My 2009 December Outlook.

Issued: 11/27/09
by: Ruggie Weather

For The Delaware Valley and Mid-Atlantic Regions

As we head into the first official month of Meteorological winter, we usually find the month of December being quite variable and can be very tough to predict. A month that can bring a wide range and swing in temperatures. Overall, I see this upcoming month being quite interesting to say the least, but nothing real extreme this go around.

In a typical weak to moderate El Nino winter season, most of the time December can be on the mild side, but also quite variable with both temps and snowfall. In my 2009-2010 Winter Forecast, I've forecasted December to be "Near to Slightly Above" or 0 to +2 and snowfall to be near average for the month.

As we head into December, we are in the beginning stages of what I call a seasonal step-down of the Jet stream. We've seen a progressive pattern for much of November that was driven by a fairly moderate Pacific jet with temps averaging on the mild side for many. As this seasonal step-down progresses during the first 10 days of December, this Pac/jet will become more suppressed and relaxed, which will allow for some colder air to enter the CONUS. This will be gradual at first, and temp anomalies will be near to slightly above avg for the first week of the month or days 1-7.

As we head into days 8-15 of the month, I'm seeing signs that we'll be in a colder and stormy pattern. Key teleconnections will start changing during the beginning of the month, and we should start to see the effects of these changes as we approach this 8-15 day time-frame. The NAO which has been mainly neutral for the past several weeks, will begin to go slightly negative, this will allow for some blocking to develop in the Atlantic, but nothing major. The AO which is now slightly Neg. will begin to really tank and this will allow for a huge discharge of colder air to head south and east towards the CONUS. The PNA is near neutral and heads slightly POS for the first half of December. This will allow for ridging to develop from the NW states into the Western Canadian coast, which will also aid in forcing the colder air southeastward towards the States.

With all this being said, colder air will initially filter into the western and central CONUS during the first week of the month, and then shift more eastward with the mean trough developing and setting up over the central and eastern CONUS, as we head towards the middle of the month. The big question is how long does this last ? At this time I'm thinking a good week or so, mainly between Dec 10th - 17th, then We'll see a relaxation period that will result in near normal temps (Seasonably cold) as we close out the month from Dec 18th - 31st. So we can expect more seasonably cold conditions for the later 1/3rd of the month.

I'm looking for an active and stormy pattern to develop between the 8th and 15th of the month for much of the Central/Southern US and also the Eastern half of the country. Temperature will average below normal during this period. The coldest temps for the east will be for the SE and M/A States with the greatest negative departures or Anomalies. We could be looking at -5 to -10 departures for a few days in these areas, but overall -2 to -5 degree departures can be expected.

For the second half of the month Dec 16th -31 things will become more variable with some days above normal and some below, but nothing extreme, when all is said and done we should be close to normal during this second half stretch. With the huge discharge of cold air due to the AO going big time Negative, this second period will be re-load time for the colder air to build back up, so watch out come January for the big Pattern change and snow chances to increase considerably.

So will it snow and how much ? I'm going to make this call for The Delaware Valley Region<>

My December 30 day Outlook:
For the Eastern CONUS.

For the NE and M/A:

Temperatures:
-1 to +1 (Near Avg.)

Precipitation:
Near to Above normal.

Snowfall potential for The Delaware Valley/PHL & NYC Only:
2 or 3 Events. (2) minor events of less than 2" and (1) event of 1"-3" or 2"-4"

Total snowfall for the month:
3" to 5" for both the PHL and NYC areas.

For the SE States:

Temperatures:
-1 to -3 (Below Avg.)

Precipitation:
Above Normal Precip.

Take Care,
Ruggie