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Posted at 7:25 PM ET, 01/25/2011

At weather expo, top snow expert shares views

By Andrew Freedman
NOAA's Louis Uccellini.

At the American Meteorological Society annual meeting in Seattle, the crowd of top weather and climate scientists has been buzzing about the potential East Coast storm event. Today I caught up with one of the foremost experts on winter weather forecasting, Dr. Louis Uccellini, to get his take on the situation. In addition to getting his thoughts on the forecast, I also asked him why the computer models have struggled to agree on projections for several winter storms so far this season, especially compared to last winter.

Uccellini, is the longtime director of NOAA's National Centers for Environmental Prediction (NCEP), located in Camp Springs, Maryland. Numerous forecasting centers are housed within NCEP, including the Hydrometeorological Prediction Center (HPC), which plays a key role in predicting winter weather events. Along with former Weather Channel winter weather expert Paul Kocin, Uccellini wrote what is essentially the bible of East Coast winter storms. Uccellini is also President-elect of the American Meteorological Society.

On how confident he is that this storm will result in a moderate to heavy snowfall in Washington, D.C. and nearby suburbs:

Uccellini hesitated to say he is very confident in the computer model projections at this point, even though they seem to have largely come to a consensus. Instead, he said the "trendline towards confidence is clearly pointing up, in terms of a significant snow event."

He explained that the models are catching onto a key reason why there may be just enough cold air available during this storm than was initially thought. "We're clearly trending toward a colder solution since yesterday, and I think it makes physical sense," he said.

A storm that moved through New England today, dropping 2-4 inches of snow in the process, has helped reinforce what is known as a "confluent zone" to the northeast of Washington. "That upper level confluent zone is important for keeping cold air in for us," he said, adding, "It's not a textbook confluent zone, but it's an added factor that's helping to keep the colder air in."

As we've noted in our forecasts, the earlier computer model runs had shown warm air moving northwards at the middle and lower levels of the atmosphere, keeping precipitation as rain or mixed rain, sleet, and snow. But the latest projections don't show such a warm air intrusion.

"I'm more confident that the storm will produce heavy precipitation over the mid-Atlantic region, and that it would produce snow," Uccellini said.

He said the models have been showing the possibility of a period of very heavy precipitation as the storm intensifies near the Washington metro area. "I'm not absolutely convinced that for this event... we're going to get that, but the model is certainly pointing to a slantwise convection aspect and we have to pay attention to that."

On the model flip-flops leading up to this, and other, recent winter storms:

Uccellini said that when he examines computer model projections, and their fluctuations from run to run, he tries to determine "if they're occurring within an envelope of solutions... [which are] usually established several days in advance." He noted that if the models depart from those bounds (as at least one model run did on Sunday evening), "then you really know you have a serious [forecasting] problem." In this case, by and large the models were flip-flopping within a particular envelope.

On why predictions for this year's storms have had less skill, especially further in advance, compared to last year's major storms:

Uccellini said he is not surprised that computer models are having a tougher time handling this year's storms, and that the perception that there are more "predictability issues" this year is probably valid. "In terms of storm track and precipitation amounts, there was definitely more consistency" last winter, he said.

But why such a difference from one year to the next, if the computer models have essentially remained the same, and forecast skill is thought to be improving with time?

Uccellini pointed to differences between the origin of winter storms, and the large-scale weather patterns in place, this year compared to the winter of 2009-10 as accounting for much of the disparity.

Last winter was dominated by a strong El Nino, which led to a very intense southern jet stream that transported waves of low pressure off the Pacific, marched them across the country, and then directed them up the eastern seaboard. This year, there is a strong La Nina in place, and the northern branch of the jet stream is dominating the weather in the eastern half of the country.

"My sense is that last year's very strong El Nino, and very strong southern jet stream... that the way the waves [of low pressure] developed in the southern stream and remained essentially coherent along the southern states, lent itself to a more consistent forecast from run to run."

Simply put, Uccellini said, the models don't have as much skill at dealing with storms that originate from within the northern branch of the jet stream versus southern stream systems. "I've not been surprised about the challenges in predictions of these storms versus last year," he said.

The reasons for this are somewhat complicated, but they can partly be traced back to the lack of observational data in the polar regions. Without accurate and timely observational data, computer models won't necessarily simulate realistic projections of future weather conditions (the old "garbage in, garbage out problem").

"I've said all along... that the cross polar flow regime was less predictable than the flow coming in off the Pacific," he continued. "We actually observe the Pacific quite well."

Uccellini said that this year, storm development is much more dependent on the phasing between weather systems embedded in the northern jet, with energy rippling along the weaker southern jet stream. The computer model vacillations often relate to aspects of this phasing process, which, as we saw during the Dec. 25-26 event, can be critical to determining whether Washington receives any snow at all.

"If you have to rely on northern stream systems for east coast storms you're going to have a lot more uncertainty," he said. "I have felt for the last ten years very strongly that we haven't paid enough attention to the northern stream, and this year is proving that point."

By Andrew Freedman  | January 25, 2011; 7:25 PM ET
Categories:  Freedman, Latest, News & Notes, Science, Winter Storms  
Save & Share:  Send E-mail   Facebook   Twitter   Digg   Yahoo Buzz   StumbleUpon   Technorati   Google Buzz   Previous: Winter storm on the way: Timeline, snow accumulation map, impacts, & FAQs
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Posted by: benzachr | January 25, 2011 7:34 PM | Report abuse

In otherwords - even the experts don't know.

Posted by: barbnc | January 25, 2011 7:35 PM | Report abuse

Ha! I really did it! Whatever. I still like our odds.

Posted by: benzachr | January 25, 2011 7:36 PM | Report abuse

Ahhhhh so he weather Guru left me more confused as to whether or not we will ge punished tomorrow night.

Posted by: RJ16 | January 25, 2011 7:36 PM | Report abuse

Great piece Andrew, fortunate to hear from one of the godfathers of NE snowstorms. How is the mood among DC area patrons? I figure there might be a support group or something. ;)

Posted by: Ian-CapitalWeatherGang | January 25, 2011 7:39 PM | Report abuse

i.e., D.C. is the LA of east coast weather.

The pivot point for LA is one hundred miles or so for the winter to be a drought or a deluge.

For D.C.-- it's the snow.

Posted by: thealaskan | January 25, 2011 7:39 PM | Report abuse

CWG, what are the odds that the models flip on us and we get less snow than expected? How about vice versa?


Posted by: BobMiller2 | January 25, 2011 7:40 PM | Report abuse

To echo @Bob's question, what does CWG make of the models - sure they're moving this way and that way, but what do you guys make of it all?

Posted by: prickles1009 | January 25, 2011 7:46 PM | Report abuse

Thanks Mr U! Ya'll are all SO GOOD at hedging just that little bit of doubt into every critical forecast.
I would do exactly the same thing in your shoes.

Posted by: FIREDRAGON47 | January 25, 2011 7:54 PM | Report abuse

Great stuff Andrew, for you it must be like to talking to the weather gods!

"the lack of observational data in the polar regions."

How can this be improved in the future?

Posted by: jaybird926 | January 25, 2011 7:54 PM | Report abuse

@BobMiller and prickles-

There's not a ton of variation in the models with this one. We've written about that in several posts today. The devil is in the details-where exactly is the heaviest snow going to set up...which is essentially unpredictable. In the last post, we have specific FAQs about whether the storm could overperform or underperform. The answer is yes to both... probably 20% chance it could be significantly less than what we're calling for and 20% chance it could be significantly more.

Posted by: Jason-CapitalWeatherGang | January 25, 2011 8:02 PM | Report abuse


Posted by: SNOWLUVER | January 25, 2011 8:02 PM | Report abuse


Not cool.

Posted by: natsncats | January 25, 2011 8:07 PM | Report abuse

"trendline towards confidence is clearly pointing up, in terms of a significant snow event"

Translation: Maybe it will, maybe it won't.

Gosh - do I love experts.

The Washington Post - if you don't get it, it's because nobody else does, either.

Posted by: hofbrauhausde | January 25, 2011 8:22 PM | Report abuse

SNOWLUVER, why are you telling children not to do their homework? Don't you realize that if they follow your advice and blow it off, they could be in trouble with their teachers because of you??

Kids, don't listen to him/her. The CWG already gave their schoolcast, to which I recommend you refer. It's always best to do your homework NOW and get it over with.


Posted by: BobMiller2 | January 25, 2011 8:23 PM | Report abuse

This may have been addressed before, but is it the case that data on each storm get fed into the models, thereby adding to the models' store of information on how the atmosphere behaves and therefore at least to some extent making them smarter for the next time?

Posted by: SnowLovr | January 25, 2011 8:28 PM | Report abuse


I'm not a schmuck or a moron, thanks all the same. This site gets overrun by kids who clog up the threads with inane comments whenever a storm is in the offing -- who do you think SNOWLUVER is anyway?

Posted by: natsncats | January 25, 2011 8:36 PM | Report abuse

Here is my predictin, since I'm always right on poit. DC gets 7 inches. Upper Moco gets 13 inches.

Posted by: RJ16 | January 25, 2011 8:39 PM | Report abuse

TheJoeGreene - there are lots of kids who participate in this blog.
Very big & grizzled kids to be sure......
I'm not doing MY homework. Need to save strength for heavy cardio workout tomorrow I am not fortunate enough to have access to a snowblower.

Posted by: FIREDRAGON47 | January 25, 2011 8:40 PM | Report abuse

I'm confused about the SchoolCast and other symbols--Many times there are half apples, half planes and half Capitols, but the Legend doesn't define what a half symbol means. So, may I recommend that you either update the Legend, or stop using undefined symbols?
Small thing, I know, but let's have a little certainty somewhere on this site!

Posted by: StevenMark | January 25, 2011 8:41 PM | Report abuse

hofbrauhausde - Suggest you take a class on reading comprehension, you're clearly struggling to understand sentences.

Btw - It's easy to criticize...I have yet to see you submit a forecast or prove anyone wrong. Oh how smart you are! I bet you feel so great about yourself.

Posted by: parksndc | January 25, 2011 8:44 PM | Report abuse

C'mon Bob--it was funny: "cyber-panties" "living vicariously through a weather blog." Good stuff!

Posted by: StevenMark | January 25, 2011 8:45 PM | Report abuse

here's the full usa radar. pan over to our area. our moisture is in kentucky/tennessee now.

Posted by: walter-in-fallschurch | January 25, 2011 8:47 PM | Report abuse

Glad 2 c this storm is suppose 2 take the prefect track. Still leery of big accum. when a change over is required. Looks like some areas may get 6+ while others may end up with 2-3". How soon the cold area filters in will b the key 2 how much snow everyone gets, when that happens is very hard 2 predict.
This storm still has a lot of variables, so I would advise all of us snow lovers 2 temper our expectations a little bit.
Caught 20 stripers 2day, including 12 between 24-29lbs & a 31,34 & 39 pounder.

Posted by: VaTechBob | January 25, 2011 8:54 PM | Report abuse

Ian - there is definitely a support group forming here, and way too many people are looking at the latest model runs during peoples' presentations.

jaybird926: the lack of observational data in polar regions is an issue in atmospheric sciences but also for biologists and many other disciplines that are racing to keep track of changing Arctic conditions. For meteorologists, in situ atmospheric measurements from weather balloons, aircraft, and other instruments would be very helpful, as well as more surface monitoring stations. Satellites help a lot, and are doing a great job, but a denser network of other measurements are needed too. The National Science Foundation has been funding some work towards this end, but more on the climate change side of things rather than weather forecasting (there may be benefits for both though).

Posted by: afreedma | January 25, 2011 8:58 PM | Report abuse

walter in falls church

thanks for the link cool radar view

Posted by: VAresident2 | January 25, 2011 8:59 PM | Report abuse

Will all you kids that have absolutely nothing to add to the discussion stop polluting this threads every single day?

And can I get a BobMiller2 ignore feature?

Posted by: LoudounSouth | January 25, 2011 9:01 PM | Report abuse

Folks- I'm going to delete comments with name calling and consider blocking commenters who do it if there are repeat offenses. We're really busy try to cover this storm and having to police comments here means less analysis and less content we can provide.

Finally, please comment only on the content within the post. If you'd like to comment about the forecast, accumulation, schools, etc, we have a post directly underneath this all about that.

Posted by: Jason-CapitalWeatherGang | January 25, 2011 9:01 PM | Report abuse

hey everyone this is a blog it isn't a Yale class on meteorology. Enjoy it for what it is, a great source of weather and insight along with the occasional oddity

Posted by: VAresident2 | January 25, 2011 9:04 PM | Report abuse

yeah all you stinkin kids need to skedaddle! Go play xbox or club penguin or something.

Posted by: SNOWHATERRR | January 25, 2011 9:05 PM | Report abuse

Was listening to AM 650 this evening, waves bouncing off the ionosphere from 400 miles away. Nashville under Winter Storm Warning with 3-5" expected.

Posted by: spgass1 | January 25, 2011 9:06 PM | Report abuse

Folks, stop the name-calling. That's against our comments rules and will get posts deleted.

Re the question "This may have been addressed before, but is it the case that data on each storm get fed into the models, thereby adding to the models' store of information on how the atmosphere behaves and therefore at least to some extent making them smarter for the next time?" No, the computer models don't build up a memory of each storm in a winter season. Rather, they are run, or "initialized", based on observations of weather conditions from the oceans to the upper atmosphere. This data gets fed into the models, which then use that to make a projection based on mathematical representations of how different atmospheric processes work.

Perhaps in the future there will be a computer model that will teach itself to learn with each storm, but that might put us out of a job...

Posted by: afreedma | January 25, 2011 9:09 PM | Report abuse

If I work in Bethesda and live in Manassas, what time do you think I should leave work tomorrow in order to avoid a hellish commute?

Posted by: aaf314 | January 25, 2011 9:11 PM | Report abuse

state of the union or live blogging? haha

Posted by: vtjudyz | January 25, 2011 9:13 PM | Report abuse

andrew, you said,
"No, the computer models don't build up a memory of each storm in a winter season."

that's surprising/disappointing. nonetheless, there MUST be some kind of "keeping track", by computers or humans, of how these models do? right? maybe a comparison of "runs" to actual outcomes 12, 24, and 48 hrs out?

Posted by: walter-in-fallschurch | January 25, 2011 9:17 PM | Report abuse

OK.. NWS just reduced accumulations by 2"!!!!! WTF?!?!?!?!?!

Posted by: SnowDreamer | January 25, 2011 9:17 PM | Report abuse

The 00Z NAM came in slightly drier, a tad slower than the 1*Z but otherwise is pretty similar, It still suggests the changeover for dc would be somewhere around 4PM or slightly after that time but that by 4PM the precipitation would be either snow or snow and sleet if the model sounding were taken as gospel.

Posted by: wjunker | January 25, 2011 9:21 PM | Report abuse

I've been reading/commenting for quite a while. I could do without the constant spamming and nonsense from the blogs few pet favorites. I'm definitely in favor of that block bobmiller2 function.

It would be great if we could read the info, have people ask legitimate questions, and not have the 2-3 teachers pets ranting about anything and everything and jumping on people about posts that are VERY OBVIOUSLY MEANT TO BE HUMOR like SNOWLUVER's comment was.

Keep up the great work CWG and work on getting a block/ignore feature so we can just weed out the human spambots.

Posted by: TheJoeGreene | January 25, 2011 9:25 PM | Report abuse

I like the new NAM!

Posted by: BobMiller2 | January 25, 2011 9:26 PM | Report abuse

Wes - I know you can't speak for other weather outlets, but I've noticed the NWS just significantly scaled back snow accumulations. As another poster commented. Would you think its b/c of the latest NAM model? If you had to guess. Also, does the model make you feel like we'll be on the lower end of your previous forecast?

Posted by: parksndc | January 25, 2011 9:27 PM | Report abuse

@SnowDreamer yeah looks like 4-5" is now forecast instead of 5-6" inside Beltway. It's a small change though...I'm sure we could perhaps still get 6" here. But I guess 3" is still possible as well. So...looks like the 3-6" forecast from CWG for the immediate metro area is still spot on...but fingers crossed for 6 instead of 3!!

Posted by: BH99 | January 25, 2011 9:29 PM | Report abuse

Walter, there are statistics measuring the accuracy of the model forecasts of various fields and there are fields that are bias adjusted to a certain extent (MOS output). However, the atmosphere is a fluid and the physics/ equations that govern the atmosphere don't really change. How we represent process such as convection which occur on such a small scale that you can't explicitly handle it may be different from model to model but the basic physics pretty much stays the same and is not changeable from run to run.

Posted by: wjunker | January 25, 2011 9:29 PM | Report abuse

I like our current graphics and don't see any reason to change them. I doubt the NWS changed the forecast based on the new run since they are just digesting it.

Posted by: wjunker | January 25, 2011 9:32 PM | Report abuse

Got it...thanks!

Posted by: parksndc | January 25, 2011 9:45 PM | Report abuse

The latest prediction show that the NAM and the latest run have it at 15"-20"+ in northwest Virginia, and it looks like the NWS has forecasted 20"+ inches for the DC metro area

Posted by: joshwarhawk | January 25, 2011 9:53 PM | Report abuse


REALLY could the amounts change that much!?!?!?!

I am only 25 miles south of DC.....OH PLEASE OH PLEASE!!!!! I would love it if that really happens!!!

Posted by: Hopin4Snow | January 25, 2011 10:00 PM | Report abuse


I just looked again to make sure, and I didn't see a significan't scaleback of snow on NWS. Where are you talking about, and how much did they cut back on? For Leesburg, they actually INCREASED from 4-9 to 5-11 inches.

Posted by: 4seamed | January 25, 2011 10:08 PM | Report abuse

My models are now showing the cold air arriving later than originally predicted. The changeover will occur later, the storm will move through faster. I am forecast 2" max for DC, 1.5" for Springfield, Lorton, Woodbridge, and lesser amounts east. 2.5" for Western Fairfax. 3" for IAD and Upper Montgomery.

Sorry snow lovers, but there will be no big heavy snow accumulation from this storm for us.

Posted by: Firedude | January 25, 2011 10:15 PM | Report abuse

@ Firedude....You know how to KILL a buzzz!!
One can still hope!!!!!

Posted by: Hopin4Snow | January 25, 2011 10:22 PM | Report abuse

The nws just came out with a new run. Instead of the original three to six we will actually only be getting -2 to -3 inches of snow. It's too bad I don't ever think were going to get a good snow

Posted by: joshwarhawk | January 25, 2011 10:37 PM | Report abuse

like 4-5" is now forecast instead of 5-6"
.... .... .... .... ....


must go and roll up my socks now.

Posted by: thealaskan | January 26, 2011 12:22 AM | Report abuse


Just a great interview and summary.
Thank you.
I learned a great deal from it all.

Best wishes to you.

Posted by: bcastner | January 26, 2011 6:23 AM | Report abuse

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