Over the next week, the weather pattern will rearrange itself from mild and fall-like to unambiguously winterlike. Colder-than-normal temperatures are on the way and, yes, above-normal chances for snow.
And the cold pattern could last for a while, possibly even into Christmas, although confidence is very low in the outlook beyond eight to 14 days.
It is too soon to pin down snow opportunities, as computer models don’t do a good job simulating the timing, location and strength of storms more than five to seven days out. But there are likely to be opportunities. The question is: Can they gel?
Both the GFS and European models suggest the season’s first opportunity for light snow or flurries Wednesday night, in the wake of the big cold front. They probably wouldn’t accumulate.
Whether more meaningful snow potential develops in the following days is still to be determined.
The main culprit for the pattern change is a massive dome of high pressure that is forecast to develop over Alaska and extend south along the Pacific Coast, shown in orange and red in the map below. Such a strong high pressure ridge along the West Coast usually leads to a downstream dip in the jet stream across the eastern half of the country.
That downstream dip promotes Arctic air flow from the northwest southward into the eastern United States. Meanwhile, all the above-normal pressures extending across northern Canada to Greenland (orange shades) strongly suggest that the Arctic Oscillation (AO) will become strongly negative, another good sign for establishing a cold pattern across our region. Most simulations from the American model keep the AO strongly negative through mid-December.
Why is our confidence growing about this cold pattern despite our constant harangues about models not being that good so far into the future? Because the average forecasts from the various American and the European model simulations essentially show the same pattern. Importantly, each of the 50 individual European model simulations are pretty unanimous about the high-pressure ridge developing in the West — see how the green lines representing different simulations are closely clustered together and form the same general shape.
Guessing how long the pattern change will hold is always tricky.
Both the European and American models hold onto this cold pattern through mid-December. Note how similar the patterns portrayed for Dec. 15, below, are to the forecast for Dec. 8, above. The models are predicting a fairly stable pattern. That said, individual simulations do show increasing divergence as you go out in time, suggesting that not every day is guaranteed to end up colder than normal.
The North American Ensemble System gives an idea of the most likely range of temperatures — the gray-shaded area — suggesting that the maximum temperatures from Dec. 8 to 14 are mostly likely to average between the upper 30s and mid-40s, and the minimum temperatures are likely to fall in the mid-20s to low-30s range.
What about Christmas? That’s way beyond the time frame when any models have much skill, but for those who like fantasizing, the long-range European simulations maintain the same basic pattern to Christmas Eve.
What about snow? The pattern from Dec. 8 through Christmas, according to the models, looks like an above average chance for snow, although it’s important to remember that there are many Decembers when we fail to get an inch. Around here, a good pattern needs to be combined with luck to actually cash in on a storm.
