Austin / Travis County and South Central Texas Weather Update #2
Prepared by UT University/Incident Meteorologist Troy Kimmel
830pm CT – Wednesday / 01 May 2019
… Oh Where, Oh Where Did Our Rain Go ?? …
In the wake of my blown forecast today, I thought I’d take a second or two to
explain what went wrong today with the weather forecast and why it’s important for us to
continue to be prepared for potential stormy and wet weather for the next couple of days.
As I attempt to explain what went wrong, understand the three basic building blocks for
thunderstorms… moisture, lift and instability.
Over south central Texas.. here’s what was going on today..
(1) Moisture, no worries, we had that covered. Aircraft soundings from Austin Bergstrom
International Airport indicated we have a rich layer of low level moisture from the surface up
to about 7500-8000 feet with the atmosphere drying out a bit above that. There was enough of this
low level moisture for a few rain showers today in areas along the IH35 corridor and even a
few thunderstorms in areas east of the IH35 corridor earlier today.. even though, where they
developed, you might have noticed that they struggled to maintain intensity (see below to know
why..).
(2) Lift was problem #1 in today’s forecast. The passing upper air low pressure disturbance
traveled from the west southwest and moved by well to the north of the area. The problem
with that is that the best dynamic lift stayed well north of the area.
(3) Instability was our primary problem today, however. To get a truly an unstable air mass you want
the air temperature, with height in our atmosphere, to get colder quickly as you go up. Today’s
aircraft soundings out at ABIA consistently showed a layer of warmer air aloft (what we call a
temperature “inversion”) that persisted through much of the day between 6,000 and 8,000 feet up.
This temperature inversion simply acted like a “lid” on the atmosphere; only in the last few hours
has it weakened a bit (but not completely gone).
The long and short of it is that warmer and more stable air aloft primarily kept my forecast from
verifying with the secondary lack of dynamic lift a feature in the “demise” of my forecast.
… Our Days Ahead … A Heads Up …
I still believe that we’ll see rain showers and thunderstorms develop over the area, although the
activity will remain rather scattered over the area tonight into tomorrow with the most focused
activity most likely to our north and northeast.
By late afternoon tomorrow through tomorrow night and Friday, however, much more widespread rain
showers and thunderstorms will develop overhead as we see the a much more unstable atmosphere
takes hold. More dynamic lift will develop as a stronger upper air low pressure disturbance, approaching
from the west out of northern Mexico, takes direct aim on our area from the west and an associated surface
cold front moves southeastward into central and north Texas. This activity will persist over the area
until the surface cold front sluggishly moves southeastward across the IH35 corridor around or just
after daybreak on Saturday.
In addition to the possibility of heavy rain, we could see a few stronger thunderstorms tomorrow night
and into Friday. NWS/Weather Prediction Center guidance is suggesting 2 to 4 inches of rain over our
area in this period. The NWS/Storm Prediction Center outlooks, from 7am tomorrow until 7am Friday,
has all of our area in a “marginal” to “slight” severe weather risk area with larger hail and damaging
thunderstorm winds being the primary risk. Of course, frequent dangerous lightning will be featured
as well. In summary, I urge you to continue to closely watch the period from 7pm tomorrow night
through sunrise Saturday and continue to be weather aware.
I’m continue with this series of updates as we move into this upcoming inclement weather episode.
Have a good night.. I’ll update you tomorrow.
tk