Roll or Arcus Cloud
Supercell Thunderstorms
Storm split 1
Storm Split 2
Storm split 3
Squall Lines
Bow Echoes
and Derechos
DC Derecho: June 10, 2013
Often Associated with Strong
Straight Line Winds Known as
“Derechos”
• These straight-line winds may exceed 100
miles per hour, reaching 130 miles per hour
in past
eventshttp://www.youtube.com/watch?v=E
GJmOeDEBtw
• Great Derecho Website:
http://www.spc.noaa.gov/misc/AbtDerechos/d
erechofacts.htm
Climatology (Events over 19802001
Major Derecho on June 2012
June 2012 Derecho
• Wind gusts increased substantially, peaking
as high as 91 mph (147 km/h) in Fort
Wayne, Indiana
• Extremely hot and highly unstable
atmosphere with CAPE values in excess of
5,000 J/kg. Temperatures on the south side
of a stationary front were in excess of 100F.
Derecho Prediction
• Warm season derechos in the Northern
Hemisphere form in west to northwesterly
flow at mid levels with moderate to high
levels of instability (CAPE).
• Derechos form within environments of lowlevel warm air advection and significant
low-level moisture
Numerical Simulation of
Convection
• High resolution simulates cable of explicitly
resolving convection have been run in
research mode.
• It appears that such numerical model can
provide great insights into the conditions
necessary for convection and how varying
environments influence convective
evolution.
METED Convective Storm
Matrix
• http://www.meted.ucar.edu/convectn/csmatr
ix/
• Allows you to experiment with instability
and shear and view how the storms evolve.
High Resolution Numerical
Prediction of Convection
Explicit Convective Prediction
• Requires high resolution (4km or less grid
spacing)
• Requires high-resolution analysis of current
situation, using radar, surface observations
and all other assets.
• NCAR (WRF model) and CAPS
(Oklahoma, ARPS model) are two leading
efforts.
Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX)
Using the WRF Model
Goal: Study the lifecycles of mesoscale convective vortices and
bow echoes in and around the St. Louis MO area
10 km WRF forecast domain
4 km WRF forecast domain
Field program conducted 20 May – 6 July 2003
Real-time WRF 4 km BAMEX Forecast
Initialized 00 UTC 9 June 03
Reflectivity forecast
Composite NEXRAD Radar
Real-time WRF 4 km BAMEX Forecast
Valid 6/10/03 12Z
4 km BAMEX forecast
36 h Reflectivity
4 km BAMEX forecast
12 h Reflectivity
Composite
NEXRAD Radar
Real-time WRF 4 km BAMEX Forecast
Initialized 00 UTC 10 June 03
Reflectivity forecast
Composite NEXRAD Radar
Real-time 12 h WRF Reflectivity Forecast
Valid 6/10/03 12Z
4 km BAMEX
forecast
10 km BAMEX
forecast
22 km CONUS
forecast
Composite
NEXRAD Radar
Real-time WRF 4 km BAMEX Forecast
Initialized 00 UTC 30 May 03
Reflectivity forecast
Composite NEXRAD Radar
Real-time WRF 4 km BAMEX Forecast
Valid 5/30/03 23Z
23 h Reflectivity Forecast
Composite NEXRAD Radar
Line of
Supercells
Realtime WRF 4 km BAMEX Forecast
Valid 6/23/03 06Z
30 h Reflectivity Forecast
Composite NEXRAD Radar
6” hail 00Z
Squall line
Realtime WRF 4 km BAMEX Forecast
Initialized 5/24/03 00Z
Reflectivity Forecast
Composite NEXRAD Radar
12 h
Squall line
24 h
Persists
Dissipates
Preliminary BAMEX Forecast Verification
Mode for corresponding convective systems
For Convective Mode 2 or 3
Cases Observed
Yes
No
Cases Yes
Predicted No
61
25
16
21
Probability of detection (POD) = 79%
False alarm rate (FAR) = 29%
(Done, Davis, and Weisman)
A High-Resolution Modeling Study of the 24 May 2002
Dryline Case during IHOP
(Xue and Martin 2006a,b MWR)
Goal:
Understand exactly
WHEN, WHERE, HOW convection is initiated
Time and Location of Initiation
(Loop time: 17UTC – 22 UTC)
From Wakimoto et al.
(2006 MWR).
Surface
analysis
+ satellite
images1900
2100
2000
2200
18 UTC May 24, 2002 I.C.
3 km / 1km grid
Model Configurations
1200 UTC
CI ~ 2000UTC
1800 UTC
0000 UTC
3km
ADAS
0006 UTC
1km
ADAS
• ARPS model with full physics, including
ice microphysics + soil model + PBL and
TKE-SGS turbulence
t=3h, 2100 UTC
sfc. winds, qv, and composite reflectivity
t=4h, 2200 UTC
t=5h, 2300 UTC
t=3h, 2100 UTC
2000 UTC
2015 UTC
2030 UTC
2045 U
t=2h
t=2h 15min
t=2h 30min
t=2h 45min
C
B
A
C
A
B
B
A
B
C
A
Bottom Line
• High resolution NWP can often predict the mode of
the convection correctly, even a day ahead (supercell,
bow echo, scattered convection).
• Skill in predicting the magnitude and location of
convection fades out quickly after only a few hours.
• Predictability is lengthened when there is strong, large
scale forcing (e.g,. front or dry line)
The Future of Convective
Forecasting
• Clearly, there is substantial uncertainty that must be
considered.
• A major requirement is for there to be large
convection-resolving ensembles run operationally
(25-100 members), with varying initializations and
physics.
• Need for better initializations to describe the detailed
3D configuration of the lower atmosphere (using all
assets: commuter aircraft, mesosnets, satellite data,
etc.)
• http://www.spc.noaa.gov/exper/sseo/
Storm Prediction Center
Ensemble of Opportunity
• Based on 7
high-resolution
deterministic
forecasts run by
a variety of
groups.
Another Major Advancing Tool: High Resolution Rapid
Refresh: Particularly for Next Few Hours.
The U.S Storm Prediction Center
Storm Prediction Center
• Main U.S. entity responsible for severe weather
forecasting.
• Coordinates between NWS forecast offices, who
also important players for their areas.
Forecasting of Convection
Summary
• The big challenge is to predict the
environment in which convection will
develop.
• Parameters such as vertical instability
(CAPE), wind shear and helicity, low-level
thermal and moisture structures, CIN, etc.
• These can change rapidly with large
mesoscale variations.
Major Ingredients for General Convection
• Convective or conditional instability
– Lifting turns convectively unstable sounding to a
conditionally unstable sounding
– Negative LI
– High CAPE
– Low LFC
– CAPE is more useful than LI
• Moist layer near the surface
– Generally Td > 53F needed.
• An initiator
– Source of upward motion (front, dry line, sea breeze front)
• Low or moderate CIN