This year already has been the deadliest for tornadoes since 2011, due to repeated outbreaks in the Southeast. April was the deadliest month for tornadoes since May 2013, and April also was the most active such month on record since 2011, with more than 300 preliminary tornado reports.
The Storm Prediction Center estimates the nation has seen more than 500 twisters on the whole so far this year, one of the most robust starts to the year on record and just five dozen tornadoes shy of the 563 tornado year-to-date record. The April 12 and 13 outbreak, during which more the 150 tornadoes touched down, proved one of the most vicious tornado swarms on record.
While the Southeast has caught the brunt of severe weather so far this severe season, signs point to a shift west and north in storm risk during the course of the next month.
But May this year looks to enter suspiciously quiet, a quiescent period that belies the month’s ferocious potential. While that relative calm should last for a couple of weeks, there are signs a return to an active period could be on the horizon.
Month at a glance
Early May is expected to be less active compared to average. Severe thunderstorms still will develop in some areas — especially over the eastern Plains and Mississippi Valley — but the odds of widespread severe thunderstorm or tornado activity over the central and southern Plains look to be lower than during an average start to May.
During this time frame, storm chances and heavy rain and flooding potential could linger over the Southeast for longer than is typically seen during spring.
A ramp-up in storm activity could be on its way around May 15 to 22, with a number of features aligning to boost potential tornado and severe thunderstorm chances late in the month. Early indications suggest near-average or above-average storm potential over the southern and central Plains during this time frame, bleeding into the High Plains by the end of May and into June.
It is also important to note that an extremely warm Gulf of Mexico will boost the amount of water vapor available to thunderstorms from the Southeast to the Plains states.
Late in the month, and especially into June, could also feature slightly above-average chances of severe thunderstorms in the Ohio Valley.
The first few days of May will be relatively quiet, with perhaps one or two gusty hailstorms east of the Rockies on Saturday in Colorado. Nationally, Sunday looks quiet.
By Monday, May 4, low pressure over the Northern Plains associated with an approaching upper-level disturbance will drag a cold front south and east. That could trigger a few severe thunderstorms with the threat of large hail and a secondary risk for strong to damaging winds and a few tornadoes. The areas to watch include eastern Kansas, northeastern Oklahoma, northern Arkansas and much of Missouri.
Thereafter, the severe thunderstorm threat will shift south and east, potentially affecting portions of the South, Southeast and perhaps even the Gulf Coast by Tuesday and Wednesday. Heavy rains could fall in the Tennessee and central Mississippi valleys, which have already seen excessive amounts of spring rainfall.
By the end of next week, a return to southerly winds over the Plains will favor an increase in temperatures and humidity, possibly fueling additional severe storms as an upper-level disturbance approaches into next weekend. Beyond that, things become increasingly uncertain, but we can look at the prevalence of certain weather patterns for clues.
Mid-May players point to a rude awakening
Severe weather typically shifts northwestward from the Gulf Coast states to the Plains during May. The bull’s eye of climatologically favored severe weather activity meanders toward Oklahoma, lifting to the High Plains by June as warmth builds from the south and the jet stream retreats toward Canada.
Great Plains storms feed off a basic clash of air masses — namely spurts of high-altitude cold air from the west that are ejected from a more persistent cold air mass over the Rockies. Nestled in bursts of jet stream winds, these punches of cold overspread near-surface warm, humid air steaming north from the Gulf of Mexico. These ingredients, each benign on its own, together can set the stage for vicious storms.
This May, however, the pattern is flipped — the warm “ridge” of high pressure is in the west, while an energetic orb of low pressure hovers relentlessly over the Northeast. That’s been keeping the Mid-Atlantic and New England socked in beneath low clouds, frequent rains and a chilly rawness to the air.
For the time being, this pattern — unfavorable for widespread severe weather over the Plains — will hang around. That’s good news for the Interstate 35 corridor, which will likely see below-average severe weather activity for the first two full weeks of May. That’s not to say severe thunderstorms won’t occur, but the threat for larger-scale outbreaks is lower than usual.
What may cause the late-month storm uptick
Around about May 15-20, however, a number of computer models suggest this pattern will break down and shift to a more favorable severe weather pattern. With upper-level support banked to the west and a long-overdue return to warmer conditions in the East, severe weather chances will return to at least average levels by the last two weeks of the month.
Existing research points to an uptick in severe weather activity when a “Phase 2” pattern of the MJO is achieved — something the European model, among others, highlights as increasingly likely by about May 20.
During some years, the presence of an El Niño or La Niña event in the tropical Pacific Ocean offers valuable predictive insight about how severe weather season may play out. However, ENSO, or the El Niño Southern Oscillation, is close to neutral right now — and likely won’t have much an influence for the remainder of spring severe weather season. An anticipated developing La Niña may be a player for a more active hurricane season, however.
Typically the placement of severe weather evolves late in May into early June, with the risk diminishing to the south and east as the bulk of severe weather activity lifts into the High Plains, especially in eastern Colorado, Nebraska, western Kansas and the Dakotas. It looks like the timing of the upper-air pattern, coupled with MJO influence, could give a boost to the High Plains season.