You've probably heard about the widespread drought still in place over the nation's mid-section.
What about a drought most people love? With April coming to an end, it may be safe to say the U.S. is in a short-term tornado drought.
U.S. Tornado Stats
According to Severe Weather Expert, Dr. Greg Forbes (Facebook | Twitter), only 67 tornadoes have touched down in the U.S. so far this April, according to preliminary data through April 28.
Over the past 10 years (2003-2012), the April average through April 28 is 203 tornadoes. If the month ended already, it would be the quietest April, tornado-wise, since 1992, according to Dr. Forbes.
Contrast that with the record-setting April 2011, when an incredible 758 tornadoes recorded, including the massive "Superoutbreak" from April 25-28. That's over 11 times the number of April 2013 tornadoes!
(RECAP: Anniversary of a Superoutbreak)
This comes on the heels of the least tornadic March in the U.S. in 35 years.
When taking into account "tornado inflation", the increased reporting of tornadoes over the last few decades due to technological improvements, increased awareness, and the growth of spotter networks, the March-April period may deliver one of the lowest tornado counts on records, relative to what you would expect, according to Dr. Forbes.
With that said, we have had some notable tornadoes in 2013, including...
- Adairsville, Ga. (Jan. 30)
(RECAPS: Adairsville timeline | Outbreak recap) - Hattiesburg, Miss. (Feb. 10)
(RECAPS: Maps, timeline | News/photos) - Killer Miss./Ala. Tornado. (Apr. 11)
(RECAP: Facts, figures)
Recently, it's been winter storms and flooding that have taken center stage, rather than severe weather outbreaks.
(MORE: Winter Storms A to Z)
So, what factors have limited the severity and areal extent of severe weather outbreaks over the past six weeks?
First and foremost, it's been the dominance of cold air from the Deep South through the Plains states.
At right is a map of surface temperature anomalies since March 1. We've seen a persistent parade of cold air masses plunge far south, relative to what is considered "average" for March and April. Note the impressive cold anomalies from the Dakotas to the Southeast.
(MORE: Coldest March since 2002 |"Marchuary")
These impressive cold plunges sweep any warm and humid air out of the Plains and Deep South into the southern Gulf of Mexico or western Caribbean Sea, making it more difficult for sufficiently warm and humid air to become entrenched ahead of the next front.
Looking at the upper-level wind flow pattern in April (at right), we have seen the bullish southward dips, or troughs, in the jet stream needed in significant severe weather outbreaks.
A key contributor in severe outbreaks is for these jet-stream troughs to surge east over the warm, humid air near the surface. It's the superposition of these upper-air and surface features that triggers the large, widespread severe weather outbreaks.
There have also been times since March 1 when the upper-level pattern is not at all conducive to thunderstorms, much less severe thunderstorms, in the eastern two-thirds of the nation.
A northward bend in the jet stream, or ridge, of high pressure in the West, and a broad jet stream trough in the East is one of those "non-conducive" patterns in the spring. Instead, you see repeated intrusions of cold, dry air at the surface I mentioned earlier.
Despite all this, there's a key point to take away, here: This recent "tornado drought" says nothing about May and June, typically two of the most tornadic months of the year in the U.S.
No comments:
Post a Comment