Gridded Warnings The next generation of NWS warning product?

Gridded WarningsThe next generation of NWS warning product? Contributions: David Andra Harold Brooks Don Burgess Charles Kerr Jim LaDue Mike Magsig Kiel Ortega Kevin Scharfenberg Travis Smith Greg Stumpf Lou Wicker HWT/EWP Participants: Brad Grant Les Lemon Dan Miller Steve Nelson Patrick Burke Liz Quoetone Dave Sharp Steve Zubrick

Introduction • This idea and project are nascent. • This work must consider many intersecting disciplines: • Meteorology • Technology • Sociology • Human Factors

Recap from 1st Wkshp • Warnings (forecasts) are for areas • Reports (events) are points • Probability of Detection (POD): • points within polygons / total points • False Alarm Ratio (FAR): • polygons without points / total polygons • Measures of skill (e.g., CSI) cannot be calculated • These definitions are inconsistent • No measure of correct null forecasts • Warning area sizes are inconsistent • Verification really affects areas, not points

Storm versus County • NWS implementing “Storm-based Warnings” for FY08. • Characterizes the area to be threatened with (supposedly) no regard to geopolitical boundaries • But there are still issues

Area-Based WarningShortcomings • Storm-based warnings are still area forecasts verified by point events. • The polygons are just “differently shaped” • The threat information for the polygon remains “monotonic” • Each location inside polygon is under exact same threat for the exact same time period • Each location inside (outside) polygon is given 100% (0%) certainty of event

Vision • Warnings and Reports are GRIDDED • Consistency between forecasts and events • Consistency with other forecast products • Allows for growth (added detail) • Can be aggregated to simpler formats

OB8.1 WarnGen Limitations • Allow toggling individual counties off/on • Automatically clip small portions of counties within an area threshold setting • Not provide for situations when threats straddle adjacent CWAs • Not match the edges of adjacent warnings where there was manual or automated county clipping • Not provide a easily-identifiable preview of the actual polygon until the after the warning was transmitted

Polygon Slivers This storm produced a damaging tornado that went nearly down the unwarned “sliver” that the arrow points to!

Was this really intended?

Downstream Warnings • Very little overlap between adjacent warnings • Many times, a new polygon is issued only when storm is just about to exit the previous polygon. • This gives inconsistent lead time to all points within each polygon, with some points getting much less and sometimes zero lead time.

No Warning for User B A B t=0

< 5 min lead time for User B A B t=50

< 5 min lead time for User B A B t=55

Area-Based WarningShortcomings • WarnGen has a motion estimator (“Drag Me To Storm”): • But, once any vertex in the swath polygon is changed, the storm motion and swath become decoupled • e.g., if swath is lengthened or shortened, it no longer matches the storm speed and warning time. • Forecaster has no control over initial swath area provided by WarnGen (point and line motion only) • Forecaster has no control over motion uncertainty values (speed and direction) • Therefore, the forecaster can only guess the result of these variables when adjusting the swath polygon

Hypothesis • Instead of the forecaster guessing at the swath… • …we propose that much more robust warning information can be derived if the forecaster instead determines • the initial threat area at time=0, along with • the motion vector, and adds • motion uncertainty information

Drivers for growth • This information used to integrate a more accurate swath • Continuously advecting threat areas • Meaningful guidance on time of arrival and time of departure • Allows for the addition of forecaster uncertainty information (e.g., probabilities)

Deriving Swath User defines: Initial Threat Area at time=0 time = 0

Method User: Drag Me To Storm Use cursor to reposition threat area time = -20

Method Motion Vector Calculated time = 0, 15, 30, 45, 60

Method Threat Area Advected

Method User Defines: Motion Uncertainty ± 5 m s-1 ±15°

Method Speed uncertainty stretches threat along motion vector ± 5 m s-1 ±15°

Method Direction uncertainty stretches threat perpendicular to motion vector ± 5 m s-1 ±15°

Method Integrated threat area across warning time provides swath

Swath properties • Greater (lesser) motion uncertainty leads to wider (more narrow) swath • Larger (smaller) initial threat area leads to wider (more narrow) swath • Neither of these factors can be controlled in WarnGen. • Swaths are truly independent of geopolitical boundaries.

Advecting threat • As threat area advects downstream, the area affects different grid points at different times. • For each grid point in the warning, the intersection of the threat area over time provides information about • time of arrival • time of departure

Advecting Threat Area x A B t=0 TOA TOD NOTE: No warning (yet) For User B 1 Warn? 0 A B Time 

Advecting Threat Area x A B t=5 TOA TOD TOA TOD NOTE: > 40 min lead time For User B 1 Warn? 0 A B Time 

Advecting Threat Area x A B t=10 TOA TOD TOA TOD 1 Warn? 0 A B Time 

Advecting Threat Area x A B t=35 TOA TOD TOA TOD NOTE: Point A already In “all clear” 1 Warn? 0 A B Time 

Threat Type • Each severe weather type can be depicted on separate grids • Hail • Wind • Tornado • Lightning • Threat subtypes could also be depicted • Hail size • Wind speed • Lightning Density

Gridded Warnings The next generation of NWS warning product?

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