Major Severe Weather Outbreak Possible This Week

The atmospheric ingredients are aligning for what could be intense severe weather this week, especially on Wednesday and Thursday. A powerful upper-level low is expected to bring the storminess to the Plains, Midwest, and South as it slowly makes its way eastward. The low is now pushing into California, where its power is being put to good use: providing much-needed snow in the Sierras and rain at lower elevations. From 4” to 8” of snow was observed above 3500 feet in the Sierra over the weekend, and winter storm warnings are in effect for 8-16” of higher-elevation snow late Monday into Tuesday, with local two-foot amounts possible on the highest peaks. The San Francisco Bay area could get anywhere from 0.5” to 2” of rain. While this won’t come close to breaking the severe multi-year drought across California, it’ll at least add a few drops to the bucket and give residents a psychological boost. The cold upper-level air may even lead to severe thunderstorms over central California on Tuesday, with a tornado or two possible.


Figure 1. Dew points as of 9:00 am CDT Monday, April 6, had already risen above 55°F across Texas and Oklahoma, with 65°F values moving north from the Gulf of Mexico. Higher dew points indicate richer moisture near the surface; most severe weather occurs with dew points of at least 55°F. Image credit: NCAR/RAL Real-Time Weather Data.


Ahead of the upper low, high-level southwesterly winds will overspread much of the country over the next several days. This flow over the Rockies will help maintain surface low pressure over the high plains of Colorado and Kansas, and the circulation will pull in plenty of moisture. As shown in Figure 1 above, dew point temperatures (the temperature to which the air needs to be cooled in order for the relative humidity to reach 100%) are already near 70°F along the Texas coast--not too far from typical summertime values. This rich moisture will surge north through the week, with mild, humid air possibly making it as far north as Chicago and Cleveland by Thursday. The juxtaposition of muggy low-level air and the cold upper-level storm will produce strong instability over a wide area. However, a very warm, dry layer sandwiched between the two—an atmospheric “cap”—should keep storms from becoming widespread on Monday and Tuesday. Any storms that do manage to break through the cap could quickly become severe, especially along a dry line from Texas to Missouri. Late Monday and again late Tuesday, overnight storms could produce severe wind or hail across parts of northeast Kansas and Missouri.


Figure 2. NOAA’s Storm Prediction Center has placed much of the southern Plains in a slight risk of severe weather for this Wednesday, April 8. Image credit: NOAA/SPC.


Computer models are in strong agreement that the situation will become more volatile by Wednesday afternoon, as the upper low pushes into the Great Basin and pieces of energy swing around it onto the Plains. The unusually rich moisture for early April will lead to CAPE values perhaps exceeding 3000 J/kg, which is more than sufficient for supercell thunderstorms. (CAPE is “convective available potential energy,” or the amount of energy that could be unleashed as rising motion if a storm begins to develop.) In addition, the dry line will sharpen and begin moving east, and a warm front will also sharpen east of a surface low in the vicinity of central Kansas. As air converges along the dry line and front, it will be forced upward, encouraging storm development. Another crucial factor will be the strong vertical wind shear, or the variation from low-level southerlies to much stronger southwesterlies just above the surface. Strong wind shear imparts a spin to air parcels, and as they feed into a supercell thunderstorm, their spin (or vorticity) can become stretched and concentrated, enhancing storm rotation and possible tornado formation. (For an excellent depiction of this process, see this National Geographic interactive animation, which illustrates the classic supercellular tornado process. Weaker, shorter-lived tornadoes can form in other ways.)

All of the factors noted above are strongly associated with classic severe weather outbreaks, and their expected intensity is such that significant severe weather is is possible, as mentioned by NOAA’s Storm Prediction Center in its Day 3 outlook for Wednesday, issued early Monday. The area most at risk for tornadic storms (see Figure 2) is along the dry line, anticipated to extend from central Oklahoma into central Kansas, and along the warm front through eastern Kansas. Since the storms may not be exceptionally numerous at first, SPC’s current outlook for Wednesday calls for only a “slight” risk of severe weather, even though tornadic supercells are quite possible where storms do form.


Figure 3. The red zone on this map indicates an elevated risk of supercell thunderstorms at 6 pm Wednesday, based on Monday’s 1200 GMT NAM model run. The supercell composite parameter combines several measures of instability and vertical wind shear. The wind flags show the contrasts in wind speed and direction between the 500 mb (blue) and 850 mb (black) levels. Strong vertical wind shear is evident across much of Oklahoma, southern Kansas, and southwest Missouri, where powerful westerly winds at 500 mb are flowing above more southerly flow at 850 mb. Image credit: College of DuPage NeXT Generation Weather Lab.


The upper low will sweep into the central Plains on Thursday, pushing the risk area for severe storms well eastward. Much will depend on where storms develop on Wednesday night and how much they persist into Thursday morning. Where the air is not extensively rain-cooled, very intense storms could develop along the surging cold front and just east of the surface low, perhaps reaching eastern Iowa, Illinois, and Wisconsin by evening. Upper-level winds will be even stronger than on Wednesday, and any tornadoes that form could be moving rapidly, adding to the threat. By Thursday night, a large complex of severe storms may bring high winds, large hail, and very heavy rain from east Texas into the Tennessee and Ohio valleys. These storms are likely to weaken somewhat as they move further from the upper low into the Gulf and Atlantic coastal states by Friday. The system may also bring additional rain to flood-hammered parts of Kentucky. According to the Weather Channel’s Nick Wiltgen, Louisville, KY, saw its fourth-wettest calendar day on record last Friday, April 3, with 5.64” at Standiford Field (the city’s official reporting site) and 8.03” at the Louisville NWS office. The city has received more than a foot of rain and melted snow since March 1. More than 100 water rescues were carried out in the Louisville area, and Kentucky governor Steve Beshear declared a state of emergency on Saturday.


Weather and climate talks, hands-on science this weekend in Nebraska
I’ll be delivering a talk and signing copies of my book “The Thinking Person’s Guide to Climate Change” this Saturday, April 11, at the Central Plains Severe Weather Symposium and Family Weatherfest in Lincoln, Nebraska. This free event draws several thousand people each year from throughout the state and region. The symposium, organized by University of Nebraska meteorology professor Kenneth Dewey with a variety of sponsors, began in 1999 and features a half-day of speakers from eastern Nebraska and beyond. Representatives from the Nebraska Office of Emergency Management will discuss the state’s response to the disastrous twin tornadoes that struck the town of Pilger last June 16. Joining me for the book signing will be Nancy Gaarder, a top-notch weather reporter at the Omaha World-Herald. Nancy has just released “Nebraska Weather,” a 200-page guide with dozens of photos from the World-Herald archives. The concurrent Family Weatherfest is modeled after a similar event held at the annual meeting of the American Meteorological Society. Local TV weathercasters will be on hand, and kids can deliver forecasts on the same type of “green screens” used by the pros.

Bob Henson