I hear many people talking about how they're sick of the chilly weather we've been seeing over the past couple of weeks. North and east winds have been making life near the lake shore somewhat uncomfortable lately, as temperatures during the day have averaged around 45°, and when combined with the strength of the lake breezes, it can easily feel like 10-20 degrees less. Unfortunately for those who have not enjoyed this stretch of weather, it looks like it's going to stick around for another 7-10 days. But cheer up - this isn't going to last forever, and when the change comes, it's going to be quick and it's going to be dramatic. In a month or two, you may be missing this weather, especially when you're trying to sleep.
Before I get into the summer outlook, I want to first take a look at the shorter range outlook, specifically the weekend rain.
The weekend will be not unlike the past week temperature wise, with temperatures in the 40s and 50s in most areas, and likely cooler near the lakeshore. We will have a slightly larger rain storm on our hands, though, which should drop anywhere from ¾ to an inch of rain over the southern half of lower Michigan. This rain should arrive late Saturday evening or early Sunday morning.
After the storm passes, temperatures are going to drop again. More chilly, plant killing nights may be ahead, too; temperatures are forecast by multiple models/runs to drop below freezing in some areas. The chill should be especially felt in areas near the lake shore as the strong high pressure system over Canada continues to make its presence felt.
Now, onto the more interesting stuff - that is, how long will this abnormal cold last? And what kind of summer are we looking at?
First, I want to talk about something called the North Atlantic Oscillation. This is another one of those wonderful oceanic phenomenon (like El Niño, a.k.a "El Niño Southern Oscillation" or ENSO) that heavily influence our seasonal weather. Unlike El Niño, the NAO is an entirely atmospheric phenomenon. It is based on the strength of two semi-permanent atmospheric features present over the Northern Atlantic ocean: an area of low pressure parked over Iceland - the Icelandic Low - and an area of high pressure similarly parked over the Azores Islands - the Azores High. When these features are strong, the NAO is considered to be in a "positive" phase. When they weaken, the NAO is considered to be in a "negative" phase.
The fact that it's been particularly strong several times this year means that the NAO is in a particularly strong positive phase. This waxes and wanes, of course; as it stands, the NAO is slightly negative right now and will likely be for the next ten days. However, this year, it has been positive more than it has been negative. That means that it's likely to return to a positive phase by the second half of May.
So what impact will a strongly positive NAO have on us this summer?
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| Photo courtesy of MLive.com/Mark Torregrossa |
You may have heard me and others talk about the Bermuda High. Well, this is actually the same feature as the Azores High. In late summer, the Azores High tends to drift west, and becomes the Bermuda High. The wacky thing is that this it has already drifted west, this early in the season! When the NAO becomes positive this year, expect the Bermuda High to manifest strongly. When it forms, it will stream warm, tropical air into the eastern U.S. This has a particularly powerful impact on Michigan. Warm, moist Gulf air plows northward right into our area. During the summer, this means 90°+ days, with dewpoints sometimes reaching the mid 70s. Very hot, very humid air.
Another factor contributing to our summertime weather will be our old friend, Mr. El Niño himself. Or rather, I should say, his lacking - El Niño is quickly vanishing, and being replaced by La Niña; there is actually such a thing as a La Niña Watch, and it has recently been issued. Looking at the image above, you can just begin to see hints of the orange colors of El Niño beginning to fade to blue around the west coast of South America. This is a sure fire indication that La Niña is on the way, and with it the textbook more-amplified jet stream that accompanies it. A more amplified jet stream during the summer tends to mean that high pressure ridges will be particularly powerful - the jet stream will curve its way up to Canada, allowing the full power of the southern heat dome to rise northward into our neck of the woods.
So my opinion? Same as it was when Spring was beginning: expect a very hot, humid summer.
How about severe weather and precipitation?
Well, severe weather looks like it's going to have a couple things going for it, and a couple things going against it. During La Niña years, severe weather tends to ramp up all over the country. This is due to a more amplified jet stream whipping up and creating powerful upper level disturbances which then encounter equally powerful high pressure ridges. If something like this were to occur when the hot humid air that the Bermuda High brings to our neighborhood is in place, watch out. An example of such an event was the May 1998 Great Lakes Derecho. You may have heard me mention this one before. This system produced an outbreak of supercells (one of which spawned an F4 tornado in South Dakota) which consolidated into a line, and began to propagate new storms off of the previous' storms outflow boundaries like a nuclear chain reaction. It went on to plow across six states, doing its heaviest damage in Michigan, where it produced category 4 hurricane-force winds of 130+mph. This thing quite literally generated a storm surge on Lake Michigan and sank several boats, on top of knocking down five 345 kilovolt transmission towers. It killed 6 people and injured hundreds.
Now, I'm not saying that something like this is likely to occur again this year, but I do think we'll see one or two major severe weather events. Originally, I was of the opinion that we were only as likely to see as many, if not fewer than, the number of severe events we saw last year, but after some research I'm thinking we actually may see more severe days. The main thing going against severe weather is that during summers like these, the jet stream tends to stay to the north, and we don't get as much cold air interaction as we typically do. Last year, the jet stream was "zonal" - that is, more east to west in flow - and pretty nearly parked over Michigan for most of the summer. This lead to a lot of thunderstorms, if you recall, and the June 22nd/August 2nd events, but not too many severe days in between. If the jet stream stays north, you can kiss thunderstorms - and really, precipitation of any kind - goodbye. Years in which we transition from El Niño to La Niña definitely tend to be drier. Less precipitation usually means less thunderstorms. However, with the type of air we're going to have in place, when storms do manage to form, they will probably be very strong. I've been going over analog years to get more indication on this. So far, I've noticed an average to slightly above average number of tornadoes in most of the years mentioned as analogous to this one by the weather community. I'm looking at 1998 in particular, as it was coming off the heels of the strongest El Niño on record, and we're coming off the heels of a similarly strong one. Michigan had 21 tornadoes touch down that year, most of which were weak (F0 or F1). One F2 touched down in Berrien County (crossing into Cass County before crossing the state line). By contrast, we had 14 tornadoes last year. Our average is 16. Combined with the May derecho, 1998 was a much more potent year for severe weather than was last year. So despite my earlier Facebook post, I'm reversing my position - I now think that we'll be looking at a fair amount of severe weather, possibly even significant severe weather this year. I'm hoping some experts other than me will talk about this and perhaps go over the full list of analog years detailing significant severe weather that occurred. Aside from the May 98 storm, nothing really stands out in my memory.
My final talking point is going to be rain. As stated above, summers like this tend to be on the dry side. It's going to be tough to say how dry, but if spring is any indication, we're probably not going to see much more stratiform precipitation this year. This type of rain is what we'll be getting this weekend. The storms that have come have not carried a whole lot of moisture and have moved off pretty quick. Despite thunderstorms likely being more intense than last year, I think we're going to see a lot fewer general thunderstorm days than last year.
To summarize:
• The state of Michigan is looking at a hot, humid, drier than normal summer thanks to a La Niña transition and a strongly positive NAO/strong Bermuda High
• Severe weather events are likely to be a bit stronger and more numerous than last year, though I'm still waiting to hear more from the experts
• The first half of May is likely to be colder than normal
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