Last year, just prior to starting this blog, we had a miniature tornado outbreak on June 22nd across Mid Michigan/the Thumb Area and into southeast Michigan. This was the day of the Millington EF2, which was the strongest tornado in the state last year. It could've been a lot worse, considering; it was the same level of "bust" that occurred in the Great Plains a couple weeks ago. June 22nd was looking like a potential high risk day up to a few days out, but it didn't quite manifest up to what the models were predicting, mainly due to early-day thunderstorms eating up some of the energy, and the timing of the cold front not quite coinciding with peak heating. If you were following me on Facebook prior to me beginning this blog, you'll remember how much I was talking about June 22nd in the weeks and days leading up to it. This was because the models began to "whisper" about this event several weeks before it took place. The GFS, in particular, was hinting at something potentially catastrophic occurring at our general latitude in the weeks leading up to it.
I stress "our latitutde" for a couple of reasons. Everyone knows how unpredictable severe weather can be. It's often very difficult to highlight an exact region where severe thunderstorms will form even days in advance (as evidenced by last week's moderate event that didn't quite pan out - the SPC was VERY GOOD at tempering their enthusiasm, though). When you get into weeks, it's pretty much impossible. However, the models can predict the general weather patterns pretty well even this far out. Based on mathematical calculations, and the presence and strength of atmospheric (like the NAO) and oceanic phenomenon (such as El Niño), the Climate Prediction Center can put together a pretty good picture of what things will look like in the weeks ahead. This includes the climate for thunderstorms.
You see me post a lot of images, and 90%+ of those images come from the College of DuPage (a fairly prestigious school for a community college), which is known for its meteorology department. Dr. Vittorio Gensini, a professor there, has been pouring research into the relationship between these atmospheric wave phenomenon and tornado activity in the United States. His evidence is no real surprise: the more amplified the jet stream, the more likely severe weather and tornadoes in particular are to form. These amplifications are how "wavy" the jet stream is - the more ridges and troughs in the jet stream, the more amplified it is. You've probably heard me talk about positive and negatively tilted troughs, too, and if you've read my posts on this you'll know that the most significant severe weather tends to occur with negatively tilted storm systems. When such a trough occurs, the most significant severe weather tends to occur along the latitude directly west and southwest of its position. Dr. Vittorio has created an experimental tornado activity forecast system based on when long-range models predict jet stream amplification and the associated movement of air masses/distribution of temperatures and dewpoints that ensue. If there is a sharp temperature or moisture gradient predicted along with particularly strong trough amplification, tornadoes are likely to form - thus this system would indicate "above average" tornado activity.
Though Dr. Gensini's system only goes out about three weeks (and at present tornado activity is expected to be average to below average for that time), the Climate Prediction Center's models can also be used to indicate severe thunderstorm activity based on the same characteristics of ridging/troughing and temperature/moisture distribution. By interpreting their models and looking for patterns, one can estimate the likelihood of severe weather at a given latitude several weeks in advance. I've identified such a pattern which has persisted across several model runs between the week of May 22nd and June 5th. Now, 4 weeks seems like fairly WIDE grace period, but when you're talking this far out, it's actually pretty narrow. Persistence is the key word - the scenario has shown up in several runs of the NCEP CFSv2 model. Essentially, conditions for severe thunderstorms would be particularly high during this period. The key is that some very warm air would be on deck to stream into the northern U.S. from the Gulf of Mexico. Ample moisture would be present, and significantly high dewpoints (60-70°) would be characterize this airmass. Temperatures could reach the upper 80s or even low 90s. That alone should tell you that there'd be plenty of fuel for big storms should the air mass above it be significantly cooler/drier, which it should be. This is what's known as an "EML plume" - temperatures in the mid levels of the atmosphere (the mixed layer) would be expected to cool very rapidly with height. The "steeper" these lapse rates, the more unstable the airmass. Couple this significant instability with any kind of wind shear (air increasing in speed with height), and you have the ingredients for severe thunderstorms. Couple it with forward veering wind shear (air switches direction from east/southeast/southwest to south/southwest/west/northwest with height - also known as "right moving" wind shear), and you have conditions for supercells and tornadoes.
At or around this time, the CFSv2 has persistently predicted these types of conditions at our latitude. This means it has a good chance of occurring somewhere in the country, and that our area could be the "somewhere" it occurs. The chance of it occurring in our area increases the more meridians are included in the area of interest, how often the conditions appear over the course of each run (i.e. once, twice, three times per run), and for how many hours during each run that they persist. The latest run was the most aggressive so far with these features. As you can see in the image above, it predicts ample severe weather parameters over a very widespread portion of the northern plains, upper Mississippi Valley and Great Lakes. It suggests these conditions would persist for several hours and reemerge over the course of several days. Our chances of "getting hit" by a severe weather outbreak go up when this is the case. Most severe weather is "one and done". Situations do arise, though, when the same region or multiple regions can be struck several times over the course of a day or over the course of a few days. This is beginning to produce hints that such an event could manifest.
Our severe weather season tends to be most intense from early June to mid July, but it's not unheard of to see significant severe weather in our state in late May, either, especially not in a year like this one. I mentioned in previous posts that our state's climate this year may be very similar to that of 1998 - current atmospheric and oceanic phenomenon are undergoing similar transitions as to that year, and we all know what happened that year in late May. It's impossible to tell what we're looking at, if anything, just yet, but it's definitely worth remembering that severe weather season is coming up, and now is the time to rehearse plans with your family. Or, in my case, keep plenty of gas in the car and lots of space for photos and videos on the phone, too!
To summarize:
• Long-range models are hinting at a potentially strong/multi-day severe weather event occuring at our latitude between late May and early June
• These "rumblings" are reminding me a bit of the weeks leading up to the June 22nd event last year - wide longitudal dispersion exists and it's enough to make me concerned that Michigan could see something out of it
• Our year is analogous to 1998, which saw one of the most powerful storm complexes in state history strike Michigan in late May
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