Significant Severe Threat for Midwest on Wed; Danielle is Done; SW Still Cooking

Fierce thunderstorms capable of spawning significant tornadoes and very high winds are possible Wednesday afternoon and evening across an east-west corridor that will lie near or across the Chicago area. NOAA’s Storm Prediction Center is predicting a moderate risk of severe weather for Wednesday PM extending from southeast Wisconsin and northeast Illinois to southeast Michigan and northwest Ohio (see Figure 1). “Moderate” is actually the second-highest category in the five-level SPC hierarchy of risk. It’s fairly unusual for SPC to issue such a high threat level on the morning before an event, which speaks to the volatility of the set-up expected to take shape.


Figure 1.. WU depiction of NOAA/SPC severe weather risk areas as of late Tuesday morning, June 21, 2016, valid on Day 1 (left, Tuesday) and Day 2 (right, Wednesday).

Tuesday is “the day before the day,” as a weak surface boundary extending from Nebraska into the Missouri and Ohio Valleys begins edging northward while a powerful upper-level impulse approaches from the west and a surface low organizes in the Central Plains. SPC has slight-risk zones for Tuesday afternoon and evening along the surface boundary from the western Dakotas all the way to the mid-Atlantic. Where and when storms unfold in the Midwest on Tuesday night will help influence the scenario on Wednesday. A mesoscale convective system, or MCS (a large, persistent thunderstorm complex) may develop in South Dakota late Tuesday and move toward the Chicago area overnight, a frequent early-summer track. If this happens, it would leave a boundary of rain-cooled air somewhere just to its south—perhaps reinforcing the current surface boundary—with extensive clouds to its north. Any east-west surface boundaries could serve as focal belts for Wednesday afternoon’s storms, helping to concentrate the low-level “spin” (vorticity) that can feed tornadic supercells. It’s too soon to know exactly where the zone of greatest risk will end up, but the Chicago area is certainly a possibility.

The period of greatest concern is Wednesday afternoon and evening, when the upper-level impulse will approach the Great Lakes and a strong surface low will begin accelerating eastward along the surface boundary. Very rich low-level moisture will be in place, and southeasterly low-level winds will be overtopped by strong westerlies at middle and upper levels. These classic ingredients for supercell thunderstorms are projected by models to reach unusually intense values by late Wednesday. One possible scenario is for one or more long-lived supercell thunderstorms—potentially with significant tornadoes—persisting for several hours, most likely focused over northern Illinois and/or southern Wisconsin into southern Michigan and northern Indiana. These storms might then morph into an MCS that could tear eastward overnight into Ohio and Pennsylvania with very strong surface winds. If the storms consolidate quickly, it is certainly plausible we could have a Wednesday night derecho—a long corridor of damaging surface winds extending over several hours. Derechoes are extremely difficult to predict, as they hinge on dynamic and thermodynamic features that are often not clear until the event is at our doorstep, but the situation certainly bears a close watch.


Figure 2. The 4-km version of the NAM model run at 12Z (8:00 am EDT) Tuesday, June 21, 2016, valid at 9Z (5:00 am EDT) Thursday, June 23, 2016, indicates a large area of thunderstorm-driven winds at 850 mb (about a mile above the surface) exceeding 64 knots (74 mph). In such a situation, high winds would likely mix to the surface in some areas. Any model depiction of thunderstorm activity this far out involves great uncertainty, so this map should be seen not as a literal forecast but as a rough depiction of one of many potential scenarios. Image credit: tropicaltidbits.com.

Although severe weather is typically more frequent further to the south, the Chicago metro area is one of the nation’s most vulnerable to tornadoes and other severe threats. We looked at the potential threat of a violent, long-track tornado in Chicago in the post Big Wind in the Windy City. Victor Gensini, an associate professor of meteorology at the College of DuPage in the Chicago suburb of Glen Ellyn, is quite concerned about Wednesday’s outlook. “I would argue that this forecast severe weather environment is likely the most impressive (at least from an ingredients standpoint) ever to take shape over one of the nation’s biggest three metro areas,” he said in an email. Gensini’s group is planning to launch a special radiosonde on Wednesday afternoon in northern Illinois, where the ingredients may come together in an area poorly sampled by the standard NWS radiosonde network.


Figure 3. Michael Martinez drinks a bottle of water at a Salvation Army hydration station in an effort to beat rising temperatures in Phoenix on Monday, June 20, 2016. Phoenix set daily record highs of 118°F on Sunday and 116°F on Monday, which were the earliest readings above 115°F in city history. Image credit: AP Photo/Ross D. Franklin.

All-time heat record set on Tuesday in Blythe, CA
The atmospheric oven remains set on “high” across the Desert Southwest, where a fresh batch of records was seared into the weather annals on Tuesday. Chief among them was the 124°F at Blythe, CA, breaking the city’s all-time high of 123°F set most recently on July 28, 1995. Records in Blythe go back to 1948. Another standout in the Lower Colorado Valley was the 125°F reading at Needles, CA, the highest temperature ever observed there in June (previous record of 123°F was on June 29, 2013). Tuesday tied with the 125°F observed on July 17, 2015, for the highest temperature ever recorded in Needles, where records go back to 1941. Thanks to WU weather historian Christopher Burt for this tidbit. Needles is often the hottest location in the contiguous U.S., and back on August 14, 2012, Needles had a trace of rainfall at a temperature of 115°F, producing what appears to be a global record for the “hottest rain on Earth.” The hot spot on Monday was Death Valley, CA, which set a new daily high temperature record of 126°F (old record for the date: 125°F in 1961.) On top of these local records, the high temperature in Laughlin, Nevada hit 125°F. If verified, this would be a tie for the hottest temperature ever measured in state--a 125°F reading at Laughlin on June 29, 1994.

The most exceptional aspect of the Southwest heat wave is its location on the calendar, days earlier than similar heat has ever been recorded in many spots. On Wednesday morning, Phoenix scored its earliest low of 90°F on record, beating June 22, 1990. The intense upper-level high in place over the Southwest will be weakening slightly as the week rolls on, so this particular heat wave may have now crested, although excessive heat warnings continue in place for much of southeast CA, southwest AZ, and southern NV. Even with the heavy media attention paid to the hazards of this heat wave, it has taken an unfortunate toll: at least five people died from the heat on Sunday in Arizona. Several were young, in-shape hikers or bikers, a reminder that heat exhaustion and heat stroke can affect anyone, especially those without adequate access to water.

Another byproduct of the record heat atop multiyear drought is increased fire risk. Two major fires broke out on Monday very near each other in Azusa and Duarte, just north of the heavily populated San Gabriel Valley about 25 miles east of Los Angeles. The uncontained fires had grown to 5400 acres by Tuesday morning, prompting evacuations of more than 750 nearby residents.


Figure 4. Smoke from wildfires burning in Angeles National Forest fills the sky behind the Los Angeles skyline on Monday, June 20, 2016. Located several miles apart, the wildfires devoured hundreds of acres of brush on steep slopes above foothill suburbs erupted in Southern California as an intensifying heat wave stretching from the West Coast to New Mexico blistered the region with triple-digit temperatures. Image credit: AP Photo/Ringo H.W. Chiu

Danielle dissipates after making landfall as a 40 mph tropical storm
Tropical Storm Danielle made landfall near 7 pm CDT Monday ten miles north of Tuxpan, Mexico as a tropical storm with 40 mph winds. The rugged terrain of Mexico severely disrupted the storm, and Danielle dissipated early Tuesday morning. Lingering moisture from Danielle will continue to affect Mexico, bringing some areas of total rainfall amounts of 8 - 12" along the coast between Tampico and Veracruz, causing dangerous flash flooding and mudslides through Tuesday evening. So far, the highest rainfall amounts appear to be the  8+ inches that have fallen at Pozo Rica de Hidalgo; Tuxpan received 4.98", and Puebla, 5.33". No significant damages or casualties have been reported from the storm thus far. Danielle was a tropical storm for just twelve hours. Danielle's formation date of June 20 was the earliest appearance on record for the Atlantic's fourth named storm of the season; the previous record was June 23, 2012, when Tropical Storm Debby formed.

The tropical Atlantic looks like it will be quiet for the remainder of the week, with none of our reliable tropical cyclone genesis models showing anything developing.

Bob Henson and Jeff Masters


Figure 5. Tropical Storm Danielle nearing landfall on Monday afternoon, June 20, 2016, as seen by NASA's Aqua satellite. At landfall, Danielle had top sustained winds of 40 mph. Image credit: NASA.