DERECHOSDERECHOS

Jennifer BelgeJennifer BelgeAndAnd

Mike BakerMike Baker

ESC 452ESC 452

4/20/064/20/06

DefinitionDefinition

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

Identification of a DerechoIdentification of a Derecho Concentrated area(400km) of convectively Concentrated area(400km) of convectively

induced wind damage and/or convective gusts induced wind damage and/or convective gusts >50kts.>50kts.

Nonrandom pattern of occurrence (single swath Nonrandom pattern of occurrence (single swath or series of swaths).or series of swaths).

Within the area there must be 3 reports Within the area there must be 3 reports separated by 64km with of F1 damage and or separated by 64km with of F1 damage and or convective gust of ≥65kts. convective gust of ≥65kts.

No more than 3 hours can elapse between wind No more than 3 hours can elapse between wind damage events.damage events.

Wind damage events must be part of the same Wind damage events must be part of the same MCS.MCS.

Serial DerechoSerial Derecho Associated with a well Associated with a well

defined migrating low defined migrating low pressure system.pressure system.

Extensive squall line Extensive squall line parallel to mean wind parallel to mean wind directiondirection

Series of Line Echo Wave Series of Line Echo Wave Patterns (LEWPs) and Patterns (LEWPs) and bow echoesbow echoes

Most common in late Most common in late winter/springwinter/spring

Speed: Speed: ≤15 ms≤15 ms-1-1 perpendicular to mean perpendicular to mean flowflow

Progressive DerechoProgressive Derecho Associated with a Associated with a

stationary front or stationary front or stagnant weather patternstagnant weather pattern

Small squall line of Small squall line of restricted length restricted length perpendicular to the perpendicular to the mean wind directionmean wind direction

Bulging in the general Bulging in the general direction of the mean direction of the mean wind flowwind flow

Most common during Most common during warm seasonwarm season

Speed: Speed: ≤23 ms≤23 ms-1-1

Johns and Hirt 1987

Derecho ClimatologyDerecho Climatology

Most common in an area starting from the Most common in an area starting from the northern Great Plains extending southeast northern Great Plains extending southeast to the mid-Atlantic statesto the mid-Atlantic states

Late winter-warm seasonLate winter-warm season79% of all derecho events begin at night 79% of all derecho events begin at night

(1600 -0400 UTC)(1600 -0400 UTC)Average duration: Average duration: ~9 hours~9 hours

Derecho ClimatologyDerecho 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).

The Bow EchoThe Bow Echo Derechos usually form in an Derechos usually form in an

environment composed of environment composed of several bow echoes at one time several bow echoes at one time (depending on type of (depending on type of derecho).derecho).

The bow echo has a crescent The bow echo has a crescent like shape and is a result of like shape and is a result of downbursts (Fujita 1978). downbursts (Fujita 1978). Usually about 20-120 km long.Usually about 20-120 km long.

Develops from a Mesoscale Develops from a Mesoscale Convective System(MCS) and Convective System(MCS) and indicates convective indicates convective organization.organization.

Usually associated with squall Usually associated with squall line type and can be called Line line type and can be called Line Echo Wave Patterns(LEWP’s)Echo Wave Patterns(LEWP’s)

Fujita 1978

Bow Echoes cont... (RIJ)Bow Echoes cont... (RIJ) Weak echoes behind the Weak echoes behind the

apex are indicative of the apex are indicative of the Rear Inflow Notch/Jet.Rear Inflow Notch/Jet.

When elevated RIJ descends When elevated RIJ descends to the ground, that’s when the to the ground, that’s when the damaging winds tend to damaging winds tend to occur.occur.

Strong cells along the leading Strong cells along the leading edge will create a stronger edge will create a stronger warm pool aloftwarm pool aloftenhances enhances horizontal vorticityhorizontal vorticity intensifies RIJ.intensifies RIJ.

The cold pool at sfc. And The cold pool at sfc. And warm air aloft cause the warm air aloft cause the meso-low at mid-levels which meso-low at mid-levels which creates convergence which creates convergence which produces the RIJproduces the RIJ

RIJRIJ

Bow echo system starts off upright or downshear-tilted because Bow echo system starts off upright or downshear-tilted because of strong ambient shearof strong ambient shear causes cold pool to strengthen. causes cold pool to strengthen.

As the system then starts to tilt upshear, the CAPE will help to As the system then starts to tilt upshear, the CAPE will help to strengthen the warm pool aloft which with the help from the strengthen the warm pool aloft which with the help from the already strengthen cold pool will create an even stronger rear already strengthen cold pool will create an even stronger rear inflow jet.inflow jet.

If the RIJ stays elevated for a long period of time and comes 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 closer to the leading edge of the system, it can help strengthen the system’s updraftsthe system’s updrafts strengthens the cold pool even more strengthens the cold pool even more strengthen the entire systemstrengthen the entire system

Example of RIJ (RIN)Example of RIJ (RIN)

http://www.crh.noaa.gov/dlh/science/event_archive/summer_archive/http://www.crh.noaa.gov/dlh/science/event_archive/summer_archive/1999blowdown/1999blowdown.php1999blowdown/1999blowdown.php

Mid-Altitude Radial ConvergenceMid-Altitude Radial Convergence

The MARC signature can often be a precursor to damaging winds before The MARC signature can often be a precursor to damaging winds before the “bowing” occurs.the “bowing” occurs.

Areas where strong inbound and outbound velocities are next to one Areas where strong inbound and outbound velocities are next to one anotheranother creates radial convergence in the mid-levels. creates radial convergence in the mid-levels.

Horizontal extent of the MARC signature can be as long as 120 km with a Horizontal extent of the MARC signature can be as long as 120 km with a depth of 4-7 km.depth of 4-7 km.

Velocity differentials tend to be at or greater than 25 msVelocity differentials tend to be at or greater than 25 ms-1-1.. This signature can lead to better leads times on when damaging winds This signature can lead to better leads times on when damaging winds

may occur. may occur.

Radar SignaturesRadar Signatures

Radar cont..Radar cont..

Forecasting ChecklistForecasting Checklist(Johns and Hirt 1987)(Johns and Hirt 1987)

Is there a quasi-stationary boundary almost parallel to 500 mb Is there a quasi-stationary boundary almost parallel to 500 mb flow?flow?

Is the 500 mb flow greater than or equal to 240Is the 500 mb flow greater than or equal to 240oo ? ? Is the 850 mb / 700 mb warm advection within 20 nm of the Is the 850 mb / 700 mb warm advection within 20 nm of the

area of interest?area of interest? Is the low to mid-trophspheric wind speed 25 kts or greater?Is the low to mid-trophspheric wind speed 25 kts or greater? Relative humidity in low levels less than 80% ?Relative humidity in low levels less than 80% ? A Lifted Index of -6A Lifted Index of -6oo C or less? (More negative C or less? (More negative more more

severe)severe) CAPE values anywhere from 2200 J kgCAPE values anywhere from 2200 J kg-1 -1 to 4500 J kgto 4500 J kg-1-1

(Evans and Doswell )(Evans and Doswell )

Derecho damage and impact on Derecho damage and impact on societysociety

Can be as substantial as Can be as substantial as hurricane and tornado damage.hurricane and tornado damage.

From 1986 – 2003 derechos From 1986 – 2003 derechos have caused about 150 fatalities have caused about 150 fatalities and over 2500 injuries.and over 2500 injuries.

In a case from a 1995 derecho, In a case from a 1995 derecho, loses were estimated to be loses were estimated to be about $230 million!!about $230 million!!

(Walker and Mote 2005)(Walker and Mote 2005)

ReferencesReferences http://www.meted.ucar.edu/convectn/mcs/index.htmhttp://www.meted.ucar.edu/convectn/mcs/index.htm http://www.spc.noaa.gov/misc/AbtDerechos/derechofacthttp://www.spc.noaa.gov/misc/AbtDerechos/derechofact

s.htms.htm http://www.crh.noaa.gov/lsx/science/marc/MARC.phphttp://www.crh.noaa.gov/lsx/science/marc/MARC.php Johns, Robert, & Hirt, William (1987). Derechos: Johns, Robert, & Hirt, William (1987). Derechos:

Widespread Convectively Induced Windstorms. Widespread Convectively Induced Windstorms. Journal Journal of Weather and Forecasting, 2, 32-49.of Weather and Forecasting, 2, 32-49.

Ashley, Walker, & Mote, Thomas (2005). Derecho Ashley, Walker, & Mote, Thomas (2005). Derecho Hazards in the United StatesHazards in the United States. AMS, . AMS, 1577-1592.1577-1592.

Evans, Jeffry, & Doswell, Charles (N.D.). Examination of Evans, Jeffry, & Doswell, Charles (N.D.). Examination of Derecho Environments Using Proximity SoundingsDerecho Environments Using Proximity Soundings. . Journal of Weather and Forecasting 15 p.Journal of Weather and Forecasting 15 p.