April 28, 2011

Are the Eastern U.S. tornadoes and our cold weather connected?

We are about to finish the coldest April in at least 60 years. And it appears that April will bring the all-time record number of tornadoes to the U.S., including yesterday's catastrophic event with over 250 deaths.

Are they connected? I think one can make a very plausible case that they are.

But before I discuss this connection, let me note that around 6 PM tonight a funnel cloud was observed near Mt. Vernon. You can tell how unstable the air is around here with all the towering cumulus and cumulonimbus. Here is a video of the Mt. Vernon funnel and a good still picture, courtesy of KOMO TV:

Returning to the April records. It is now clear that we will break the all-time-record for the coldest average maximum April temperature at Sea-Tac airport---and plenty of other local observing locations will report similar records.

There is a particular and highly persistent flow pattern that has been associated with our cold and I have illustrated that below. This figure shows the flow at 500 mb last night--actually it gives the height of the 500 mb pressure surface above sea level. Winds are nearly parallel to those lines. Yellows indicate low heights in troughs. You will notice one trough just east of us that has brought cold, unstable air into the Northwest. The places where the lines are close together are associated with strong winds--which we call the jet stream when it is strong and extensive enough.

Anyway, with a trough over us the jet stream heads farther south than normal--directed towards the southeast U.S. The upper flow is wave-like so that a trough over us generally means a ridge over the Rockies and a trough somewhere over the eastern U.S. This trough...which is clearly evident in this image...is ideally situated to provide all the ingredients for severe convection over the SE--moist air from the south, strong uplift, large wind shear in the vertical. This flow pattern has developed repeatedly this month.

When I saw this configuration setting up, it provoked me to suggest in my previous blog that severe weather was a possibility over the eastern U.S., and I was not the only one to see this. But no one knew it would be this bad .

Thus, I believe that there was a direct and compelling connection between our historic spring cold wave and an historic tornado month over the eastern U.S.

An interesting note is that the largest tornado event in U.S. history, the super tornado outbreak of April 3 - 4, 1974, occurred in a La Nina year, as we are this year.

Finally, let me note that the National Weather Service did a very good job in forecasting and nowcasting (providing constant updates) on these severe storms. The fact that so many people have died shows the unique severity of this outbreak and how much effort is still required in spreading warnings and insuring that people take shelter. Another issue is the vulnerability of mobile homes to strong winds.


  1. This lifelong West Coast girl learned something new tonight! At 6PM I was driving East towards Mt. Vernon with the sun behind me. I saw some pretty weird-looking clouds, slanting down in a strange way from the sky.

    "That's a funny looking cloud," I thought. "Too bad I forgot my camera! It looks neat!"

    Doop dee doo! Kept on driving! Later I get home and find the NWS alert in my email in-box.

    Lesson #1: Funnel clouds are a thing. Avoid them!

    Lesson #2: Always bring the camera!

    P.S. at about 6:15PM we had snow in downtown Mt. Vernon. SNOW.

  2. I was in Mt. Vernon and watched as the funnel cloud lowered from the sky. After about ten minutes it began to dissipate and then it lowered again. There were sun-breaks where I was, but a mile north it was snowing and hailing. It was just hard enough to accumulate a little on the grass and the cars, but quickly disappeared when the storm passed.

  3. I'm glad you mentioned the South's tornadoes tonight. I've been in a funk today thinking about their situation.

    The connection absolutely makes sense. Besides our cold weather playing a factor, I know its been moist for you on the westside, but by the time this moist pacific air moves over multiple Mt. ranges, it becomes super dry.

    It was ironic but eerie to see how stable and calm it was today, which was a factor in the outbreak.

  4. Quite interesting that you left the comment the other day which I read at the time, "that there was a direct and compelling connection between our historic spring cold wave and an historic tornado month over the eastern U.S". Very, and sadly accurate.

  5. I wonder if this really was the strongest tornado outbreak ever. It seems to have been the most lethal since 1937, but back in 1937 there was essentially no warning. So for this many people to die despite the warnings being everywhere, it would seem that the storms must have been worse.

    But on the other hand, the area is far more settled now and the population density is higher. So greater population density might be balanced by greater warning accuracy, leaving us still uncertain....

    Science is hard!


Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

Thunderstorms Return to the Northwest

 Thunderstorms have been relatively rare this summer, but today will see some boomers over the Cascades and eastern Washington. In fact, the...