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Large, Sparsely VerifiedSevere Thunderstorm Warnings:
Can we do better…and do we want to?
Eric Lenning and Ben Deubelbeiss NWS Chicago2014 GLOMW
MOTIVATION
Goal: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 FOCUS
Consider “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
QUESTIONS
Can 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
War
nin
g V
erif
ied
Nu
mb
er o
f L
arge
War
nin
gs
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 Characteristics
Fast 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
RESULT
4-5 August 2008:
Warning 163 from 0018z-0130z
68% areal verification
Widespread damage
29 June 2012:
Warning 54 from 1544z-1645z
8% 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
CONCLUSIONS
Use 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 cells
Strength of 3km shear relative to cold pool