Snoverkill forecast: Good overview, details messy
Weather Checker is an outsider's analysis of CWG's forecast accuracy. See previous Weather Checker posts.
By Jamie Yesnowitz
There had to be significant fatigue setting in on the CWG in the wake of Snowmaggedon, when the opportunity to forecast Snoverkill presented itself. Though CWG correctly communicated the big-picture risks of the second crippling storm in less than a week, it was a hair late in sounding the alarm about Snoverkill, and some of its more detailed predictions did not materialize.
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The initial predictions from Brian Jackson's SLCB on Sunday morning, just after the end of Snowmaggedon, had the timing of the onset of the storm on track (Tuesday afternoon or night). The "iffy" accumulation estimates offered in this forecast (25% chance of less than 1 inch, and 85% chance of 8 inches or less) clearly did not verify. Neither did Dan Stillman's forecast Sunday night, which tweaked the accumulation numbers a little bit (15% chance of less than 2 inches and 75% chance of 8 inches or less) -- although it did highlight the potential for "significant accumulation."
Early Monday morning, Jason Samenow's forecast pinpointed a possibility that points north and east of Washington would receive the most snow, but such snow was only characterized as moderate. There was no appreciable change to accumulation amounts (15% chance of less than 1 inch and 80% chance of 8 inches or less). Jason did include a most likely amount of 3-6 inches for the immediate metropolitan area, with more amounts north and east, and less amounts south and west.
By late Monday morning (about 30 hours before the storm started), CWG's forecast started moving in the right direction. Jason indicated a "best bet" of 5-10 inches in the metro area, and 8-14 inches north and east to Baltimore, with 3-6 inches for points south and southwest of Fairfax County. CWG also correctly identified the potential for "serious snow/wind" and for the first time highlighted the potential for blizzard conditions.
The 3:30 pm team forecast came in with the first map with medium confidence. The map tweaked the late-morning forecast slightly, and included a swath of 8-16 inches covering the northern tier of suburbs including practically all of Montgomery County, a little piece of Fairfax County, skirting D.C. proper, and points north and east of D.C.; 5-10 inches for other parts of the metro area; and less than 6 inches with mixed precipitation to the south and extreme east of the city.
CWG updated this map later in the evening. The updated map extended the 8-16 inch zone to the upper half of Washington, most of Arlington, and other points in nearby Northern Virginia. Importantly, CWG did not modify its accumulation after that point, unlike the National Weather Service and many media outlets whose forecasts wavered significantly.
Early Tuesday morning, CWG released its storm timeline, predicting temperatures of 26-31 degrees throughout the snowstorm, an initial period of snow Tuesday afternoon and evening (this verified), a snow /sleet line that would impact Washington and points south and east for a time on Tuesday night, and heavy snow/wind late Tuesday night into Wednesday, that would taper to flurries on Wednesday afternoon.
The temperature forecast did not verify during the second half of the storm (though it did for the first half). In the heart of the blizzard conditions that overtook the area on Wednesday, temperatures actually dropped to 20 degrees at Reagan National and Dulles airports at 1 p.m., and 21 at Baltimore-Washington International at 4 p.m. Temperatures actually moderated back toward 30 at night after the storm ended.
As for mixed precipitation impacting the District and points south and east Tuesday night, that forecast came true, although the mixed precipitation clearly moved into south-central Montgomery County (further north than forecast), hearing the classic pings from my vantage point for several hours that evening. CWG correctly predicted the precipitation would let up for a time late Tuesday night and that the snow and wind would pick up in earnest early Wednesday morning.
However, the endgame of Snoverkill was missed. Several bands of snow, enhanced by blowing snow that had previously fallen, along with gale-force gusts, raked the Washington area beyond the late-afternoon time frame forecast. The last flakes did not depart the region until 10 p.m. Wednesday night.
In looking at the reported accumulation totals from the area, while the accumulation predictions verified in many locations, particularly on the updated map (issued Monday night), there were some exceptions. BWI, contained in the 8-16 inch zone, actually received over 19 inches of snow, with portions of Baltimore, Carroll and Frederick counties receiving as much as two feet. IAD verified on both maps issued by CWG with a snowfall of over 9 inches, as did most areas south and west of the District. However, accumulations of more than 6 inches were recorded in portions of Calvert, Charles and St. Mary's counties, areas in which CWG forecasted more mixing, and less snow.
Beginning more than a day before the storm, CWG clearly captured many of the big-picture aspects of this complex storm correctly, including the overall evolution. However, they missed certain details and kind of let the storm sneak up on them, probably because they were still recovering from the last one.
It's been a tough winter season to say the least, and though the challenge of forecasting a major winter event must always be of interest to the CWG meteorologists, I can't imagine them, or many others, really wanting to see any more of the white stuff until December.
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.
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