Friday, January 31, 2020

Weather Whiplash

In Wednesday's post, we mentioned the likelihood of unseasonably warm temperatures in the mountains this weekend (temperatures in valley and bench locations will also be mild, but how warm will depend on how stingy the valley cold pool is). 

We are on track for that as the NAM continues to bring in +4˚C air at 700 mb (10,000 ft) for Saturday afternoon. 

As I mentioned, I think this will cause a touch of spring fever.

However, not far behind that warmth is a cold trough that will drop temperatures greatly by Monday.  The NAM forecast for Monday afternoon local time shows a deep trough over Utah with 700-mb temperatures around -14˚C over Salt Lake.   

Looking at the numbers right from the model grid over Salt Lake shows 700-mb temperatures of 3.5˚C at 5 PM MST Saturday, 5.5˚C at 5 AM Sunday, -3.7˚C at 5 PM Sunday, -9.9˚C at 5 AM Monday, and -14.1˚C at 5 PM Monday.

If that's not cold enough for you, the GFS is even colder and eventually drops the 700-mb temperature to -20.1˚C by 8 AM Tuesday.

I have a rule that I call the 20/20 rule.  700-mb temperatures at Salt Lake City above 20˚C or below -20˚C are unusual and get my attention.  Below -20˚C is not exceptional (the all-time record is -30˚C and the record for February is -25.9˚C), but it is cold.  Usually it ensures sub-zero temperatures at upper elevations. 

The models are also generating precipitation with and following frontal passage.  The SREF ensemble gets things started near or following 0000 UTC 3 February, so late Sunday or Sunday night and by 0000 UTC 4 February (5 PM MST Monday), all but two members generate at least 8 inches of snow and many are over 10 inches. 

Thus, early next week is looking good, but Tuesday is going to be COLD. 

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