Hot in December? You're Probably Not Reading These Maps Correctly
One of the biggest (and most welcome) weather stories today is that a warm-up is slated to descend over the United States during the next two weeks. The maps we use to show the possibility of above-average temperatures are being misread by much of the public, and that's a problem.
The above map shows the latest temperature anomaly forecast from NOAA's Climate Prediction Center for the period between December 9 and December 15. These medium range maps serve as a guide to tell you the odds that temperatures will be above average or below average. Odds that are 33% or greater either way show that forecasters expect temperatures to deviate from normal.
The latest medium range forecast from the CPC shows a huge chunk of the country from the West Coast straight across to the Great Lakes sitting at a 70% chance of above-average temperatures. When many people on social media see these maps making the rounds, they get excited because it's going to get so warm, because look at all that red!
That's not always the case.
These maps just show the odds that temperatures will be above or below average, not what the temperatures will actually be on any particular day during the forecast period. Higher percentages (darker reds for above, darker blues for below) indicate higher probabilities that temperatures will deviate from normal. Even if temperatures are a few degrees above average, their forecasts verify, but that's still not warm for this time of the year. Today's average high in Chicago is around 40°F, for instance, and in Denver and Kansas City it's 45°F.
For the past month or two, the eastern two-thirds of the United States has seen one cold snap after the other thanks to a steep ridge of high pressure over western North America, allowing big dips in the jet stream over the east. These dips in the jet facilitate cold air flowing south from the Arctic. What's expected to happen over the next couple of weeks is that the jet stream will "even out," so to speak, and we'll enter a period of relatively zonal flow, or upper-level winds flowing from west to east with less extreme wobbles to the north and south. The absence of that intense ridge over the west will keep cold air from coming far enough south to make people below the Canadian interior miserable, walking freezy pops. At least for a little while, anyway.
The models have consistently shown a storm system swinging through the Midwest and into the East Coast around Wednesday the 10th, which will bring cold air with it, but after that the weather should start to calm down and warm up. How warm are we talking? It's too early to tell, but a good number of cities (especially in the Midwest and Plains) could be looking at temperatures double-digits above normal. With average highs ranging from the 30s in the north to the 40s in the mid-section of the country by the middle of the month, temperatures possibly reaching the 50s (or even 60s) aren't classically "warm," but it beats worrying about nose icicles.
You can access the latest these forecasts (and more) from the Climate Prediction Center's website. (For mapping geeks, here are their shapefiles.) The agency has a new website coming out soon that should make it easier for both the general public and weather geeks to access their products—the redesign is available in beta mode right now. Remember that the CPC is only telling you the odds of anomalous temperatures or precipitation occurring over the extended range (more than a week out). If you want hard temperature or precipitation forecasts, stick with the CPC's sister agency, the National Weather Service.