A look into early December: cold likely
Pattern may also support snow chances
The beginning of meteorological winter (December 1st) is just on our doorstep . The Christmas shopping season is getting in full swing. Many readers are probably wondering how the weather might affect their shopping plans during the next couple of weeks. This article will attempt to explain what we know, or think we know, about the weather pattern that is expected to prevail during the first couple of weeks of winter. The start of December certainly appears to be following the script of the Capital Weather Gang's winter outlook.
The pattern early in December definitely is trending colder than normal. The picture is a murkier in terms of our chances of seeing an early December snow but the odds of getting snow appear to be increasing.
The temperatures across the East look like they will start to gradually fall after a rainy day on Wednesday (Dec. 1) and by December 5 or 6, temperatures across the region should be well below normal. At such time ranges, it's usually smart to give a probabilistic forecast rather than saying "I know definitely what will happen".
The Climate Prediction Center's (CPC) temperature forecast (above) for the period Dec. 5-11 reflects this innate uncertainty. Despite various weather models showing a clear signal for cold, the CPC forecast gives our area a 60% to 70% chance of having temperatures averaging below normal for the period (see also: additional information how to interpret the map).
My own feeling is that the probability of colder than normal temperatures is even higher than those shown on NOAA's CPC based on the mean pattern forecast for the period.
You can get a feeling for how similar the models are in the overall pattern by looking at the 168 to 240 hour mean 500 mb pressure (or height) patterns from the European and NCEP global models as shown below. When analyzing a weather pattern, a mean chart like the one below has one distinct advantage over looking at a daily chart. The smaller scale waves which are faster moving and harder to accurately predict often disappear due to the averaging process so the more important features tend to stand out more clearly.
In this case, the models now are showing northwest flow across south central Canada which should help deliver cold air into the region. Both models are also depicting a strongly negative North Atlantic Oscillation (the combination of having an area of higher than normal heights and pressures, red area over Greenland and eastern Canada, with below normal heights to its south and west in blue). The odds of snow usually increase when the NAO is in the negative phase compared to when it it positive as we've discussed in a previous article.
During the period from December 5-9 the probability of snow looks like it will be higher than normal for the Washington area. However, it's important to remember that odds of getting an inch of snow during that 5-day period is low and has only happened 19 times in the last 122 years. So even with an above normal probability, the odds of getting snow during the period are still probably below 50%.
For those who like to fantasize about snowstorms, last night's GFS model had a significant storm for DCA at 216 hours (see below) but then the next run of the same model took it away.
Forecasts at such long time ranges are usually incorrect and should be taken with a grain of salt. The European center model from last night also had a fantasy storm but a little over a day later than the GFS storm shown below. There is no way to tell which if any of these storms will come to fruition. However, the pattern definitely is becoming interesting enough for snow lovers to monitor daily for the next ten to twelve days.
| November 29, 2010; 10:30 AM ET
Categories: Latest, Winter Storms
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