Premium: November Outlook and pattern evolution

Changes in the stratosphere have been well documented and explained over the past several days and, despite the fanfare, have not yet had their dramatic impact on the weather pattern across the Continental United States. But in due time, they will, and it will likely begin — ironically — at the very start of the month of November. A much more amplified pattern in the higher latitudes will kick off changes in the Pacific Ocean, starting with a much more amplified Pacific Jet.

This Pacific Jet stream will flood the United States with warm, maritime air as a large vortex forms in the Gulf of Alaska. This is an important development for several reasons moving forward, but in the interest of November alone, its short term significance will be verified by a surge of much above normal temperatures into the Central and eventually Eastern United States.

The impressive Pacific Jet and the resultant vortex in the Gulf of Alaska will essentially act to amplify the ridge over the Central Untied States. Southwesterly winds at multiple levels of the atmosphere will surge past the Rockies and into the Plains, eventually moving towards the Ohio Valley and Northeast United States by the middle of November’s first week. Temperature anomalies will be extremely impressive, especially in the Plains towards the Mississippi River Valley.

5-Day Averaged temperature anomalies will approach 10 degrees F and exceed 15 degrees F over a large area. This air will slowly seep eastward toward the Appalachian Mountains, and while its effects may be muted, it will still be quite notable. By and large, the Continental United States will be void of below normal temperature anomalies through November’s second week.

November’s Second Half 

While all of the aforementioned warmth is ongoing, dramatic changes are likely to be occurring in the higher latitudes. Forecast models are in good agreement that, even after its split, the stratospheric vortex will remain under fire. It’s reconsolidating will allow warming to continue from Eurasia and Siberia, allowing a cross-polar flow to attempt to become established.

There is some suggestion that a retrogression of the pattern will occur as a result of this. The weakening of the stratospheric polar vortex essentially allows individual perturbations to occur, trailing from the Pacific into the Continental United States while blocking ridges remain over Greenland and Arctic. The big trough in the Gulf of Alaska — discussed at length above — would then retrograde to a position south of the Aleutian Islands.

Such a development would be extremely impactful on the pattern across the United States. Forecast models, including the GEFS, ECMWF, and ECMWF Weeklies, suggest this development will allow ridging to build along the US West Coast and into British Columbia. A cross-polar flow could, then, allow cold air to seep into the Central and Eastern United States.

While confidence is certainly low in the development of widespread below-normal temperatures, one thing is for sure: The hemispheric pattern looks very likely to change right around the middle part of the month, with the warmer temperatures succumbing to colder air. The main question, at this juncture, is the longevity of the cold. If the Pacific Jet remains active and strong, a progressive pattern may act to keep any sustained cold from becoming established.

The Big Picture and Wrap-Up

A very warm start to the month is expected country-wide, with the exception being the Northeastern US during the first half of the months’ first week. After 10+ days of above normal temperatures across a large area, a hemispheric pattern change is anticipated by the middle to latter half of the month. This should support temperature anomalies falling toward or below normal across the Central and Eastern United States from the end of November into December.