Over the last several weeks, unusually warm weather has gripped the United States. In fact, a large region of the United States had temperatures run 4 to 6 degrees above average for the month of October with quite a few regions making it in the 6 to 8 range. There were even some areas that had temperatures over 8 degrees above average for the month. Now, that’s hot!
Many have been wondering for quite some time if the current pattern that has caused such warmth over the United States will eventually break and if this change will occur before we reach the winter months. In short, the answer is yes; however, November will act as a transition month to a colder and more active pattern this winter for the central and eventually the eastern U.S. Firsthand Weather’s winter forecast has not changed since early August, so please read give it a read if you’re wanting a broad overview of what to expect this winter. We have more details that will be released on the site in the coming weeks.
In this article, I want to primarily focus on this month and give a timeline on how our pattern will evolve with time. In order to do this, it’s important to understand what has caused such warm temperatures over the United States. An unusually strong Pacific jet stream has extended across the entire northern Pacific, often reaching the West Coast of the United States (Figure 1). This alone has transported warm, Pacific air into the U.S. Although the flow of this jet has been mostly zonal (west to east flow), the jet has often curved cyclonically in the Gulf of Alaska. A strong mid-level trough has persisted near the Gulf of Alaska, and downstream of this trough, strong ridging has been present (Figure 2). The region under the influence of this ridging has been much of the U.S.
Figure 1: A strong west-to-east component of the jet stream was present through much of October.
Figure 2: A persistent trough has been present near the Gulf of Alaska along with quasi-stationary ridging over much of the U.S.
Because of the extension and strength of the Pacific jet stream, storm systems have been able to readily ride the westerlies straight to the West Coast. This is another process that has led to the persistence of ridging over the U.S. With such a setup, given the strength of the Pacific jet and strong Gulf of Alaska trough, large amounts of latent heat have helped strengthen the downstream ridge, thanks to the low pressure systems making it to the West Coast and releasing this latent heat into the ridge. Taking all of this into account, it’s no surprise that stubborn record-warmth has been quite persistent. Just when one ridge weakens, another just builds right behind it.
What To Expect Through Thanksgiving (Broad Overview):
The big question becomes. . .how long will this pattern persist? First, let’s address what needs to happen for a pattern change to occur. The anomalously lower heights (the trough over the Gulf of Alaska) needs to weaken and retrograde west some. In addition to that, the Pacific jet stream needs to calm down quite considerably. It needs to have more of a meridional (north to south) component to it, which would open the door for more ridging to build along the West Coast and extend into Alaska. This, in effect, would allow Arctic air to spill into the United States east of the Rockies, as troughing would be more present over those regions.
My argument over the last week or two hasn’t been that the pattern isn’t going to change into something more seasonable, but that there is going to be a transition period before the more “permanent” change. In other words, there’s going to be quite a bit of volatility to the pattern with swings in temperature, so if you’re looking to get persistent cold in the central or eastern United States, I don’t expect that to occur until later November closer to Thanksgiving or even later. However, the colder intrusions that push into the U.S. before Thanksgiving will be a huge shock to the system, and some could even be potent. Even once we get past Thanksgiving, I’m not ruling out potentially another warmup, especially across the Southeast and parts of the East Coast. Again, all of that is discussed in the Firsthand Weather winter forecast.
What Will Transpire This Week (Broad Overview):
Volatility in the pattern is not a bad thing if you’re looking for some real action. Behind a ridge that is currently moving from west to east across Canada and the U.S., another ridge is going to build over the same regions early to mid week (Figure 3). This will cause temperatures to soar by mid-week across the western third of the U.S. into the Northern Plains. However, a wave that will move across southern Canada and the Upper Midwest ahead of this ridge will pave the way for a front to move through the Plains and eventually eastward. This won’t be an Arctic front by any means, but it will help to alleviate the really hot temperatures across the Southern Plains and some regions east of that.
Figure 3: Another ridge will build into the U.S. and Canada midweek.
The building midweek ridge will eventually break later in the week (imagine a wave in the ocean building and crashing), which will shove a disturbance into the Southeast. It may help cool temperatures down slightly, but this won’t be a rain-maker. I don’t expect any significant rain chances across the Southeast until at least mid-November. I’ll have a separate post on that soon.
Yet another ridge will build over parts of the U.S. and Canada late in the week and will also break (Figure 4). Downstream of the ridge, this will push much cooler temperatures into the Great Lakes region, the Northeast, and parts of the Mid-Atlantic. This cooler air will spill down into the Tennessee Valley and parts of the Southeast (Figure 5). The air mass will be much more potent farther north, but this will seem like a big improvement even farther south, given how hot it has been. The lake effect snow machine will start to crank up this week, and I expect the lake-effect snow to be a big deal (above average) late fall and early this winter.
Figure 4: Yet ANOTHER ridge will build into the U.S. and Canada late week.
Figure 5: A downstream trough will develop, allowing for much cooler weather over the Northeast, Great Lakes, Mid-Atlantic, and even southward next weekend.
This will be followed by more warmth, and this cycle will pretty much continue through Thanksgiving. Forecast models do indicate that ridging will begin to build over the West Coast after mid-month, which could open the door for additional and potentially colder air masses to drop southward into the central and eastern U.S. after that point. In fact, some models have a stronger low pressure system developing in the Gulf of Mexico mid-month, which could bring relief to the drought-stricken Southeast and help usher in colder, more seasonable air across the central and eastern U.S.
I realize that I jumped around quite a bit in this article, but the goal was to show everyone the volatility of the evolving pattern. Forecast models often don’t handle pattern transitions well, so keep that in mind if you rely on weather apps or weather forecasts that are based heavily on model data.
Also keep in mind that with the kind of warmth that will be building over Canada over the next week or two, this will not allow much snow cover to advance over that region. This could act to modify some of the cooler air masses that drive southward into the U.S. and keep them a bit milder than they otherwise would be.
Despite the back and forth in temperatures that many of you will experience through Thanksgiving, this should excite many of you fall/winter lovers out there. I will continue to keep you updated as the pattern evolves with time, and let you know how this will impact your weather.