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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!!