After a long summer of heat, the first harbingers of fall weather have begun to arrive across the southern plains: cold fronts. Several pushes of cooler air have arrived across Kansas, Oklahoma, and Texas in the last two weeks, and these will only become stronger and more frequent as the intensity of the sun across the Northern Hemisphere continues to fade. As is often the case during the summer-to-fall transition, however, continental cold fronts are often displacing air that’s very warm and moist (and sometimes even tropical). A prime example of this phenomenon unfolded across portions of the lower Red River Valley this past weekend. The Red River Valley makes up much of the east-west border between Texas and Oklahoma, the states’ panhandle regions aside. Generally, it dominates the region between the Dallas-Fort Worth metro area and the Oklahoma City metro area. This past weekend, the valley was also the site of a confluence of deep tropical moisture; a strong, slow-moving cold front; and an upper-level trough that acted to lift and focus the moisture into waves of heavy thunderstorms.

Beginning on the evening of Thursday, Sept. 20th, a lee cyclone over eastern Colorado and western Kansas began deepening, pushing a cold front southward along the Rockies. Very warm air overspread Texas and Oklahoma ahead of the front, with daytime high temperatures in the 90s °F with dewpoints of around 70°F. Meanwhile, moisture associated with the post-tropical remnants of Tropical Depression Nineteen-E, which had made landfall the previous day on a relatively sparsely populated stretch of the Sonoran coast of Mexico, streamed northeastward towards Texas and Oklahoma (Figure 1). This tropical low pressure system was quickly shredded apart by the mountains of northern Mexico, but atmospheric moisture levels 2-3 standard deviations above normal (roughly 95th to 99th percentile) can be seen stretching from the Gulf of California on into the northern plains.

GFS pwat loop

Figure 1. GFS model analysis of precipitable water anomalies over Sept 20-21, 2018, with the envelope of moisture associated with TD 19-E highlighted.

As this robust moisture streamed northeastward, scattered strong thunderstorms developed primarily across northern Texas and southwestern Oklahoma, producing almost no reported severe weather but bringing a round of gusty winds and pockets of brief heavy rain. This area of rain gradually filled in overnight as the upper-level trough approached, and by late Friday morning (Sept 21st), moderate to heavy rain covered much of the state of Oklahoma (Figure 2).

southplains radar 20180921 1625z

Figure 2. Composite radar image showing widespread moderate-to-heavy rain across OK and parts of central TX at 16:25 UTC (11:25am local time) on Sept. 21, 2018. Image courtesy of UCAR’s radar imagery archive (http://www2.mmm.ucar.edu/imagearchive/).

The line of storms that set up across southern Oklahoma was particularly persistent, and the National Weather Service in Norman, OK issued numerous flash flood warnings to highlight the imminent danger of quickly rising waters. Rainfall of 3″ to nearly 9″ occurred in just 6 hours on Friday morning across parts of southern Oklahoma, with the heaviest being recorded in the Fittstown vicinity. As if that weren’t enough, rainfall continued in these same areas for another 8-12 hours, further compounding the flood problem. Meanwhile, the southern extent of the slow-moving line of storms pushed into Texas during the afternoon hours of Friday the 21st, producing upwards of 10″ of rain in just 12-18 hours across a region stretching from the Dallas metro area through Denison, TX. This prompted water rescues and at least one fatality in the Arlington area. While much of the area of southeastern Oklahoma that was hardest hit by flooding rain is fortunately less populated, there is likely to be substantial damage to crops and numerous livestock deaths across the heavily agricultural region, as in this tragic example of an entire herd of cattle swept away in floodwaters. When all was said and done on Saturday morning, heavy rain was reported across the entire Red River Valley region, with isolated pockets of 15″ to nearly 20″ of rain (Figure 3).

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Figure 3. Storm-total rainfall as analyzed by MetStorm from 7am local time on Thursday, Sept. 20, 2018, through 7am Sunday, Sept. 23, 2018.

These staggering rainfall totals occurred mainly over a 36-hour period, but the bulk of the rain fell in just a 24-hour period. We therefore compared the maximum 24-hour precipitation calculated at each grid cell of the MetStorm analysis to local frequency estimates for 24-hour rainfall to come up with an Average Recurrence Interval (ARI), or the average number of years one would typically expect between rainfall events of a similar magnitude at a given point. Unsurprisingly, maximum 24-hour rains of up to 1000 years were observed in south-central Oklahoma and northeast Texas (Figure 4). Rains of around a 10-year to 50-year recurrence were fairly widespread across the region (yellows and oranges in the figure below).

metstorm2018092206 24hr max ari detailedmap

Figure 4. Average Recurrence Interval (ARI), in years, associated with the maximum 24-hour precipitation at each grid cell.

Given the localize ARI values in excess of 1000 years, it may come as no surprise to hear that the Oklahoma state rainfall record may have been closely challenged during this event. The current 24-hour rainfall record as accepted by the Oklahoma Climatological Survey is 15.68″ at Enid on Oct. 11, 1973. The National Weather Service in Norman received a rainfall report from an emergency manager of 15.50″ from 6 miles south of Stonewall, OK at 5:52pm CDT on Sept. 21, but the report did not provide a start time for the accumulation. Based on radar and surrounding gauge reports, it seems plausible that all or most of this precipitation fell within 24 hours, but without precise timing information it is impossible to definitively say. We can therefore conclude only that it is possible that Oklahoma may have seen one of its top few heaviest 24-hour rainfalls of all time during last week’s storm. If we receive additional information as to the veracity of this report, we will be sure to update this post. Regardless of whether the record was challenged or not, this was a devastating storm for many across the Red River region, and we wish everyone the best in their recovery efforts. Continue to watch this space for more analyses of extreme rainfall events as they happen.