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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”.

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Storm split 1

Storm Split 2

Storm split 3

often associated with strong straight line winds known as derechos
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 events
  • Great Derecho Website:

june 2012 derecho
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
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 low-level warm air advection and significant low-level moisture
numerical simulation of convection
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
METED Convective Storm Matrix
  • Allows you to experiment with instability and shear and view how the storms evolve.
explicit convective prediction
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.

10 km WRF forecast domain

4 km WRF forecast domain

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

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



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




Preliminary BAMEX Forecast Verification

Mode for corresponding convective systems

For Convective Mode 2 or 3



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

surface analysis satellite images
Surface analysis + satellite images

From Wakimoto et al.

(2006 MWR).





model configurations
Model Configurations

CI ~ 2000UTC

  • ARPS model with full physics, including ice microphysics + soil model + PBL and TKE-SGS turbulence

1800 UTC

0000 UTC

1200 UTC

0006 UTC






t=3h, 2100 UTC

sfc. winds, qv, and composite reflectivity


2000 UTC 2015 UTC 2030 UTC 2045 UTC

t=2h t=2h 15min t=2h 30min t=2h 45min












bottom line
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
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.)
storm prediction center ensemble of opportunity
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
Another Major Advancing Tool: High Resolution Rapid Refresh: Particularly for Next Few Hours.
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
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
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