Published: 2008-11-24 08:59:00
Updated: 2008-11-24 10:10:38
Posted November 24, 2008
By Mike Moss
Last week turned out to be fairly eventful in terms of the weather, in a more benign way, in the wake of the severe weather that occur ed on Saturday. Strong cooling behind the frontal system and upper level trough axis that helped trigger tornadic storms was reinforced a time or two through the week as cold fronts passed through followed in some cases by very steep low level lapse rates (decrease of temperatures with height above the ground) and very col pockets of air aloft. This led to a midday and afternoon round of scattered flurries and snow showers on Tuesday (Nov 18th), and then another strong upper disturbance made it's way across the area on Friday morning (the 21st), this one with a band of snow and in some warmer spots rain.
While the snow on Tuesday was fairly early in the season for our area, it didn't claim any records, as a trace was recorded at the RDU airport, and we have seen a trace of snow there as early as November 2nd back in 1954. Our earliest measurable snowfall at the same location was .6 inch, which fell in 1953. We did set a record with the snow that swept across the airport on Friday morning, as four tenths of an inch was recorded, the first time measurable snow has fallen at RDU on that date (we did have a trace there on the same date 2006, however).
Another record fell about 24 hours later, as some very cold and dry air that poured into the region during the remainder of Friday settled in under clear skies Friday night into Saturday morning, with winds dropping off and allowing strong radiative cooling to take the temperature down to a new record low for November 22nd of 21 degrees. That surpassed the old mark of 23 that was set in 1987.
We've now gotten into a bit of an extended spell of cool to cold temperatures, although when we look back over the end of October and all of November so far (first image above, from the Raleigh NWS Office) we're reminded that we have had some periods of warm weather during the month as well, so that overall we're only a little over a degree below normal on our mean temperature for the month through the 23rd.
We'll probably see that departure increase a little before the month is done, since it appears a pattern that frequently results in troughing aloft over the eastern U.S. and a cool to cold flow for us, will continue to dominate through the Thanksgiving weekend. A 6-10 day outlook from the Climate Prediction Center (second image above) is unusually emphatic on the likelihood of below normal temperatures for central and eastern parts of the country as we finish this month and enter December. Bear in mind, though, that the dark blue is not reflective of how cold it will be, just the likelihood that temperatures will fall into the "below normal" category. We're actually thinking that temperatures through that period will only run about 3-6 degrees shy of normal, compared to the 15-20 degree anomalies we've seen at times over the past week or two. The associated precipitation outlook for that period (third image) has our state in a "near normal" category. The precipitation that may result in near normal amounts appears, for now, most likely to fall somewhere between late Friday and early Sunday morning as a wave of low pressure passes by just to our northwest.
I also posted recently updated Climate Prediction Center monthly outlooks for December temperatures and precipitation. For us, there isn't a strong enough climate level signal for anything beyond "equal chance" of below, near or above normal regarding temperatures for the month, while we are on the northwestern edge of an area expected to have a better chance of below normal precipitation. Current model projections of large scale pattern indices like the North Atlantic Oscillation and the Pacific North America pattern would seem to bode for a colder than normal period for at least the first few days of the new month, but beyond that we'll just have to wait and see.