March 10, 2000 Supercell

CORYELL AND BELL COUNTIES
I had been mentioning the severe storm threat for Friday, March 10th for over 36 hours here on the homepage by the time the event actually got underway. As luck would have it, I also worked Thursday night at KWTX-TV, filling-in for our chief weather anchor, who was under the weather, so I had an opportunity to mention it on-the-air, too. Early Friday morning, after examining the output from the ETA model (based on data from 6pm CST 3-9-2000) and from the RUC model (based on data from 6am CST Friday), I wrote:

A stalled front draped across Central Texas and an approaching vigorous upper air system will combine to produce an enhanced threat of severe storms and possible tornadoes across Central Texas today (3-10-2000). I am curtailing the maps normally available in this section in favor of emphasizing the official Storm Prediction Center and National Weather Service products. In my view, this threat is going to become focused in the central and eastern portions of Central Texas during the afternoon hours. My advice to all is to monitor official weather information sources closely. I will try to update this section with any additional information which becomes available later this morning (Friday).

I continued to monitor the weather situation through the day Friday, and at 3:30pm Friday afternoon, I wrote:

My surface analysis based on 3pm CST data shows the stalled front is along a Shreveport-Waco-north of Junction-north of Del Rio line and is nearly stationary, although in place it is oscillating north and south as outflow from morning storms north of the front competes with warmer, moist air just to the south. The main upper air jet max appears to be entering the Texas Panhandle now and divergence associated with this feature is spreading into the western parts of north central Texas near Stephenville. A mesolow (small scale surface low) is just east of San Saba and is likely to move eastward or east-northeast toward Waco over the next 2  to 3 hours. Strong to severe storms are likely to develop along the stationary boundary from east of San Saba to just west of Waco by 6pm.
Other, more isolated storms may form to the south toward Austin and San Antonio.

I should mention that I generally analyze surface pressure (PMSL) and draw isobars every two (2) millibars. The standard practice for decades has been to analyze and drawn contours every four (4) millibars when looking at the synoptic scale. It has been suggested by several authors that mesoscale analysis is facilitated by the two-millibar analysis and I find that it sometimes helps to indentify smaller features or new features which are in an incipient stage.

Here are three of the computer forecast maps which indicated the development of the general threat as early as Thursday morning. These are from the ETA model which was run on data collected at 6am on Thursday morning, 3-9-2000 and available to me by 10am the same morning.

The icing-on-the-cake was this map, which shows the upper level wind speeds at 250mb (which is roughly 35,000 feet above sea level. A strong velocity maximum (jet max) within the jetstream flow was shown (charcoal shading) diving southeast toward west central Texas by noon Friday. These upper air features are known to produce strong divergence aloft, which permits air near the surface to rise if it is already unstable. Although the most favored areas for ascent with these features is the left front and right rear quadrants, the strength of the incoming jet max suggested that our area would experience this divergence aloft at least for a time on Friday afternoon. The position of the system also suggested that surface pressure should be falling over west central Texas, which would tend to pull the moist, warm, unstable air westward across Central Texas.

By Thursday evening, the front was stalling near the DFW area. This suggested that the threat area for severe storms might actually be closer to Central Texas by Friday afternoon, so I mentioned this on the evening weathercasts, but said that I still thought that the main threat would be southeast of our area.

The images below show output from the RUC model based on data from 6am CST Friday, and valid at 6pm CST in the afternoon.

Well, I looked at a lot of additional output from the 6am CST RUC but these maps pretty well tell the story. The threat area had definitely moved westward into Central Texas! By 9am Friday, it was time to monitor the evolution of the system and compare what was actually being reported to what the models were forecasting.

Strong thunderstorms developed across northern Texas and southern Oklahoma during the morning hours, mostly in the area north of the stalled front. These storms were what are termed "elevated", meaning that they are drawing on moisture and instability that is not "surface-based". In other words, the moisture and instability feeding these storms was aloft, above the cool air north of the front. Put another way, the front was shallow. Despite being "elevated", these storms produced an impact at the surface: heavy rain and hail feel from some of them, and they all produced cool outflows which served to reinforce the temperature gradient across the stalled front, which actually began sinking southward again, fueled by the renewed cooling at the surface.

By 12 noon CST, the surface map looked like this.

There are many different ways to monitor the evolution of a system like this one: one can use surface observations, or radar, or satellite imagery, or profiles of the winds aloft (generated by either vertical profilers or by the nexrad radars).  The best approach is probably to use all of these. Of particular value on this date was the high-resolution visible imagery generated from the geostationary satellites (GOES), which provide frequent images showing objects as small as .6 mile in diameter (i.e. 1km). The next images is a high-resolution visible image taken at 2010UTC (2:10pm CST) and centered on Waco, Texas. At this time, the GOES system had been placed in rapid-scan mode and was providing a new image every 5 minutes.

Also notice these other important features: (a) the relatively weak storms forming in McCulloch and Mason counties along the dryline, with the tops of these storms being blown to the east-southeast, giving a hint that the storm-relative winds at upper levels are from the west-northwest. This would be a very important indicator that conditions conducive to supercell storms were evolving. And (b) the relatively cloud-free area just east of the mesolow in at the triple-point. This clearing permitted increased solar radiation to reach the surface, destabilizing the atmosphere rapidly in that area.

Remember the EHI parameter? Well, it's not only a forecast parameter, some analysis programs can compute an estimate of EHI in realtime. The Storm Prediction Center does just that, and the map below shows the 2pm CST computed EHI considering only the change in wind speed and direction (the helicity part of the equation) below 1 kilometer above the ground (that's roughly from the surface to 3,000 feet AGL). This is a particularly important parameter for tornado prediction, because recent studies suggest that it is the increase in velocity and directional turning in this layer which most affects whether a tornado forms. Here's the analysis:

Another parameter I like to use for situations where convection appears likely is moisture convergence. Remember the RUC forecast map above which showed forecast divergence (convergence) of the wind. It is possible to perform this type of analysis with respect to both wind and the moisture it contains. This is called moisture convergence, and again, various computer programs can perform this analysis. The ones I've chosen so show you come from the Center for the Analysis and Prediction of Storms (CAPS) site at the University of Oklahoma. I have several of them to show you, but in the interest of saving loading time, I'll only put two in-line and let you "click" to see the others.

Here is a zoomed portion of my 3pm CST surface analysis map. It's a little messy because it was drawn in some haste as the storms became more active. The handwritten numbers below and to the right of some stations show the 2-hour pressure change in millibars and tenths (i.e. -25 would be a 2.5mb decrease in surface pressure over two hours time). There is a zone of >2mb pressure falls over this period stretching from Waco to Lufkin. When other factors are also present (as they were in this case), the zone of maximum pressure falls may also give an indication of where storms will intensify.

Well, when you get to this point in the day and storms are developing, the very best forecasting (actually, nowcasting) tool is probably the weather radar. So, let's finish this up with a look at selected radar images. Unfortunately, I wasn't able to obtain any images from the closest nexrad site (the DoD radar at Granger, controlled from Fort Hood) but you can see what happened using the Fort Worth NWS nexrad, too.

Here's the first image, from the KFWS (Fort Worth) site at 4:33pm CST:

Here's the second image from the KFWS radar at 5:03pm CST:

Here's the 5:33pm CST image from KFWS:

Here's the 6:03pm CST image from KFWS:

Here's the 6:32pm CST image from KFWS:

And to sort of tie this all back to a weather map, click here to see the 6pm CST surface analysis.

    CAPS is The Center for Analysis and Prediction of Storms (CAPS), located at the University of Oklahoma in Norman, and is a National Science  Foundation Science and Technology Center whose mission is to demonstrate the practicability of small-scale numerical weather prediction with an emphasis on deep convection.

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