Positive PNA and negative EPO signals colder and quieter start to the new year; California and Southwest become stormy in early January

The pattern in most of December 2015 has led to multiple storm systems across the Lower 48 with the negative PNA signal that developed during the second week of December but has flipped to a positive signal during the last few days. The negative PNA combined with the El Nino-driven split-flow pattern across the country has also led to widespread above-normal temperatures, especially over the eastern U.S.

Some of the Ohio Valley, northeastern U.S. and eastern Canada has experienced average temperatures a full 8-9 C (15-17 F) above the normal temperatures for December. It has even been above normal in the northwestern U.S. and Rockies, but not by as much. California and the Desert Southwest has run near to slightly below normal overall. The mean 500 pattern on the right demonstrates the negative PNA with the trough axis in the Rockies and the strongly split-flow pattern locking the colder Arctic air up in northern Canada and bringing multiple storm systems in the West and central U.S. which has brought everything from heavy snow, to ice, to flooding rainfall, to unusual severe December thunderstorms and tornadoes. The stormiest areas have been over the northwestern U.S. and the Plains/Midwest, the storm tracks in the negative PNA pattern.

Our most recent storm pushed out of the Plains through the northeastern U.S. during the past few days while the overall pattern is in the process of transitioning to a quieter positive PNA pattern.

The upper ridge is starting to build along the west coast of North America and the flow pattern remains quite split across North America.

As the PNA index goes strongly positive and stays there right into early January, we should expect a much quieter pattern as compared to December along with colder temperatures in the central/eastern U.S. and some temperature moderation in the Northwest. Why not the Southwest? That's because the El Nino-driven split-flow regime keeps a southern jet stream alive over California, the Southwest and northern Mexico which can bring significant rain and mountain snow to those who have been waiting for the strong El Nino to bear fruit so to speak. Some Arctic air mass intrusions could reach the central/eastern U.S. as the ridge amplifies north through the Yukon and Alaska as the EPO signal goes negative.

The changes in the pattern are in the forecast for the next two weeks.

As the strong PNA develops, we see a strong upper ridge building over the northwestern U.S. and western Canada.

There can be snow showers in the higher elevations of the interior West on Wednesday and some light insignificant snow with a weak disturbance sliding through the Corn Belt and Upper Midwest; otherwise, the weather looks quiet through the end of this week into the weekend. Temperatures across the western and central U.S. which are running colder than normal will start to moderate some over the northern Plains.

For next week, we can see the southern jet stream coming under the strengthening positive PNA ridge over the northwestern U.S. and western Canada. Models differ on exact strength and timing of southern stream systems coming into California and the Southwest but agree on the potential for significant precipitation at last! First southern-stream system is expected to bring significant rain and mountain snow to Southern California into the Desert Southwest on Sunday night through Monday night. What we see on the day-8 ensemble means is a stronger system coming into this region on Tuesday into Wednesday making for two decent systems as the El Nino finally starts to pay off for California and the Southwest! A cold front representing the leading edge of an even colder air mass will push across the Plains and the Midwest during the middle of next week bringing some light precipitation with it.....just a brief interruption in the quiet pattern. The northwest looks mainly quiet next week, but a weak front may bring some light rain/light mountain snow to Washington and Oregon late next week.

The positive PNA/negative EPO pattern is well established late in the period. With the ridge building all the way from the northwestern U.S. to the Arctic, we see the potential for some Arctic highs to press as far south as the northern Plains to the northeastern U.S. Notice the "cross-polar" flow down through central Canada. The Northwest should have near- to above-normal temperatures thanks to the ridge but the outlook is good for more storm systems to ride the southern jet stream through California and the Southwest and farther east across the southern U.S. Temperatures will tend to be colder than normal from the Rockies across the Plains and Midwest, but there will be little if any storminess with the possible exception of a clipper-type system or two.

Have a great day!

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