Tranquility Reigns across Tornado Alley; Cyclone Pam Rages in Southwest Pacific

There’s a calm across the nation’s tornado-prone regions, a quietness that seems odd when you consider the calendar. By crunching numbers from the monthly and yearly severe weather summaries produced by the NOAA Storm Prediction Center (SPC), it becomes clear that tornadoes are just part of the story.

Through March 9, the U.S. has racked up only 28 preliminary tornado reports, compared to an average of 95 for the same time span (Jan. 1 to Mar. 9) during the years 2000 – 2014. (Note that the 2014 data remain preliminary.) We’ve had quiet starts to tornado seasons before--in 2002, there’d been only 5 tornadoes by this point in the year--but 2015 stands out even more for its utter lack of strong thunderstorms. The most notable convective excitement of the year has been Jim Cantore’s amped-up encounters with thundersnow in New England (now Auto-Tuned for posterity). If you happened to encounter ice falling from the sky, it was more likely sleet than hail.

The dearth of severe weather comes into focus when we look beyond tornadoes. Through March 9, we’ve seen 119 preliminary reports of severe wind, compared to a 15-year average of 708. Even more striking is the almost-complete absence of large hailstones. The 15-year average up to this point in the year is 362 reports of severe hail (at least 1” in diameter), but in 2015 thus far, we’ve had only two such reports, both occuring in northern Louisiana on Feb. 1.


Figure 1. Total counts of tornadoes, severe hail, and severe wind for the period Jan. 1 – Mar. 9 in each year from 2000 to 2015. The national criterion for severe hail was raised from ¾” to 1” in 2010. Image credit: Jerimiah Brown, Weather Underground; data from NOAA Storm Prediction Center.

It’s pretty easy to see what’s caused the severe-weather drought of early 2015. A stubborn upper-level trough over Hudson Bay has kept northwest flow dominant across the eastern half of the country, shunting potentially unstable air masses well out to sea before they have a chance to generate thunderstorms. Even high-contrast fronts, like the one that plowed through the South last week, haven’t enough upper-level support ahead of the cold air in order to produce severe weather. The large-scale patterns have been so clear-cut that even the ambiguity that might prompt a tornado or severe thunderstorm watch has been in short supply. So far this year, SPC has issued just four watches (all tornado watches). This is the latest we’ve gone without at least one severe thunderstorm watch, and the lowest total number of watches through March 9, in records that go back to 1970. On average, more than 30 watches have been issued by this point. The first nine days of March 2015 didn’t see a single severe weather report or a single watch; no March in the official record has gone past March 10 without at least one watch.


Figure 2. Persistent northwest flow kept severe weather set-ups to a minimum across the central and eastern U.S. from late January through February. Image credit: The Weather Channel.


Variability in tornado seasons is increasing
After the catastrophic tornado season of 2011, which took more than 550 U.S. lives, the pendulum swung hard in the other direction. The period 2012 – 2014 saw the fewest tornadoes of any three-year period going back to 1950, when reliable tornado records began. As for 2015, the odds appear slim for any major severe outbreaks in the U.S. over at least the next week. “I would, in fact, not be surprised if we do not see a significant tornado in the month of March,” tornado researcher Victor Gensini (College of DuPage) told me in an email. It’s important to remember that a season that gets off to a slow start can still become active by spring, as noted by the Weather Channel’s Jon Erdman in this overview. And even a below-average season, such as 2013, can feature a few devastating days, such as the deadly outbreaks in late May 2013 that struck in and near Shawnee, Moore, and El Reno, Oklahoma.



Figure 3. Cloud-to-ground lightning near Tulsa, OK, on May 31, 2013, a day that also brought deadly tornadoes and flash flooding to the Oklahoma City area. Image credit: wunderphotographer mrwing13.

If it seems like tornado seasons are getting more variable, your impression is backed up by research. Harold Brooks (NOAA National Severe Storms Laboratory) led a study published last autumn in Science that details the increased ups and downs in tornado occurrence on a variety of time scales. Looking only at twisters with at least F1/EF1 strength on the Fujita/Enhanced Fujita damage scale, the study found that the number of days with at least one F1/EF1 tornado has dropped since the 1980s, while the number of days with at least 30 such tornadoes has risen dramatically. As a result, twisters are becoming more concentrated into a few high-intensity days each year. About 20% of all tornadoes in the decade 2004 - 2013 occurred on the three biggest days of each year, whereas this was the case for only 10% of all tornadoes before that decade. The reasons behind the shift aren’t yet clear, but the authors observe, “If the variability continues to increase, it could lead to an even greater concentration of tornadoes on fewer days.”

Seasonal timing is becoming more variable too, according to the study. For example, in all but three years from 1954 to 1996, the 50th F1/EF1 tornado of the year was reported between March 1 and April 10. But in the subsequent period (1997 – 2013), just 6 of 17 years saw the 50th twister occur in that early-spring interval, which implies that seasons are getting off to faster and/or slower starts. We’ll have more on climate change and tornadoes in an upcoming post.

Pam becomes a powerhouse in the South Pacific
In the South Pacific Ocean about 1,800 miles east of Australia, Tropical Cyclone Pam has quickly intensified to a Category 1 storm with top sustained winds of 90 mph as of 8 am EDT Tuesday. Pam has generated quite a bit of hype over the past few days, thanks to eye-popping model projections by the GFS and European models which show the cyclone intensifying into a Category 5 monster with a central pressure less than 880 mb by late this week. If this forecast verifies, it would make Pam one of the strongest tropical cyclones ever recorded, world-wide. However, these models are not known for making reliable intensity forecasts, and are generally disregarded by NHC for intensity forecasts in the Atlantic and Eastern Pacific. The HWRF model, which is one of our better intensity forecast models, predicted with its 06Z (2 am EDT) Tuesday run that Pam would reach a central pressure of 902 mb by Friday, which would make it a still-formidable Category 5 cyclone. The Tuesday morning official intensity forecast from the Joint Typhoon Warning Center (JTWC) goes along with this idea, making Pam a Category 5 storm by Friday. Pam is an unusually large cyclone over extremely deep warm water, with widespread surface temperatures above 30°C (86°F). With wind shear a moderate 10 - 20 knots and expected to be in the low to moderate range this week, Pam should be able to undergo a period of rapid intensification into at least a Category 4 storm. Fortunately, no major populated areas are in the projected path of Pam, although a westward shift in track could threaten the islands of Vanuatu.


Figure 4. MODIS satellite image of Pam taken at 23:00 UTC Monday March 9, 2015. Image credit: NASA.

Rare subtropical depression off the coast of Brazil
A rare subtropical depression, with characteristics of both a tropical and a non-tropical system, has formed in the South Atlantic waters off the coast of Brazil, according to the 8 am EDT March 10, 2015 analysis by the Navy Hydrographic Center in Brazil (thanks go to wunderground member Tropicsweatherpr for alerting us to this.) The unnamed storm has top wind speeds near 35 - 40 mph, according to an 8:20 am EDT pass from the ASCAT satellite. The surface pressure was near 1008 mb, and a modest amount of heavy thunderstorm activity was on its east side, in a band well removed from the circulation center. Sea surface temperatures are near 27°C, which is about 0.5°C above average, and 1°C above what is typically needed to support a tropical storm. Phase space diagrams from Florida State show the storm has a warm core at low levels which should get better defined through Wednesday, but by Thursday the storm will begin losing its tropical characteristics as it moves over cooler waters, and it is unlikely the storm has time to become fully tropical. The unnamed storm is not a threat to make landfall.


Figure 5. Surface analysis from 8 am EDT Tuesday March 10, 2015, from the Navy Hydrographic Center in Brazil showing a 1008 mb subtropical depression off the coast of Brazil.

South Atlantic tropical storm history
Tropical and subtropical storms are so rare in the South Atlantic that until 2011, there was no official naming of depressions or storms done. In 2011, the Brazilian Navy Hydrographic Center instituted a naming system with nine names, of which two have been used so far (Arani in 2011, and Bapo in 2015.) If this week’s storm becomes a subtropical storm, it will be called Cari. Brazil has had only one landfalling tropical cyclone in its history, Cyclone Catarina of March 2004. Catarina is one of fewer than ten tropical or subtropical storms to form in the South Atlantic, and the only one to reach hurricane strength. An unnamed February 2006 storm may have attained wind speeds of 65 mph, and a subtropical storm brought heavy flooding to the coast of Uruguay in January 2009, killing fourteen people. Tropical cyclones rarely form in the South Atlantic Ocean, due to strong upper-level wind shear, cool water temperatures, and the lack of an initial disturbance to get things spinning (no African waves or Intertropical Convergence Zone exist in the proper locations in the South Atlantic to help spawn tropical storms).

Climate change and South Atlantic storms
It is uncertain whether climate change may cause an increase in South Atlantic tropical storms in the future. While today's storm formed over waters that were about 0.5°C above average in temperature, Catarina in 2004 formed over waters that were 0.5°C cooler than average. Sea surface temperature is not the main limiting factor inhibiting these storms--wind shear is. How climate change might change wind shear over the South Atlantic has not been well-studied.

Bob Henson and Jeff Masters