Thursday, December 13, 2018  4:10 AM

Time for Winter Cleaning (of the Air)

Leftover Damp and Dirty Air to Be Cleaned Out by Afternoon

The clammy and murky conditions continued through yesterday and overnight. Temperatures never got milder than 28 degrees during the day ( see NWS: Last 72 hours of St. Cloud Observations) and didn't drop below 24 degrees overnight. The low clouds and southeast winds continued to reinforce the damp air mass (see Shortwave Albedo from Colorado State satellite slider menu) and the air pollution levels remained high (see hourly Air Quality Index from the Minnesota Pollution Control Agency) with air quality indices over 100 in the Twin Cities area.

Some Fog, Light Snow or Mixed Precipitation Could Put Some Ice on Roads This Morning

However, note the cleaner air over Marshall. That's one sign of a new air mass that has been working its way towards western Minnesota overnight (see UCAR Dakotas surface chart). This cleaner air is behind a weakening front (see NWS WPC Latest North American zoom-in surface map). There is dense fog across southwestern and southern Minnesota early this morning, so the National Weather Service has a dense fog advisory out from Benson, Willmar, Redwood Falls, and Mankato south and west. Expect a slippery morning commute with some patchy freezing drizzle on untreated surfaces (see MnDOT Minnesota road reports). On top of that, there are a few light snow showers and some mixed precipitation from the new front on radar (see NWS Upper Mississippi Valley radar loop) and some of this is actually making it to the ground in the form of both light snow and some light mixed precipitation. We may see a little light snow or mixed precipitation this morning. This will grease up some roads for a while.

Conditions Calm and Warm by Late Morning

We will get into the clearing air behind this front by late morning. That and a strong northwest wind will finally get rid of the dirty air and ease any slippery roads as temperatures climb above freezing. We could see a high in the upper 30's to perhaps near 40.

Next Warm-Up Friday into Saturday, But Not as Dank or as Dirty

We will repeat the cycle of warmer air moving (see Unisys N. Hemisphere 850 mb chart) over the top of the leftover air near the ground tomorrow and Saturday. In fact, the air aloft will be even warmer than what we have now, as temperatures could rise to over 40 degrees at 5,000 feet above the ground. The hope is that getting the clear air at the ground from off the Rockies will mean that it will take longer for moisture to build up in the lower atmosphere. So, I am hopeful of seeing sunshine after some early morning frost or rime tomorrow. Highs should climb above freezing. Temperatures will remain milder on Friday night, thanks to the milder air, some high clouds and a persistent southwest wind, so readings will remain in the 20s. It is possible that we could see some thicker low clouds blowing in from off the melting snow cover in Nebraska, but I'm hopeful the fresh start will lead to fewer cloud problems.

Upper 30's or 40 on Saturday??

We should begin Saturday with some fog and low clouds, but I am hopeful of seeing some afternoon sunshine. Temperatures will climb to the upper 30's to even near 40. And, without the usual weekday traffic, I am hopeful that the air pollution levels will stay lower than they have been the past two days (see hourly Air Quality Index from the Minnesota Pollution Control Agency).

Cooling Trend Sunday into Monday

A stronger cold front will push in late Saturday night. There might be some spotty light snow or flurries with it towards Sunday morning, but that will do it. Temperatures will remain in the 20's to near 30 on Saturday night, then recover to around freezing or a little higher on Sunday. Significantly colder air will push in on Sunday night, so lows will fall back into the teens by Monday morning. High temperatures on Monday will be back in the 20's where they are supposed to be this time of year.

We could have another mild interlude through the middle of next week.

Confidence Level: "The Bulb on That Hard-To-Reach Light in the Foyer Will Burn Out"

Thursday 12/13/2018: Cloudy with a chance of morning light snow or flurries, then partly cloudy in the afternoon with a bit of a breeze. Milder. High: between 34 and 38. Winds: SW 5-10 MPH in the morning, becoming NW 10-15 MPH in the afternoon. Chance of measurable snowfall: 30%.

Confidence Level: "Every Time I Try to Put in a New Bulb, the Light Fixture Will Try to Detach From the Ceiling"

Thursday Night: Partly clear, light winds, and perhaps some fog by morning. Low: between 15 and 22. Winds: light W evening, becoming SW 5-10 MPH late at night. Chance of measurable snowfall: 10%.

Friday 12/14/2018: Partly to mostly sunny, light winds, and seasonably mild. High: between 35 and 40. Winds: SW 5-10 MPH. Chance of measurable snowfall: 10%.

Confidence Level: "The Only Way I'll Be Able to Change the Bulb and Hold the Fixture in Place Will Be to Yell At It

Friday Night: Partly clear and continued seasonably mild. Perhaps some fog. Low: between 17 and 22. Winds: SW 5 MPH evening, 5-15 MPH by early morning. Chance of measurable snowfall: 10%.

Saturday 12/15/2018: Sunshine through high clouds, breezy, and milder. High: between 37 and 42. Winds: S 10-20 MPH. Chance of measurable snowfall: 10%.

Saturday Night: Partly clear, and mild. A slight chance of a flurry. Low: between 25 and 30. Winds: W-NW 8-15 MPH. Chance of measurable snowfall: 20%.

Sunday 12/16/2018: Partly sunny, breezy, and a shade cooler. High: between 32 and 36. Winds: NW 10-20 MPH. Chance of measurable snowfall: 10%.

Sunday Night: Partly clear with diminishing wind late and colder. Low: between 10 and 15. Winds: N 5-10 MPH. Chance of measurable snowfall: 20%.

Monday 12/17/2018: Sunshine through high clouds, light winds, seasonably cold. High: between 24 and 30. Winds: E 5 MPH. Chance of measurable snowfall: 10%.

Extended: Another mild interlude the first half of next week??

Forecast Confidence (10 - "The Rabbits Will Thump, Objecting to Me Yelling at the Light Bulb"; 0 - "I Will Be Hosting a DIY Show on Home Repairs"): 7 Wednesday, 6 Wednesday night and Thursday, 5 Thursday night, 4 Friday, 3 Friday night through Sunday.

Yesterday's High: 27°F; Overnight Low (through 4 AM Thursday): 23°F
St. Cloud Airport 24 Hour Precipitation (through 4 AM Thursdayay): None; SCSU Precipitation (through 4 AM Thursday): None

Next Update: Friday, December 14, 2018 8 AM (or as needed)

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