Thinking it's been particularly cold lately? It has been and here is why!
Fall…it conjures up thoughts of pumpkin (spice lattes), changing leaves, sweaters and fireplaces. After a long and hot summer, it can be a welcome change. However, early this November, the first cool snap descended upon us and we jumped right into winter and forgot all about fall!
The month started off above average, but that trend didn't stick around.
The average daily high for D.C. in November starts off at 63° on the 1st and falls to 52° by months’ end. That’s a loss of 11 degrees over a span of 30 days…so a gradual decrease.
But, that is not how you’d describe the drop we saw this past weekend when all major climate reporting stations in the region all recorded their 1st freeze of the season on the same day! We tied the record low downtown of 26° (set back in 1973) and the afternoon high only made it to 39°. There had not been a day downtown where the temperature never made it out of the 30s since back on March 15th!
So you get it, you feel it…it’s been darn cold, which begs the question: why? I set out to find some answers and employed my friends at our local National Weather Service for some meteorological advice. I’ll try and explain this to you in the simplest terms possible.
High pressure generally brings nice weather. As of late, high pressure has been parked out west. This ridging drives the Jetstream north into the colder climates – up and around the top of the ridge. Then it sinks south over the eastern half of the U.S. bringing the cooler air with it. This extended area of low pressure, is known as troughing. Mother Nature is like the great equalizer as she is always trying to balance the earth’s atmosphere.
This upper level “river” of air taps into cold Alaskan and Canadian air and brings it into the portions of the lower 48.
To explain why this pattern has been so persistent, we have to look north and east to the Davis Strait. There is a persistent blocking high pressure over this area (northern parts of the Labrador Sea, Mid-way between Greenland and Canada's Baffin Island.) This essentially “locks” the cool trough in place over the eastern half of the county, preventing us for warming and thus continuing the cold pattern.
Are you still with me?! I know it’s a lot to understand but hang in there. In addition, we look at air patterns called the NAO (North Atlantic Oscillation) and the AO (Arctic Oscillation) to key us into patterns and pattern chance. When these begin to turn more negative our area remains in a cooler pattern. Negative AO equates to weaker high pressure over the Arctic leading to a decrease in the winds that circle the top of the globe. Stronger winds act like a fence trapping the cold air farther north, when the winds are weaker, it allows the cold arctic air to sink southward toward us.
Latest trends point to a more negative AO for the coming days and thus we continue our stretch of below average temperatures. While some days may be slightly below normal there will be extremes. Latest Climate Prediction Center's 6-10 day temperature forecast resembles this pattern quite well!
So keep your warm clothes on the ready and who knows, if we could time out the cold with some solid moisture, we *could* see some snow!