Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Austin/San Antonio Texas
1021 am CDT Thursday Oct 30 2014
Update...forecast on track for today and tonight with no major
changes in thinking at this time. We are still anticipating
showers and thunderstorms to develop ahead of the cold front
across North Texas late afternoon and evening...moving south into
the northern County Warning Area around midnight and into central and southern
areas of the County Warning Area between 3-6 am Friday. Decent pop chances tonight...
that may be trended further upward in the afternoon package...
especially along the I-35 corridor and west...if 12z runs continue
their trends. Storm Prediction Center has highlighted areas roughly along and north of
a Rocksprings to San Antonio to Bastrop line in a marginal risk
for severe storms...with main threat being a storm or two that
could produce strong to severe straight-line winds and hail. The
severe weather potential statement as been updated to mention this.
Previous discussion... /issued 626 am CDT Thursday Oct 30 2014/
Aviation discussion...for 12z taf package...
early morning stratus deck not showing as much coverage on infrared
satellite this morning. Expect MVFR ceilings to be intermittent
through middle to late morning before improving to prevalent VFR.
Otherwise VFR for the remainder of the day.
Cold front will move through kaus by 08-09z based off hi-res
models and bring -shra to all terminals by morning. MVFR visible may
be a result as some moderate rainfall potential exists. Included
this possibility at all terminals.
Previous discussion... /issued 353 am CDT Thursday Oct 30 2014/
Short term (today through friday)...
light easterly winds continue across the area at the present time
but will become more southerly today ahead of the next system. At
500 mb...there is a broad trough that covers the eastern half of
the Continental U.S. While a weak but broad ridge covers the western half of
the Continental U.S.. a much stronger low center will move into the northern
Continental U.S. Late this afternoon and evening...which will cause the upper
ridge across the western Continental U.S. To quickly become more amplified.
This places the Central Plains in strong north/northwest flow
aloft. This pattern will send a surge of cold air south into
south-central Texas this evening and overnight into Friday morning.
Beginning last night...models have been trending wetter with this
frontal passage. Forecast soundings show a decent moist layer between the
surface and 700 mb. Upper dynamics are mostly lacking which would
normally limit widespread rain chances. The focus for lift then
shifts to any mesoscale features that may be present which could
locally enhance lift. 850 charts from all models show a Theta-E
ridge at 850 mb just ahead of the 850 mb front. The 850 front is
stacked nearly vertical over the surface front and any lift
associated with the Theta-E ridge would be juxtaposed with locally
enhanced lift associated with the surface frontal convergence. The
moisture available is sufficient for convection to develop with
the lift as the front moves through the County Warning Area tonight. Instability
is enough to warrant the mention of thunder...but severe storms
are not expected. The 00z NAM is showing some heavy rainfall
tonight...but there appears to be some convective feedback at 500
mb in the vorticity fields. Sometimes...convective feedback
although wrong...can be a hint at something....but overall
moisture levels in the atmosphere should not support widespread
heavy rain. Isolated locations could receive about an inch of
rain. Went with 30/40 probability of precipitation across much of the area...but these
may need to be raised further on the day shift if confidence
continues to increase.
All precipitation should be south of the forecast area shortly after
daybreak Friday. The rest of Halloween is expected to have clearing
skies...breezy northerly winds...and temperatures struggling to
reach the middle 70s for most locations. You made need to a jacket
or blanket for the trick or treaters as temperatures around sunset
will be dropping into the lower 60s and even the 50s by 9 PM.
Long term (friday night through wednesday)...
southerly winds will return on Saturday after lows reach the 40s
for much of the area...with some 30s possible in the Hill County.
Increasing moisture and warmer temperatures are expected by
Sunday/Monday ahead of the next storm system. Otherwise...the
weekend looks great for any outdoor activities. The GFS is showing
warm air advection shower and thunderstorm activity on Monday. The
GFS normally tends to overdo these sort of things...but with
surface dewpoints quickly reaching the 60s...cant rule out
anything. Will continue with the 20 probability of precipitation for Monday.
The focus of the rest of the forecast then shifts to the strong
upper trough or low center expected to be over the western Continental U.S.
On Monday and its eventual track into Texas. Models had been
having a hard time agreeing on whether the pattern would be
progressive as the GFS has been showing or the low would cut off
and slow as the European model (ecmwf) has shown. These two models continue to have
the same disagreement....but it seems the European model (ecmwf) is slowly coming
into line with the GFS. The European model (ecmwf) now has a front moving through
the entire County Warning Area...although 12 hours slower than the GFS. This is a
sign that the parent upper trough is going to remain strong and
progressive to our north. The GFS continues to show the southern
end of the longwave trough open and progressive before becoming
closed over Texas Wednesday. The European model (ecmwf) continues to cut the low
off early in the week and then move it over Texas Thursday/Friday.
However...the GFS solution remains preferred as there is
considerable agreement between it and the Canadian and the North
American ensemble forecast system. Upped probability of precipitation on Tuesday to 50/60
percent for much of the County Warning Area...with 50s continuing Tuesday
night....and 30s/40s for Wednesday. The GFS and Canadian continue
to show a very favorable pattern for rainfall exceeding 1-2 inches
across much of the County Warning Area. This will be in part to Pacific moisture
from a remnant tropical system meeting the front and strong upper
low at the same time. It is still too early to get into the
specifics of mesoscale features along the boundary...but
confidence is increasing that most of south central Texas will
receive much needed rainfall. Stay tuned.
Preliminary point temps/pops...
Austin Camp Mabry 81 61 76 46 67 / - 40 - 0 0
Austin Bergstrom international Airport 80 57 76 41 67 / - 40 10 0 0
New Braunfels Muni Airport 83 62 76 46 69 / - 40 10 0 0
Burnet Muni Airport 78 57 73 42 64 / 10 40 - 0 0
del Rio International Airport 85 64 75 51 71 / - 30 10 - 0
Georgetown Muni Airport 78 57 73 42 65 / - 40 - 0 0
Hondo Muni Airport 82 60 77 42 71 / - 40 10 0 -
San Marcos Muni Airport 81 60 76 44 68 / - 40 10 0 0
La Grange - Fayette regional 80 59 76 45 67 / - 40 10 0 0
San Antonio International Airport 84 64 77 49 69 / - 40 10 0 -
Stinson Muni Airport 84 63 79 49 70 / - 40 10 0 -
public service/data collection...32