THUNDERSTORMS THUNDERSTORMS ANDAND

WIND SHEAR WIND SHEAR

Angela LeseAngela Lese

National Weather Service LouisvilleNational Weather Service Louisville

OUTLINEOUTLINE

Basic thunderstorm mechanicsBasic thunderstorm mechanics

Wind shearWind shear

How do we forecast wind shear?How do we forecast wind shear?

Preparation (examples in TAFs)Preparation (examples in TAFs)

SummarySummary

The ThunderstormThe Thunderstorm

What is it?What is it?– Rapidly rising column of airRapidly rising column of air– Warmer than its surroundingsWarmer than its surroundings

IngredientsIngredients– MoistureMoisture– InstabilityInstability– Forcing/lifting mechanismForcing/lifting mechanism

FrontFront Low pressureLow pressure Other boundary (outflow, lake breeze)Other boundary (outflow, lake breeze) Upslope/OrographyUpslope/Orography Daytime heatingDaytime heating

Thunderstorm StagesThunderstorm Stages

All updraftAll updraft No rain/wind yetNo rain/wind yet

Cumulonimbus/anvilCumulonimbus/anvil Heavy rain/wind/hailHeavy rain/wind/hail

All downdraftAll downdraft Light rain/weak windsLight rain/weak winds

Thunderstorm FrequencyThunderstorm Frequency

Thunderstorm HazardsThunderstorm Hazards

IcingIcing

LightningLightning

IFR and LIFR conditionsIFR and LIFR conditions

Turbulence/Wind shearTurbulence/Wind shear

Wind ShearWind Shear

What is turbulence?What is turbulence?– Wind currents that vary greatly over short Wind currents that vary greatly over short

distancesdistances

What is wind shear?What is wind shear?– A change in horizontal wind speed and/or A change in horizontal wind speed and/or

direction with heightdirection with height

Two types of wind shearTwo types of wind shear– Convective wind shearConvective wind shear– Non-convective wind shearNon-convective wind shear

Convective Wind ShearConvective Wind Shear

Convective wind shearConvective wind shear

– Also known as Also known as microburstmicroburst Short-livedShort-lived, , ground-basedground-based, , small-scalesmall-scale event event Associated with thunderstormsAssociated with thunderstorms

– Will be included in any TAF if:Will be included in any TAF if: Thunderstorms are expected at or in the Thunderstorms are expected at or in the

vicinity of the airport vicinity of the airport

Under the storm: Under the storm: IFR/LIFR/VLIFR, IFR/LIFR/VLIFR, MicroburstsMicrobursts

Hard edges: Hard edges: Turbulent up to 20 miles awayTurbulent up to 20 miles away

Fuzzy anvil: Fuzzy anvil: IcingIcing

MICROBURST LIFE CYCLEMICROBURST LIFE CYCLE

FORMATION

Evaporation and precipitation drag forms downdraft

IMPACT

Downdraft quickly accelerates and

strikes the ground

DISSIPATION

Downdraft moves away from point of

impact

MicroburstMicroburst

X

The aircraft encounters point X, where it enters the microburst zone, and a headwind causes it to rise above the normal glideslope. At the center of the microburst, point Y, there is a downdraft causing the aircraft to sink. The aircraft now enters the most dangerous zone, point Z, where a sudden tailwind causes the aircraft to lose airspeed and potentially crash. F-7-14-54

Y

Z

Glideslope

MicroburstMicroburst

Non-convective Wind ShearNon-convective Wind Shear

Non-convective wind shearNon-convective wind shear– Also known as Also known as low-level wind shear (LLWS)low-level wind shear (LLWS)

Longer-lived, large-scale velocity change with heightLonger-lived, large-scale velocity change with height Usually associated with nocturnal low-level jetUsually associated with nocturnal low-level jet

– LLWS will be included in any TAF if:LLWS will be included in any TAF if: One or more PIREPs are received that include LLWS One or more PIREPs are received that include LLWS

within 2,000 feet of surface, at or within vicinity of within 2,000 feet of surface, at or within vicinity of airport, causing an air speed loss or gain of 20 knots airport, causing an air speed loss or gain of 20 knots or moreor more

Wind shear of 10 knots or more per 100 feet in a Wind shear of 10 knots or more per 100 feet in a layer more than 200 feet thick are expected/reported layer more than 200 feet thick are expected/reported at or within vicinity of airportat or within vicinity of airport

Non-convective Wind ShearNon-convective Wind Shear

1,000 feet1,000 feet

500 feet500 feet

SurfaceSurface

WSW 50 knotsWSW 50 knots

SW 30 knotsSW 30 knots

SE 10 knotsSE 10 knots

Forecasting Wind ShearForecasting Wind Shear

How are we forecasting microbursts?How are we forecasting microbursts?– Severe: greater than or equal to 50 knot gustsSevere: greater than or equal to 50 knot gusts– Strong: roughly 30-40 knot gustsStrong: roughly 30-40 knot gusts– Average: roughly 20 knot gustsAverage: roughly 20 knot gusts

How are we forecasting LLWS?How are we forecasting LLWS?– If winds will be If winds will be >> 40 knots in lowest 2,000 feet with 40 knots in lowest 2,000 feet with

surface winds surface winds << 10 knots ( 10 knots (>> 30 knot change) 30 knot change)– If wind direction will be If wind direction will be >> 60 degree difference between 60 degree difference between

the surface and 2,000 feetthe surface and 2,000 feet

Question: how does this criteria suit you?Question: how does this criteria suit you? Would you like to see more liberal criteria?Would you like to see more liberal criteria?

What can we use?What can we use?– For convective wind shear. . .For convective wind shear. . .

Radar velocity dataRadar velocity data Surface observationsSurface observations PIREPsPIREPs

– For non-convective wind shear. . .For non-convective wind shear. . . RAOB soundingsRAOB soundings Radar VWPRadar VWP ProfilersProfilers ACARS/model soundingsACARS/model soundings PIREPsPIREPs Model forecastsModel forecasts

Forecasting Wind ShearForecasting Wind Shear

Radar Radar VelocityVelocity

RAOBRAOB VWPVWP ProfilerProfiler Surface Surface

ObsObs ACARS ACARS

soundingsounding PIREPsPIREPs Model Model

forecastsforecasts

165165oo at 5 knots – 1100 feet at 5 knots – 1100 feet

225225oo at 40 knots – 2200 feet at 40 knots – 2200 feet

PreparationPreparation

Prepare ahead of time for your flight!Prepare ahead of time for your flight!– Monitor the TAFs at the appropriate airport Monitor the TAFs at the appropriate airport

and at surrounding airportsand at surrounding airports– NWS homepage: NWS homepage: weather.gov/louisvilleweather.gov/louisville– Aviation Digital Data Service: Aviation Digital Data Service: adds.aviationweather.govadds.aviationweather.gov

But to be prepared, you must know the difference But to be prepared, you must know the difference between convective and non-convective wind between convective and non-convective wind shear in TAFsshear in TAFs

TAF ExamplesTAF Examples

Convective wind shearConvective wind shear– FM1600 21008G20KT P6SM VCTS BKN040CBFM1600 21008G20KT P6SM VCTS BKN040CB– FM1900 22010G35KT 3SM TSRA BR BKN035CBFM1900 22010G35KT 3SM TSRA BR BKN035CB– FM2100 VRB20G55KT 1/2SM +TSRA FG BKN015CBFM2100 VRB20G55KT 1/2SM +TSRA FG BKN015CB

Non-convective wind shearNon-convective wind shear– FM0900 19004KT P6SM SKC WS015/19040KTFM0900 19004KT P6SM SKC WS015/19040KT– FM0800 VRB02KT P6SM SCT010 WS010/22035KTFM0800 VRB02KT P6SM SCT010 WS010/22035KT– FM0200 17005KT P6SM SKC WS020/23055KT FM0200 17005KT P6SM SKC WS020/23055KT

SummarySummary

Thunderstorms produce convective wind shear, Thunderstorms produce convective wind shear, which is short-lived, and localizedwhich is short-lived, and localized– Microbursts (FAA may refer to this as ground-based Microbursts (FAA may refer to this as ground-based

wind shear)wind shear) Non-convective wind shear is longer-lived and on Non-convective wind shear is longer-lived and on

a larger scale a larger scale – LLWS not associated with a thunderstormLLWS not associated with a thunderstorm– Low-level jet (often occurs in morning-nocturnal)Low-level jet (often occurs in morning-nocturnal)

WS in TAF – non-convective wind shearWS in TAF – non-convective wind shear TS and gusts in TAFs – microburst TS and gusts in TAFs – microburst LLWA at airport – microburst LLWA at airport – microburst Help us by sending more PIREPs!!Help us by sending more PIREPs!!