There have been plenty of times in recent weeks when it wasn’t obvious that winter was upon us. The current pattern, which has occasionally supported temperatures in the 60s in places used to the 30s, has produced strikingly little snow nearly everywhere in the country outside of the mountains and very narrow swaths downwind of the Great Lakes. It hardly invokes images of the past two supremely harsh winters.
Sure, cold weather has descended upon locations east of the Mississippi in the past two days. As I predicted last week, highs have been 10-15°F below average from New York to D.C. to Raleigh in this cold spell. In fact, subfreezing air plowed its way through Florida just last night, with Jacksonville reaching 23°F this morning and Naples, FL 36°F. And freeze watches and warnings continue to cover the almost the entire Florida peninsula until Wednesday morning.
But with temperatures poised to soar once again into the potentially record-busting 60s all over the Central and Northern Plains tomorrow (including Kansas City, Des Moines, Rapid City) and then get back above normal by Friday in the East, winter weather continues to be elusive.
However, since late last week, the global weather models have been hinting that at least one true Arctic blast will impact the Lower 48 by the middle of the month.
But relying on this kind of extended-range guidance from these models for daily weather forecasts is, at face value, basically useless. The atmosphere presents way too much complexity, there are too many approximations in the models, and too much uncertainty in the data that drives them to yield skillful day-to-day forecasts beyond about a week.
But when viewed through the wide-angled and blurry lenses of longer-range prediction systems, and within the context of the expected behavior of ‘external’ influences that poke our midlatitude patterns on an intraseasonal basis (such as the Madden Julian Oscillation), the tendencies of the global weather models at week two, in certain circumstances, can indeed provide useful insight
With this in mind, it appears that over the next 1-2 weeks, high-altitude westerlies will slow down a little over the Pacific in middle latitudes, probably the most since November. This is ostensibly in association with a convergence of wave activity near the dateline and an attendant amplification of a tall ridge over the Central Pacific. The image to the right shows a 12-day forecast (use with caution given the long lead time) of the jet stream flow (highlighted by the red arrow) and the mid-Pacific ridge.
It is not unusual for this kind of process to occur. In fact, it happened a lot during the past two winters with simultaneous blocking over Greenland. And we all know how that tandem affected the United States. This time around, however, the jet stream will not likely buckle acutely over the North Atlantic for any sustained period anytime soon. It will remain largely on a westerly course over that part of the world without blocking in its way.
Still, the shape of the circulation in the picture above is quite a departure from what we’ve seen over the past month. It no longer features a strong westerly jet over the Pacific Basin that douses North America with mild maritime air. Instead, it contains a north wind out of the Arctic immediately downstream of the ridge axis (dashed line).
One of the questions to be resolved, assuming that some derivative of this forecast will verify, is where the ridge will set up shop and how strong it will be, and in particular, where the north winds on its east edge will blow. If they set up offshore of North America, the likelihood of a widespread Arctic strike on the Lower 48 is reduced. In this case, much of the frigid air will drain southward over the ocean. And though pieces of it could still slide across the Northern Tier, overall a less cold west wind would flow over the United States.
If the ridge develops a little farther east, it could deliver the Northern Plains an Arctic punch that could spill to the Gulf Coast and eventually the Northeast. Multi-day temperature anomalies in this scenario could easily be 20°F below average.
But the general circulation does in fact carry a monthly memory – one that shows up statistically (in low-frequency autocorrelation calculations). And since the past few months have favored a mild pattern without North Atlantic blocking, I suspect any penetration of Arctic air into the States this month will last days, not weeks.
Components of the ocean/atmosphere system that have so far helped to shape this season’s mild climate will probably not change dramatically enough over the next several weeks to flip the pattern for the long haul. So, moving forward, I expect winter to continue to tease us from time to time, with infrequent, short-lived visits of Arctic air amidst an otherwise milder than average climate.