder in for the
next 3 hours...but removed after midnight. We have seen only 1 or
2 strikes over the past 3 hours and with the surface getting
colder and no evidence of strong lift coming in from the
west...feel that the risk of thunder is dropping quickly. Have
also reduced pop out west to 50 percent based on current trends
and lack of enhancement on infrared satellite. All the mesoscale models
including the hrrr shows scattered rain areas overnight without
much in the way of organization. Feel that our current quantitative precipitation forecast looks
good with an additional 1-2 inches overnight in some spots.

Front is just about out of our County Warning Area with it being over southern
Atascosa County and over the southern sections of our coastal
counties. Areas that have already see the frontal passage will get
near steady temperatures overnight. Areas in the western Hill Country
have seen some middle to upper 30s already but all models keep the
area above freezing so do not expect any ice at this time...just a
cold rain. With all the metars showing near saturation and rain
expected to continue...there is not much room for wet bulbing.
Will continue to monitor.

Previous discussion... /issued 233 PM CST Friday Nov 27 2015/

Short term (tonight through Saturday night)...
as of 3 PM...the cold front was located along a line from near
Lexington to Austin to del Rio. Temperatures ahead of the boundary
are in the middle to upper 70s with temperatures behind the front quickly
dropping into the 40s. The front is slowly pushing south and
expect it to continue to do so through the rest of the afternoon
and evening. The front should be at our southern border around
midnight and then clear the area shortly after. Some of the model
guidance is coming in with slightly higher totals for our area
for tonight just behind the frontal boundary. The GFS is showing
some 2 inch in 6 hour rain totals for the I-35 corridor behind
the front. Am a bit skeptical of amounts this high for this
area...but it is Worth noting. The GFS seems to have initialized
the precipitable water values too high with 2 inch values across the southeastern
half of the area. The Storm Prediction Center mesoanalysis only shows 1.8
inches...which is still abnormally high for this time of the year.
Do think rain will fall for the I-35 corridor and even if 2 inches
fall over 6 hours...rain rates should be less than an inch per
hour as the bulk of the heavy rain will fall in the cold sector
and the best warm-rain processes will be shut down.

Continue to think the higher amounts of quantitative precipitation forecast may actually fall
across the western counties....between del Rio and San Antonio.
The various WRF models and now the hrrr continue to forecast a weak
surface trough to develop near del Rio...then track to the east-southeast in
the evening hours. This feature would likely help locally enhance
the lift to support better precipitation rates. The low will also
allow for slightly warmer temperatures to persist in the area as
some of the air becomes wrapped around the surface low also
allowing for slightly better precipitation rates. In addition to the
expected surface trough...850 mb moisture transport vectors are
pointing to this region as well for the best potential for
heavier rainfall. We are already seeing more enhancement in the
reflectivities across the western counties versus the eastern and
this is likely the reason. Our storm total precipitation graphic for this
evening through Sunday will show the highest amounts along an axis
from Zavala County up into Medina/Uvalde and northeast into The
Hill Country and I-35 corridor. Am expecting 1-2 inches of
additional rain for this area with isolated amounts reaching 4+.

While there are abnormally high amounts of moisture in the
atmosphere and a front...there are several ingredients lacking for
a more significant rainfall event for our area. The first is deep
forcing for lift in the atmosphere. The area remains between the
upper low centered over Utah and a ridge in the Gulf of Mexico.
The best shear axis /lift/ remains well north of the area. This
means our main lifting mechanism will be warm air advection over
the frontal inversion. Second...our region is also underneath the
right exit region of the upper jet which also isnt favorable for
deep convection. Third...we do have a cold front in place but
this front is not expected to stall and thus will not serve as a
focus for training of convection. Lastly...the 850 front will
remain north of the area...detached from the surface front which
will not enhance frontal lift along the boundary. To
summarize...we are still expecting some higher rain amounts in
localized places...but we are lacking mechanisms for intense
rainfall rates out of the expected activity.

Lows tonight will drop into the 30s/40s for much of the area and
highs tomorrow will not warm much from those values. Overrunning
conditions are expected to persist tomorrow afternoon and will
continue with fairly high rain chances. Precipitation amounts should be
less tomorrow as the middle-levels dry out from today. The middle-levels
will continue to dry Saturday night...lowering rain amounts even
further. The higher rain chances and precipitation amounts are expected
to be north of closer proximity to the shear axis. Lows
tomorrow night will drop down into the 30s and 40s once again.

Long term (sunday through friday)...
for Sunday...high temperatures will rebound a bit...but not by
much as north winds remain at the surface. Southwest flow aloft
will also persist and expect low probability of precipitation to continue for light
showers. Upper flow is expected to become more westerly by Monday
which should shift the rain chances to the eastern half of the
area...and these probability of precipitation will only be 20 percent.

The strong upper low currently over Utah will finally push east
and eject into the plains by Monday afternoon. This will send
another reinforcing cold front into the area Monday night into
Tuesday night. This will allow north flow to continue for the
remainder of the forecast which should equate to continued below
normal temperatures. Large scale lift arrives to the area Tuesday
night into Wednesday as an open trough approaches the area. This
should allow for decent isentropic lift to set up over the area
and we could see more widespread rain develop. This activity looks
to be much lighter than the current event and more typical for
the winter season. The forecast will dry out for Thursday and
Friday with continued below normal temperatures.


Preliminary point temps/pops...
Austin Camp Mabry 44 46 43 49 45 / 90 80 70 60 30
Austin Bergstrom international Airport 43 47 44 49 44 / 90 80 70 60 30
New Braunfels Muni Airport 45 47 44 51 45 / 90 80 70 60 20
Burnet Muni Airport 41 44 40 47 42 / 100 80 70 70 20
del Rio International Airport 41 47 44 57 45 / 80 60 50 30 -
Georgetown Muni Airport 40 44 41 48 42 / 90 80 70 70 30
Hondo Muni Airport 43 48 44 53 46 / 100 80 70 50 10
San Marcos Muni Airport 44 46 44 50 45 / 90 80 70 60 30
La Grange - Fayette regional 50 50 44 51 48 / 80 70 60 60 30
San Antonio International Airport 47 48 45 51 47 / 100 80 70 60 20
Stinson Muni Airport 49 50 46 53 48 / 100 80 70 50 20


Ewx watches/warnings/advisories...



National Weather Service Glossary of Abbreviations