Pacific breezes return, January thaw kicks in

Minnesota has spent much of the past week in the grips of the coldest stretch of weather so far this winter season. Yes, when Minnehaha Falls is frozen solid, you know it's cold in Minnesota.

Now milder Pacific breezes blow in, and many savor temperatures in the 30s above zero instead of wind chills in the 30s to 50s below zero.

Feeling 70 degrees warmer in one week in mid-winter?

Priceless.

Air mass transfusion

A modified pacific air mass that crossed the Rockies spills east into Minnesota Thursday. Temperatures above freezing ride all the way up the lee of the Rockies well into Canada. Here's a look at the upstream inbound milder air from the University of Illinois earth sciences web server, WW2010.

Here's the surface map. Minnesota rides the boundary of milder Pacific (not arctic) fronts the next few days. It turns out all cold fronts are not created equal. Friday's weak cool front is still of relatively milder Pacific origin. The same front pushes back north Friday night as a warm front.

The European Centre for Medium-Range Weather Forecasts model continues to be the most aggressive with the coming warm up. The National Oceanic Atmospheric Administration's Global Forecast System has backed off a few degrees in the past two days. Given the nature of the warmth to the west I am inclined to go with the Euro notion of warmer temperatures. I won't be shocked to see us get a good temp boost well into the mid 30s Thursday and upper 30s to near 40 Saturday afternoon.

January thaw: Myth or reality?

Growing up as a young weather geek (weather enthusiast) in Minnesota I always heard about the legendary "January thaw." It turns out it's more reality than legend. Temperatures have reached 32 degrees in all but three years in the Twin Cities since 1891.

A loose definition of a January thaw? Two or more days at or above 32 degrees in January. That's a 92 percent historical probability of a January thaw in the metro. It's closer to a 50-50 proposition as you move north to International Falls.

Here's more on the January thaw from the Minnesota Climate Working Group.

Looking back to 1891 in the Twin Cities, there have been only three years when the thermometer did not reach 32 degrees at least once in January. The years that did not have a high temperature of 32 or greater in January in the Twin Cities are: 1912, 1978 and 1979.

But what about a January thaw? A January thaw is loosely defined as having TWO daytime highs greater than 32 degrees in the month of January.  Using this definition, there was not a "January Thaw" in the Twin Cities in 2009. The last (previous) time the Twin Cities failed to have a "January Thaw" (Based on two or more days with daytime highs greater than 32 degrees F) was in 1982.

Hers' more from University of Minnesota climate guru and MPR colleague Mark Seeley on the historical frequency of January thaws in Minnesota.

Based on two or more days with daytime highs greater than 32 degrees F the historical frequency of January thaws for various locations in Minnesota shows great reliability in most of southern Minnesota, and even parts of central Minnesota, but more like a 50/50 probability in the northern sections of the state.

Historical frequency of January thaws at various locations since 1948

(here defined as two or more days with daytime temperatures greater than 32 F)

• Twin Cities 92 percent

• Rochester 95 percent

• Pipestone 92 percent

• Fairmont 93 percent

• St Cloud 87 percent

• Morris 80 percent

• Crookston 62 percent

• Duluth 60 percent

• International Falls 50 percent

It is interesting to note that the three consecutive years of 1977, 1978, and 1979 brought no January thaw to most of Minnesota. All three were dominated by abundant January snow cover and are ranked among the top 15 coldest in state history.

The last year without a January thaw in most places was 1994.

Enjoy the warm up, but keep an eye out for icy spots with freeze-thaw cycles the next few days.