NORTH CENTRAL GREAT PLAINS DERECHO PRODUCING MESOSCALE CONVECTIVE SYSTEMS (DMCSs): A FORECASTING PRIMER

 

 

 

 

 

Mace L. Bentley
Climatology Research Lab
The University of Georgia
Athens, Georgia

 

 

Thomas L. Mote
School of Aerospace Sciences
University of North Dakota
Grand Forks, North Dakota

 

 

Stephen F. Byrd
National Weather Service Forecast Office
Omaha/Valley, Nebraska

 

 

INTRODUCTION

Synoptic-scale patterns favorable for producing derechos in the Northern Plains are examined with the goal of providing pattern-diagnosis techniques for identifying environments within derecho activity corridors. Nineteen derechos were identified across the Northern Plains region during, 1986-1995. These systems formed during the warm season months of May through August. The synoptic environment at the initiation, midpoint, and decay locations of each derecho was evaluated using surface, upper air and re-analysis datasets.

Results suggest that the low-level synoptic environment is critical in initiating and maintaining derecho producing mesoscale convective systems (DMCSs). Through the lifting of potentially unstable air and Ekman pumping, circulation around surface low pressure increased the instability gradient and maximized storm-relative convergence in the initiation region of nearly all events regardless of DMCS location or movement. Other commonalities in the environments of these events include: the presence of a thermal boundary, high convective instability, and a layer of dry low to mid-tropospheric air. The synoptic environment in place downstream of the MCS initiation region determines the movement and potential strength of the system. Idealized models are provided in order to further illustrate the synoptic-scale environment and to assist meteorologists in recognizing conditions favorable for derecho formation.

Much of the analysis in this investigation was conducted for a COMET Partner's Grant between The University of Georgia and the Omaha/Valley National Weather Service Forecast Office.

SYNOPTIC SITUATIONS FAVORABLE FOR DMCSs

(Idealized Models (Map Provided for Scale Purposes) )

A. Southeast Moving (NWFL) DMCSs - Nine Events Identified

Note: - Upper-level ridge/trough location
- LLJ orientation
- ULJ orientation
- ULJ = upper-level jet
- MLJ = mid-level jet
- LLJ = low-level jet

B. Northeast Moving Central Plains DMCSs - Six Events Identified

Note: - Upper-level ridge/trough location
- LLJ orientation
- ULJ orientation

C. Southeast Moving Central and Southern Plains DMCSs (Southward Burst) - Four Events Identified

Note: - Upper-level ridge/trough location
- LLJ orientation
- ULJ orientation
- Both A and B events initiate in WSW mid-and upper-level flow

THE THERMODYNAMIC ENVIRONMENT

A. Southeast Moving (NWFL) - Nine Events Identified

  Temperature °C Dewpoint °C

 

Wind Direction
Wind Speed m s-1
 
Initiation
Near
Midpoint

Initiation
Near
Midpoint

Initiation
Near
Midpoint

Initiation
Near
Midpoint
850 hPa 24 22 10 14 223 223 8 8
700 hPa 13 11 -2 -1 252 270 13 13
500 hPa -9 -8 -18 -24 253 277 18 18
250 hPa -45 -45 - - 262 286 28 29

Time CAPE J kg-1 Lifted Index
Initiation 3437 -3.0
Near Mid-point 3620 -8.4

Note: - Increased instability downstream
- Mid-level dry air
      - Warm, moist low-level air

* Proximity soundings were used to construct the thermodynamic environment. A sounding was included if it was within 250 km and taken within three hours of the first reported severe convective wind gust.

B. Northeast Moving Central Plains - Six Events Identified

  Temperature °C Dewpoint °C Wind Direction Wind Speed m s-1
 
Initiation
Near
Midpoint

Initiation
Near
Midpoint

Initiation
Near
Midpoint

Initiation
Near
Midpoint
850 hPa 22 23 14 12 190 208 6 14
700 hPa 12 11 -3 -4 296 250 7 14
500 hPa -10 -9 -14 -19 268 266 14 19
250 hPa - -44 - - 261 261 28 28

 

Time CAPE J kg-1 Lifted Index
Initiation 1780 -5.4
Near Midpoint 3478 -5.6

 

C. Southeast Moving Central and Southern Plains (Southward Burst) - Four Events Identified

  Temperature °C Dewpoint °C Wind Direction Wind Speed m s-1
 

 

Near Midpoint

 

Near Midpoint

 

Near Midpoint

 

Near Midpoint
850 hPa 25 12 203 9
700 hPa 13 3 260 8
500 hPa -8 -20 318 15
250 hPa -44 - 318 15

 

Time CAPE J kg-1 Lifted Index
Near Midpoint 2724 -4.6

 

COMMON HODOGRAPH SIGNATURES

(Heights Are in Meters)

A. Southeast Moving (NWFL)

B. Northeast Moving Central

Note: - Winds are uni-directionally WSW in initiation region.
- Winds turn NW downstream in A.
- Low-level shear is moderate to strong in initiation region.
- Bow is oriented normal to the mean shear vector (primarily in the mid-levels).

C. Southeast Moving Central and Southern Plains (Southward Burst) - Four Events Identified

Note: - Moderate shear oriented normal to the bow (from 1.2 to 6 km).

* Proximity soundings were used to construct the hodographs. A sounding was included if it was within 250 km and taken within three hours of the first reported severe convective wind gust. For Southward Burst DMCSs, there were no proximity soundings available to sample the initiation environment.

SURFACE ANALYSIS FEATURES

(Idealized Models (Map Provided for Scale Purposes))

Note: - DMCS initiation takes place along a theta-E gradient north of surface low pressure.
      - The DMCS then moves along the gradient into the greatest instability.
      - Storm-relative convergence is maximized along the gust front until the system reaches the decay region.
      - Very warm, moist surface air is present at the surface in all events.

A. Southeast Moving (NWFL) DMCSs

- shaded region represents location of DMCS initiation.

B. Northeast Moving Central Plains DMCSs

C. Southeast Moving Central and Southern Plains (Southward Burst) DMCSs

Note: - Similarities with southeast moving NWFL events.

OTHER USEFUL ANALYSES

Figure 1. 850 hPa moisture transport (magnitudes and vectors, g kg-1 m s-1, dashed lines and shading), heights (dm) and theta-E (K, dotted lines) for 0600 UTC 12 July 1995 (DMCS initiation). Black line indicates the derecho track.

Figure 2. Conceptual diagram of the orientation of an 850 hPa theta-E ridge to the track of southeastward moving northern tier derechos.

A. Vertical Theta-E Gradient and Wind Evolution

Figure 3. Vertical cross section of theta-E (K, dotted), mixing ratio (g kg-1, shaded) and winds (knots) for 0600 UTC 13 July 1995 (initiation). Derecho track and location for vertical cross-section illustrated with solid arrow.

Figure 4. Vertical cross section of theta-E (K, dotted), mixing ratio (g kg-1, shaded) and winds (knots) for 0000 UTC 9 July 1993 (initiation). Derecho track and location for vertical cross-section illustrated with solid arrow.

Figure 5. Vertical cross section of theta-E (K, dotted), mixing ratio (g kg-1, shaded) and winds (knots) for 1200 UTC 7 June 1994 (initiation). Derecho track and location for vertical cross-section illustrated with solid arrow.

B. Predicting MCS Mode and Motion

Figure 6. Schematic of the vector approach. a) Used when MCS is not expected to move faster than the mean wind. b) Used when MCS utilizes system-relative convergence and moves faster than the mean wind. Note: Difference in computed speeds and direction when using storm relative inflow versus the low-level jet (after, Corfidi 1998).

Note: - In this investigation, DMCSs were found to travel on average 30% faster than the 700 hPa to 500 hPa layer averaged wind speeds.

GLOSSARY

Advection: The horizontal transport of a quantity. Advection = -V s
Cold Pool: The large dome of cold air underneath a mature MCS. The cold pool is produced by evaporative cooling, momentum transport, and precipitation drag. Cold pool strength is essential for the maintenance of DMCSs
DMCS: Derecho producing Mesoscale Convective System. The parent MCS that is producing the derecho.
Moisture Transport: A moisture inflow field calculated by multiplying the wind vector by the mixing ratio. The magnitude of this vector field illustrates regions of high or low moisture inflow.
NWFL: Northwest Flow. Southeastward moving northern tier events have been correlated with the synoptic pattern favorable to northwest flow severe weather outbreaks.
Southward Burst: A southward propagating MCS primarily occurring in the Southern Plains. Movement of this type of MCS appears related to instability and propagation of the cold pool.
Wind Shear: The vertical change of the wind vector as measured on the hodograph. Moderate shear (10 - 18 m s-1), Strong shear (>18 m s-1). The strength and orientation of the 0-3 km shear with respect to the bow echo are important factors in determining derecho potential.

REFERENCES

Bentley, M. L., 1997: Synoptic Conditions Favorable for the Formation of the 15 July 1995 Southeastern Canada/northeastern United States Derecho Event. Natl. Wea. Dig., 21, 28-36.

____________, and T. L. Mote, 1998: a Climatology of Derecho Producing Mesoscale Convective Systems in the Eastern United States, 1986-1995. Part I: Temporal and Spatial Distribution. Bull. Amer. Meteor. Soc., 79, 11, in press.

Bluestein, H. B. and M. H. Jain, 1985: Formation of Mesoscale Lines of Precipitation: Severe Squall Lines in Oklahoma During the Spring. J. Atmos. Sci., 42, 1711-1732.

Corfidi, S. F., 1998: Forecasting Mcs Mode and Motion. Preprints, 19th Conf. on Severe Local Storms, Minneapolis, AMS (Boston) 626-629.

Evans, J. S., 1998: an Examination of Observed Shear Profiles Associated with Long-lived Bow Echoes. Preprints, 19th Conf. on Severe Local Storms, Minneapolis, AMS (Boston), 30-33.

Johns, R. H. and W. D. Hirt, 1987: Derechos: Widespread Convectively Induced Windstorms. Wea. Forecasting, 2, 32-49.

____________, K.W. Howard, and R. . Maddox, 1990: Conditions Associated with Long-lived Derechos - an Examination of the Large-scale Environment. Preprints, 16th Conf. on Severe Local Storms, Alberta, Canada, AMS (Boston), 408-412.

____________, and C.A. Doswell, III, 1992: Severe Local Storms Forecasting. Wea. Forecasting, 7, 588-612.

____________, 1993: Meteorological Conditions Associated with Bow Echo Development in Convective Storms. Wea. Forecasting, 8, 294-299.

Porter, J.M., L.L. Means, J.E. Hovde, and W.B. Chappell, 1955: A Synoptic Study of the Formation of Squall Lines in the North Central United States. Bull. Amer. Meteor. Soc., 36, 390-396.

Przybylinski, R.W., 1995: The Bow Echo: Observations, Numerical Simulations, And Severe Weather Detection Methods. Wea. Forecasting, 10, 203-218.

Weisman, M. L., 1993: The Genesis of Severe, Long-lived Bow Echoes. J. Atmos. Sci., 50, 645-670.

 


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.