Winter returns, later sunsets start this week

Minnesota gets its winter Mojo back this week.

December returns to form the rest of this week as a more seasonably chilly Canadian air mass surges south. The departing low pressure system leaves some light snow in its wake in the metro, with heavier totals to the north and west. Canadian high pressure builds in the rest of the week with colder temperatures and more abundant sunshine.

Most of the overnight snow is gone by the Tuesday morning commute, but beware of some residual slick spots as temperatures drop below freezing for the first time since early Friday morning. The quick synopsis for the rest of this week features highs in the 20s and lows in the teens for most of Minnesota.

Free winter vacation?

Our long weekend December thaw was like a (free) winter vacation. Usually Minnesota snowbirds head south for milder temperatures in winter. This year milder temperatures came to us. Not everyone likes the winter warm up, but if you do you can probably thank a budding El Ni&ntiled;o for what amounted to a free mid-December winter vacation.

Expect the unprecedented

Our mild December air mass is not just unusual, it is unprecedented. We set some eye-opening records during this mega-thaw. Bottom line? We've never had an air mass this warm...or this moist in mid-December in the Twin Cities. Check out some of these incredible numbers.

• 51 degrees – record high at Minneapolis-St. Paul International Airport Monday at 4:45 a.m.

• First Twin Cities record high ever recorded between 4 a.m. and 5 a.m.

• 3rd straight day at or above 50 degrees at MSP

• 49 degrees – highest dew point ever recorded so late in the year at airport

The first time the Twin Cities has ever recorded a record maximum temperature between 4 and 5 a.m.? The Twin Cities highest water vapor content ever recorded this late in the year?

When my my MPR colleague and University of Minnesota climate watcher Mark Seeley sends a note during an unusual stretch of weather, you know something freaky has happened.

The 51°F at MSP airport at 4:45 a.m. this morning breaks the record high of 49 °F set on this date in 1923, but more than that I think it is the only time in the Twin Cities climate history that a maximum temperature record has been set between 4:00 and 5:00 a.m.

Further over the last three days the Twin Cities has set new daily records for highest dewpoints: 44°F on Dec 13th; 48°F on Dec 14th; and 49°F on Dec 15th.  The last reading is the highest water vapor content ever measured in the Twin Cities so late in the year.

Mark

Sunsets start getting later this week across Minnesota.

According to my Minnesota Weatherguide Calendar the sun will set at 4:33 p.m. Tuesday evening. That's slightly later than Monday evening's 4:32 p.m., where sunset has been stuck since December 4. The precise sunset time varies according to your location, and different calculating authorities like the U.S. Naval Observatory. But you get the picture. Thanks to a quirk in the elliptical nature of earth's orbit around the sun, sunsets start getting later this week even before the winter solstice, which occurs at 5:03 p.m. Sunday.

Here's a good explanation of why sunsets get later before the winter solstice from the Washington Post's Capital Weather Gang.

What causes this discrepancy? To begin, it helps to understand the two "competing" forces that determine local sunrise and sunset times: 1) Changes in the sun’s declination, or height above the horizon throughout the year, and 2) The changing time of solar noon.

Solar noon occurs when the sun reaches its maximum height in the sky on any given day. At any location on Earth, the time of noon slowly oscillates back and forth by several minutes throughout the year (in other words, a sundial would not consistently show noon occurring at the same time as your wristwatch). These shifts are due to the earth’s elliptical (non-circular) orbit and axial tilt, and are summed up in a complex relationship called the equation of time (for simplicity, let’s call it the "solar noon effect").

The reason the earliest sunset occurs before the winter solstice has to do with the later shift in solar noon “outweighing” the effect of the sun’s decreasing height and length of time above the horizon. For example, in D.C. solar noon is at 11:57 a.m. on December 1, but drifts 14 minutes later – to 12:11 p.m. – by December 31. This forward shift means that it takes a few seconds more than 24 hours for the sun to complete a full circle between its maximum noontime height from one day to the next. Meanwhile, as we approach the solstice, the sun’s declination is no longer changing as rapidly, which causes the days to shorten at a slower pace.

Enjoy the later sunsets this week as our inbound Canadian air mass brings sunnier skies.