© 2019 Terry Swails

SIGNS OF WINTER....

December 20, 2018

Are you ready for some changes? Like them or not, they are coming to the North American weather pattern. The split flow that's brought dry and mild weather since early December is breaking down, at least for awhile. The polar jet is amplifying and winter will return to nation in the 6-10 day period.

 

Here's a look at the split flow we are currently in at 500mb

Look at the warmth covering North America.

You can see extensive low pressure throughout Canada, the harbinger of warmth.

Stepping forward in time, now look at what the operational GFS is showing at 500mb  December 28th. Cold air is pouring into the NC United States. 

There's a complete reversal of temperature departures from positive to negative. Arctic air resides over SC Canada pressing southward into the Dakotas and Minnesota.

The low pressure that covered Canada has been replaced with high pressure. A huge flip from today.

Also note in the graphic above the strong low pressure that's centered over Iowa. That is a significant winter storm that will pack a punch as it travels northeast December 26th-27th. Its departure opens the door for the cold air to surge into the Midwest by the end of next week. This is what the storm looks like as it's passing through Iowa surrounded by wind, snow in the cold sector NW, and rain to the SE of the low.

The GFS depicts this for 10 day precipitation...most coming from this vast storm.

 

This has the potential to be a prolific snow producer over the upper Midwest. Very early in the game, the GFS has this for snow totals.

Following the storm, the cold blasts into the central U.S. These are GFS forecast highs Saturday December 29th. In Manitoba readings as cold as 32.9 below zero are shown. 20 below in NW Minnesota. I will say, this is the first run of extremes like this so I would say exercise caution using the temperature maps below. These could end up far too cold. As for now this is what's shown. 

These are the lows Sunday morning December 30th. 29 below in NC Wisconsin.

Dangerous wind chills up to 40 below are shown. 61 below in parts of central Canada. 

There's one other piece of business and that's a system the EURO brings into the central Midwest Christmas Eve with some snow. It's been showing the potential for several days. The GFS on the other hand has shown no such storm. I've been waiting for one of the models to blink since Monday but so far both are sticking to their guns.

 

Since the energy is still in the Pacific it's not well sampled and obviously each model is initializing it differently. One is clearly on the wrong track but until I can see some consistency all I can say is, it's wait and see time. Roll weather...TS

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