Gradual warm up; Extreme “negative” AO; “Good” winter for northern forests?

Making Progress

The weather maps continue to look encouraging for those who want some warmer weather in Minnesota.

It’s a slow waltz, but all signs suggest our weekend 30s should give way temps near or above 40F by next weekend.

That’s progress, even though our average high in the Twin Cities is 48F by next Friday.

We can finger a near record “negative Arctic Oscillation” for our chilly March weather.

Minnesota’s deep snow pack holds plenty of water these days. That’s creating elevated concerns about spring flooding, especially on the Red River.

In this Updraft we scan the weather horizon for warmer days ahead and get an update on Red River flood concerns.

Is it April yet?

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Feeling chilly? Blame the record “negative” Arctic Oscillation

If you’re looking for clues to our unusually chilly March weather pattern, I have a strong lead.

This week’s near record “negative” AO values are a reflection of the unusual upper air pattern over the northern hemisphere. AO values reached -5.6 on Wednesday. That’s the 5th lowest value on record.

The dreaded “Greenland Block” is in full force.

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We call it “wave spacing” in weather. When there’s high pressure over Greenland, there is usually a deep “Hudson bay Low” feeding frigid air into the Upper Midwest. That’s just the way upper waves in the atmosphere “space out.”

That means it’s been warmer in Greenland than in Minnesota this week.

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40s in balmy southern Greenland? Who know spring break in the Arctic could be so nice?

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Rick Grow with Capital Weather Gang has a nice explanation on why a negative AO means a chilly March for much of the USA.


Just how low has the AO tanked? The AO index plummeted to -5.6 on Wednesday, a historically low value. It’s possible that the measurement taken today, tomorrow or over the weekend will be even more extreme, nearing or surpassing the most negative daily values appearing in the top half of the table below.

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Good winter for Minnesota’s northern forests?

Only in Minnesota would you consider temps below -40F and deep snow “good weather.”

But if you’re a tree in northern Minnesota, that’s exactly what you’d be thinking now about the “real winter” of 2012-’13.

Extreme cold below-40F can kill “invasive pests” that harm our northern conifers. Deep snow cover helps protect root systems from the deep freeze.

Northern Minnesota has endured multiple nights of -40F or colder this winter.

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I asked Lee Frelich, the Director of UM’s Center for Forest Ecology if our colder weather is a good thing for the forests “up north.”


Paul:

Yes, deep snow and cold temperatures will give the boreal forest a brief reprieve from the warming temperatures we have had in recent years. Such winters could delay the arrival of insect pests not in Minnesota yet, such as mountain pine beetle coming from the west and Balsam fir wooly adelgid coming from the east. It should also limit the spread of emerald ash borer from southern to northern MN, where hundreds of millions of black ash trees in swamps form an important habitat for wildlife and native plants.

The deep snow will prevent root freezing, which can ironically be very damaging to boreal trees. A few years ago we had some Hobos buried 10 cm in the soil at Seagull Lake (end of the Gunflint Trail) including a very cold winter with temperatures as low as -45 at Seagull Guard Station, but with 20 inches of snow, it stayed about 30 degrees F in the soil all winter. The snow also insures a relatively late spring compared with recent years, with good soil moisture, so we probably won’t see the damage to spruce and other conifers we had from the dry early spring last year.

Unfortunately, -40 is not cold enough to stop red maple from invading the boreal forest in the Boundary Waters. Maples can deep supercool their phloem sap down to about -45 or -48, thereby avoiding freezing injury. So, the replacement of the boreal conifer forest by temperate deciduous forest we have been observing in the last decade will continue.

Lee

Thanks Lee. It’s good to know the deep cold does some good!

Warmer days ahead:

The overall weather pattern continues to look promising for (gradually) warmer days in the next 2 weeks.

High pressure overhead will steer a weekend storm south of Minnesota, so we’ll dodge the bullet on weekend snow here.

The sun this weekend is as strong as on September 17th, so the wintery chill can’t fight the season too much longer.

High climb into the 30s this weekend and into next week.

Slightly milder temps will ease in later next week…and 40 is looking more likely by next Thursday & Friday.

The longer range maps still suggest a break in our frigid weather pattern the 1st week of April.

A string of 40s…and maybe some 50s may arrive by then.

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The AO signal and long range maps burned us once before this month on a potential warm up, so call me “cautiously optimistic” about the prospects for much milder temps by around April 4th-7th.

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Elevated Red River Valley flood risk:

The deep snow…and high Snow Water Equivalent (SWE) from 3″ to 8″ in western Minnesota means there’s plenty of water available to run though Minnesota’s rivers this spring.

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The NWS at Grand Forks has the latest on why that could mean another year of “major flooding” on the Red.

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The shallow “hydrology” of the Red…and the fact that it flows north into the often still frozen basin acts like a natural dam…and often increase the flood risk.

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Here’s a link to stay updated on Red River flood prospects.

Lake Superior: Not much ice again this year

Lake Superior is mostly “ice free” again this March.

Thursday’s MODIS Terra shot shows the big lake with plenty of open water, with ice confined to most of the bays and a few ice floes in the lake.

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Image: NASA MODIS Terra via UW Madison

I asked Dr. Jay Austin with the Large Lakes Observatory in Duluth about ice conditions this year.


Hi Paul-

Thanks for writing. There’s very little ice out there. The last decent MODIS satellite image I could find was 13 March, at which point the fringing bays (e.g. Chaquamegon, Keweenaw, Whitefish, Nipigon, Thunder, and pardon my spelling) were all socked in, as expected, plus the blown ice we have here in the western arm- the wind has been surprisingly steady out of the NE this month- but basically no open lake ice. The last day or so we’ve gotten some thin locally formed ice as it’s been surprisingly cold. I don’t have the stats yet but this will rank as yet another very low ice year- probably not as low as last year, but that’s not saying a lot.

I still think that Lake Superior in 2012 is a great climate story- it was a truly remarkable year. I’m working with a group right now putting together a worldwide database of lake temperatures as a novel view of the climate story. Lakes, for a number of reasons, are a pretty ideal way to study climate variability (i.e. high heat capacity, immune to landscape changes, and limited geographical scope).

Jay

Thanks Jay!

PH

  • Clint Forseth

    I had not considered far negative AO as the reason for the cold. Makes a lot of sense. Hopefully that will settle out and we can get a slow warming trend (there is hope!) without tons of flooding. Thanks for going into detail!