REVIEW OF WINTER 2010-11March 17, 2011 Severe Weather WorkshopMike 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 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

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 (late at night)

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. 7… struck in morning

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

Feb. 9: During the day

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, KYNWS Photo – Mike York

Feb. 7 - Satellite Precip Estimate:

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