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Area forecast discussion 
National Weather Service Austin/San Antonio Texas
409 am CST Friday Nov 21 2014

Short term (today through saturday)...
cloudy skies and patchy fog have overspread south central Texas
overnight as continued southerly flow pumps in warmer temperatures
and higher moisture into the area. The weak lift associated with
the warm air advection has led to isolated shower development
since yesterday and this will continue mainly across the eastern
zones early this morning. Currently...the low-level flow is south-southwest in
the lowest 4 km of the atmosphere. By the afternoon most guidance
members are backing the winds to the south-southeast and increasing the low-
level jet in response to the approaching upper low which is
currently over Southern California per latest water vapor
analysis. The backing of the wind and increasing of the speed will
lead to a strong push of positive Theta-E advection beginning
in the southeast zones...near the coastal plains this afternoon.
The nose of the increasing Theta-E values will then push northwest
to the I-35 corridor by 6 PM and then The Hill Country and Edwards
Plateau by midnight. Positive Theta-E advection is a direct
measure of increasing temperature/moisture which is lift. This
lift can also be seen on isentropic charts on constant Theta
surfaces. Expect about a 50 to 60 percent coverage of showers and
thunderstorms in the form of a broken line beginning this
afternoon across the southeast and then moving northwest through
midnight.

As we get into tomorrow morning...continued weak lift on
isentropic charts will result in isolated to scattered activity
across much of the area. Also by tomorrow morning...the upper low
is prognosticated to be near northern Mexico...entering West Texas.
Models are in fairly good agreement with the placement and
strength of the low as it progresses through the area. This low is
expected to transition from a positive tilt to either a neutral or
slightly negative tilt as it moves over the area Saturday
afternoon. A negative tilt to an upper trough is more conducive
for thunderstorm development as upper diffluence commences ahead
of the trough. Models are forecasting the center of the strongest
lift of the upper low to move across The Heart of our County Warning Area as is
evident by the peak tropopause fold to 430 mb over San Antonio
Saturday night. This should equate to substantial upper forcing
for ascent and will lead to widespread shower and thunderstorm
activity across much of if not all the County Warning Area. The question then
turns to the instability/shear parameters and any mesoscale
features that come into play for severe weather chances.

Models are in fairly good agreement that there will be around 750
to 1500 j/kg of surface based cape across much of the area
Saturday. This amount of instability in addition to 40 knots of
0-6 km shear will be sufficient for organized convection. The main
limiting factor will be the amount of activity and what effect
that will have on severe chances. If there is too much of a
coverage of activity...it will be difficult to realize the peak
instability values. Nevertheless...the most likely scenario will
be widespread activity with embedded rotating thunderstorms as the
activity moves from west to east during the day Saturday. Large
hail and damaging winds will be the main threats. The sherb index
is a forecast tool to help with these high shear/Low Cape events
and current forecasted values are above 1.0 which is favorable for
severe convection...mainly in the form of a qlcs or individual
line segments. This seems reasonable and can somewhat be seen on
the high-res models. The tornado threat is non-zero given the
amount of low-level shear expected over the area. Would like to
see more discrete activity and a focused surface boundary over the
area to expect anything greater than isolated tornadoes.

Models mostly agree that the activity will shut off from west to
east pretty rapidly in the afternoon. By 6 PM...the back edge of
the convection should be just east of the I-35 corridor. Dry west
winds from the surface to 850 mb will move in ahead of the upper
low and this will end things. The outlier is the NAM which is
developing another line of storms around 6 PM west of the I-35
corridor...but the NAM 850 winds are unrealistic. It keeps the
west wind shift directly underneath the 500 mb low...while the
rest of the guidance members have the wind shift ahead of the low
which is more realistic.

The last thing to focus on is the heavy rain potential across the
area. The highest rain totals will be across the eastern and
northern zones. Moisture should pool up nicely in that corridor
as a weak surface low develops just north of our County Warning Area. Precipitable water values
will be around 1.5 or about 2 Standard deviations above the mean
for this time of the year. These areas will likely receive 1 to 2
inches of rain with isolated locations picking up 4 inches.

&&

Long term (saturday night through thursday)...
the rest of the thunderstorm activity will be exiting the area by
midnight Saturday night. Dry west winds will be left in place
Sunday and this should be a warm and sunny day for the area with
highs reaching lower to middle 70s. A longwave trough will pass
through the plains Sunday and will send a dry front into the
region by Monday morning cooling things off once again. The rest
of the week should remain dry with highs in the 60s and lows in
the 30s/40s. Thanksgiving looks dry and seasonal.

Hampshire

&&

Preliminary point temps/pops...
Austin Camp Mabry 72 63 70 53 76 / 40 30 100 20 0
Austin Bergstrom international Airport 73 62 72 51 76 / 40 30 100 20 0
New Braunfels Muni Airport 74 63 72 53 77 / 40 30 80 10 0
Burnet Muni Airport 71 61 68 50 73 / 30 40 100 10 0
del Rio International Airport 71 62 74 50 76 / 10 30 50 - 0
Georgetown Muni Airport 71 62 69 52 74 / 40 40 100 20 0
Hondo Muni Airport 72 62 72 47 75 / 20 30 60 - 0
San Marcos Muni Airport 73 62 71 52 76 / 40 30 90 20 0
La Grange - Fayette regional 73 63 74 56 75 / 50 30 90 30 0
San Antonio International Airport 72 65 73 52 76 / 30 30 70 10 0
Stinson Muni Airport 74 65 74 51 76 / 30 30 70 10 0

&&

Ewx watches/warnings/advisories...
none.
&&

$$

Mesoscale/aviation...03
synoptic/grids...29

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