Transcript Watching your rear

Forecasting Storm
Duration
Neil I. Fox
David Jankowski, Elizabeth Hatter and Liz Heiberg
Dept. Soil, Environmental and Atmospheric Science
University of Missouri - Columbia
Considering rear edge propagation
velocity in flash flood forecasting
Why worry about your rear
 How your rear moves compared to
your middle
 Using knowledge of your rear to
forecast rainfall totals
 Stop the rear jokes

Why worry about your rear?
Current nowcasting tools (e.g. SCIT
tracks) concentrate on arrival time
 Excellent for Severe Weather
warning
 Flash flood forecasting: Interested in
total duration of precipitation
 Event management / Emergency
services like to know end time

This study looked at

The use of three measures of storm
velocity as indicators of flash flood
potential
• 1/vc
• 1/vr
• (vc-vr)/vcvr

The last of these is defined as the
‘Storm Duration Factor’
Storm duration factor (SDF)
(vc  vr ) 1 1
 
vc vr
vr v c
Duration (D) over a
point at distance x :
D  SDF .x
Rainfall accumulation
(Ra) at x assuming
steady-state rainfall
rate R:
Ra  SDF.x.R
Data
Initially data was taken from a
number of cases where (flash)
flooding occurred
 A range of storm types, locations and
situations
 Not all storm cells observed caused
flooding
 Then more data for more cases

Analysis
Centroid velocities found using the
NSSL algorithms (SCIT)
 Rear edge velocities found by
locating position from tracing
centroid vector backward until Z falls
below threshold

Analysis

The three measures were plotted
against rainfall accumulations
• for the subsequent 60 minutes
• 0 km and 25 km ahead of storm center
location
• Greater distances saw very little rain
(storms don’t move that fast or
dissipate within the distance)
Comparison of vc & vr
1/vc & precip accumulation
1/vr & precip accumulation
SDF & precip accumulation
Results
All correlation coefficients are
horrible
 If you squint you can kind of see
what you want to see
 More work required!!

Next

This could be because
• We don’t consider
development/dissipation
• We don’t consider size of storm
• We don’t look at sensible distances or
have good rainfall data
Accounting for storm size
Tried a “pure” measure
(vc  vr ) 1 1
SDF 
 
vc vr
vr v c
Unsuccessful – so try measure based
on velocity and storm size
( x  x) x
D

vr
vc
Test

Rainfall accumulation versus duration
based on
• centroid velocity (x+Δx/vc)
• rear edge velocity (x+Δx/vr)
• Both (reduces to the others for x = 0)
( x  x) x
D

vr
vc
Rainfall total vs vc/Δx
Rainfall total vs vr/Δx
Problems

Radar rainfall accumulations
• use gauge

Rear edge velocity determination
• automate – make robust

Mixture of storm types
• stratify
Thanks

Parts of this work have been funded
by the COMET Partners Program and
The University of Missouri Research
Council