review of winter 2010-11
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Transcript of review of winter 2010-11
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REVIEW OF WINTER 2010-11March 17, 2011 Severe Weather WorkshopMike York (Forecaster / Winter Weather Program Leader)
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How did we do? Preliminary Verification Statistics:
Very good, but is that the whole story?•Issued 70 county Winter Storm Warnings•False Alarm Ratio: 26 percent•Probability of Detection: 80 percent•Average Lead Time: 5.1 hours
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Stats compared to average of past several seasons:
Average number of warnings: 70 vs. 177 Average false alarm ratio: .26 vs. .33 Average prob. of detection: .80 vs. .88 Average lead time: 5.1 hrs vs. 21 hrs
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What is lead time?
The time between warning issuance time and the time 4” is on the ground
Lead times are not computed for watches.
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Why the short lead times?
Snow amounts were under forecast until the storm was in progress.
Why?
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After the 4th under forecast snow event, the boss was not happy.
Science team tasked with investigating why
Preliminary results still not complete
What we do know… will follow shortly
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Four events under review:
Dec. 24 (Christmas Eve – Paducah area)
Jan. 25 (Pennyrile region) Feb. 7 (Western Kentucky) Feb. 9 (Tennessee border)
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Dec. 24… Heavy snowfall rates for a few hours after dark Around 1” per hour Total was around 4” in Paducah and nearby areas
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Dec. 24 Preliminary Findings “Split flow” pattern: Moist southern
branch of the jet played a greater role than expected
Band of moisture/heavy snow streamed northeast faster than expected
Warm pavement temps were a non-factor due to heavy snowfall rates
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Jan. 25-26 (late at night)
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Jan. 25-26 Prelim. Findings 48-72 hours in advance: All models
showed system bypassing region to the south
Models then trended slowly north
Within 12 hours, NAM and RUC caught onto a deformation zone but missed the location
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Feb. 7… struck in morning
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Feb. 7 preliminary findings Deformation zone played a key role
in heavy snow
Models began picking up on this zone about 12 hours prior
Warm pavement temps again a non-factor
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Feb. 9: During the day
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Feb. 9 preliminary findings 30-48 hours prior, forecasters
suspected models were too weak based on 2/7 system
Liquid to snow ratios were a concern (dry and powdery vs. wet and heavy)
Banding was not anticipated
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Common thread: Mesoscale bands of heavy snowfall Bands from 4 to 40 miles wide Sometimes accompanied by thunder
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Mesoscale Bands:
Difficult to forecast because of their size
Computer models cannot explicitly forecast these bands
Conditions favorable for banded snowfall can be forecast
BUT not precisely!
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Forecaster options:
At longer time ranges, use the caveat “locally higher amounts possible”
At shorter time ranges, satellite imagery is an excellent tool for first identification
Feb. 5, 2004 Near Paducah, KYNWS Photo – Mike York
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Feb. 7 - Satellite Precip Estimate:
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Common threads of these events: Computer models under forecast
precipitation amounts Unforecast “deformation zones”
caused intense snowfall rates Warm pavement temperatures
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What next?:
Science team is looking at snow to liquid ratios (dry snow vs. heavy wet snow)
Science team is looking at what role banding played and how to anticipate it
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Summary:
We are still researching “what went wrong”
More than one factor played a role Computer model limitations were
one factor Forecaster ability to troubleshoot the
models may be a factor Forecasting snow to liquid ratios may
be a factor