NBC10 First Alert Weather: A Cold and Windy Thanksgiving Week - NBC 10 Philadelphia

NBC10 First Alert Weather: A Cold and Windy Thanksgiving Week

Even though Thanksgiving is more than a week away, it is clear that it will be on the cold side.

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    NEWSLETTERS

    Sunny skies are ahead this week with a chance of slight showers Wednesday night into Thursday. NBC10 First Alert Weather chief meteorologist Tammie Souza takes us through the roller coaster weather we can expect in our 10-day forecast.

    (Published Tuesday, Nov. 14, 2017)

    BETTING ON COLD-BUT HOW COLD?

    Sometimes, weather patterns become obvious enough that we can make pretty accurate predictions more than a week in advance. This has happened more and more in recent years, as computer models get better and better.

    Even though Thanksgiving is more than a week away, it is clear that it will be on the cold side. In fact, all of next week will be colder than normal. Some of the days will be waaaay below normal.

    Here is the world famous (and world’s best) European model for Thanksgiving:

    The purple area covering the entire Eastern U.S. represents much below normal air overhead-at about 5000 feet. Meteorologists use this level, since it smooths out differences due to elevation and day vs. night. We can follow the warm and cold air masses clearly. Also notice the huge area of waaaaay above normal temperatures over much of Canada. This could have implications for December, as we’ll see later.

    THE CANADIAN AND U.S. MODELS AGREE

    Below are the Canadian map for the same time as the one above, and the main U.S. model (GFS) for a day later, since it is moving everything more slowly:

    That’s a lot of agreement nearly 10 days in the future. If only there was so much agreement all the time!

    The bottom line: Thanksgiving Day, and all of Thanksgiving weekend, will feature well below normal temperatures. The “normal” high in Philadelphia is 53, so I would expect highs in the 40s. Some of the days could see them closer to 40. Add the wind, and it’ll feel like winter.

    ANY PRECIP. WITH THE COLD?

    While the idea of a colder than normal Thanksgiving week and weekend seems clear, predicting precipitation that far ahead is a different story. Sure, we have to put weather symbols on the 10-day forecast, but there is more uncertainty with timing of the rain than the general temperature outlook.

    Overall, it looks like a rather dry period between next Monday and the Monday after Thanksgiving. There could be some showers as a weak front moves through, perhaps Wednesday night, but it’s still in the “chance” category. The rest of the holiday weekend should be dry.

    AND AFTER THANKSGIVING WEEKEND?

    The pattern that will give us the cold Thanksgiving week and weekend is the result of a “blocking pattern” in the upper atmosphere. Here is the map for next Monday:

    This is a pretty extreme pattern for late November. The red areas are above normal pressures aloft; the blues are below normal. The dark reds and blues are the most extreme.

    The blue over the Eastern U.S. allows the cold air to come in. And the “block” keeps it around for a while. The red area above that-near Greenland, is part of the block. Another block, even more extreme, is in the North Pacific.

    But it may not stay that way. Winds at that level of the atmosphere blow along the lines. So look at the line that goes through Philadelphia, and trace it back. That shows you where our air is coming from. And that line extends into the Pacific Ocean, NOT the Arctic, Northern Canada, or even Central Canada. This would eventually push mild air into the western half of the U.S. and Southern Canada. So, without another blocking pattern, December could start mild across much of the country. Stay tuned-it’s a tricky pattern.