Large, Sparsely VerifiedSevere Thunderstorm Warnings:

Can we do better…and do we want to?

Eric Lenning and Ben Deubelbeiss NWS Chicago2014 GLOMW

MOTIVATIONGoal:An increased understanding of the types of severe weather systems (lines/clusters) that prompt large warnings.Concern is large polygons with few if any reports:

• Training?• Verification?• Population?• Radar sampling?• Workload?• Fear?

Conclusion:Environment + Radar Signatures Better Decisions

NARROW THE FOCUSConsider “LARGE” SVR polygons (> 7000 sq km).

Consider “SPARSELY VERIFIED” polygons (< 20% coverage).

Over 72% of large polygons from GL offices (139 of 191) were sparsely verified from 2008-2013.

LSR 15KM BUFFERPolygon Area: 2088 sq km

1 LSR and 43% Areal Verification

Polygon Area: 12538 sq km

5 LSRs and 21% Areal Verification

QUESTIONSCan we better distinguish between high-end and low-end lines or clusters of storms?

** Use both environment and radar signatures **For lower-end lines/clusters, can we focus the warning area?Are we willing to take a few hits to avoid over-warning?Is there an SPS or IBW approach?Is over-warning even a problem???Are we missing something else?

LARGE (>7000 SQ KM) POLYGONS: 08-13

2008-2013

J2 I1 L2 L1 J3 L3 I2 J10

4

8

12

16

20

24

28

32

36

40

44

48

0

1

2

3

4

5

6

7

8

9

10

11

12

LOT Large (> 7000 sq km) Warnings & Verification Percentage by Forecaster

AVERAGE AREA VERIFIED NUMBER OF LARGE WARNINGS

Ave

rage

Por

tion

of W

arni

ng V

erifi

ed

Num

ber

of L

arge

War

ning

s

2008-2013

WHAT DO WE KNOW ABOUT DAMAGING LINES/CLUSTERS?Wind is main concern with large polygons?Favorable Environments

Marginal to large (sfc-based?) instability.Marginal to large shear (0-6km or 0-3km?).Cold pool potential, LFC height, wind/shear orientation…

Radar CharacteristicsFast motion, tight reflectivity gradients, rear-inflow

jets, rotational couplets, MARC signatures, cell intersections, leading/trailing/parallel stratiform, bookend vortices, reflectivity tags….

FAMILIARRESEARCHCorfidi, 2003

MORE RECENT RESEARCHSchaumann and Przybylinski, 2012

2008 AUG 4VS2012 JUN 29

20080804 23 UTC 20120629 14 UTC

3km VGP and 0-3km Shear Vector

RESULT4-5 August 2008:Warning 163 from 0018z-0130z68% areal verificationWidespread damage

29 June 2012:Warning 54 from 1544z-1645z8% areal verification (Still a HIT!)~60 mph gust, power lines down

VGP and 0-3km Shear Vector06/12/2013 23Z

VGP and 0-3km Shear Vector07/03/2012 01Z

CONCLUSIONSUse large polygons based on morphology, not geography.Large polygons best suited for derecho / bowecho types.A large but ordinary line or cluster may warrant warnings, but only for smaller portions.Environmental factors:

Orientation of 3km Shear Vector to lines of cellsStrength of 3km shear relative to cold pool