Thursday, September 29, 2011

Cool Outflow Boundary Interactions Across Central Texas

The above image was just taken from the New Braunfels area doppler radar. It shows heavy shower and thunderstorm activity across portions of southcentral and southeast Texas.  If you look closely, you can see a few outflow boundaries.  These are like mini cool fronts that depict where rain cooled air is blowing out away from the thunderstorm activity.  I have highlighted them for you on the next image:

The Southern outflow boundary is moving toward the South, and the Northern one is moving toward the North.  Isolated thunderstorms are beginning to develop on the Northbound boundary, to the Northwest of Georgetown.

In a post earlier today, I pointed out that the HRRR model (an experimental, high resolution computer forecast model) was calling for widespread 1-2 inch rains with localized 3-4 inch amounts across much of this same region.  Below is the HRRR total rainfall forecast image from that post, valid 7pm CDT this evening:

On the same image below, I've zoomed-in on a section along the I-35 corridor from Austin to San Antonio.  Look at the forecast from the computer model for about 1 inch of rain within the purple circled area (which is over central and southern Hays County):

Now, lets take a look at the latest storm total rainfall estimate from the New Braunfels radar:

I have encircled approximately the same area of Hays County on the radar image, this time in white.  Note how remarkably accurate the HRRR was in estimating the location where the heavier rain would fall in such a small geographic area.  While it didn't do so hot on the exact amount of rain (the radar estimates about 3 inches, which is 3 times more than the HRRR forecast), it absolutely nailed the geographic area in which it took place.  I should point out that the computer model forecast was generated at 9am CDT this morning, some 6 hours before the event began.

As I often point out in situations such as this, it's more important to look for trends in the model data, instead of a precise answer.  This is a perfect example, where the model scores a 100% on identifying the area to watch, but a lower grade on the exact outcome of the event.  Still, a remarkable step forward in weather science from even just 1 or 2 years ago...

The HRRR rainfall forecast image above was valid through 7pm CDT this evening.  I'll make another post later this evening to show you how the final outcome of the event compared to the model forecast, rainfall wise...

***Updated 7:30pm CDT 9/29/11:

You have to use your imagination a little bit to fill in the blanks, but the above images are the storm total rainfall estimates from the New Braunfels, Houston and Del Rio radars, respectively, through shortly after 7pm CDT.

Overall, I'd say the HRRR model from earlier today (see images above from earlier post), did well with respect to the areal coverage of the rain.  It underestimated the intensity of the rain in some instances, but did tend to focus on the right areas that received heavy rainfall.

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