DERECHOS

1. DERECHOS Jennifer Belge And Mike Baker ESC 452 4/20/06

2. Definition • A widespread long lived windstorm produced by a family of downburst clusters within a Mesoscale Convective System (MCS)

3. Identification of a Derecho • Concentrated area(400km) of convectively induced wind damage and/or convective gusts >50kts. • Nonrandom pattern of occurrence (single swath or series of swaths). • Within the area there must be 3 reports separated by 64km with of F1 damage and or convective gust of ≥65kts. • No more than 3 hours can elapse between wind damage events. • Wind damage events must be part of the same MCS.

4. Serial Derecho • Associated with a well defined migrating low pressure system. • Extensive squall line parallel to mean wind direction • Series of Line Echo Wave Patterns (LEWPs) and bow echoes • Most common in late winter/spring • Speed: ≤15 ms-1 perpendicular to mean flow

5. Progressive Derecho • Associated with a stationary front or stagnant weather pattern • Small squall line of restricted length perpendicular to the mean wind direction • Bulging in the general direction of the mean wind flow • Most common during warm season • Speed: ≤23 ms-1 Johns and Hirt 1987

6. Derecho Climatology • Most common in an area starting from the northern Great Plains extending southeast to the mid-Atlantic states • Late winter-warm season • 79% of all derecho events begin at night (1600 -0400 UTC) • Average duration: ~9 hours

7. Derecho Climatology The approximate number of times "moderate and high intensity" (MH) derechos affected points in the United States during the years 1980 through 2001. Areas affected by 3 or more derecho events are shaded in yellow, orange, and red. (modified from Coniglio and Stensrud 2004).

8. The Bow Echo • Derechos usually form in an environment composed of several bow echoes at one time (depending on type of derecho). • The bow echo has a crescent like shape and is a result of downbursts (Fujita 1978). Usually about 20-120 km long. • Develops from a Mesoscale Convective System(MCS) and indicates convective organization. • Usually associated with squall line type and can be called Line Echo Wave Patterns(LEWP’s) Fujita 1978

9. Bow Echoes cont... (RIJ) • Weak echoes behind the apex are indicative of the Rear Inflow Notch/Jet. • When elevated RIJ descends to the ground, that’s when the damaging winds tend to occur. • Strong cells along the leading edge will create a stronger warm pool aloftenhances horizontal vorticity intensifies RIJ. • The cold pool at sfc. And warm air aloft cause the meso-low at mid-levels which creates convergence which produces the RIJ

10. RIJ • Bow echo system starts off upright or downshear-tilted because of strong ambient shear causes cold pool to strengthen. • As the system then starts to tilt upshear, the CAPE will help to strengthen the warm pool aloft which with the help from the already strengthen cold pool will create an even stronger rear inflow jet. • If the RIJ stays elevated for a long period of time and comes closer to the leading edge of the system, it can help strengthen the system’s updrafts strengthens the cold pool even more strengthen the entire system

11. Example of RIJ (RIN) • http://www.crh.noaa.gov/dlh/science/event_archive/summer_archive/1999blowdown/1999blowdown.php

12. Mid-Altitude Radial Convergence • The MARC signature can often be a precursor to damaging winds before the “bowing” occurs. • Areas where strong inbound and outbound velocities are next to one another creates radial convergence in the mid-levels. • Horizontal extent of the MARC signature can be as long as 120 km with a depth of 4-7 km. • Velocity differentials tend to be at or greater than 25 ms-1. • This signature can lead to better leads times on when damaging winds may occur.

13. Radar Signatures

14. Radar cont..

15. Forecasting Checklist(Johns and Hirt 1987) • Is there a quasi-stationary boundary almost parallel to 500 mb flow? • Is the 500 mb flow greater than or equal to 240o ? • Is the 850 mb / 700 mb warm advection within 20 nm of the area of interest? • Is the low to mid-trophspheric wind speed 25 kts or greater? • Relative humidity in low levels less than 80% ? • A Lifted Index of -6o C or less? (More negative more severe) • CAPE values anywhere from 2200 J kg-1 to 4500 J kg-1 (Evans and Doswell )

16. Derecho damage and impact on society • Can be as substantial as hurricane and tornado damage. • From 1986 – 2003 derechos have caused about 150 fatalities and over 2500 injuries. • In a case from a 1995 derecho, loses were estimated to be about $230 million!! (Walker and Mote 2005)

17. References • http://www.meted.ucar.edu/convectn/mcs/index.htm • http://www.spc.noaa.gov/misc/AbtDerechos/derechofacts.htm • http://www.crh.noaa.gov/lsx/science/marc/MARC.php • Johns, Robert, & Hirt, William (1987). Derechos: Widespread Convectively Induced Windstorms. Journal of Weather and Forecasting, 2, 32-49. • Ashley, Walker, & Mote, Thomas (2005). Derecho Hazards in the United States. AMS, 1577-1592. • Evans, Jeffry, & Doswell, Charles (N.D.). Examination of Derecho Environments Using Proximity Soundings. Journal of Weather and Forecasting 15 p.