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When Models Go Wrong: The December 20-21, 2010
Cape Cod Snowstorm
Joe DelliCarpini Frank Nocera
NOAA/NWS Taunton, MA Cape Cod Times Photo
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What Happened 10-13 inches of snow
Up to 0.75” liquid
Storm Total Snowfall 24hr Precipitation Ending 12z Dec 21
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3:27 am Forecast Discussion
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10:49 am Forecast Discussion
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3:11 pm Forecast Discussion
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6:50 pm Forecast Discussion
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GFS Analysis: 00z Dec 21
MSLP 850 MB
700 MB 500 MB
Vertically stacked low east of New England
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GFS Ensemble 12 hr QPF: 12z Dec 20
Good agreement in keeping axis of heaviest precipitation offshore Most members show 0.60” to 0.90” on Cape Cod
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Model 24 Hour QPF: Ending 18z Dec 21
NAM GFS
ECMWF SREF
Consensus is 0.25 to 0.50” with higher amounts offshore ECMWF forecast as much as 0.75” on the outer Cape
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SREF Snowfall Probabilities: Ending 03z Dec 21
Highest 12hr probabilities for >1 and >4 inches Greatest totals focused offshore
>1 inch >4 inches
>8 inches >12 inches
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96hr CIPS Snowfall Analogs
December 6, 1981 – Over a foot of snow in southeast Massachusetts
http://www.eas.slu.edu/CIPS/ANALOG/analog.php
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310Ke Isentropic Surface
Strong lift east of New England at 00z Dec 21 Tight gradient along E MA coast implies presence of upper level boundary
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310Ke Isentropic Surface
Strong thermal gradient shows location of mid level warm front (TROWAL)
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700 mb Frontogenesis: 23z Dec 20
Axis of frontogenesis along coast – farther west than models indicated -EPV located offshore (shading)
SPC Mesoanalysis
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Pettersen 2-D Frontogenesis
NAM GFS
ECMWF SREF
Weak surface frontogenesis over eastern MA at 00z Dec 21 ECMWF is focused offshore
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NAM Snow Growth Cross Section
Strongest lift 400-500 mb in “cold” air on Cape Cod Weaker lift in -12C to -18C max snow growth layer Some low level lift present BOS HYA ACK PYM
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Chatham Sounding: 00z Dec 21
Deep and nearly saturated ice-producing layer and snow growth region Well aligned northerly low level flow 17 C delta T between surface and base of inversion
0 C
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NAM Simulated Reflectivity
Highest reflectivity (35-40 dbz) well offshore at 00z Dec 21 Some ocean enhancement is evident on Cape Cod
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Water Vapor Satellite Loop
Courtesy of Sheldon Kusselson, NESDIS
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Taunton, MA Radar Loop
Courtesy of Mike Ekster, NWS Gray, ME
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Summary • Strong mid level lift (TROWAL) combined
with low level instability to produce heavy snow on Cape Cod (modeled too far offshore)
• Cold air aloft helped to overcome “warm” surface temperatures
• Good case where forecasters need to examine satellite and radar trends to adjust model QPF
• Demonstrates importance of nowcast tools to maintain high situational awareness – Radar, satellite, observations, webcams
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When Models Go Wrong: The December 20-21, 2010
Cape Cod Snowstorm
Contact Information: [email protected]
[email protected] Cape Cod Times Photo