Severe Storms Looming for Early April

The first ten days of April could produce more severe weather than the modest amount racked up so far across the U.S. in 2015. The same upper-level pattern that kept the West warm and the East cold through most of the winter has also kept severe weather to a minimum, as northwest flow and a series of cold fronts pushed warm, unstable air off the U.S. mainland. This year through March 30, we’ve seen a preliminary total of a mere 38 tornadoes, compared to a January-March average over the preceding three years of 163. As of Tuesday morning, NOAA’s Storm Prediction Center had issued just four tornado watches and four severe thunderstorm watches for the year thus far. This compares to a long-term Jan.-Mar. average (1970 – 2014) of around 39 tornado and 24 severe thunderstorm watches, according to SPC’s Greg Carbin.


Figure 1. Severe-weather outlooks issued by NOAA’s Storm Prediction Center on Tuesday morning, March 31, valid for Tuesday (top), Wednesday (center), and Thursday (bottom). Image credit: NOAA/SPC.


The 2015 numbers look destined to rise over the next few days, as a major circulation change takes place over North America. The long-prevailing northwest flow is being replaced this week by more zonal (west-to-east) flow, with an embedded upper-level impulse reaching the Midwest on Wednesday and another targeting the Southern Plains on Thursday. A moist low-level air mass is already in place, with dew points close to 60°F from Tulsa, OK, to Birmingham, AL. Surface low pressure generated by the upper-level impulses should continue to pull the moisture pool northward and generate unstable conditions. SPC placed a broad belt from Oklahoma to Georgia under a slight risk of severe weather for Tuesday afternoon and evening, with large hail possible as the moisture continue to flow north.

A large swath of the Great Plains is under an slight risk for Wednesday, from northwest Oklahoma to southern Minnesota, with an slight risk in place for Thursday across part of the Southern Plains and mid-Mississippi Valley. The predominantly west-to-east upper flow combined with southerly low-level flow will enhance vertical wind shear, a key ingredient in the formation of supercell thunderstorms. The main threats appear to be high winds and large hail, although tornadoes can’t be ruled out. As was the case last Wednesday, when F2 tornadoes struck the Tulsa suburb of Sand Springs and the Oklahoma City suburb of Moore, the corridor from Interstate 44 into east central Oklahoma could be a particular focal point for supercell formation by late Thursday. Severe weather may continue into the lower Mississippi Valley on Friday before the associated cold front moves into the Gulf of Mexico.

Forecast models indicate a strong upper-level trough will settle across the Great Basin by early next week, setting the stage for what could be a more extensive multiday round of severe weather beginning as soon as Sunday. A strong surface low should develop over the High Plains by late in the weekend, pulling unstable air back northward across a large area beneath west-southwest upper flow. Strong thunderstorms could spread across the Midwest and South by early next week, with several days of focused severe weather possible.

Is El Niño about to make its presence known?
Surface waters have warmed dramatically over the far eastern tropical Pacific over the last several weeks, and the water temperatures are now assuming a more classic El Niño configuration that’s been absent for the last few months, with prominent warming just off the coast of South America. Computer models are remarkably consistent on projecting a strengthening of El Niño conditions over the next few months. All eight international models surveyed by the Australian Bureau of Meteorology on March 16 indicated that at least moderate El Niño conditions should be in place by August (i.e., sea-surface temperatures at least 1.0°C above average over the Niño3.4 region). However, in a March 31 update, the BOM cautions: “Model outlooks spanning February to May . . . have lower confidence than forecasts made at other times of year. Some models currently show some spread in their outlooks for tropical Pacific Ocean temperatures, indicating that while further warming is indeed very likely, there remains some ambiguity about the amount of warming expected.”

One hallmark of El Niño across the U.S. is split flow, with the polar jet stream retreating to the north and the subtropical jet stream intensifying across the southern tier of states. This pattern tends to keep unstable air shunted toward the Gulf Coast, hiking the chance of severe weather there (especially in Florida) but reducing the odds over the nation’s heartland.

Long-range models for mid-April are suggesting an El Niño-flavored pattern may emerge, with prominent split flow (see Figure 2, below). This is partly related to an intense Madden-Julian Oscillation event, the strongest on record (see our March 18 post), whose impacts are now reaching the eastern tropical Pacific and reinforcing the trend toward El Niño conditions. On its heels, a strong downwelling (or warming-phase) oceanic Kelvin wave will be pushing slowly eastward within the eastern equatorial Pacific over the next month, according to WSI’s Michael Ventrice. “This should favor increased organized thunderstorm activity over the eastern tropical Pacific basin, which will act to accelerate the subtropical jet stream over the U.S. through the end of spring,” says Ventrice. “This spells heavy precipitation threats (including some severe weather) across the southeastern tier of the nation over the upcoming months under the developing split-flow type pattern.”

Unfortunately, next week’s strong western trough may bypass California, further reducing hope of an “Awesome April” that might take a dent out of the severe drought and heat now plaguing the state. If a strong El Niño were to develop and persist, it could increase the likelihood of substantial rain in California during the 2015-16 wet season.



Figure 2. The GFS ensemble forecast issued at 1200 GMT on Monday, 30 March, and valid at 0000 GMT on Monday, 13 April, shows a pronounced split-flow pattern at the 200-millibar height (roughly 40,000 feet), with much of the United States lying between the polar and subtropical jet streams. Image credit: Michael Ventrice, WSI.


New insight on how El Niño, La Niña shape severe weather risk
A paper published this month in Nature Geoscience elaborates on how the odds of U.S. severe weather in late winter and spring tend to be boosted by La Niña and diminished by El Niño. The authors, led by John Allen (International Research Institute for Climate and Society, or IRI), acknowledge that it’s difficult to examine the connection between the El Niño/Southern Oscillation (ENSO) and severe weather. The datasets are imperfect (not all tornadoes or severe hailstorms get reported), and there’s a great deal of variability from year to year. “Trying to tease out an ENSO signal from both the natural noise and the human noise becomes quite complicated,” said coauthor Michael Tippett (Columbia University) in an IRI news release. “You can’t get a robust correlation using the observational record alone.” In this new study, the state of ENSO from 1979 to 2012 is compared not only with actual severe reports but also with the environmental factors associated with severe weather, such as instability and vertical wind shear, thus enabling the results to be analyzed more comprehensively. The study is the first to examine ENSO’s relationship to severe hail.


Figure 3. When ENSO is in a warm, or El Niño, phase (top), the frequency of springtime tornadoes goes down. When it is in a cool, or La Niña phase (bottom), tornadoes increase (indicated by red areas). The effect is strongest in the boxed area. Image credit: IRI, from Allen et al., Nature Geoscience, 2015.


In line with previous work by others, the largest influence found by the IRI team in winter (December-February) is across southern Texas and Florida, where the risk of tornadoes is roughly doubled during El Niño events. Prior studies had been inconclusive for springtime, but the IRI group found a significant ENSO influence focused across parts of northern Texas, Oklahoma, and Arkansas (see Figure 3), where the risk of tornadoes and severe hail rises dramatically during La Niña and drops during El Niño. There’s some asymmetry in this response: while not every El Niño event puts a dent in the region’s severe weather, La Niña events almost always push the likehood of tornadoes or severe hail above the climatological norm. “Naturally, this is only a simple model for the influence of ENSO on hail/tornadoes, and there needs to be more complexity added moving forward,” Allen told me in an email. He and colleagues are now looking into how variations in the strength of ENSO across a severe weather season might influence the outcome.

Given the weak El Niño event now under way, the IRI team is calling for slightly enhanced odds of a less-active severe season than usual. Allen explains the forecast in a video clip on IRI’s website.

Maysak maintains Category 5 strength
Jeff Masters posted a full report earlier today on Super Typhoon Maysak, now plowing across the Northwest Pacific east of the Caroline Islands. As of 8 am EDT Tuesday, Maysak’s top sustained winds were at 160 mph, according to the Joint Typhoon Warning Center. This makes the system one of only three Category 5 typhoons ever observed in the Northwest Pacific prior to April (the other two are Super Typhoon Ophelia of January 1958 and Super Typhoon Mitag of March 2002, both with 160-mph winds). The JTWC predicts some further strengthening of Maysak, with a projected top wind speed of close to 180 mph by Tuesday night. Any eyewall replacement cycle, if one occurs, could keep Maysak from getting stronger. Fortunately, Maysak is tracking north of Yap, the most populated of Micronesia’s Caroline Islands, and cooler water temperatures should lead to a fair amount of weakening by the time Maysak approaches the Philippines this weekend. At least 5 deaths and extensive damage have been reported on Chuuk State (Micronesia). The storm also passed just north of the sparsely populated islands of Fais and Ulithi while close to its top strength.

According to intensity estimates from the Joint Typhoon Warning Center, 2015 is the first year on record to have three Category 5 storms form in the Pacific Ocean during the first three months of the year. The other two Category 5 storms in 2015 were Tropical Cyclone Pam (165 mph winds), which devastated Vanuatu in mid-March, and Tropical Cyclone Eunice (160 mph winds), which affected ocean areas in the South Indian Ocean. Reliable satellite records of Southern Hemisphere tropical cyclones extend back to the early 1990s, so we only have about a 25-year period of good records for global tropical cyclones.

We’ll have a new post by Wednesday morning.

Bob Henson


Figure 4. An infrared image of Super Typhoon Maysak from 0444 GMT on Tuesday, March 31. Image credit: NOAA/NASA and RAMBB/CIRA, courtesy Stu Ostro (The Weather Channel).