Brief Discussion On Early to Midweek Weather:
We’re about to briefly shift out of the pattern that has been responsible for the recent colder weather across the eastern United States and warmth out west. The pattern has previously been characterized by an amplified ridge that has extended from the western U.S. through western Canada and into Alaska. Downstream of the ridge, deep troughing has persisted and in fact, even brought colder conditions all the way into Florida. Remarkably, parts of the panhandle of Florida and over the open waters of the Gulf had snow on the backend of the system that dumped copious amounts of snow across parts of the Southeast. This amplified pattern is going to flatten out next week, allowing for relatively warmer conditions to prevail across most of the U.S. through mid-week. With that said, there will be a quick-moving trough that’ll trek across the Northern Plains early next week and then eastward over New England by mid-week. That’ll bring a quick intrusion of cold air from the Northern Plains over into the Great Lakes and over parts of New England, but that cold will remain mostly confined to those regions. We will eventually have to discuss that system on Firsthand Weather, since it will most likely be bringing a swath of precipitation (rain) across the eastern third of the U.S. Parts of the Northern Plains, Great Lakes, and inland regions of New England could get some snow out of the system, so we’ll have to watch that.
Overview Of Long-Range Forecast Discussion For Late Next Week Through Just Beyond Christmas:
Let’s go ahead and fast forward to the end of next week through the Christmas timeframe and just slightly beyond that. That’s really the entire purpose of this article, and I want to begin digging into how the pattern could begin to evolve after the early to mid-week warmup. Temperatures have mostly been below average across the majority of the eastern third of the nation over the last seven days, while the core of the warmth (relative to climatological averages) has been centered over the Northern Plains and has extended westward into Montana, Wyoming, and Colorado. Additional parts of the western third of the U.S. have also been quite warm. Now if you read Firsthand Weather’s winter forecast that was published about a month ago, you probably noticed that we were bullish on the cold across the Northern Plains and into the Rockies, so if you’re located across any of those locations, you might be wondering what’s going on with the forecast. Get ready, changes are on the way!
It’s not all that uncommon to hear a lot of talk about the North Atlantic Oscillation (NAO) and the Arctic Oscillation (AO) in many of the medium and longer-range forecast discussions that get published on the internet; however, the eastern Pacific Oscillation (EPO) is a lesser known index that honestly should be discussed more often than it is in the U.S. The negative phase of the EPO is characterized by ridging over Alaska with troughing to its south. It’s simply a dipole pattern similar to the NAO, but its location is in the northeastern Pacific. Any atmospheric feature(s) that is present upstream of the U.S. can have important implications for how our weather in the U.S. may evolve with time.
It’s important to note that simply determining whether the EPO is negative or positive doesn’t give the whole picture. It’s just as important to determine the position of the Alaska ridge or trough, its orientation, and its amplitude. For example, a ridge can extend well into Alaska, but its position may be far enough offshore that the downstream trough centers itself over the western and central U.S. This, in effect, can open the door for ridging to develop across the Southeast and along the East Coast. The moral of the story is that a negative EPO pattern is one to watch closely in the U.S. since it can oftentimes signal widespread cold, but it’s important to dig a bit deeper to fully determine where those coldest anomalies will be located. That’s what we’ve been in the process of doing at Firsthand Weather and will continue doing through the weekend and early next week.
How Will This Impact The Weather In The United States:
While the GFS model is generally more aggressive with the development of Southeast ridging, the European features a weaker ridge. Nonetheless, the pattern that will likely evolve from very late next week into Christmas will feature widespread cold across the heartland of the country. The coldest anomalies should be centered over the Northern Plains and upper Midwest, and it won’t be uncommon for that Arctic air to spill southward into the Rockies and the central/southern Plains. Additional regions to watch for very cold conditions will be the Ohio Valley, northwestern parts of the Tennessee Valley, and Northeast (especially inland regions away from the coast). The Pacific Northwest could also get in on some of these colder conditions, and that colder air could spill into the Mid-Atlantic at times, too. To summarize this into one sentence, this pattern will likely feature widespread cold, which differs quite drastically from the eastern-focused cold earlier in the month.
The next image from the Climate Prediction Center (NOAA) shows the probability that temperatures will be above or below average from December 21 to December 27. That’s not a bad compromise across the Southeast, given that the guidance does tend to develop ridging across the Southeast but at times, colder air will make it into those regions. Notice that the probability for below-average temperatures is depicted over a large region of the U.S. Keep in mind that this particular graphic tells you nothing about the magnitude of the cold. Its purpose serves to show you which regions could simply have above or below average temperatures.
The next analysis takes GEFS forecast model data and produces a temperature anomaly map (from December 23 to 25, in this case) based on previous similar pattern setups. Based on this kind of setup, you’d generally expect storm systems to swing southeastward through the Rockies, into the central/southern Plains and then hook northeastward across the mid-south, the northwestern Tennessee Valley, Ohio Valley, and across inland regions of the Northeast. Even though I didn’t draw in that storm track, I mostly agree with this analysis. The region to watch most closely for wintry weather (including ice) over this period will generally extend from the central/southern Plains over into the Ohio Valley and into inland regions of the Northeast. That puts parts of the Tennessee Valley and the mid-south (parts of Arkansas, western Tennessee, far northward Mississippi) in a zone that could swing either way; however, it’s a region to watch nonetheless. To make it easy, take a glance at the map, and if you’re relatively close to the storm track drawn on the map and located on the northern/northeastward side, take notice. This is meant to give you a general overview of what the dominant storm track could look like, so please understand that this isn’t depicting a particular system.
I suspect that most of the Southeast (the red-shaded zone across parts of the South in the image above) will transition to a predominantly wetter and a warmer pattern through Christmas and maybe just a bit beyond. With that said, some of the guidance does have the colder air spilling into the Southeast at times through the period (notice the back and forth in temperature anomalies depicted in the two images below), which seems realistic to me. Essentially, this is a pattern that favors wetter and warmer conditions, which can then be followed by colder and drier conditions, and then the cycle continues. If you’re located just east and southeast of the Appalachians, it’s worth noting that surface high pressure moving across the Northeast can result in surface temperatures being at freezing just outside of the mountains across those locations as precipitation is moving across the Southeast. That’s a scenario to watch for with this kind of pattern and can cause icy conditions (not snow though); however, that’s a very localized threat that often doesn’t even reach into cities like Birmingham or Atlanta and doesn’t occur across locations west and southwest of the Appalachians. Unless the ridging that is expected to build into Alaska is farther east than I’m anticipated, I expect most of the wintry weather to occur across the locations I specified in the previous paragraph.
Chris is planning on posting an article for the Pacific Northwest and one for the Southern Plains sometime soon. In the meantime, I’ll also be working on regional forecasts across additional areas. Hopefully this article gives everyone a general overview of the pattern we’re going to be dealing with, and we’ll get into the nitty-gritty specifics throughout the month. Of course, any changes to the overall pattern could require me to make revisions to this forecast, so most definitely be aware of that! I don’t have all of the answers and do make mistakes, so please follow Firsthand Weather continuously.