Presentation on theme: "March 17, 2011 Severe Weather Workshop Mike York (Forecaster / Winter Weather Program Leader)"— Presentation transcript:
March 17, 2011 Severe Weather Workshop Mike York (Forecaster / Winter Weather Program Leader)
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
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
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.
Why the short lead times? Snow amounts were under forecast until the storm was in progress. Why?
After the 4 th 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
Four events under review: Dec. 24 (Christmas Eve – Paducah area) Jan. 25 (Pennyrile region) Feb. 7 (Western Kentucky) Feb. 9 (Tennessee border)
Dec. 24… Heavy snowfall rates for a few hours after dark Around 1” per hour Total was around 4” in Paducah and nearby areas
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
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
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
Common thread: Mesoscale bands of heavy snowfall Bands from 4 to 40 miles wide Sometimes accompanied by thunder
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!
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, KY NWS Photo – Mike York
Common threads of these events: Computer models under forecast precipitation amounts Unforecast “deformation zones” caused intense snowfall rates Warm pavement temperatures
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
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