Temperature Extremes Across the U.S. This Week

Politics is not the only place in which extremes can exist in our country--- the weather is showing that it is no slouch in this domain.

During the last week, there has been an extraordinary contrast of temperatures across the nation, with temperatures hitting 100F or more in the Southeast, but lows in the teens/twenties and heavy snow in portions of the Northwest. 

To illustrate, here are the mean temperatures for October 1-2 (top), with the difference from normal (anomalies) for the same period on the bottom.  Huge warm anomalies over the eastern U.S., reaching nearly 20F.   And nearly equal cold anomalies over the upper Plains and the West down to the Mexican border.

Many records have fallen on both sides of the country:  cold records in the west, warm records in the east.  For example, here are daytime records on October 1.  Wow.  There are not only daily high temperature records, but monthly records as well.  THAT is impressive.

On the other hand, the same morning produced low-temperature records
over the west, including breaking daily records, and in some locations, monthly ones.  Several locations, such as Spokane, broke daily/monthly snowfall records.

So why the simultaneous extremes on differing sides of the nation?  It has to do with a highly perturbed and anomalous upper level wave pattern, which caused a large undulation in the jet stream.

To illustrate, here is the upper level (500 hPa--about 18,000 ft) analysis for 5 AM PDT September 29th (Sunday).  Blue and purple indicate much lower than normal heights (a trough), and orange/red are higher than normal heights (ridge). 
You can see a very disturbed, undulating pattern, with a ridge over the Northeast Pacific, a very deep trough over the west, and a ridge over the eastern U.S. 

Troughs are associated with cold, low-level air, ridges with warm air.

Why?  Such a pattern moves warm air into the western side of ridges (southerly flow) and ridges have sinking (which warms air by compression).  Thus ridges are warm.   Just the opposite for troughs.

The same chart on October 1, the day of the records shown above, is very similar:

 

The pattern today has the same general idea, but a bit weaker.

But I have some news.  It looks like ANOTHER major push of cold air will surge into the Northwest on Tuesday, with more snow at higher elevations (see map below for Tuesday at 5 PM PDT). 

If we don't shake this pattern, November could bring snow to the lowlands.  But don't worry...this weekend should be decent, and Sunday will be perfect.  Sunny, no precipitation, and mid-60s in the western lowlands.

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/10/temperature-extremes-across-us-this-week.html