In order to compare any particular year to the average year, we can see how far ahead or behind that year is to the average for each day of the year. In other words, if there are 500 tornadoes at the end of May, that would be about 100 behind the long-term average of about 600. If there are 700, it would be about 100 ahead of the average year. A time series of this departure through the year gives an idea of how far ahead or behind the season is throughout the year.

Tornado anomalies through the year

On this graphic, positive departures indicate above average and negative departures are below average. If the line is horizontal, that period of the year is exactly average. The years shown include the biggest years in the tornado record (2004, 2008) and the smallest years (1987, 1988), as well as 1973 and 1992, which demonstrate different ways that a big year can happen. In 1973, there aren’t any really large periods for tornadoes, but a steady increase ahead of normal throughout the year. In 1992, the early part of the year looks a lot like the really slow years of 1987 and 1988, reaching more than 300 below normal by 14 June. After that, a short period of frequent tornado occurrence got the season back to normal within a month and then ending up more than 200 ahead by the end of the year.

The black dots represent 2009 at the end of each month and mid-June. Reports after March 2009 have been adjusted as discussed here. The gray line is for 1996, the closest analogue to 2009, where the season was ahead of normal through early May, then fell almost 100 below normal by the middle of June before finishing about 50 ahead.

In reality, despite the slow period that coincided with the VORTEX2 field project when, for a 5-week period, there were over 100 tornadoes fewer than normal, the 2009 season has stayed within 100 of the normal throughout the year.  Normally, by 22 June, more than 60% of the median 1290 tornadoes per year have occurred.  We’ll have to wait to see how 2009 turns out.

Yesterday, May 21, 2009, marked only the second time that a May 21st has had no reports of severe weather (tornado or hail or wind event) since 1955. Surprisingly, the last time no recorded tornado, hail, or wind event occurred was just six years ago, on May 21, 2003.

Every other May 21st in every year since 1955 (the first year that NWS started official records for severe thunderstorm hail and wind events) has had at least a single report in the database for this day and most May 21sts have had numerous reports given this is about the peak in severe weather activity during the annual cycle.

How about severe thunderstorm and tornado watches issued by the NWS Storm Prediction Center (SPC)? If the SPC does not issue a watch today or tomorrow, that would be a substantial “first” for this particular week in May. Since 1970, there has not been another May 17th-23rd period without at least 6 watches issued (either a severe thunderstorm watch or a tornado watch somewhere in the country). The SPC has not issued a watch since Saturday evening, May 16, 2009. Since 1970, there have been about 28 watches issued during this week in May. The greatest number of watches ever issued during this week-long period was 59 (averaging almost 9 a day!), in 1989. It now appears likely that SPC will break the previous minimum number of watches issued for the week, 6 in 1976, by Saturday, achieving a record of 0 watches for the week of May 17th-23rd, 2009. That has not happened before and can be considered quite unusual.

If we look at the first three weeks in May with respect to SPC watches, then this unusually quiescent week is masked by watches issued earlier in the month. Through the first three weeks of May 2009, SPC has issued 93 watches. This is a little above the long-term average for the period of 88 but well short of the maximum number issued (a remarkable 226 watches in three weeks), in 2003. The fewest number of watches ever issued for the first three weeks of May was 28, in 1976.

What is the cause for such tranquility in the atmosphere? The overall flow regime has quickly transitioned into one more characteristic of July or August, or even September. Fast mid-level flow is confined to the U.S./Canada border and very weak flow is left across the rest of the continental U.S. There has been a highly anomalous low pressure system drifting slowly westward from Florida over the past few days. This low formed in the wake of the last significant weather system to bring severe weather to the Central and Eastern U.S. last week. Low level moisture has persisted along the Gulf Coast, and is starting to return to parts of the Southern Plains as the Florida/Gulf low moves westward, yes westward. At the same time, and also characteristic of July/August, an early monsoon pattern has started in the Southwest U.S. This will result in an increase in thunderstorms across that region, and northward across the interior west and Rockies, in the days ahead.

It’s still possible that the overall flow pattern can revert to a more typical and active late May weather pattern, thus resulting in more widespread severe thunderstorms and a greater potential for tornadoes. However, the chances of that happening decrease as the summer months approach and the weather pattern, as established, becomes more typical for the time of year we find ourselves in.

While yesterday’s data remain preliminary, here are the official top 10 wind report days occurring in either the month of January or February since 1955:

01/29/08 333
02/21/97 217
01/03/00 203
02/10/90 189
02/05/08 186
02/06/08 184
02/22/03 178
01/18/96 173
02/11/99 172
02/24/01 160

It’s no real surprise that all these days have occurred since the 90s. The NWS has undergone dramatic restructuring during this time and the emphasis on documenting severe storm events has likewise increased. That, in combination with an increase in population and a technological revolution in communications technology (cell phones, the web, etc.) have led to a rise in storm event reporting.

However, severe weather events during the winter months are not that common. Significant thunderstorm wind events during January and February are even less common (refer to the list). The first and only time until yesterday that more than 300 severe thunderstorm wind reports occurred on a single day in January or February was just last year on January 29. Wednesday’s event will be the second time we’ve equal or exceeded that number on a January or February day since 1955. Even more remarkable is that 4 out of the top 10 January or February thunderstorm wind days have all occurred in a little over the past year.

Some of this is obviously due to the factors discussed above. How much of it is due to a real trend in severe weather during the cold season remains an open question. Or, do these episodes occur every so often in the long-term record and we are only beginning to see them due to the increased density of the reporting network?