Monday, November 23, 2009

Weather menu: Thanksgiving slush

October was one of the 5 coolest on record, according to climatologists at NCDC, the National Climactic Data Center in Asheville, North Carolina. November is trending much warmer than average (that hardly comes as a headline) but I was intrigued to see a global snapshot of temperatures worldwide since January 1. Granted, 9 months does-not-a-trend-make, but you don't have to be a rocket scientist to see that, across the entire planet, temperatures are trending warmer than the long-term average. We're on track to see the 5th warmest year, worldwide, since 1880 (in spite of consistently cooler temperatures for the northern tier states of the USA). Again, it's so tempting to look out the window and ASSUME that this is what everyone is experiencing. Keeping that true (global) perspective is problematic, even for seasoned meteorologists. There's still lingering confusion between "weather" (ie CNN Headline News) and "climate" (ie the History Channel). October's chill was an example of weather, so is our mild November, for that matter. You really have to step back and examine trends over decades to reach any conclusions about climate trends. The bottom line: even though Minnesota experienced a somewhat cooler summer (with fewer tornadoes!) and October was unseasonably chilly, the global trend has still been one of consistent warming, not just 2009, but a steady upward trend in temperature since the early 80s. Have temperatures leveled off a bit since 1998? Yes. But there is absolutely no sign of "global cooling" or any other data points that would suggest climate scientists are colluding, plotting or scheming to skew the data in any way, at least none that I can find. There will always be conspiracy theorists, and people who just don't want to face the (potentially unpleasant) facts, some politicians will always cherry-pick data and try to do their spinning, but the next time it snows (and it will), or the next time you squeeze into a parka, muttering about "that damn global warming" under your breath, remember it's weather, not climate. To get a grip on what's truly happening, planet-wide, try to keep that (global), long-term perspective. I know - easier said than done.

No sign of "global cooling". This is snapshot of temperature departures from normal since January 1, for the entire planet. The blue dots represent cooler-than-average, the red dots show warmer-than-average readings. The larger the dot the greater the departure from normal. The data is stunning: yes, the northern tier states of the USA have been cooler than normal, but the rest of the planet is trending significantly warmer than the long-term average (since 1880), according to NCDC, the National Climatic Data Center, a division of NOAA. As of October 2009 is the 5th warmest year on record, the average global temperature a little more than 1 degree F warmer than average, but across much of Africa, Asia and northern Russia/Siberia temperatures are trending as much as 3-5 degrees F above average. [I know, there's a typo in the title of the graphic, it should read "Temperature Anomalies" - nothing I can do about that. Yes, even the government makes mistakes.] For more details click here.

I still think we'll wake up to a white Thanksgiving - but there's no cause for panic-in-the-streets. The bulk of precipitation with this latest storm will continue to fall as rain, through the day today and tonight. But tomorrow as all levels of the atmosphere begin to cool down I still expect a changeover to wet snow. During the day there may be a little slushy accumulation on some lawns and fields (roads should stay wet during the daylight hours Wednesday with temperatures staying above freezing). But after sunset tomorrow, after 4-5 pm, give or take, some of that slushy snow may start to stick, especially on secondary roads and side streets. By the time it's cold enough for snow MOST of the moisture will be south/east of town, we're talking 4-8 hours of light snow at the tail-end of this storm, most of it coming Wednesday evening/night. How much? Best bet is under 1", although I could still envision as much as 1-3" across western Wisconsin and parts of southeastern Minnesota, closer to this "cold core low" sliding off just to our south and east. With temperatures falling into the upper 20s Wednesday night many roads will become slushy and slick, and I think many of us will wake up to a coating of slush Thanksgiving Day. Much of the snow will melt during the day Thursday, roads mainly-wet after 9 or 10 am. If you're out Wednesday night or FIRST THING Thanksgiving Day you will probably encounter some slippery roads. But getting around from midday Thursday into Saturday should be just fine as skies slowly clear and temperatures moderate a bit, reaching well into the 40s again Saturday (the nicer, brighter/drier day of the weekend). A reinforcing cold front squeeze out a few hours of very light snow and flurries on Sunday (a coating of slush can't be ruled out with highs close to freezing). No heavy snow accumulations are imminent, but within 24-36 hours there will be NO doubt in your mind that it's late November. It was nice while it lasted, but now it's time to face reality. Ugh.

Air pollution in late November? According to the Minnesota Pollution Control Agency (MPCA) the air over much of Minnesota and western Wisconsin is unhealthy, a temperature inversion (warmer air aloft) trapping pollutants at ground level. Dense fog has been reported over the MN Arrowhead and much of Wisconsin. People with risk factors (lung/heart conditions) may want to consider cutting down on strenuous outdoor activities today - a rush of colder air will break up the inversion tomorrow as rain mixes with, and eventually changes over to a period of wet snow.

Thanksgiving Day Weather (courtesy of the MN State Climate Office)

Because Thanksgiving Day occurs at the transition period between autumn and winter, Thanksgiving weather can be balmy to brutal. A typical Thanksgiving Day in the Twin Cities has high temperatures in the 30's and at least a bit of filtered sunshine.

Having a mild day in the 50's on Thanksgiving Day is relatively rare, looking at the historical record back to 1891. A maximum of 50 or more has happened only nine times in 115 years, or about once every 13 years or so. The warmest Thanksgiving Day is a tie of 62 degrees set in 1914 and 1922. The mildest recent Thanksgiving Day is 58 degrees on November 26, 1998.

On the other side of the spectrum, it is common to have a high temperature below 32. The average Thanksgiving Day temperature is right around freezing. What about extremely cold Thanksgivings? It is about as likely to have a minimum at or below zero on Thanksgiving Day, as it is to have a maximum of 50 or above. It has occurred eight times in the past 115 years. The coldest Thanksgiving Day temperature is eight degrees below zero. This has happened three times, 1893, 1905 and 1985.

Measurable snow fell on 25 of the past Thanksgivings back to 1891, about every five years or so. The most snow that fell on Thanksgiving was five inches in 1970.

Historically, about one in three Thanksgivings have at least one inch of snow on the ground. The deepest snow pack is a tie with 1921 and 1983, both with 10 inches on the ground by Turkey Day.

It occasionally rains on Thanksgiving Day as well. In 1896, a two-day event in the Twin Cities doused Thanksgiving travelers with nearly three inches of rain.

To check out Thanksgiving Day weather, year by year (since 1891) click here.

Paul's Outlook for the Twin Cities

Today: Periods of rain, a bit cooler (but still much warmer than average for late November). Winds: N 5-15. High: 49

Tonight: Rain, mixing with some wet snow over central/western MN - most metro roads should remain wet, but a little slush can't be ruled out on some lawns/fields by morning, especially west of Monticello and Cokato. Low: 36

Wednesday: A period of wet snow, mixed with a little rain at times - mainly wet roads. High: 38

Wednesday night: Light snow and flurries, a coating to an inch or two possible. Low: 29

Thanksgiving Day: A slushy start to the day. Leftover clouds and flurries, partial clearing late. High: 39

Friday: Dry and sunny for power-shopping and travel. High: 42

Saturday: Nicer day of the weekend. Fading sun, a bit milder. High: 45

Sunday: Cloudy with a little light snow/flurries - coating of slush can't be ruled out. High: 37

Monday: Average for late November. Lingering clouds and flurries. Coat-weather returns. High: 35

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