Friday, November 24, 2017

Long-Range Forecast: November 24th - December 21st, 2017

This is a long-range forecast for the November 24th thru December 21st, 2017.

Weeks 1 and 2

We begin with the weekly forecasts from the CFS model, looking at the Week 1 and 2 timeframe.
The top panel shows 500-millibar geopotential height anomalies from November 24th thru November 30th, with the same parameter shown on the bottom panel from December 1st thru December 7th.

We first analyze the top panel, and see a quite warm pattern set up for the country through the end of the month. This warmth stems from the jet stream being forced north as a strong blocking pattern, present since late October (as seen below), has set up in the Pacific just south of the Aleutian Islands.
This blocking pattern has allowed a persistent roughing pattern to emerge in the Gulf of Alaska into the Pacific Northwest, which as you can see from November 9th onward in the above animation, has pumped a ridge up across the South U.S. into the central and eastern states. The East has been increasingly unaffected by this, as the last handful of frames in the animation show a negative North Atlantic Oscillation (NAO) pattern emerge as a strong ridge has blossomed over Greenland. This has allowed a trough to build down through southern Canada into the Northeast and Great Lakes region, keeping that area relatively cooler.

The Week 1 forecast, then, is more or less a continuation of this pattern, but the CFS anticipates the ridge in the South to bleed east and overrule that trough that's trying to push into the Northeast thanks to the negative NAO.

In Week 2, the bottom panel of the top image, we see a dramatically different pattern. The blocking pattern we are currently enduring in the northern Pacific has broken down in Week 2, with the CFS forecasting a strong low in the Bering Sea and a ridge well south of Alaska to promote a stronger Pacific jet stream and more zonal flow. With this ridge seen blossoming east towards the West Coast, that trough currently in the West U.S. is expected to weaken, leaving the Pacific jet with a red carpet rolled out to sweep into the U.S.

As is typically seen in this sort of Pacific pattern, a broad ridge is then forecasted to set up in the Central & Eastern U.S., focused in the Southeast (like one may typically see in a La Nina pattern). What we also see, however, is a very strong negative NAO pattern now having evolved, with a strong ridge over Greenland.
The actual temperature forecast from the CFS for these two weeks reflects the deductions we've made above:
Weeks 3 and 4

Weeks 3 and 4 show much less meridional flow over the Northern Hemisphere, albeit with a reduced magnitude in all troughs and ridges as the members of this ensemble diverge, diluting the mean forecasts shown here.
Most interestingly, we see a relatively strong ridge positioned right in the Arctic Circle, which would suggest a displacement of the (now infamous) polar vortex to the south. Where it goes if it was to be displaced south is very much to-be-determined, but early approximations in that top panel show below-normal anomalies (stormier conditions) in the Northeast U.S., northern Europe and Japan up through the Bering Sea.

Week 4 brings a similar picture to Week 3, though with a weaker ridge over the Arctic Circle and a resurgent ridge in the northeast Pacific and West Coast in tandem with a trough in the Gulf of Alaska. This again seems like a pattern where the Pacific jet stream may dominate, especially with that slight ridge shown well to the east of Florida that may resemble a Southeast Ridge if it keeps showing up in future forecasts. By this time, however, confidence is substantially lower, although the corresponding temperature graphics again are rather similar to what we've discussed above:
Interestingly enough, the CFS isn't biting too hard on the possible cold wave in Week 3 as that ridge establishes itself in the Arctic Circle. We do see some light cooler-than-normal anomalies in the Southeast in this period, but the country as a whole is broadly above-normal.

To Summarize:
- Warmer than normal conditions are expected for the majority of the country through the end of November and the first week of December.
- Colder than normal conditions may present themselves in the Eastern U.S. by the second week of December with the emergence of a ridge in the Arctic Circle, but a strong Pacific jet stream may not permit this cooler weather to stay for a prolonged period of time.


No comments: