COOL, CALM, AND COLLECTED...
Considering the weather a couple of days ago, things were pretty pleasant around the region Thursday. Sunshine and temperatures of 55-60 helped me forget about that little reminder of winter we experienced Halloween. Fortunately the cold moved out fast and at least for now, there's none of that in the forecast for some time to come.
In fact, I was looking at the long range EPO off the EURO Weeklies (eastern Pacific Oscillation) and it is shown predominately positive through the first week of December.
As the name implies, the EPO is a variation in the atmospheric flow pattern across the eastern Pacific, as well as Alaska. It's considered one of the most important pattern oscillations for North America. When the EPO is in a positive phase, mild Pacific air flows straight into the West Coast of North America. While such a pattern can bring significant rain and snow to the western portions of the country, the air flow warms and dries as it descends down the eastern slope of the Rockies. This results in a warmer than normal pattern across most of the central and eastern portions of the U.S. and southern Canada, particularly during the winter season. The upper level flow pattern and the distribution of warm and cold air during a positive EPO pattern is shown in the image below.
Here's what the EURO Weeklies show for the EPO through December 18th. That's a very healthy stretch of positive PNA.
With the El Nino cranking, this should ensure brief and limited incursions of cold, potentially much of November. You can see the mild sea surface temperatures in the Pacific below that are driving the El Nino. One thing to note, is that the warmest water is building west and no longer is centered off the coast of South America in region 1+2. If the trend continues and the warmest waters set up in 3.4, that gets us into a Modoki El Nino. That can have far different circumstances as far as a cold winter is concerned. Things could flip and that potential has always been on the table later in December and beyond.
Short term though, the 500mb mean jet is projected to look like this on the weeklies. That certainly has a mild appearance to it the next 3-4 weeks. You can see the source region of our air masses being the Pacific, far warmer than if the flow was out of Canada. I do need to stress, this is a smoothed average and there will be ups and downs but in general this would promise warmer than normal temperatures during much of the period.
As you would expect, the next 30s days the weeklies do indicate temperature anomalies that look like this.
However, by mid-December the weeklies show the pattern breaking and the flow is no longer coming off the Pacific.
That portends a whole different brand of weather with the possibility of some impactful cold. Below you can see it sinking into the Midwest December 16th.
Something else that may become a factor is a "stratwarm" that is forecast on the EURO. You can see the warmth over the pole the second week of December. The warming is happening at the highest levels of our atmosphere (in this case the stratosphere). Physically, that forces the intense cold that typically resides there to be displaced out of the pole towards the surface and mid-latitudes. Maybe that's what the weeklies are showing with the colder pattern at the end of the run December 18th.
If nothing else, after showing a mild November, the weeklies show a major thermal transition in December and there is snow over much of the Midwest before Christmas. That would be nice.
There's certainly some things to think about in the long range, not the least of which is the milder weather that builds in over the weekend. Seasonal highs in the 50s are expected through Sunday. By Monday, 60s are back on the table. Outside of a few sprinkles late Friday, the weekend is now shown dry on all the models. A trend I was anticipating. Have a good one and roll weather....TS