Evolving to Polygon Warnings: An Effective False Alarm Solution for the National Weather Service...

Evolving to Polygon Warnings:An Effective False Alarm Solutionfor the National Weather Service

Pete Wolf, SOO

NWS Jacksonville FL

False Alarms• Two Key Types:

– “Unavoidable” FA’s• “FIRE!”, with smoke pouring into

the room.

• Our mission requires warnings, regardless of verification.

– “Unnecessary” FA’s• “FIRE!”, with no smoke or fire

(no actual threat).

• This is what we aim to reduce with polygon warnings.

• False Alarms are a Factor.– Affect credibility, thus response.

Options for Reducing False Alarms• Option #1: Issue fewer warnings (GPRA FAR goal)

– Change warning methodology that leads to fewer warning issuances (e.g. only strong meso or TVS for tornado warnings).

• For 3000/4000 unverified warnings…FAR = 0.75• For 2300/3300 unverified warnings…FAR = 0.70 (GPRA met)

• For 1500/2500 unverified warnings…FAR = 0.60

– Risk of significant unwarned events increases.• Harold Brooks presented POD-FAR relationship. • A “bad idea” from “risk management” perspective.

• Option #2: Issue smaller warnings– Can reduce false alarm area by 50% or more. – Still issue warnings for perceived threats.

• From “risk management” perspective, this is a much better idea.

Polygon Warning DefinedSimply put, a polygon warning (PW) is the the following:

PW = WCW – FAA (“unnecessary”)

where WCW is the whole county warning and FAA is the

area where no perceived imminent threat exists.

- =

In the polygon approach, all we’re trying to do is reduce alerted territory where no imminent threat is perceived.

FAR vs FAA (Not the Same Thing!)# FAs Average FA size Total FAA

1000 800 sq mi each 800,000 sq mi

1500 266 sq mi each 400,000 sq mi

If you increase FA’s from 1000 to 1500, you increase your FAR by 50%. But, at the same time, if you reduce average warning size from 800 sq mi to 270 sq mi each (1/3 original size), you actually decrease FAA by 50%.

In a national county- vs polygon-based warning study, from 2004 to 2005,FAA decreased by about 10%, despite a year-to-year increase in FAR.

Comparing Counties to Polygons

• Field evaluation was completed in 2005.– Involved over 20 WFOs across the country.

• Results confirmed those from local studies at Wichita and Jacksonville.– 70% reduction in TOR FAA with polygon

warnings compared to whole-county warnings.• Larger in WR offices, smaller in ER offices.• Reduction is “unnecessary” false alarm area.

– Some negative impact on POD.• More so than anticipated based on concept.• Training and software issues.

T T

Polygon Warning Issues• Issue #1: Describing polygon aerial coverage for text products/audio broadcasts (NWR).

– Text products will look similar to today.• Likely will shift focus from “pathcasts” to “locations in warning” in text product.

– Any issues due to polygons also exist today with partial county warnings.

A B G H

C D I J

E F K L

#1 #2

What do we provide today?

The National Weather Service in Jacksonville has issued a…

Severe Thunderstorm Warning for the following locations…

Southern Nassau County in Northeast FloridaWestern Duval County in Northeast FloridaNorthern Clay County in Northeast Florida

until 700 pm EST.

At 610 pm EST…Doppler Radar indicated a line of severethunderstorms extending from 6 miles southweest of St. George to near Baldwin to 7 miles south of Lawtey…or extending from 15 miles west of Callahan to 18 miles west of Downtown Jacksonville to 15 miles southwest of Middleburg…moving east at 40 mph. The storms are capable of producing damaging winds up to 60 mph, heavy rainfall and deadly cloud-to-ground lightning.

The storms will be near…Bryceville at 620 pm ESTNassau Village and Jacksonville Heights at 635 pm ESTJacksonville International Airport at 650 pm ESTRiverside at 655 pm EST

This is a dangerous line of storms…with a history of producing damaging winds. Take cover now!

Polygon Warning Issues• Issue #2: How will we verify polygon warnings?

– Shift to polygon-based verification (including GPRA measures) planned by FY08.

• WFOs will be encouraged to shift from counties to polygons in FY07.

– Concern about Polygon-based FAR leading to larger polygons and more FAA (not less FAA). Polygon size needs to be incorporated.

– Report accuracy is critical to for polygon warning verification.• Will increase post-event work.

TORNADO WARNING

Polygon Warning Issues• Issue #3: Software

– Private Sector: game of “chicken” …who goes first?

• Technology companies may wait until NWS shifts to polygons before upgrading their technology

• NWS may be waiting to shift to polygons until private companies upgrade their technology.

• After the NWS takes the lead, the private sector will rapidly follow.

– Who wants to be the only one not providing polygon warnings?

– NWS: • WarnGen • LSR• Changes in support of polygon warnings are

planned in OB8 (Spring/Summer ’07)

Polygon Warning Issues• Issue #4: Forecaster Training…“Polygonology”

– Areas in gap between polygons are not warned.– Overlap not as critical of a problem but should be minimized as

much as possible.– County borders are not important, but CWA borders are! – Confusion from issuing too many warnings.

• Dealing with multiple threats.

What single change could you make today at your WFO to get us started in the right direction?

Avoid Drawing Whole County Polygons

Stop drawing polygons to match county borders!Let the polygons outline the threat areas.

(Let’s review an interesting sr-supercell case…March 12, 2006.)

Warning-By-Polygon Benefits

• Focus warning on the true threat area – Reduce the area alerted that’s not threatened

• Easier handling storms moving along county borders

• Supports weather radio evolution to alert areas smaller than county• Supports efforts to warn on smaller events we may typically miss/ignore.• Supports development of graphical/web based products that show threat area.

Probabilistic Guidance: The Next Step Beyond Polygon Warnings

Case #1: Central Mississippi, April 6, 2003Major severe weather event during the afternoon/early evening hours.

Blue Polygon: Low Probability(warning may be needed if further intensification/advection occurs)

Magenta Polygon: High Probability(deterministic warning polygon)

(Note: focus is just on “damaging weather”, not tornado vs wind vs hail.)

2127z

2127z

2247z2247z

2332z2332z

2337z2337z

0117z

P A U S E

Probabilistic Guidance: The Next Step Beyond Polygon Warnings

Case #2: Northern Florida, June 16, 2005

Strong (isolated severe) pulse t-storms during the afternoon.

Green Polygon: Low Probability(warning may be needed if further intensification/advection occurs)

Magenta Polygon: High Probability(deterministic warning polygon)

1845z

SVR issued for Suwannee Co

1855z


1905z


2025z

SVR issued for St. Johns County

2035z


2045z


2055z


2105z


2115z


2125z

SVR issued for St. Johns County P A U S E

Latest Polygon Warning Implementation Information

• Polygon Evaluation Team Polygon Implementation Team– Approved by Ops. Committee of Corporate Board

• Forecaster Training Planned through CY07– Intro concept module/teletraining– WES-based scenario training

• Needed software improvements Planned starting in OB8 (CY07)

• Operational implementation Planned for FY08

• Implementation of polygon Planned for FY08 verification in GPRA

Final Thought

“One doesn’t discover new lands without

consenting to lose sight of the shore for

a very long time.” --- Andre Gide

Any thoughts/questions?

Pete Wolf, SOO NWS Jacksonville FL

Evolving to Polygon Warnings: An Effective False Alarm Solution for the National Weather Service...

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