Cold snap comes and goes

We start the day unseasonably warm and end it unseasonably cold by the time everyone is back home from school and work as seen in 3km NAM model forecast map above.

Temperatures drop 10-15 degrees today to the low 50s by the lunch hour and low 40s before sunset with lows tonight 29-31 and windy at times behind the morning rain showers.

A dip (trough of low pressure) in the jet stream is ushering out the mild air and ushering in another cold snap with temperatures normal to a little below normal returning through the weekend. Sunshine returns in force tomorrow and Thursday but the chance of rain returns by Sunday.

But the jet stream is what we call progressive (troughs and ridges keep moving rather than locking into place) and zonal (largely West to East carrying in mostly mild air from the Pacific and occasional storms from the Pacific Ocean every 3 days or so. This pattern is expected to continue for most of the rest of the month and bodes ill for lovers of a White Christmas or at least fireplace weather. Santa may pack short sleeves.

Warmer than normal Decembers have been a common feature across much of the country in the 2000s.

GFS model ensemble 500mb jet stream change between today and Christmas:

This WGN graphic shows the result of the jet stream pattern change as we head into Christmas week:

GFS ENSEMBLE MEAN DECEMBER 25TH SNOW ESTIMATE:

I got this from a Christmas Shoppe in Asheville while on vacation:

GFS ENSEMBLE TEMPERATURE GUIDANCE:

It currently looks mild and dry but obviously Christmas is far enough away for the forecast to change one way or another.

Longer term below-normal temperatures will try to make a come-back after we get into the new year.

For more follow me on Twitter @MellishMeterWSB.

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