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March 9, 2010

December snow, and season, lose 3 inches

The winter of 2009-2010 will still go down in the history books as the snowiest on record for Baltimore. But in the end it will be three inches less stupendous than we thought.

Meteorologists at the National Weather Service have just finished adjusting their snow tallies to account for measurement problems at BWI-Marshall Airport. Officially, at least - the winter delivered 77 inches, not 80.2 inches as the weather service first reported.

And the big storm in December will be recorded officially as an 18-inch snowfall, not 21.1 inches as the first reports stated. December's monthly total has been similarly reduced from 23.2 inches to 20.1 inches, according to the Baltimore-Washington Forecast Office in Sterling.

The changes don't affect any of the records broken in December. The Dec. 18-19Snow in Baltimore snowstorm remains the biggest December snow on record for Baltimore, and the month remains Baltimore's snowiest December.

And even at a mere 77 inches, it's still the snowiest winter on record for the city. The annual average for Baltimore is 18.2 inches.

The reduction in some winter snow totals was made late last week as Sterling reviewed each of the season's snowfalls to adjust for measurements that were not in compliance with the weather service's protocol.

Until the problem was discovered in the wake of the Feb. 5-6 blizzard, contract observers working for the Federal Aviation Administration were making only hourly snow measurements, and taking storm totals after the flakes stopped falling - called "snow depth" measurements.

The technique, which complies with FAA rules, is considered invalid by the NWS for climatological data, because it does not allow the snow to compact.

The weather service requires that snow measurements for climatological purposes be made one every six hours. Because of compaction, the totals are usually smaller. That's what the FAA Snow in Baltimorecontractor was supposed to have been supplying to the weather service.

So, with no six-hour data, Steve Zubrick, Sterling's science and operations officer, elected to use the FAA's snow depth data instead of the hourly measurements, because it is the most conservative solution.

He and forecaster Jared Klein combed through the data and made the changes. Some snowfalls were unaffected. Most turned out smaller. A few increased due to rounding of snow depth numbers to the nearest inch.

Here are the original and revised numbers for BWI:

December: Original:  23.2 inches  Revised:  20.1 inches

January:  Original: 7.5 inches  Revised:  6.9 inches

February:  Original: 49.5 inches  Revised:  50 inches

Season:  Original: 80.2 inches  Revised: 77 inches 

Major storm totals:

Dec. 18-19:  Original:  21.1 inches  Revised:  18.0 inches

Feb. 5-6:  Original:  24.8* inches  Revised:  25 inches  

Feb. 9-10:  19.5 inches (no change; measured properly) 

* This was the original snow depth measurement. The FAA's total from one-hour measurements was 28.8 inches. 

(SUN PHOTOS: Top: Algerina Perna/Bottom: Karl Merton Ferron)

Posted by Frank Roylance at 4:20 PM | | Comments (9)
Categories: By the numbers


Hi Frank,

At one point, using the old date, Baltimore held first place as the snowiest city in the U.S. Where do we stand now, near the end of the season and after the data revision?

FR: Looks like Baltimore ended the season in 5th place, behind Syracuse, Erie, Rochester and Philadelphia.

Awww, now are we going to have to root for another three inches to get our 80" total back? ;)

FR: Will you take it in rain inches? I think we could oblige.

Thanks for the update. My back feels better already!

Hi Frank- Does this mean that all previous winter storm totals were correctly handled by the measuring system to NWS standards and don't need to be adjusted?

FR: Not at all. These errors could go back to 1998, when the FAA contractors took over from the NWS at BWI. They have already said they measured the same way during the February 2003 blizzard. This story is not finished yet.

This is a joke. Many arean near BWI had 24 inches with the Dec storm. Many areas near BWI had 30 or more with the first February blizzard. These numbers are grossly understated in the new reportings, and seemed low to begin with.

Shouldn't they find some nearby reports from spotters and average them out and figure that's the total?

WHAT?! In the words of David Puddy: "No way, man! This is BOGUS!"

Is there any way to find out the season snow totals for other areas around the region such as Randallstown, Ellicott City, and other places which receive more snow than the airport which is south of the city?

FR: The NWS does produce snow maps for these storms. I'll try to find the links.

How in the world do you throw out hourly measurements because they were not taken in 6 hour periods? That makes absolutely no sense! To me it sounds like the FAA contractors were taking more accurate measurements than what the NWS wanted. This sort of behavior and inconsistency is the same thing that reduces the credibility of weather and climate forecasters. I get the impression they are deliberately reducing snowfall totals that already seemed lower than the area average.

FR: They did not throw them out. The FAA contractor measures hourly for aviation purposes, so that pilots know when the snow rates are increasing. They measure, wipe off the board and measure again an hour later, repeating until the snow stops. The NWS protocol calls for them to let the snow pile up for 6 hours, measure, then wipe clear and start again. It allows for some compaction and a more realistic total for climatological purposes. Since the six-hour totals were not taken, they have to use the more conservative "snow depth" numbers the FAA reported, which are the measurements of the final, total, snow pack - which adds even more compaction due to weight and melting, and is therefore smaller than either the 6-hour ot 1-hour numbers. It's a science thing.

Wow, negative snow... what a concept!

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About Frank Roylance
This site is the Maryland Weather archive. The current Maryland Weather blog can be found here.
Frank Roylance is a reporter for The Baltimore Sun. He came to Baltimore from New Bedford, Mass. in 1980 to join the old Evening Sun. He moved to the morning Sun when the papers merged in 1992, and has spent most of his time since covering science, including astronomy and the weather. One of The Baltimore Sun's first online Web logs, the Weather Blog debuted in October 2004. In June 2006 Frank also began writing comments on local weather and stargazing for The Baltimore Sun's print Weather Page. Frank also answers readers’ weather queries for the newspaper and the blog. Frank Roylance retired in October 2011. Maryland Weather is now being updated by members of The Baltimore Sun staff

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