After a pleasant finish to the weekend with bright skies and high temperatures in the mid 50s, we'll head into the last week of 2009 with temperatures on the cool side of normal thanks to a cold front that moved into the area last night, and behind which we'll hover in the low to mid 40s for highs the next few days.
While it's not set in stone just yet, there are some pretty Strong indicators that we'll get hit with an even colder airmass funneling into the region for the first weekend and into the first week of the new year. Some of the large scale patterns that give a sense of overall temperature trends have been showing a moderately negative "North Atlantic Oscillation" pattern (first attached image, from NOAA's Climate Prediction Center) and a strongly negative "Arctic Oscillation" signal (second image), both of which tend to correlate to cold temperatures across the eastern U.S., while there is a near-neutral outlook for the "Pacific North America" pattern that trends toward weakly positive (third image). In contrast to the first two, the last pattern tends to be inversely related to temperatures across the eastern U.S., so that positive PNA values often translate to cooler than normal temperatures here.
A couple of other images may help illustrate the cold air that will likely head our way. The fourth and fifth images (from NOAA'S Environmental Modeling Center ensemble page) show the locations of two different isotherms (lines of equal temperature) marking the location of air at about 5000 feet above the ground that is at 0 degrees C (freezing) and -25 degrees C, first at 1 pm today, Monday 28 Dec 2009, and then at 1 pm on Saturday 2 Jan 2010. The 0-degree line is generally farther south of course, and you can see that with fairly cold air moving into the area last night and today, the first image shows the 0-degree line a little to our south and east. By Saturday, though, as a colder blast of air makes its way in from the northwest (likely on gusty winds), that same line is a good ways farther south of us, indicating we're much deeper into the chilly airmass.
You'll notice that on those two images there area actually many lines that are all pretty close together on the first one (which is a forecast for 18-hours into the future) and spread around a good bit on the second (a 138-hour forecast). That's because we're looking at results from an "ensemble" forecast in which two basic models, one from the U.S. National Centers for Environmental Prediction and the other from the Canadian Meteorological Centre are run numerous times, with the initial data or some of the basic physics of the model tweaked slightly to simulate some of the inherent errors in measurement of atmospheric variables and in our understanding of how best to formulate the equations used to predict future states of those variables. When many or most of the model "perturbations" agree closely in their output, confidence in that solution is pretty high, and vice versa. As you can see in this example, the locations of the -25 and 0 C isotherms agree quite well among the forty-some model runs involved after just 18 hours, but not nearly as much by Saturday afternoon. Still, there is a decent indication that air will be plenty cold Saturday into Sunday, with a close look showing that many of the U.S. models (solid lines) are showing the cold air farther south than the Canadian ones (dashed lines). While I didn't show it here, a model from the European Center for Medium Range Weather Forecasting that tends to be a pretty strong performer, is in good agreement with the colder U.S. model results.
All in all, it appears for now that we'll have some partial sunshine on Saturday and lots of sun on Sunday, but despite the sun we'll have stiff northwest winds that combine with highs in the mid 30s to around 40 both days (compared to a normal of around 50), with lows Sunday and perhaps Monday morning in the upper teens to low 20s. This will probably not reach record-setting proportions, as we've had highs on January 2nd and 3rd as cold as 32 and 28, respectively, but it's still quite the chill, so be prepared to bundle up as needed!
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