Tornado Risk Amping Up This Week and Beyond

Spring began in the Northern Hemisphere at 6:28 am EDT Monday, and the U.S. atmosphere seems to have gotten the memo. A multi-day stretch of severe weather should kick off by late Thursday or Friday. There are hints that the pattern will remain stormy off and on through next week, with upper-level energy expected to remain fairly progressive and the Gulf of Mexico supplying ample moisture.

Widespread damage from Southeast freeze
At least 90 percent of the peach crop in South Carolina (the nation’s top peach producer behind California, with a typical crop value of $90 million) was wiped out by freezing temperatures late last week, according to the state’s agriculture commissioner. The state’s wheat and corn fields also suffered heavy damage, reported WISTV. A less severe freeze in Georgia may have ruined anywhere from 25 to 75 percent of that state’s peach crop. Blueberries across the Southeast also experienced major damage, as summarized by Louisville, KY, broadcast meteorologist John Belski. It dropped to 25°F in Gainesville, FL, on Thursday morning, the coldest reading for so late in the year in more than a century of Gainesville records. Jacksonville’s 28°F was also a record for so late in the year. Update: Total crop losses in South Carolina and Georgia could approach $1 billion, according to an AP report filed Monday afternoon.

Fruit trees that budded and blossomed weeks ahead of schedule took a major hit across the mid-South during last week’s freeze. Temperatures that dipped to 21°F on Wednesday and 22°F on Thursday in Louisville spelled a hasty end to the city’s pear blossoms. “I have never seen the blooms go from white to brown while still on the trees,” Belski said.

In Washington, D.C., a large portion of the renowned Tidal Basin cherry blossoms were toasted by the deep freeze, but some of the less-developed buds (about half of the total, according to the National Park Service) apparently made it through. The survivors are now expected to transition quickly into peak bloom later this week, well ahead of the March 25 opening ceremony of the National Cherry Blossom Festival.


Figure 1. Visitors make their way through a deflating lineup of flash-frozen cherry blossoms in Washington, D.C.’s Tidal Basin on Saturday, March 18, 2017. Cold weather killed half of the blossoms on Washington's famous cherry trees just as they were approaching peak bloom. Image credit: AP Photo/Alex Brandon.

Major pattern shift will usher in severe weather
Marginally severe storms are possible across parts of Indiana and Illinois on Monday and the Tennessee Valley on Tuesday, as a spoke of energy rotates around a upper-level low sweeping through eastern Canada. The bigger event will come later this week as the upper-level pattern shifts back toward a Pacific-dominated regime. Several inches of rain and major mountain snows are headed for parts of California, Oregon, and Washington as one storm swings through on Tuesday/Wednesday and a stronger one around Friday/Saturday.

The first upper-level wave in this sequence will reach the Great Plains by late Thursday. Low-level moisture will be rapidly returning from the Gulf, but it’s not yet clear whether enough instability will be on hand to support severe weather. If there is, the focal point would be along a strong dryline expected to be over the High Plains of western KS/OK/TX by late Thursday. A more robust severe threat appears likely for Friday over eastern TX/OK into AR/LA, and on Saturday across parts of MS/AL/GA/FL, as the system marches east into more-unstable air. The NOAA/NWS Storm Prediction Center has outlined both regions with a 15% likelihood of severe weather for Friday and Saturday. I’d expect those odds to rise as the timing and locations become clearer through the week.


Figure 2. The 7-day precipitation forecast issued on Monday morning, March 20, 2017, reflects increased Pacific energy that is expected to generate heavy rain and mountain snow on the U.S. West Coast and fire off one or more rounds of severe weather across the Southern Plains and Southeast. Image credit: NOAA/NWS Weather Prediction Center.

The next multi-day round of severe weather will likely erupt with the second wave in the series, in tandem with the gradual establishment of a upper-level trough in the western U.S. Models are struggling with the evolution of these features, although recent runs of both the GFS and ECMWF models tend to agree on a pattern that would favor severe thunderstorms over the Southern Plains of TX/OK for at least a day or two early next week.

Wildfire also a threat this week; Boulder dodges a bullet
Fire danger may hit critical levels on Thursday and Friday as high winds and warm, dry air sweep across parts of NM/CO/TX as part of the first central U.S. storm. The landscape is drying quickly in this region following a very warm, dry February and early March. Moderate to severe drought now covers most of eastern CO, western KS, and northern OK, according to last week’s U.S. Drought Monitor.

After setting a record high of 80°F on Saturday, Boulder, CO, had a major scare on Sunday: a fire that erupted just west of town on Saturday night swelled to just over 100 acres before it was largely contained by late Sunday. More than 1000 people were under mandatory evacuation orders, and a pre-evacuation zone extended nearly to downtown. Nationwide, the 2.06 million acres burned in wildfires from January 1 to March 17 is by far the largest amount burned by mid-March in data going back to 2007.


Figure 3. Smoke rises from a wildfire on Sunday morning, March 19, 2017, near the west edge of Boulder, CO. Image credit: Seth Frankel via AP.

Extending severe weather outlooks to three weeks: Year 3
Meteorologists in a multi-institution effort based at the College of DuPage have embarked on their third year of providing generalized guidance on the likelihood of U.S. severe weather up to three weeks in advance. The Extended Range Tornado Activity Forecasts (ERTAF) are released each Monday, featuring outlooks for week 2 (the week beginning the following Monday) and week 3. For each forecast week, ERTAF indicates whether the likelihood of U.S. tornadoes is above, near, or below the climatological average, together with a confidence rating (high, medium, or low).

The technique is based on atmospheric angular momentum (AAM), which relates to the pace at which momentum imparted by Earth’s spin is being transferred to higher latitudes (see Figure 4 below). Gensini and colleagues employ an AAM-related index called the global wind oscillation (GWO), which is broken into eight phases similar to the daily Madden-Julian oscillation index. When AAM is relatively low, we’re more likely to see upper-level troughs in the U.S. West and ridging in the Southeast, a favorable setup for springtime severe weather.


Figure 4. Angular momentum is transferred from the tropics to midlatitudes as air rotating more quickly at the tropics (because of Earth’s larger diameter) ascends and then descends at midlatitudes, transferring momentum to the surface. Image credit: UCAR/COMET Program.

Gensini and colleague Alan Marinaro (Northern Illinois University) demonstrated the utility of their approach in a 2015 Monthly Weather Review paper. That same year, they introduced the ERTAF, which performed very well: 10 of 16 two-week outlooks, and 10 of 15 three-week outlooks, correctly specified whether activity would be above, below, or near the climatological norm for that week (with “normal” defined as between 75% and 125% of the weekly average number of tornadoes). The forecasts were a bit more challenging in 2016, but 6 of the 13 two-week outlooks and 5 of 12 three-week outlooks were on target, and only 4 of the 25 outlooks erred by more than 50% (e.g., by calling for above-average activity when below-average activity occurred, or vice versa). The ERTAF website includes all of the verification statistics for 2015, 2016, and 2017 thus far, based on SPC preliminary tornado totals.

For the week beginning March 26, ERTAF’s six forecasters are calling for an above-average likelihood of tornadoes with high confidence. “We all agreed week 2 is going to be above average. It was a slam dunk,” Gensini told me. The current week-3 outlook, valid April 2-8, is also for above-average activity but with low confidence. “At that range, we’re using primarily statistical analogs, but you only have the realm of what’s been historically observed. Week 2 is where we can couple the statistical and dynamical approach. In terms of subseasonal forecasting, this is really low-hanging fruit.”


Remembering Matt Parker
The U.S. meteorological enterprise suffered a major blow on Friday with the untimely loss of Matthew Parker (Savannah River National Laboratory), who died in his sleep on Wednesday night. Matt had just begun a one-year term in January as president of the American Meteorological Society. However, he had been heavily involved as president-elect in 2016 and was a key player in many other AMS activities before then. A native of Ohio, Matt spent more than 27 years at DOE’s Savannah River National Laboratory after completing his bachelor’s and master’s degrees at North Carolina State University.

I always enjoyed running into Matt at AMS annual meetings, though we never had a chance to work closely together. "He was not just a colleague but a close friend that has stayed in my family's home," said Dr. Marshall Shepherd, past-president of the AMS and host of The Weather Channel's WxGeeks, in a weather.com article by Jon Erdman. "He was as committed as anyone I know to the weather enterprise and bringing academia, government, and private sector together,” Shepard added.

“This is an enormous loss not just for the AMS family but for the entire scientific community,” said Keith Seitter, AMS Executive Director. “Matt was deeply admired for his commitment to the AMS community. He was a leader and a friend, and we will all miss him tremendously.” 

Succeeding Parker at the helm of AMS for the remainder of this year will be Dr. Roger Wakimoto (National Science Foundation), who was this year’s president-elect for 2018.

We’ll be back with a new post on Tuesday.

Bob Henson