By Joseph D’Aleo, CCM
Tuesday was an active severe weather day with 29 reported tornadoes (preliminary).
But for the season thus far, we are at the bottom end of the frequency as of the date. When it was active in 2011, the ignorant and opportunists blamed it on global warming. Last spring was very warm and activity was way down. This year, spring ranked among the coldest and activity was very low and only recently has picked up as warmth finally began pushing north to clash with the colder air to the north.
It is in the region of CONTRAST that the jet stream is energized and severe weather occurs. When it is too warm or too cold from north to south, activity is depressed. Global warming would diminish the contrast north to south and result in weaker storms, a weaker jet stream, fewer tornado outbreak and strong EF3-5 tornadoes.
We are passing the peak of the severe weather season. The delayed shift north of the jet stream and alternating cooler and warmer weather will allow for some catch up.
See the activity year to date this year
See how suppressed the 300mb jet stream has been on average.
See the activity last year (whole year).
See how far north and weak the jet stream was. Tornadoes were above normal in southern Canada, below in the CONUS.
In 2011, coming off the strongest La Nina since the 1950s or early 1970s, the jet stream was ideal for severe weather with a southern jet and northern jet. Tornadoes were favored to the left front of the southern jet and right rear portion of the northern one. That is where upper divergence is favored.
See how the negative PDO which favors La Ninas by 2 or 3 to 1 and favors stronger (EF3-5) tornadoes (and more big outbreaks) correlated well with the EF3-5 counts.