Mother Nature has all of a sudden become quite shy under the intense scrutiny of a national field program to uncover more of her secrets with regard to the formation of severe thunderstorms and tornadoes. Storms this past week have essentially been non-existent and the uneasy quiet should continue into the weekend.
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.
