More heavy rain fell in the south plains last weekend, continuing a rather long cycle of flooding and dangerous storms across the southern plains over the past couple of months (take a look at our previous MetStorm analyses for April storms in Texas, as well as this excellent NASA write-up of widespread rainfall in Texas and Oklahoma from late-May to early-June). Radar composite imagery for Texas and Oklahoma over the course of June 12-13th is shown below.
radar images via http://www2.mmm.ucar.edu/imagearchive/
These storms were oftentimes slow-moving, especially in Oklahoma, and frequently went through dissipating and re-development stages. South of the Dallas/Fort Worth radar station (you can easily spot this radar station as the black dot in the center of the circular area of radar “clutter” in the north-Texas region near Dallas/Fort Worth), observe the line of thunderstorms that seemingly remain stationary from about 6 to 11 UTC (1 to 6 am central time) the morning of the 13th. This area experienced what is known as “training” in meteorology, in which thunderstorms consistently develop in the same area and then move in a similar direction as they mature and eventually dissipate. Areas underneath training thunderstorms thus see significant amounts of precipitation, often in a relatively short amount of time, compared to nearby areas. The implications of this training event are discussed below.
Storm rainfall totals for both Texas and Oklahoma exceeded 10 inches over the course of these two days, as shown below in the MetStorm Storm Total Precipitation map. The two regions that experienced the most amount of rainfall were the areas over and just east of Lawton, Oklahoma, and south of the Dallas/Fort Worth metropolitan area in northern Texas. On top of the previous south plains storms already mentioned, this large amount of rainfall over a two-day time span spelled disaster for homes and infrastructure, particularly for Lawton, where homes needed to be evacuated and extensive road closures occurred throughout the area. An eight-mile stretch of I-45 south of Dallas was closed Monday morning due to storm waters. Check out the MetStorm map to see that one of the core areas of precipitation in Texas fell directly over I-45: the area under the training thunderstorms mentioned above.
Situations like these are often difficult to forecast and are a complicated entity from a disaster management perspective. Below are MetStorm mass curve plots of incremental and accumulated precipitation plotted for the storm centers in both Oklahoma near Lawton (above) and south of Dallas along I-45 (below). The vast majority of the rainfall near Lawton fell approximately 12 hours before the rainfall in Texas occurred, each storm system producing the most rainfall at roughly 10 UTC, or 5am central time, on their respective days. Overnight and early morning flooding events such as these are quite dangerous, as they usually catch communities at their most vulnerable times, and similar events in the south plains this year have resulted in numerous deaths.
The heavy rainfall is also reflected in river flow and discharge data acquired from the USGS National Water Information System for nearby river basins. As an example, below is a time series of water discharge along the Neches River near Neches, Texas, from mid-May to present. Observe that in the hours overnight from the 12-13th of June the rate of water discharge surged from 2,000 cubic feet per second to about 5,500 cubic feet per second. During this very short span of time, the river height rose nearly 3 feet from 13 to 16 feet. Also note past surges in water discharge in late May/early June associated with other heavy rainfall events in the area.
data via http://waterdata.usgs.gov/tx/nwis/uv?site_no=08032000&format=gif&period=31
The final MetStorm product for this storm event is a determination of the relative rareness of a rainfall event such as this. This is accomplished through the calculation of an Average Recurrence Interval, or ARI. Simply, the ARI is the probability of the occurrence of the total recorded rainfall amount over a specified duration in any given year. Here we have plotted 6-hour ARI values over our area of interest. In Oklahoma, near Lawton as well as south of Norman near I-35, 6-hour ARI values exceeded 500-year occurrence. And in Texas south of Dallas along the I-45 corridor, the maximum ARI was over 1000-years. In other words, this stretch of I-45 saw heavy rainfall over a 6-hour duration that was so large that the probability of its occurrence in any given year is only one in one thousand.
Sunnier and drier days are in the forecast for the southern plains for the days to come, as is most of the rest of the continental United States as a large upper-level ridge settles itself in for the long-run. With June halfway over we’re entering the thick of summer, which will likely be a welcome change of pace after what was an unusually eventful spring and early summer.
Please note that the maps presented here are preliminary and will be updated when new data become available. If you are interested in this product, or any other product from our MetStorm Precipitation Analysis tool, please email us or send us a message though our contacts page here.