Item 12-20: 2013 Project Update on Expressions of Uncertainty Initiative

Highlights New Probability thresholds introduced in As Examples from Isaac and Sandy will show these thresholds are too high. New Probability thresholds introduced in As Examples from Isaac and Sandy will show these thresholds are too high. PeriodUp To PWS64PWS34PWS64PWS34PWS64PWS34PWS64PWS hr 15%50%30%55% 40%50%25%45% hr 12%40% 25%45%32.5%45%20%35% hr 10%35% 20%40%25%40%15%30% hr 9%30%15%35%20%35%12.5%25% hr 8%25%10%30% 15%30%10%22.5% hr 7%20%7%25% 12.5%25%8%20% hr 6%17.5%6%20% 10%22.5%7%17.5% hr 5%15%5%15% 8%20%6%15% hr 4%12.5% 4%12.5%6%17.5%5%12.5% hr 3%10% 3%10%5%15%4%10%

3 (Isaac Example) Thresholds Too High

4 (Isaac Example – Winds/Probabilities Progression) Thresholds Too High

5 (Sandy Example) Role of Watch/Warning

6 (Sandy Example – Winds/Probabilities Progression) Let WW Modulate Output Thru Forecast

7 (During Sandy Storms) Probabilities Low Bias Tracks from which Probabilities are derived use radii-Clipper Model. Tracks from which Probabilities are derived use radii-Clipper Model. At t=0 NHC radii which is then relaxed towards climatology by 36 hours or so. At t=0 NHC radii which is then relaxed towards climatology by 36 hours or so. Statistically, this reproduces observed distributions over a large sample pretty well. Statistically, this reproduces observed distributions over a large sample pretty well. But during Sandy, observed wind radii much bigger than average (more than a factor of 2 from 24 to 72 hours compared to Clipper values). But during Sandy, observed wind radii much bigger than average (more than a factor of 2 from 24 to 72 hours compared to Clipper values). Result: Low bias in the probabilities. Result: Low bias in the probabilities. Case shown based on _0000 forecast. Case shown based on _0000 forecast. Right Image based on bias corrected radii. Right Image based on bias corrected radii. (From Mark DeMaria)

8 Additional Reasons Why Thresholds too High Error Distributions updated every year which reduces GPCE as forecast tracks keep improving. Error Distributions updated every year which reduces GPCE as forecast tracks keep improving. – GPCE correction accounts also for operational model spread. – Cases with small spread lead to tracks upon which probabilities are based more concentrated around forecast track. Result? Probabilities become higher along forecast track and lower away from the track. Result? Probabilities become higher along forecast track and lower away from the track. In the perfect forecast, probabilities would be 100% where TS/HR winds occur and zero elsewhere. In the perfect forecast, probabilities would be 100% where TS/HR winds occur and zero elsewhere. Since the thresholds we use are on the low end (less that 50%) the area covered by those is shrinking with time as forecasts keep improving. Since the thresholds we use are on the low end (less that 50%) the area covered by those is shrinking with time as forecasts keep improving. Thresholds will therefore need to be revised. One thing is certain at the present time they appear too high and therefore we are proposing to lower them for official implementation. Thresholds will therefore need to be revised. One thing is certain at the present time they appear too high and therefore we are proposing to lower them for official implementation.

In Summary Recommendation: Propose go Official for In preparation for that, reconvene Wind Tream and make FINAL decisions on: Recommendation: Propose go Official for In preparation for that, reconvene Wind Tream and make FINAL decisions on: Lowering thresholds and modifying algorithm to account for Watch/Warning throughout the forecast. Also factor in Gusts in the decision tree. Lowering thresholds and modifying algorithm to account for Watch/Warning throughout the forecast. Also factor in Gusts in the decision tree. NOTE: ER formatter team implemented GUI option to change analyzed probability to be compare against threshold. Default: Max Probability; Options based on ModeratedMax(#) designed to change sensitivity of the formatters to the probabilities. Guidance provided to implement this include: NOTE: ER formatter team implemented GUI option to change analyzed probability to be compare against threshold. Default: Max Probability; Options based on ModeratedMax(#) designed to change sensitivity of the formatters to the probabilities. Guidance provided to implement this include: – Forecast from successive adv trends towards west of 75W while south of 35N with movement that is WNW to North - stick to Max. – If track does not meet condition but forecast trends keep it W of 70W with a NW motion passed 35N stick to Max. – Otherwise use alternate ModeratedMax(#) probabilities.

In Summary ER brought up issues with the icons in point and click being entirely taken over by the tropical icon when EoU is generated. Dave Sharp proposed to consider changing the location of the EoU in the text depending whether conditions are possible or expected. ER brought up issues with the icons in point and click being entirely taken over by the tropical icon when EoU is generated. Dave Sharp proposed to consider changing the location of the EoU in the text depending whether conditions are possible or expected. Important issue lingering from 2010 season: in the short range if EoU is triggered because of probability threshold exceeded but wind speed is less than 15 to 20 knots should we create exception to drop the deterministic phrase in those cases? Counter point: customer still needs/wants that info. In Favor: such low wind speed values might negate the precautionary response that is sought after. Important issue lingering from 2010 season: in the short range if EoU is triggered because of probability threshold exceeded but wind speed is less than 15 to 20 knots should we create exception to drop the deterministic phrase in those cases? Counter point: customer still needs/wants that info. In Favor: such low wind speed values might negate the precautionary response that is sought after. If Official things to formalize: Tropical SAF issues - Are offices taking the time to set this up? If Official things to formalize: Tropical SAF issues - Are offices taking the time to set this up? If Official things to formalize: Back Up Capabilities - Training Back up sites. If Official things to formalize: Back Up Capabilities - Training Back up sites. If Official things to formalize: Public and Marine directives will need to be updated. If Official things to formalize: Public and Marine directives will need to be updated.