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Posted at 10:30 AM ET, 12/23/2009

CWG's snow forecast: Reasonable, cautious

By Capital Weather Gang

Weather Checker

* Christmas storm? Full Forecast | Evolution of a monster snowstorm *
* View CWG-user snowfall map | NWS snowfall reports (view map) *

CWG's 'final call' for snow accumulations from this past weekend's storm, issued just before 11:30 p.m. Friday night.

Weather Checker is an outsider's analysis of CWG's forecast accuracy. See previous Weather Checker posts.

By Jamie Yesnowitz

I did a lot of digging this past weekend -- first, digging out my driveway in an effort to get my family out the door on Monday with reasonable success, and then digging into the CWG forecasts to review their performance. How did CWG deal with this historic event?

Jason Samenow's weekend forecast on Monday (Dec. 14) hinted with low-medium confidence at the possibility of snow showers or flurries, and a "low probability" potential for "coastal storminess," with highs around 40 and lows in the 20s. Matt Rogers' low-confidence forecast on Tuesday for the weekend kept a 20%-30% chance of snow showers, with "the possibility of a coastal storm."

WUSA-9's forecast from Friday morning turned out to be very much on the mark.

Also on Tuesday, a Monday headline at the top of prompted Jason to post his thoughts on how AccuWeather tends to "headline low probability, low confidence events," while at CWG "we usually wait until there appears to be at least a 25-30% snow threat of at least 1" or more (and usually, that's no sooner than 5 days before a potential event)." As an ardent reader of both and CWG on a day-to-day basis, it's my opinion that Jason's Dave Kingman vs. Tony Gwynn baseball anaology at the end of his post completely captures the difference (and purpose) of these two media outlets.

Dan Stillman's Wednesday forecast continued to mention "the chance of a coastal storm" over the weekend (with low confidence), with a chance that the storm could come "close enough to bring wintry precipitation to the area" or that it could track "too far out to sea to have an impact here."

Dan's Snow Lover's Crystal Ball post on Wednesday framed the chance of a weekend coastal storm as "good enough to consider the possibility of accumulating snow," but stayed conservative with a 30% chance of 1 inch or more. Later in the day, other CWG members began to get on board in the comments section, particularly Josh. Dan himself noted the increased probability of a major snow event in a comment, and Jason followed up with an alert that the weekend snow threat was growing, highlighting why this blog format can be so valuable -- as the models evolve, the forecast can evolve in like fashion.

Josh's Thursday morning forecast increased the chance for "possibly significant" accumulating snow on Saturday to 60%, and lowered high temperatures to the upper 20s to mid-30s. Also on Thursday morning, Josh's Snow Lover's Crystal presented the first accumulation estimates, with a 35% chance of 1 inch or less, 20% chance of 1-3 inches, 25% chance of 3-6 inches, and 20% chance of 6 inches or more.

Thursday afternoon, Ian Livingston's update shifted the probabilities toward more snow with with a most likely (30% chance) amount of 6-12 inches, and 15%-20% probabilities for lower and higher amounts. Considering that Ian's forecast ultimately gave a 50% chance of less than 6 inches and a 50% chance of 6 inches or more, CWG was staying very conservative and somewhat noncommittal with its prediction.

I was hoping that CWG would have a more certain forecast soon, and they did in their team forecast (first posted just before 11:30 p.m. Thursday night) that mapped the immediate D.C. area for "7-14 +" with snow beginning Friday night. CWG then raised the accumulation estimate throughout the day Friday as the models came into better agreement on predicting a storm of historic proportions. CWG's final call of 16-24" (as the snow was beginning Friday night) ultimately verified. Likewise, the Storm FAQs, gradually added to the Thursday night post and later tweaked, generally provided accurate, detailed information.

Overall, while one could nitpick that accumulation forecasts made late Thursday and early Friday were too low, CWG dealt with this historic storm without engaging in Accuweather-style hype, and that is no mean feat. While other media outlets were predictably all over the map with their forecasts (ranging from a boldly accurate WUSA-9 forecast of 16-25 inches that worked out well, to NBC4's multiple adjustments to a very low accumulation forecast), CWG should be heralded for their reasonable, cautious approach.

In the end, CWG came to the solution that we might be affected by a significant storm very early on, they never let go of that idea, they stuck to that assessment during the storm, with consistently rising accumulation forecasts that became more accurate as the event neared, and they gave people enough warning to prepare for it.

Storm totals, as compiled by the National Weather Service. Larger image.

Jamie Yesnowitz has been interested in the weather since he rooted for school-closing snowstorms while growing up in Brooklyn and East Rockaway, N.Y. After graduating from Dartmouth College with a bachelor's degree in economics and government, his focus on the accuracy of weather predictions took hold when he moved to Coral Gables, Fla., to attend the University of Miami School of Law. Class was scheduled to begin on August 24, 1992. Hurricane Andrew had other ideas, however, shutting down the school for weeks. But what stuck in Jamie's mind was the final unpredicted swerve of the eye that saved those living in Miami and points north, and completely devastated areas about 20 miles south of Miami.

Undeterred by the hurricane, Jamie ultimately served as editor-in-chief of his law school newspaper, and earned both a juris doctorate and master's degree in taxation. Following law school, Jamie practiced corporate and securities law in New York before shifting to the state and local tax consulting world. Jamie moved from New York to the Washington area in 2003, and he is presently a state and local tax senior manager at a major accounting firm. Jamie lives in Potomac with his wife, Sandra, and their two daughters, Sarah and Carly.

By Capital Weather Gang  | December 23, 2009; 10:30 AM ET
Categories:  Weather Checker  
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Good reporting, thanks for the follow-up.

Quick note: if you are going to accuse Accuweather of early hyping whatturned out to be a pretty good early forecast, then IMO you need to balance that with notable misses like our own Bob Ryan and channel 4 Wednesday at 4:30 pm forecasting only possiblity of flurries only and great certainty that the storm would pass to our south. What a horrible bust!

Posted by: ubimea | December 23, 2009 11:11 AM | Report abuse

Here's another ananlysis of DC / BWI area snow accums courtesy NOHRSC...

Make your own @

Posted by: toweringqs | December 23, 2009 11:26 AM | Report abuse

One small nitpick, Josh said in the thread comments that the snow would be heavy and wet because storm was coming from the Gulf of Mexico. But the heaviness and wetness of snow mostly comes from the lower level temperatures, for example if warmer Atlantic air can work in from the east. As it turned out, the atmosphere was cold enough at the lower levels for a dry powdery snow.

Posted by: eric654 | December 23, 2009 11:29 AM | Report abuse

Jason and CWG,

Why do you consider a weather forecast 6 days in advance to be unreliable, but you have no doubt about the reliability of a climate forecast 100 YEARS in advance?

The science in predicting weather is far more mature and proven than the science in predicting climate. And yet you believe in a climate forecast 100 years off and scoff at a weather forecast 6 days in advance.

You will publicly take AccuWeather to task for a weather forecast less than one week out. And then turn around and advocate PUBLIC POLICY based on a climate forecast 100 years out.

That boggles my mind.

Mr. Q.

Posted by: Mr_Q | December 23, 2009 12:19 PM | Report abuse

Let me make sure I understand CWG's position -

A weather forecast 6 days in advance is something to publicly ridicule. (And never say "Mea culpa" when proven wrong.)

But a climate forecast 100 years in advance and OMG! Better pass Copenhagen NOW!

Is that an accurate summary?

Mr. Q.

Posted by: Mr_Q | December 23, 2009 12:22 PM | Report abuse

Mr. Q obviously does not understand the difference between weather and climate, especially the difference in factors that govern the predictability of each. Weather prediction is an initial value problem while climate prediction is a boundary value problem. This is not the time and place to delve into the difference. And, if Mr. Q were really interested in posting something more meaningful, he'd know the differences.

Attempting to hijack the subject of this post should be judged by all independent minded souls as totally unreasonable. Moreover, he's been warned about this by CWG, but warnings are meaningless unless enforced. Any further comments here by Mr. Q should be blocked or deleted.

I strongly recommend that noone else comment on this subject. Otherwise Mr. Q. will have succeed in changing the subject from what I'm sure a vast majority of readers are interested in

Posted by: SteveT-CapitalWeatherGang | December 23, 2009 12:49 PM | Report abuse

Yes, by all means SteveT. When all else fails, adopt the CRU and gang methodology of suppressing dissenting views.

If you look directly above the comment box, you should see -
"We encourage users to analyze, comment on and even challenge's articles, blogs, reviews and multimedia features."

Are you going to apply for a position with CRU or Dr. Tracton? Polishing up on your suppression of opposing points of view skills? I'll give you a recommendation!

Mr. Q.

Posted by: Mr_Q | December 23, 2009 12:58 PM | Report abuse

I have to agree with SteveT. I find all of Mr. Q's comments to be somewhat annoying and overly provocative. Life is too short to always be critical. The CWG isn't always perfect but they are one of the most balanced weather news (and analysis) sites online!

Posted by: snowedin85 | December 23, 2009 12:59 PM | Report abuse

I think you guys sell yourselves short a bit. For instance, I'd say you don't give your team enough credit for looking at the overall synoptic map in the days leading up to the blizzard. You noted the striking similarities in the overall maps with the 1993 Superstorm, and kept talking about the increasing "potential" for a big storm. But you didn't actually forecast it until the models came around to the solution you were suspicious of. That's the responsible thing to do. Also it's hard to predict the true jackpot areas. We got right under the comma at the moment it exploded closest to land, and the rest...will go down in history. Nice job, I really enjoyed all the posts (but not so much the distracting and pointless Accuweather-related sideshow).

Posted by: curtmccormick | December 23, 2009 1:15 PM | Report abuse


Forecast soundings depicted a deep...moist isothermal layer just below freezing with slightly colder temperatures @cloud top. The expectation for a heavy wet snowfall was reasonable given the available information.

7 AM sounding @IAD...

The 7 AM rawindsonde data from IAD on the 19th observed a much colder column...which supported the growth of low-density dendrites (hi-fluff factor). SN:H2O for IAD was 27:1. Probably a result of being under the deformation axis / cold upper LOW for an extended period.

Posted by: toweringqs | December 23, 2009 1:42 PM | Report abuse

As I've posted previously, this storm followed roughly the same track as the historic "Knickerbocker" storm, generally a virtual GUARANTEE for very heavy snow accumulations around here, PROVIDED an ample supply of cold air is anchored in place at all levels.

The "Knickerbocker" track was not a "given" as far out as Monday the 14th, and AccuWeather might be faulted for assuming it would "verify" that far in advance. As I recall, I was favoring an out-to sea track at that time.

On the other hand, Bob Ryan's advance emphasis on the out-to-sea track as late as Wednesday night could be faulted for failing to take into account the fact that the standard models and model ensembles tend to skip and jump all over the place during the five days prior to a major event. The point to watch for is that point generally around 36 to 48 hours prior to an event when the models and ensembles tend to converge on a common solution. Usually this consensus solution represents a pretty good forecast of the upcoming event.

As for those who post here, we seem to have a wide range of views ranging from Mr. Q's rightward-leaning skepticism of all things climatological to ThinkSpring's desire to dump nothing but soaking cold rain on us for the rest of the winter. Frankly, I'm hoping for a few more big snowstorms as the winter progresses. Last winter we had plenty of flurries, but nothing of substance until the very tail end of meteorological winter as March began. Hopefully we'll do better this year.

Unfortunately, our next storm seems to be trending towards the ThinkSpring end of the spectrum.

Posted by: Bombo47jea | December 23, 2009 1:47 PM | Report abuse

Just so all of you know, I've just returned home for the holidays to the snow-belt of upstate NY, and the ground is near barren. I miss the snow!

Posted by: Brian-CapitalWeatherGang | December 23, 2009 2:10 PM | Report abuse

Mr. Yesnowitz's laying out of the evolution of CWG's forecast with regard to this weather event gives me a greater appreciation for the process.

I'm so old, I remember when snowstorms like this routinely hit without warning. As late as '78, a blizzard roundly predicted to be at most "a dusting" blanketed the Northeast - no stranger to snow - shutting down the major cities for almost a month and obliging the National Guard to be called up.

In those days, the latest in forecasts, such as they were, were had by telephoning the Nat'l weather service to hear a continually updated recording. While looking with amazement at snow falling outside so fast, you couldn't see anything else, we dialed it only to hear "a dusting" had been upgraded to "light snow of 1 to 2 inches". I think got about 4 feet.

It was our first winter in Boston, where we'd moved from L.A. for college. We loved it!! Only problem was, every time thereafter, forecasts of a 1 or 2 inches found us setting aside term papers and exam preparation for celebrating the gods of snow instead, only to be betrayed by their failure to deliver all but a few inches.

Posted by: jhbyer | December 23, 2009 2:13 PM | Report abuse

Hi CWG guys,
Are flights to Dulles going to be ok on Friday afternoon/Saturday?

I have a flight from Orlando to IAD on Friday afternoon. Should I change it to Saturday or Thursday? TIA.

Posted by: RoseVA | December 23, 2009 2:34 PM | Report abuse

Interesting solutions for about a week from now are starting to emerge on the longrange maps.

The GFS suggests there might be another big player lurking somewhere off the East Coast around New Year's Eve...GFS is good at first spotting the trend, but closer to the event, the NAM is nailing the storms on the East, at least in our part of the world.

WAY too early to predict anything, but worth a mention. What a fun winter so far!

Posted by: curtmccormick | December 23, 2009 2:41 PM | Report abuse

I'm glad you did the review, but I'm surprised at the high grades you give yourselves. 52 hours before one of the biggest snowstorms in local history, and you had a 30% chance of 1" of snow. Remind me why that was a useful forecast, especially after trashing Accuweather for something that turned out to be right. Were they irresponsible but lucky, or were they better than you?

Posted by: rvodra | December 23, 2009 3:17 PM | Report abuse

One of the neat things about this early season snowstorm for me is that it greatly relieves the anxiety of having to sweat out another winter waiting for the chance of a big one and then dealing with the disappointment when it doesn't materialize.

Don't get me wrong, I'm all for another before April, but with this one in the bag, I can much more easily deal with it not happening.

Posted by: SteveT-CapitalWeatherGang | December 23, 2009 3:31 PM | Report abuse


The actual forecast was 30% chance of 1 inch OR MORE. Weather forecasting will always have some level of uncertainty, and must be expressed probabilistically or with some objective measure of confidence.

At that point in the sequence of forecasts, 30% was a very reasonable measure of the possibilities. In this case at this range, the science indicates this was the maximum information content possible - there were still many unpredictable factors which could have made the outcome something other that what happened.The real key is watching the trend in the odds which steadily increased as Fri/Sat approached, and CWG played the odds just right! The 30% might not have been useful to you, but to many, such as emergency managers, it could provide a "heads up",let's say, for contingency planning so not to be caught by surprise later and unprepared.

One thing that is certain - beyond blind luck (as for AccuWX) - it was scientifically impossible early in the game to provide a 100% certainty in either a yes or no snowstorm.

Posted by: SteveT-CapitalWeatherGang | December 23, 2009 3:55 PM | Report abuse

@rovdra, Great point on the initial low forecasts that clearly were FEET away from verifying.

That's the point of this forum though, is that we are all taking responsibility for the forecast and it's evolution. It would be easy to throw up the last snowfall forecast, say we "nailed it" and move along. But the point is to discuss it.

In all honesty, our forecasts were probably a bit too conservative early in the process. There was indicators that a historic storm was in the making, and the overall synoptic pattern supported this. However, with that said, even 54 hours before, there was a ton of disagreement between different forecasts, some of which did only bring us 2 or 3 inches. While it's easy to look back now and understand why the wrong forecasts were wrong, at the time, each options seemed reasonable.

Would it be better for CWG to delay in posting this sort of concrete, real numbers until we have an even stronger feeling of them, or maybe we can add the "forecast condfidence meter" to these predictions as well to better show the low certainty we had, even with the initial low forecast predictions?

If anything, the cautiousness in this post might be getting understated, but then again, the next time someone calls for 10 inches and we get 2, everyone will want more caution...

Posted by: JJones-CapitalWeatherGang | December 23, 2009 4:10 PM | Report abuse

There seems to be some confusion that the Weather Checker is part of the CWG team. Part of the point of Weather Checker is that it is "an outsider's analysis of CWG's forecast accuracy," as stated at the top of the post. I think if you review the past Weather Checker posts (linked to near the top) you'll see Weather Checker has been consistently fair in his assessments (several of them more harsh, and rightfully so, than this one).

Posted by: CapitalWeatherGang | December 23, 2009 4:56 PM | Report abuse

"Would it be better for CWG to delay in posting this sort of concrete, real numbers until we have an even stronger feeling of them,"

Definitely not, don't be afraid to make calls.

Posted by: spgass1 | December 23, 2009 5:05 PM | Report abuse


I think there may be some delays Friday, but you should be able to get out. I wouldn't change my plans if I were you.

Posted by: Jason-CapitalWeatherGang | December 23, 2009 7:14 PM | Report abuse

I do not believe IAD had 27:1 snow ratios.. there was clearly an automated error in their precip for the day. DCA had the same issue... no way they were 35:1+. Radar indicated more precip and BWI had a lot more. It was not a super wet snow but it was not a lake effect snow either. I would like to know if there will be any attempt to correct this date, but maybe there is no way to do so.

My own thoughts on how things played out is that we were probably a little too cautious given the setup. My forecast unaffiliated to this site was bullish 6+ days out and several of us were beating the drum in comment sections in the leadup. But it is DC, and things usually go wrong... maybe not this year though. ;)

Posted by: Ian-CapitalWeatherGang | December 23, 2009 8:19 PM | Report abuse

I don't think the issue here is being too conservative/cautious or not. It's about being objectively realistic without bias (and wishful thinking) given all available information, including personal experience and expertise.

Posted by: SteveT-CapitalWeatherGang | December 23, 2009 9:37 PM | Report abuse

So there is no misunderstanding, my comment above was not directed at any specific individual or group, but a generic statement of philosophy about weather forecasting.

Posted by: SteveT-CapitalWeatherGang | December 23, 2009 9:43 PM | Report abuse


Don't you think there's two types of forecasting? Short range, 24 hours out where you can be psyched that you forecast a snowstorm that all of the models were in agreement on.

And then longer range forecasts, which require interpretive skills and some amount of risk/assumptions to develop a point of view?

Maybe you could dedicate one member of the gang to longer range forecasts that allow us to look at how the weather might evolve out the next 7-10 days?

Otherwise, why wouldn't we just read the feed from the boys in Sterling that comes out every 4 hours?

Posted by: DHinDC | December 23, 2009 10:04 PM | Report abuse

Steve, agree generally though I think there is a danger in grouping every possibility into the same boat. Many events around here that "go bad" do so quickly and it's apparent they will. This one had numerous things going for it whether or not the models "saw" the solution outside 3-4 days. A seasoned model watcher knows their biases and can "predict" with some accuracy how they will correct themselves as an event approaches. I do recall some rather absolute comments that there was very little chance this would come together at the beginning of the week...

To me, a large number of avid CWG readers can understand when there is historical potential it may not pan out -- I guess it comes down to figuring out the best way to present that when it's not a forecast. I can certainly understand a media org not jumping all over ever threat because many do not come to pass.

Posted by: Ian-CapitalWeatherGang | December 23, 2009 10:12 PM | Report abuse

Also, just to clarify myself, I have little issue with the official CWG forecast progression and think it ended up extremely solid in the end. Though my link above was a bit of a "self back pat" there is obviously a difference between making a forecast that 10 people read and one that, potentially, hundreds of thousands will.

Posted by: Ian-CapitalWeatherGang | December 23, 2009 10:17 PM | Report abuse

I appreciate the critique - but more importantly enjoy the blog and learn from your good work.

Posted by: kaygeejay | December 24, 2009 8:55 AM | Report abuse


I agree that the larger-scale setup looked promising from at least Tuesday evening last week. Indeed, the models and ensembles showed remarkable agreement at the 500 mb level (~18,000 ft). The devil of course is in the details. These same looking large-scale patterns are consistent with a fairly wide range of details in storm track and precip. The details that really counted were in the "short wave" trough moving down the backside of the larger scale trough which eventually held the precip max and storm track from passing too far to the east (accounted for the east west elongation of low and precip max that gave us the snow).

On Tuesday and at least until Wednesday evening the phasing of the short wave was far from a sure bet. The highly skilled and experienced forecasters at NCEP and Sterling certainly recognized this. That said, this storm was considerably more predictable than most in terms of seeing a strong signal of the possibility of a Big One relatively early on.

The bottom line from my perspective is that recognizing the broader scale aspects of the circulation patterns does not uniquely determine the details relevant to surface features and precipitation. Indeed, if you go back and look at the mean circulation immediately bounding the President's day storm of 1979, I'll bet you'd never believe that snowstorm could have happened. There are many examples the other way around, namely the large scale looking good, but a big miss for a snowstorm in these parts.

Nice discussion, but this is probably getting too geeky for most readers, so I'll be glad to carry on the discussion via email (

Posted by: SteveT-CapitalWeatherGang | December 24, 2009 11:21 AM | Report abuse


Forecast ranges are nominally characterized as Short Range (1-3 days), and Medium Range (4-10 days). Useful skill, even for the large scale pattern, ON AVERAGE tends toward zero by day 6-7. The trick is knowing ahead of time the exceptions when a given forecast is likely to be reasonably accurate at the longer end of the medium range. We now have a means to make a reasonable attempt at this.

My suggestion would be to default to no forecast beyond 5 days, but highlight those relatively few cases when we believe there is a reliable indication of what's to come.

Posted by: SteveT-CapitalWeatherGang | December 24, 2009 12:08 PM | Report abuse

Thanks for the detailed response Steve, makes sense. I think part of it is that I spend a lot of time at and saw several mets who I believe to be very good getting all excited in the leadup so I probably followed their lead to some degree. I've definitely seen plenty of situations where it looked great for a storm and it did not happen right. It seems so far this season we've had luck with the wavelengths (polar and tropical jet) matching up for at least some phasing in many systems off the coast. Hoping it can continue...

Posted by: Ian-CapitalWeatherGang | December 24, 2009 4:21 PM | Report abuse

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