Progressive Pattern Makes for Active Weather and Up-and Down Temperatures Through Midmonth

The emphasis was on cold air and snow across the West Tuesday and Wednesday; however, on Thursday the problems were farther south and east from northern Texas to Oklahoma and eastern Kansas with thunderstorms along a dry line produce damaging winds, hail and a tornado or two. Rain and thunderstorms soaked areas farther north in the upper Mississippi Valley into the Midwest but no severe weather. Our pattern has become much more active here in early November due to the development of a negative PNA pattern with a trough over the west and a ridge in the east and it should last for a while.

The PNA forecast which has been consistent from run to run recently is forecast to remain negative through the first half of this month signaling a chillier and more active weather pattern across the western United States as the trough is favored over the West. So, northern stream Pacific systems will tend to amplify as they come into the West. We also will see frequent weather and temperature fluctuations as the pattern should remain progressive with systems moving on from west to east across the country. This is due to a lack of downstream blocking over the Atlantic......a positive NAO.

We have quickly transitioned into a highly amplified negative PNA pattern with a ridge over the eastern Pacific, the deep, cold trough over the West and a large, warm ridge over the East. However, since the pattern is progressive, the cold western trough is pushing out into the central U.S. in two pieces. The front-running piece already pushing into the Great Lakes helped to produce the thunderstorms Thursday with the trailing piece coming through the Rockies. Cooler air is coming down through the Plains behind the cold front associated with the front piece with a continuation of below-normal temperatures in the west and some light snow in the Rockies into Friday. The ridge building into the Pacific States is going to bring a little temperature moderation but that will be short-lived since the next wave will push in this weekend.

Otherwise, with the more progressive pattern, expect back-and-forth temperatures across much of the Lower 48, though temperatures will average above normal in the East and below normal in the West. The main challenge in the forecast will be with the exact day to day timing of weather systems and temperature changes during the next two weeks.

The much cooler air will briefly cool the central U.S. to near normal on Friday and Saturday. Enough cooling may occur for rain in the northern Plains on Thursday and Thursday night to mix with or change to wet snow before ending on Friday, but little or no snow accumulation is expected.

This storm will be followed by a very chilly air mass into the Plains and even some snow shower activity in the northern Plains. However, since the pattern is progressive, warming will already return this weekend. Expect a brief break in the cold out West as we wait for the next trough to dig in next week in the negative PNA pattern. As I eluded to before, another Pacific wave will carve out another deep trough in the western U.S. next week. As the wave enters the Northwest late on Sunday and Monday, there will be more rain and mountain snow as well as another lowering of temperatures which will lead to temperatures back down well below normal in much of the West for the bulk of next week. Ahead of the wave, another marked warmup will occur in the Plains and Midwest from Sunday into next week, as a southerly surface low develops once again. As the next trough deepens, rain and mountain snow will push into the Interior West, as well as the central and northern Rockies with rain changing to snow in Salt Lake City next Monday and Tuesday.

As the wave deepens into the west, snow will be possible in Denver on Tuesday night into Wednesday. Another storm system and cold front will cross the Plains into the Midwest on Tuesday and Wednesday, bringing more rain and storms with a possible switch to wet snow in the northern Plains and Upper Midwest around next midweek. The European Operational shows a second storm that could bring some snow as far south as the Corn Belt late next week, but the GFS operational just shows it drying out across the central U.S. Both models show it turning chillier in the Plains and Midwest the last half of next week.

The ensemble mean demonstrates more of a west to east zonal flow pattern, but it is the disagreement in the individual members on the exact location of each wave progressing through the pattern. In other words, we will likely see more amplified troughs and ridges than shown here. We are likely to see storm systems and fronts bringing the threat for rain and mainly mountain showers every three to four days or so with rising temperatures ahead of them and cold shots behind them across the western and central U.S. There might be some opportunities for some snow in parts of the Interior West as well as the northern Plains and Upper Midwest as well if enough cold air can filter south on the back side of storm systems. That being said, it is unlikely that we will see any arctic air masses yet, primarily due to the fact that we have the split-flow pattern, resulting from the El Nino. The location of the northern jet stream and westerlies across Canada will help to lock the colder arctic air to the north of the U.S. and even southern Canada for the most part.

Have a great day!

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