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Category 6 has moved! See the latest from Dr. Jeff Masters and Bob Henson here.Volatile Setup for Severe Weather in Southern Plains on Friday, Saturday
The ingredients are in place for a major multi-day outbreak of severe weather from Friday into the weekend, including the possibility of strong, long-track tornadoes. Early on Friday, NOAA’s Storm Prediction Center placed part of southwest Oklahoma under a moderate risk of severe weather for Friday, with a moderate-risk area covering a broader swath from western KS to southwest OK on Saturday. I would not be surprised to see a localized upgrade to high risk somewhere in this swath on Saturday.
Figure 1. Convective outlooks for Friday and Saturday, May 8 and 9, issued early Friday morning by NOAA’s Storm Prediction Center. Image credit: NOAA/SPC.
The tornado potential for Friday will be heavily influenced by an outflow boundary that formed late Thursday, separating rain-cooled air over Oklahoma and the Texas Panhandle (temperatures and dew points early Friday in the 50s to low 60s F) and very warm, humid air just to the south (temperatures and dew points in the low 70s). This boundary will attempt to push back north as a warm front on Friday and should be positioned somewhere near the Red River area of Oklahoma/Texas by late afternoon. Near this boundary, surface air will converge and surface winds will have a strong easterly component. With southwesterly winds expected just a few thousand feet above the surface, this will enhance the vertical wind shear that produces rotation in supercell thunderstorms. The potential for strong tornadoes is greatest when strong vertical shear is present in the lowest kilometer of the atmosphere, and when high temperatures and moisture values are also present in that lowest kilometer. Together, these tend to produce thunderstorms with low cloud bases and intense rotation near those bases. Any supercell that roots itself near the warm front and moves east or northeast along it will be a particular threat for producing tornadoes, although tornadic supercells are possible throughout the moderate-risk area. Flash flooding could be a serious threat as well, especially across parts of central Oklahoma and north-central Texas that have seen 6” to 10” of rain over the last several days (see Figure 3 below). One important caveat: several of the 00Z Friday model runs, as well as several recent runs of the hourly-updated HRRR and RAP mesoscale models, suggest that a large storm complex will form early in the day across west Texas and move across western Oklahoma, as was the case on Thursday. Storms were beginning to form west of Lubbock and south of Abilene at 8:30 am CDT, supporting this scenario. Should this activity increase, it would act to reinforce the outflow boundary and could markedly suppress the risk of supercells and tornadoes across parts of the current moderate and enhanced risk areas, though supercells could still form in pockets where the air mass is less affected. This situation may become quite geographically complex through the day, adding to the challenge for forecasters.
A sprawling severe threat for Saturday
Forecast models have agreed for the last several days that the stubborn upper low over the Southwest will finally make its move onto the Plains on Saturday. Instability may be a bit less than on Friday, but still more than adequate for high-end supercells. Moreover, howling upper-level winds and strong dynamics associated with the low will lead to very high vertical wind shear and dangerous, fast-moving storms over a broad area along and in front of a sharpening surface low in eastern Colorado and a dry line extending south across western Oklahoma into north Texas. Many cells could be moving at 40 - 50 mph. Widespread severe weather is almost a certainty on Saturday, and the risk of strong, long-track tornadoes may also cover an unusually large area (see Figure 2). Faster storm motion will help reduce the flash-flood risk somewhat, although storms rooted near the surface low will be slower-moving. Once again, if morning thunderstorms turn out to be quite extensive, the risk of significant severe weather will be tamped down.
Figure 2. Surface winds (flags) and significant tornado parameter (STP, in colored areas) at 7:00 p.m. CDT Saturday, May 9, as projected by the 0000 GMT Friday run of the NAM model. The STP is based on several measures of wind shear, instability, and cloud-base height. STP values of 8 to 11, as predicted here for western Oklahoma, are near the top end of those seen in major outbreaks, such as the Super Outbreak of April 2011 (see an example from that day). Image credit: College of DuPage.
Sunday could bring yet another round of severe weather, with the focus this time pushing toward eastern Kansas and Nebraska, Missouri, and Iowa. The risk on Sunday will be highly contingent on the amount of rain-cooled air produced by storms late Saturday toward the south. If the air mass can recover by Sunday afternoon, wind shear and instability again look favorable for potentially tornadic supercells, especially along the warm front that should be across Iowa by then.
Weather Underground is planning to cover the severe weather outbreak on Saturday in our experimental live blog, which you can access through a banner at the top of the home page. We’ve also activated a live blog on Subtropical Storm Ana. In addition, Jeff Masters and I will be posting updates at this blog on both events during the weekend. Jeff will have a complete update later today on Subtropical Storm Ana, which continues to gradually develop off the Southeast coast. Ana was being investigated by a hurricane-hunter aircraft on Friday morning.
Bob Henson
Figure 3. Flooding on Thursday night, May 7, in downtown Gainesville, TX, just south of the Oklahoma-Texas border. Photo credit: @TheMaverick21, used with permission.
Figure 1. Convective outlooks for Friday and Saturday, May 8 and 9, issued early Friday morning by NOAA’s Storm Prediction Center. Image credit: NOAA/SPC.
The tornado potential for Friday will be heavily influenced by an outflow boundary that formed late Thursday, separating rain-cooled air over Oklahoma and the Texas Panhandle (temperatures and dew points early Friday in the 50s to low 60s F) and very warm, humid air just to the south (temperatures and dew points in the low 70s). This boundary will attempt to push back north as a warm front on Friday and should be positioned somewhere near the Red River area of Oklahoma/Texas by late afternoon. Near this boundary, surface air will converge and surface winds will have a strong easterly component. With southwesterly winds expected just a few thousand feet above the surface, this will enhance the vertical wind shear that produces rotation in supercell thunderstorms. The potential for strong tornadoes is greatest when strong vertical shear is present in the lowest kilometer of the atmosphere, and when high temperatures and moisture values are also present in that lowest kilometer. Together, these tend to produce thunderstorms with low cloud bases and intense rotation near those bases. Any supercell that roots itself near the warm front and moves east or northeast along it will be a particular threat for producing tornadoes, although tornadic supercells are possible throughout the moderate-risk area. Flash flooding could be a serious threat as well, especially across parts of central Oklahoma and north-central Texas that have seen 6” to 10” of rain over the last several days (see Figure 3 below). One important caveat: several of the 00Z Friday model runs, as well as several recent runs of the hourly-updated HRRR and RAP mesoscale models, suggest that a large storm complex will form early in the day across west Texas and move across western Oklahoma, as was the case on Thursday. Storms were beginning to form west of Lubbock and south of Abilene at 8:30 am CDT, supporting this scenario. Should this activity increase, it would act to reinforce the outflow boundary and could markedly suppress the risk of supercells and tornadoes across parts of the current moderate and enhanced risk areas, though supercells could still form in pockets where the air mass is less affected. This situation may become quite geographically complex through the day, adding to the challenge for forecasters.
A sprawling severe threat for Saturday
Forecast models have agreed for the last several days that the stubborn upper low over the Southwest will finally make its move onto the Plains on Saturday. Instability may be a bit less than on Friday, but still more than adequate for high-end supercells. Moreover, howling upper-level winds and strong dynamics associated with the low will lead to very high vertical wind shear and dangerous, fast-moving storms over a broad area along and in front of a sharpening surface low in eastern Colorado and a dry line extending south across western Oklahoma into north Texas. Many cells could be moving at 40 - 50 mph. Widespread severe weather is almost a certainty on Saturday, and the risk of strong, long-track tornadoes may also cover an unusually large area (see Figure 2). Faster storm motion will help reduce the flash-flood risk somewhat, although storms rooted near the surface low will be slower-moving. Once again, if morning thunderstorms turn out to be quite extensive, the risk of significant severe weather will be tamped down.
Figure 2. Surface winds (flags) and significant tornado parameter (STP, in colored areas) at 7:00 p.m. CDT Saturday, May 9, as projected by the 0000 GMT Friday run of the NAM model. The STP is based on several measures of wind shear, instability, and cloud-base height. STP values of 8 to 11, as predicted here for western Oklahoma, are near the top end of those seen in major outbreaks, such as the Super Outbreak of April 2011 (see an example from that day). Image credit: College of DuPage.
Sunday could bring yet another round of severe weather, with the focus this time pushing toward eastern Kansas and Nebraska, Missouri, and Iowa. The risk on Sunday will be highly contingent on the amount of rain-cooled air produced by storms late Saturday toward the south. If the air mass can recover by Sunday afternoon, wind shear and instability again look favorable for potentially tornadic supercells, especially along the warm front that should be across Iowa by then.
Weather Underground is planning to cover the severe weather outbreak on Saturday in our experimental live blog, which you can access through a banner at the top of the home page. We’ve also activated a live blog on Subtropical Storm Ana. In addition, Jeff Masters and I will be posting updates at this blog on both events during the weekend. Jeff will have a complete update later today on Subtropical Storm Ana, which continues to gradually develop off the Southeast coast. Ana was being investigated by a hurricane-hunter aircraft on Friday morning.
Bob Henson
Figure 3. Flooding on Thursday night, May 7, in downtown Gainesville, TX, just south of the Oklahoma-Texas border. Photo credit: @TheMaverick21, used with permission.
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