We call them "temperature inversions" in the weather biz.

Normally temperatures cool as you move above ground. But in some situations, incoming warm fronts are especially good at blowing in warm air a few thousand feet up. In winter, with a snow pack on the ground, that can produce some big temperature contrasts aloft, as much warmer air filters in. The upside down temperature profile is good at trapping a stagnant air mass with drizzle, fog and pollutants near the ground where we live.

The stagnant air mass has caused the Minnesota Pollution Control Agency to issue an air quality alert into Sunday morning. High levels of particulate matter, small particles that come from our car exhaust pipes, furnaces and fireplaces collect near the ground. Levels have already reached the moderately unhealthy range in the metro and other sites in Minnesota.

Minnesota Pollution Control Agency

Here's more from the MPCA:

St. Paul, Minn. — The Minnesota Pollution Control Agency (MPCA) is issuing an air pollution health advisory for the southern half of Minnesota, extending from Thursday afternoon into early Sunday morning. Warmer temperatures, moisture and low wind conditions are causing fine particle formation and trapping them near the ground. Elevated fine particle levels will persist into Sunday, when a cold front is expected to move into Minnesota, shifting winds and enhancing mixing in the atmosphere. Air Quality Index levels will be in the upper moderate range, with hourly AQI readings reaching levels considered unhealthy for sensitive groups at times. An air pollution health advisory is issued when the AQI approaches but is expected to remain below 101, a level considered unhealthy for sensitive groups. The MPCA issues an air pollution health alert when air quality is expected to reach and remain above that level. As of today, this is an advisory. At-risk populations: Fine-particle levels are near a level considered unhealthy for sensitive groups. Populations sensitive to elevated fine particle levels include those with pre-existing cardiovascular or respiratory disease, the elderly, children, and people who participate in activities requiring extended or heavy exertion, both indoors and outdoors. Members of these groups may wish to postpone or reduce vigorous activities and minimize exposure to local sources of air pollution (i.e. heavy-duty vehicle traffic, wood fires and candles) during the advisory. Even individuals who are otherwise healthy may experience health effects when pollution levels increase. Health impacts: Exposure to high levels of fine particles is linked with both respiratory and cardiovascular health effects. Fine particles may exacerbate pre-existing health conditions and may cause individuals to experience chest pain, shortness of breath, wheezing, coughing or fatigue. If you experience these symptoms, contact your physician. Pollution reduction tips: Fine particles are produced from combustion activities, including fossil fuel-based energy generation, motor vehicle exhaust, gasoline-powered yard and lawn equipment and wood burning. Conserving energy, buying clean renewable power and using alternate means of transportation, such as mass transit, will all reduce your daily contribution to air pollution. During air quality advisories, residents are particularly encouraged to postpone or reduce vehicle trips and engine idling, use of gasoline-powered equipment and burning wood.

Onward through the fog

Get used to the low clouds and fog. Temperatures begin to respond Friday as milder air finally begins to mix down to the surface. The thaw peaks this weekend as temperatures in the 40s take hold — and some locations may see 50 degrees by late Saturday or Sunday.

The European model continues to be the most optimistic about the magnitude of the thaw. NOAA's Global Forecast System model trims 5 to 10 degrees of weekend numbers compared to the European solution below. Either way, it's going to be a mild, snow-melting weekend.

Weatherspark

One thing to remember as we deal with fog, mist and icy roads overnight is that road conditions can vary widely across Minnesota. There have been numerous accidents in the past few days, bot by car and from people falling on icy spots underfoot. Take it easy out there, and check pavement conditions as you travel.

California flood waters

From drought to flood in 48 hours. Anywhere from 2 to 9 inches has fallen in northern California.

Flood waters have submerged cars in Oakland.

The western storm moves inland over the next 48 hours. That pushes a warm front toward Minnesota, as we cash in on the benefits of a milder Pacific air mass.

NOAA

Seeley: 2014 weather highlights

I've often called this Minnesota's Crazy Weather Year of 2014.

The Polar Vortex drove the coldest winter in 35 years. Record spring rains and flooding. Record high lake levels in Minnetonka and other lakes. Another instant near drought in late summer and fall. The 9th coldest November on record.

My MPR colleague and University of Minnesota climate specialist Dr. Mark Seeley has some thoughts on the biggest weather events this year. Here's a preview from this week's Weather Talk post.