A Week of Storminess on the Southern Plains Dryline

With climatology in the driver’s seat, a busy week of potentially tornadic weather will take shape from Nebraska to Texas. This prototypical late-May pattern is being driven by a very slow-moving upper low centered near Montana and an upper trough extending to California. Moderately strong southwest flow ahead of this trough will persist nearly all week across the Southern Plains--a recipe for tornadic trouble this time of year, given the presence of very warm, moist low-level air streaming north from the Gulf of Mexico. The veering and strengthening of winds with height will lead to the vertical wind shear that favors tornadic supercells. In general, this week’s wind shear will not be as strong as seen in the worst tornado outbreaks, but there will be pockets of localized enhancement. In places where these coincide with areas of extreme instability, we can expect supercells to spit out very large hail, high winds, and tornadoes in some cases.

The pattern emerging this week is familiar to severe storm watchers. The persistent southwest flow at upper levels will help to perpetuate a sharp dryline separating dry air from the Desert Southwest and the moist Gulf air mass. This time of year, the dryline often “sloshes,” pushing east during the heat of the afternoon and then retreating back west overnight. Each afternoon’s push can be enough to trigger scattered severe storms along the dry line, with the locations determined by small-scale features that can be hard to predict until just hours before storms begin to develop. Storms are likely to congeal each night into sprawling, slow-moving mesoscale convective systems (MCSs) that roll across parts of the Plains. Morning cloudiness from the MCSs, and the boundaries of rain-cooled outflow air, will play a huge role in dictating where the next day’s storms will focus. Forecasters at the NOAA/NWS Storm Prediction Center and at local NWS offices will have their hands full this week pinning down those mesoscale details and identifying the highest-threat areas. Mesoscale models that are now commonly updated every hour will help greatly; for example, mesoscale runs on Monday morning have zoomed in on northwest and west-central Texas and southwest Oklahoma as one of the primary threat areas for late Monday (see Figure 3).


Figure 1. WU’s severe weather map for Sunday night, May 22, severe thunderstorm watches (yellow) and tornado watches (red) were strung along a boundary all the way from the Saskatchewan border with North Dakota to the Texas border with Mexico.


Figure 2. This large tornado, photographed in low light 15 miles east of Stinnett, Texas, around 6:30 pm CDT on Sunday, May 22, 2016, was one of several spun up by a long-lived supercell thunderstorm in northeast Texas. No major damage was reported. Photo credit: © John Monteverdi, used with permission.

The north-south spread of the activity was vast on Sunday evening: tornado and severe thunderstorm watches extended all the way from the Canadian to the Mexican border (Figure 1), with severe weather reported in every state from North Dakota to Texas. SPC logged 37 preliminary tornado reports from South Dakota, Kansas, and Texas as of Monday morning (reduced to 28 reports by late Monday morning), and hail up to 3” in diameter fell from several storms in Texas and Kansas. Fortunately, the severe weather was focused on sparsely populated areas, and no major damage or serious injuries were reported.


Figure 3. Areas of showers and thunderstorms projected by the 14Z (9:00 am CDT) Monday run of the HRRR model for 7:00 pm CDT Monday, May 23, 2016. Storm locations predicted by mesoscale models such as HRRR will vary from run to run and model to model, so forecasters watch for consistent messages across models and across time. Image credit: www.tropicaltidbits.com.


Figure 4. WU depiction of NOAA/SPC convective outlook areas in effect on Monday morning, May 23, 2016, for (left to right) Monday, Tuesday, and Wednesday, May 23 – 25. Dark orange denotes an enhanced risk of severe weather; yellow, a slight risk; and dark green, a marginal risk. The Day 1 area is updated several times each day; the Day 2 area is updated around 1:30 pm EDT. Full-sized maps can be found on the WU convective outlook page.

The outlook for Monday and beyond
On Monday, the eastward advance of a cool front is teaming up with rain-cooled air to help tamp down the threat of severe weather from Kansas northward. The air mass should have little trouble recovering in Texas and Oklahoma, where Monday’s severe threat will be focused. As of mid-morning Monday, SPC had an enhanced risk of severe weather extending southwest Oklahoma to far southwest Texas (Figure 4), with a slight risk into northwest Oklahoma and only a marginal risk further north. The big picture changes little on Tuesday, although the odds of severe weather may improve across parts of northeast Colorado and southern Nebraska as upslope surface winds increase and moisture shifts west. It now appears that a large chunk of the upper trough will swing across the Southern Plains on Thursday, which would heighten the chance of more concentrated severe weather and shift it toward more populated parts of eastern Kansas, Oklahoma, and Texas.

Localized flooding and flash flooding may become an increasing issue as the week rolls on, especially as the north-south boundary pushes into richer moisture over the eastern Great Plains. A flood watch is in effect through Wednesday morning for parts of southwest Oklahoma and northwest Texas adjoining the Red River, where additional heavy rain on Monday and Tuesday night may fall atop already saturated soils and rain-swollen creeks.

Bob Henson


Figure 5. The 5-day precipitation forecast from NOAA’s Weather Prediction Center, spanning the period from 8:00 am EDT Monday, May 23, 2016 to 8:00 am Saturday, May 28.