Three Weather Changes to Watch: Expanding Mild Temperatures, Northwest Atmospheric River, Soaking Southern Storm

World's First Chief Heat Officer's Tips

Two soaking storms and an expanding December thaw are three weather stories we are watching into the weekend.

A shift in the jet stream pattern is now aiming an atmospheric river of moisture into the Pacific Northwest. It will also allow a large area of above average temperatures to build across the Lower 48. Meanwhile, a slow-moving storm will soak parts of the South, including Florida, by this weekend.

Winter Begins With Expansive Mild Temperatures

A building area of warmer than average temperatures across the central and eastern United States is the change that will affect the most people as winter officially begins this weekend.

The jet stream will bulge northward and expand across the central and eastern U.S. into Christmas week. When that happens in winter, temperatures typically warm up since cold air is bottled up in its source regions near the Arctic Circle.

Many areas of the country will have temperatures at least 5 degrees above average for late December by this weekend or Monday. High temperatures could be up to 20 degrees above average in the Midwest.

While this doesn't mean you'll be able to break out shorts and t-shirts in the northern tier of the nation, it will be a break from what can be a cold time of year.

For example, Chicago and Detroit will have highs in the mid-40s this weekend. That's comfortably above their low-30s average highs for this time of year.

Atmospheric River Soaking the Northwest

An atmospheric river of moisture is bringing heavy rain to the Pacific Northwest, triggering flooding in some areas.

An atmospheric river is a thin, but long, plume of moisture in the atmosphere that stretches from the tropics or subtropics into higher latitudes. Atmospheric rivers are responsible for many heavy precipitation events on the West Coast each year.

Parts of Washington and Oregon will receive the brunt of the heavy rain and mountain snow from this event. Just over 8 inches of rainfall was measured near Pe Ell, Washington through Friday morning, while 7.3 inches of rain was observed near Frances, Washington.

Seattle-Tacoma International Airport received 3.25 inches of rain Friday, making it the fifth all-time wettest day, with records going back to 1945.

Heavy snowfall will pummel the Cascades, but snow levels are expected to rise as milder air from the atmospheric river moves in. An avalanche warning covered most of the backcountry of the Cascades on Friday.

Rain and mountain snow will shift southward into California this weekend, but it's not expected to produce nearly the totals it has in western Washington and Oregon.

Southern Soaker This Weekend

Low pressure is forecast to form near the Gulf Coast this weekend, and it will likely be a multi-day soaker for parts of the South.

(MORE: Heavy Rain Will Drench Parts of the South)

Rainfall should increase by Saturday near the Gulf Coast and in Florida. Periods of rain could continue through Monday in parts of Florida and other areas in the Southeast. Locally heavy rain might fall in some areas, so the potential for flooding will be monitored closely.

Winds will increase across the region, and there is also a chance of severe thunderstorms in the southern Florida Peninsula on Sunday.

Rough surf, coastal flooding and beach erosion are potential impacts along coastal areas of the northern and eastern Gulf of Mexico and off the Southeast coast. These impacts could extend to Monday and beyond.

There is still a large amount of uncertainty when it comes to the timing, location and the exact impacts from this storm, so check back for updates the next few days.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.