WHERE THERE'S SMOKE...

There are still plenty of unknowns, but it's my opinion that our weather pattern will undergo a change after the 1st of the year. I'm seeing signals that have the potential to realign the 500mb jet stream structure. That could set the table for some active January conditions over the central and eastern U.S. For now, I'm seeing smoke, but models haven't fully latched on to the fire. That means there's evidence, but it's not indisputable.

One of the first things I'll touch on is the system that zips through the Midwest with rain on Saturday. The short wave several days ago wasn't especially strong when it was shown moving SE in the long wave pattern aloft. However, we've now reached a point where the wave length has shortened and a compact 500mb low is embedded within it. This sudden shortening of wave lengths is often a precursor to some sort of pattern change. It's already altered the short term forecast by bringing temperatures early next week that are considerably colder than were shown a couple of days ago. That's a step in the right direction. Here's the energy Saturday at 500mb.

Eventually, this leads to a healthy storm over the eastern U.S. Sunday night and Monday. For us, it means a round of light rain Saturday before the pressure gradient tightens up sending in a quick shot of blustery winds and cold weather to end the weekend.

The GFS shows this for highs Monday the 18th. Some upper 20s sneak into the picture.

Most of the Midwest, Great Lakes, and Ohio Valley are several degrees "below" normal. However, this is just a short bump in the road.

As for precipitation, it's not expected to be heavy, but amounts of 1/4 to perhaps 1/2 inch are looking more likely with the stronger short wave. Here's what models are indicating. Personally, I like the EURO and its lighter totals.

The EURO

The GFS

The 3k NAM

The 12k NAM

Despite a favorable track for snow, temperatures are a few degrees too warm to get the job done. There could be few wet flakes mix in at the end but nothing that accumulates.

Following this little set-back, another round of mild weather begins to build that should have highs in the 40s much of the period Wednesday through at least Christmas. Here's the 10-day average temperature departure for the period December 15-25th. That's a holiday downer for me. Reindeer have to really be hydrated to fly in weather like that!

SHOW ME THE SMOKE....

You have to have some smoke to get the fire that brings change to this pattern. Here are some potential ways that it could happen. The key word being "potential".

We've talked numerous times about the migrating El Niño that's reaching Modoki status. The bottom right indicates tendencies for winter temperatures with a Modoki El Niño compared to the traditional east based version lower left. That's a card that needs to be played.

I've also mentioned the shortening of the wavelengths with a couple of significant storms in the 10-14 day period. This one is December 17th

Another one is shown December 26th.

These have the potential to increase pressure over SE Canada, sending back-door cold fronts that will get the ball rolling for a more seasonal brand of colder air in January. After that, more traditional high pressure may retrograde into western Canada. That's a far more traditional way of getting cold into the pattern.

Another intriguing feature to watch is a blossoming stratwarm at high latitudes. The stratosphere is the highest level of the atmosphere where a persistent vortex is found near the North Pole. You can see its structure here in relation to the Northern Hemisphere below it.

During a stratospheric warming event, the vortex warms top down and weakens. That displaces the cold polar air into the mid-latitudes. This can result in some impressive cold air, but the key to an event is the warming. Once it commences, the process of weakening the intense winds of the vortex takes place. Sometimes the whole vortex moves, other times it's stretched. It usually takes 3–4 weeks for the worst of the cold to reach the lower 48. My limited experience with stratwarms is that they are very complex and hard to project with accuracy at this distance. They can be major players when things come together right. Notice below how the warming is shown progressing in the stratosphere at 10mb between now and December 30th. That's a big difference.

Now

December 30th

One of the things to look for with a stratwarm, is the Arctic Oscillation going into a negative phase. Thursday's EURO weeklies definitely show that as a trend. The control is especially strong. Somewhere around late December to early January is when the EURO suggests the turn from positive (warm) to negative (cold).

By the way, the negative AO is tied to the weakening of upper level winds that allow cold air penetration to the mid-latitudes. Could this be tied to the warming in the stratosphere?

The positive PNA is a teleconnection that is also reflective of colder air in the central and eastern U.S. The further west the mean trough sets up, the higher the potential for cold in the Midwest. That placement is an unknown that will be hugely important to the depth of any cold air regionally.

The NAO (North Atlantic Oscillation) shown entering a negative state is often a harbinger of an active pattern across the middle of the nation and into the east, especially after Christmas. It signifies blocking over Greenland that can position troughs favorably for storms. Does the eventual set-up include us?

Then there's the good old MJO. As you can see, the U.S. climate model the CFSv2 has it in 7 late December, cycling into phases 8 and 1 by mid-January.

The temperature analogs for phases 7, 8, and 1 during that period are all point to below normal temperatures.

The weeklies temperatures were not all that cold for January, but certainly were not the blow torch version of what we have and will see through Christmas. What was impressive was the snow today's runs showed in January. This is the 46-day snow the ensemble mean shows through January 29th. Today, only 18.7 percent of the nation had any snow cover. What's indicated would be a big change, if it occurs.

The control run of the weeklies was a dream. This is very unlikely, but some version of the trend is heartening if you like snow.

If you are still with me, I just ripped off a bunch of reasons why January has the potential to be far different from December. Does that mean it will happen, of course not. In fact, despite all of these solid reasons, I still have doubts about the potential flip. Even if the transition occurs, it won't be a snap change. It will be a gradual transition that occurs over a 2-3 week period in early January. All I can say is that I have done my homework and that is where today's path leads me. I'm hoping the force is with me on this one. If not, we should know by the end of December. Additionally, if the warmth wins out, we'll all save money on heating bills, snow removal, and avoid the inconvenience of adverse conditions. There's always a silver lining. Roll weather...TS

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