Ticker for May 14, 2009

                
MESONET TICKER ... MESONET TICKER ... MESONET TICKER ... MESONET TICKER ...
May 14, 2009       May 14, 2009       May 14, 2009       May 14, 2009


Greeting Ticker subscribers ? I am Dr. Jeff Basara, the Director of Research for the Oklahoma
Climatological Survey, and as my good colleague Gary McManus noted in a recent Ticker post, I am also a
newly assigned Ticker author.  Now Gary (as well as our former Ticker author Derek Arndt) is very crafty
with words, and well, I am a scientist who tends to be straight forward and to the point.  But my hope that
I can carve my niche with Ticker offering that address relevant weather and climate phenomena that
make one wonder ?What in the world caused that to happen??.

Now, if you live in central Oklahoma, you may have had one of those thoughts during the early morning
hours on May 13th.  However, before I address the case at hand, I?d like to introduce you to a ?friend? that
will help.  If you have been a Ticker subscriber for any length of time, you are probably very familiar with
the Oklahoma Mesonet.  However, you may not know much about its young sibling ? the Oklahoma City
Micronet (OKCNET).  Completed in 2008, OKCNET was designed to observe atmospheric conditions in
and around Oklahoma City in a simple but robust manner.  This younger network, who likes the fast
pace of city life (1-minute data collection), shares the core values of its older sibling and takes pride in
quality data and products.  For more information about OKCNET, feel free to visit the website and visit
a new friend in environmental monitoring here in Oklahoma:

http://okc.mesonet.org/

With the introductions over, onto the case!  During the late afternoon of May 12th, strong/severe
thunderstorms developed across the eastern Texas Panhandle and western Oklahoma.  These storms
subsequently trekked into the central portion of the state and slowly weakened (See Figure 1 below).
However as they did so, a strong pressure gradient developed between the weakening thunderstorm
complex and a mesoscale low pressure area further north. Figure 2 shows the pressure analysis
(altimeter setting) at 12:15 with the ?red? areas contoured as higher pressure and the ?blue? areas as
lower pressure.  Now, air moves from high pressure to low pressure and you can think of the areas in
red as the top of a hill with the blue as the bottom of a valley.  As the air moves from one to the other,
it can accelerate depending on the gradient (or slope) between them.  In this case, the gradient/slope
was approximately 10 mb at 12:15 am, and the El Reno Mesonet site located near the bottom of the
?slope? was experiencing wind gusts to 57 mph.





Fast forward to 1:28 am and zoom into Oklahoma City (Figure 3).  Overall the same type of
atmospheric situation was occurring with higher pressure values across the southern portion
of Oklahoma City and lower values in a band across the northern portion.  The analysis also reveals
that the gradient was nearly 12 mb over a distance of approximately 13 miles from Westmoore High
School (south) to Wiley Post airport (north).  With such a strong gradient over such a short distance, the
winds increased dramatically from only 15 mph at Westmoore High to 65 mph at Wiley Post!  So what does
the radar look like?  Well, there is some light rain across the southern portion of Oklahoma City (Figure
4), but it is far from what one would typically expect from such intense winds.  In this case the strong
winds are analogous to rolling a ball down a steep hill ? the maximum speed occurs as it accelerates and reaches the
bottom.




Another look at the case reveals some other interesting features in the time series (meteograms ?
Figures 5-7).  For example, the strong winds lasted for an extended period (nearly an hour) with
significant decreases in the station pressure (nearly 10 mb) and dew point temperature.  Also,
considerable warming occurred which is the main reason such events are often labeled as ?Heat
Bursts?.   Of course, as the weakening thunderstorm complex exited central Oklahoma, the pressure
gradient decreased and wind speed values returned to a more uniform, ambient pattern by 4:00 am (Figure 8).






So, after all the explanation and if you experienced the event, perhaps now you have a better
understanding of what blew over your garbage can.