Spring 2015 seasonal tornado outlook
Meteorological spring is upon us, and it is time again to take a look at the spring tornado forecast. Are we going to see a repeat of the last two years with low tornado numbers? Or are we finally going to turn things around in 2015?
Not much is expected in the way of tornado activity through the first chunk of March. It’s early for much in most years anyway, but we do usually see an uptick as the month goes.
Looking at some of the teleconnection indices mentioned in the 2014 outlook and the 2013 outlook, it would seem that the near term would provide a shot or two for tornadoes. In this case, any activity is either too far north or too strung out to really get tornadic storms going.
Heading into the middle of March, the pattern looks mercilessly harsh for tornadoes, though maybe the West Coast could score a few tornadoes. After all, March is the peak climo month for tornadoes in California.
The pattern starts getting even more jumbled around once we hit the middle of March.
This type of pattern generally supports troughs and digging storm systems in the western U.S. while warmer temperatures develop over an upper-level ridge in the eastern half of the country. However, it is still far from ideal. The core of the upper-level ridge of high pressure will likely be centered over the Great Lakes, which creates easterly/northeasterly flow across the Southeast. This type of low-level wind pattern struggles to tap into the Gulf moisture.
The El Niño-Southern Oscillation (ENSO) is not expected to be a major player in the overall pattern. It is anticipated to hold onto weak El Nino or fall into neutral positive territory.
Given that fact, I focus most of my forecast off of the strongly positive Pacific Decadal Oscillation (PDO), which has kept a persistent upper-level ridge over the western U.S. through most of the winter. While December was abnormally wet in the Southwest, January and February have been fairly dry. That’s generally not a good sign for getting tornadic storms in the U.S. The PDO will likely remain positive (perhaps strongly positive) through spring.
The ongoing drought conditions in the western U.S. helps promote a positive feedback of warmer and drier conditions across that region. A lot of the big tornado-producing storm systems move across the Southwest before ejecting into the Plains and eastern U.S. So, a tendency towards drier than normal conditions, with less disturbances in the Southwest, is expected to keep the tornado numbers down a bit.
The warmer and drier conditions in the Southwest helps keep the upper-level ridge over the western U.S., forcing many systems to drop down from the Northwest and western and Canada into the Plains. Not the best storm track for tornadoes.
We may be looking at another late season for more continual instances of tornado producing systems, unless we can get disturbances to cut underneath the prominent upper-level ridge in the western U.S. Of course, last year was relatively quiet early and still had several large killer tornadoes during that time.
My spring 2015 temperature and precipitation anomaly analogs:
The only notable caveat I would add to these analogs is I think the western U.S. will end up warmer and drier than the analogs indicate.
I am expecting the tornado count for meteorological spring to be below normal again this year.
However, I do think we are in for a few good systems that move across the Southwest despite the drought and ridging. It probably won’t be enough to bring the tornado count up to near normal levels, but the season should still offer a handful of dangerous tornado days and probably some memorable periods for chasers as well.
Odds of the tornado count ending up below/near/above normal for meteorological spring:
Below normal: 50%
Near normal: 35%
Above normal: 15%
Long term forecasts, while generally providing added value to climatology, are still very broad-brush outlooks and do not offer a very consistent level of skill. This forecast only hopes to capture some of the most reliable information available to provide a best guess as to what spring may bring.