UPDATE: Patrick Marsh writes on his blog that "A yearly tornado drought like the current one would occur once every 28, 571 years."
UPDATE: My stats below are only for the last 9 years. Harold Brooks from the Storm Prediction Center also blogged that we are at a record low for 12-month tornadoes and tornado deaths. Wow! Henry Margusity also points out that we could cross the all-time minimum for tornadoes by mid-May, if no more twisters are spawned before then.
ORIGINAL REPORT: I was quoted recently in a not-yet-published story on AccuWeather.com regarding the slow start to severe weather season this year:
"U.S. tornado warnings and reports are at an all-time low so far this year, since the NWS began issuing warnings in 'polygon,' format in 2005," according to AccuWeather.com Meteorologist and Social Media Coordinator Jesse Ferrell. About 500 tornado warnings have been broadcast between Jan. and April of this year, compared to over 2,300 in 2011. Only 233 tornadoes have been reported so far this year; over 1,000 were reported in 2011 as of this date."
The graph above shows this year's new record low for tornado reports since 2005. As noted above, the number of tornado warnings so far in 2013 is also new record low for the time period examined. We've also only had 2,776 Severe Thunderstorm Warnings between January and April which ranks #2; only 2010 beat that low (see my blog from then), at 2,314. You can see both of these "Year-To-Date" totals in the graph below.
But let's delve deeper. Polygon warnings can be any size, so what about the total area? This would (in theory) be a more accurate representation of the coverage of severe storms.
January to April 2013 also has the record lowest total Tornado Warning area (543,087 sq. km), but is 4th, not 2nd, for Severe Thunderstorm Warning area. Why is this? If you look at the linear trends, the number of Severe Thunderstorm Warnings is going down, while the total area is going up (this rise in area is also evident in the yearly data)? Sources tell me that NWS staff are not penalized for warning size during the verification process, so it would be easier to make fewer warnings encompassing a larger area. This would, however, increase the false alarm rate.
So what about lightning? One assumes that, if severe weather is down overall, a measure of the number of lightning strikes (technically Vaisala measures "flashes" which contain one or more "Strokes" but effectively, these are "strikes" as the human eye would see them). Here's what the Jan.-Apr. period looks like between 2005 and 2012:
As you can see, 2010 beat us out once again, but this year ranks the #2 lowest, with "only" 1.85 million cloud-to-ground lightning strikes.
What's the reason for this year's record low severe weather? Mainly the cold air that has continued to push south from Canada. Yes, you need cold air to meet warm air to generate severe thunderstorms, but there can be too much of a good thing. With a snowstorm last week in the Midwest, the cold air has overwhelmed the weather pattern in Tornado Alley.
Our Summer 2013 Forecast indicates that severe weather will be most prominent as complexes of storms move from the Great Lakes through the mid-Atlantic and Southeast. No severe weather, of course, is great news because less injuries and property damage will occur, but is there still time to save this season for storm chasers (who have been posting pictures of themselves winter coats in snow and ice, drawing cartoons, or just rerunning photos from previous years, in Social Media)? There's a blog here with "chase season" stats for a historical comparison, but last week's shot of cold air should be the last, so they may yet be able to track down some twisters later this month.
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