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Posted at 10:30 AM ET, 11/29/2010

A look into early December: cold likely

By Wes Junker

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.

earlydec10-cpc.jpg
NOAA's Climate Prediction Center gives our area a 60-70 percent chance of below normal temperatures 8-14 days from now.

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.

heights500-early-dec10.jpg
168-240 hour average 500 mb pattern (around 18,000 feet). Source: NOAA

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.

gfs-216-0z-112810.jpg
Two successive runs of the GFS model from last night for late in the day December 7. The blue area is where the model was forecast heavy precipitation (probably snow). Note that the later run had no precipitation over the Washington area.

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.

By Wes Junker  | November 29, 2010; 10:30 AM ET
Categories:  Latest, Winter Storms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   Del.icio.us   StumbleUpon   Technorati   Google Buzz   Previous: Forecast: Wild week - Sun, storms & winter's return
Next: Introducing the Snow Potential Index

Comments

Wes-- Any updated thoughts on the 12/5 (minor) threat shown on the Euro in the last couple runs?

Posted by: Jason-CapitalWeatherGang | November 29, 2010 11:25 AM | Report abuse

C'mon December 5th snow! It's tradition now!

Posted by: CM_in_Fairfax | November 29, 2010 11:59 AM | Report abuse

We appear to have a small system exiting the region early Dec. 5th.

Not too sure about the snow risk, but flurries to a dusting Sunday morning seems definitely within the realm of possibility.

There was heavy ground frost this morning, but air temperatures at DCA may have remained above freezing.

Posted by: Bombo47jea | November 29, 2010 12:11 PM | Report abuse

Wes, I suspect one reason for CPC's week 2 forecast not calling for a higher probability for below average temperatures is the prospect in the daily forecasts for warming during the end of the period.

Note too that the CPC precip forecast chart for the same period calls for below average precip.

Too bad we just can't wish model fantasy land to become reality

Posted by: SteveT-CapitalWeatherGang | November 29, 2010 12:40 PM | Report abuse

Snow sucks.

Just say "no" to "snow."

Posted by: rmcazz | November 29, 2010 1:33 PM | Report abuse

Any thoughts on the possibility of seeing some snow on the back edge of Wednesday's rain?

Posted by: spgass1 | November 29, 2010 1:47 PM | Report abuse

I was wondering the same thing is spgass1. Several model runs has shown the 540 line passing through the DC area before the precipitation has completely moved through. Is there a chance that it will end as snow Wednesday PM?

Posted by: rapotter | November 29, 2010 1:58 PM | Report abuse

Folks-- Wes will no doubt respond to some questions later, but I'll at least this one now...

@rapotter and @spgass1

I don't think the cold air arrives in time for any kind of changeover Wednesday except in the highlands of W. Va and western Md. You rarely get snow in these scenarios east of the mountains.

Posted by: Jason-CapitalWeatherGang | November 29, 2010 2:33 PM | Report abuse

I hear you Jason, but the seems to show a chance (

Posted by: spgass1 | November 29, 2010 3:14 PM | Report abuse

Jason,

Sorry about taking so long to respond, I was golfing and had a good round so I celebrated with some friends. The European and Canadian models really like the threat for Dec 5th. I'm normally not bullish with lows that initially start in the OH valley and have to reform southeastward far enough to hold in the cold air. Boundary layer problems sometimes arise. Neither the GFS nor its ensembles are very bullish on that system. That said, the predictability is low so it's still a threat to watch. I suspect the models will continue to waffle back and forth with that threat. I guess my long winded response is I don't yet know what to think about it

Posted by: wjunker | November 29, 2010 6:11 PM | Report abuse

Steve,

You're probably right about CPCs reasoning. It is too bad we can't wish model fantasy storms into reality. I do think the pattern has more potential than climo would suggest. I'll sure be watching the model forecasts over the next 2 weeks as I'm a big fan of a negative NAO.

The 19 events in 122 years were only events that produced greater than 1 inch of snow.

Posted by: wjunker | November 29, 2010 6:15 PM | Report abuse

spgass1

I agree with Jason about Wednesday. I don't see any changeover in the DC area unless you're talking about back in the mountains of WV and western MD

Posted by: wjunker | November 29, 2010 6:19 PM | Report abuse

Posted by: wjunker | November 29, 2010 6:57 PM | Report abuse

More on the Dec 5 threat. The European model ensemble mean supports the operational European model which is very bullish on the storm. The GFS and its ensembles tend to shear the system out and keep it too weak to produce any snow. Both camps are equally likely. That said, in the past the GFS has sometimes tended to shear systems too much keeping the any low too weak to produce any snow. The threat certainly is worth watching. CWG will be watching the system over the next several days to keep everyone informed about the system's evolution.

Posted by: wjunker | November 29, 2010 7:04 PM | Report abuse

I started bouncing up and down with excitement when I saw "strongly negative NAO"... the little kid in me is coming out in anticipation of snow!
I can take it - I only have one more class next week - but I don't want anyone snowed out of final exams (however likely that may or may not be.)

Posted by: MKoehl | November 29, 2010 8:26 PM | Report abuse

The comments to this entry are closed.

 
 
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