Forecasting Thunderstorms in Forecasting Thunderstorms in Terminal Aerodrome Terminal Aerodrome

Forecasts (TAFs)Forecasts (TAFs)

Some new insightsSome new insights

Steven ThompsonSteven Thompson

National Weather Service (NWS)National Weather Service (NWS)

La Crosse, WILa Crosse, WI

PurposePurpose

Identify ways to improve TAF thunderstorm Identify ways to improve TAF thunderstorm forecasts to:forecasts to:

Reduce false alarm (FA) hours.Reduce false alarm (FA) hours.

Increase efficiency of airport operations.Increase efficiency of airport operations.

Enhance aviation safety.Enhance aviation safety.

Promote credibility of NWS aviation products.Promote credibility of NWS aviation products.

NWS TAFFM1200 18012KT P6SM SCT040

TEMPO 2024 3SM –TSRA BKN020CB

FM0000 20012KT P6SM BKN030

TEMPO 0004 VRB38G55KT 1SM +TSRAGR BKN010CB

FM0400 21010KT P6SM BKN040

PROB30 0408 3SM –TSRA BKN020CB

FM0800 24014KT 6SM HZ OVC030

PROB30 0812 2SM TSRA BKN015CB=

Where are all of the thunderstorms they forecast?

MethodologyMethodology

Primary data sources were:Primary data sources were:

Aviation Verify program.Aviation Verify program.

Aviation verification statistics.Aviation verification statistics.

Solar and Meteorological Surface Observation Network, 1961-1990.Solar and Meteorological Surface Observation Network, 1961-1990.

Thunderstorm ClimatologyThunderstorm ClimatologyApril-September, 1961-1990April-September, 1961-1990

02468

101214161820

Hourly obs

April May June July August Sept

LSE obsRST obs

Hourly observations with a thunderstorm (TS) reported at La Crosse, WI (LSE) and Rochester, MN (RST).

Thunderstorm ClimatologyThunderstorm Climatology April-September, 1961-1990 April-September, 1961-1990

2%

98%

TSNo TS

Percentage of hourly observations with and without TS at LSE and RST.

PROB TS hours PROB TS hours April-September 2003April-September 2003NWS Central Region TAFsNWS Central Region TAFs

0

5000

10000

15000

Apr May Jun Jul Aug Sep

Prob hrs

PROB TS hours verified PROB TS hours verified April-September 2003April-September 2003NWS Central Region TAFsNWS Central Region TAFs

0

5

10

15

20

25

April May June July Aug Sept

% verify

PROB TS hours vs. hours verified PROB TS hours vs. hours verified April-September 2003April-September 2003NWS Central Region TAFsNWS Central Region TAFs

1

10

100

1000

10000

100000

Apr May Jun Jul Aug Sep

Prob hrs% verify

How can we improve TS forecasts in How can we improve TS forecasts in

TAFs?TAFs? Here’s what the La Crosse NWS Here’s what the La Crosse NWS forecasters said:forecasters said:

Eliminate forecast of thunderstorms beyond 12 hours.Eliminate forecast of thunderstorms beyond 12 hours.

Limit time length of PROB and TEMPO, i.e. ≤ 2 Limit time length of PROB and TEMPO, i.e. ≤ 2 hours.hours.

Use vicinity thunderstorm (VCTS) or Use vicinity thunderstorm (VCTS) or cumulonimbus (CB) in lieu of TEMPO and PROB cumulonimbus (CB) in lieu of TEMPO and PROB groups.groups.

Keep in mind climatology, i.e. time of day, Keep in mind climatology, i.e. time of day, month.month.

Only use TEMPO for categorical events.Only use TEMPO for categorical events.

Key on Key on organizedorganized versus unorganized versus unorganized convection.convection.

Limit PROB and TEMPO groups beyond 8 to 12 hours.

““Funnel effect” philosophy of Funnel effect” philosophy of forecasting TS in TAFsforecasting TS in TAFs

TAF valid time

24 hrs

0 hr

VCTS PROB30

TEMPO VCTS

Uncertainty

PREVAILING,TEMPO

TEMPO? VCTS? CB?

CB? No TS?

Organized convection

≥50% probability of occurrence

Unorganized convection

<50% probability of occurrence

Forecast scenariosForecast scenarios

Capped summer environment with a small chance of Capped summer environment with a small chance of breaking the cap in the late afternoon/evening.breaking the cap in the late afternoon/evening.

The recommended TAF would be:The recommended TAF would be:

At 3-6hrs, with nothing currently on radar?

FM1800 18010KT P6SM SCT040

FM0300 18010KT P6SM SCT250=

At 18-24hrs, with model guidance consistently indicating a strong cap?

FM1800 18010KT P6SM SCT040

FM0300 18010KT P6SM SCT250=

Forecast scenariosForecast scenarios

TS possible, but will probably be scattered. There is a 30 TS possible, but will probably be scattered. There is a 30 percent chance of TS in the forecast grid database.percent chance of TS in the forecast grid database.

The recommended TAF would be:The recommended TAF would be:

At 3-6hrs, with scattered thunderstorms currently on radar?

FM2100 18012KT P6SM VCTS BKN040CB

FM0300 18012KT P6SM SCT250=

At 18-24hrs, with models indicating unorganized convection?

FM2100 18012KT P6SM BKN040CB

FM0300 18012KT P6SM SCT250=

Forecast scenariosForecast scenarios

Cold front moving into an unstable atmosphere and Cold front moving into an unstable atmosphere and there is wind shear to support there is wind shear to support organizedorganized convection convection along a squall line. The recommended TAF would be:along a squall line. The recommended TAF would be:

At 4-6hrs, with an actual squall line approaching ?

FM1800 20014G22KT P6SM BKN040

FM2200 20014KT P6SM VCTS BKN040CB

TEMPO 2224 VRB20G35KT 2SM TSRA BKN015CB

FM0000 31014KT P6SM SCT250=

At 18-24hrs, with model timing of cold front 22Z-00Z?

FM1800 20014G22KT P6SM BKN040

FM2200 20014KT P6SM VCTS BKN040CB

TEMPO 2224 VRB20G35KT 2SM SHRA BKN020CB

FM0000 31014KT P6SM SCT250=

Forecast scenariosForecast scenarios

Mesoscale Convective System (MCS) and moisture Mesoscale Convective System (MCS) and moisture transport maximum support transport maximum support organizedorganized convection in convection in your forecast area.your forecast area.

The recommended TAF would be:The recommended TAF would be:

At 2-6hrs, with an actual MCS approaching?

FM0600 18015G23KT P6SM BKN250

FM0800 16015G23KT 4SM –TSRA BR OVC040CB

TEMPO 0812 2SM TSRA BR BKN020CB…

At 20-24hrs, with model guidance indicating potential MCS ?

FM0600 18015G23KT P6SM BKN250

FM0800 16015G23KT 4SM –SHRA BR VCTS OVC040CB

TEMPO 0812 2SM SHRA BR BKN020CB=

CAUTION!!CAUTION!!

There is no “silver bullet” when forecasting thunderstorms in There is no “silver bullet” when forecasting thunderstorms in TAFs.TAFs.

A higher level of TAF preparation and monitoring is required to A higher level of TAF preparation and monitoring is required to utilize insights suggested in this presentation.utilize insights suggested in this presentation.

Probability of detection (POD) may suffer at the expense of Probability of detection (POD) may suffer at the expense of reducing false alarm hours.reducing false alarm hours.

TAF FA hours vs. PODTAF FA hours vs. PODApril-SeptemberApril-September

NWS La Crosse TAFs (0-12hr forecast) NWS La Crosse TAFs (0-12hr forecast)

2001 2002 2003

1

10

100

1000

TAF FA hrsPOD

TAF FA hours were reduced 71%, but POD dropped 64 %.

TAF PROB TS hours vs. TAF FA hoursTAF PROB TS hours vs. TAF FA hoursApril-SeptemberApril-September

NWS La Crosse TAFs (0-24hr forecast) NWS La Crosse TAFs (0-24hr forecast)

0

200

400

600

800

1000

1200

1400

1600

2001 2002 2003

PROB hrsTAF FA hrs

PROB hours were reduced 92%, with FA hours reduced 74 %.

ConclusionsConclusionsHow can we improve TS forecasts in TAFs?How can we improve TS forecasts in TAFs?

Limit PROB and TEMPO groups beyond 8 to 12 hours.Limit PROB and TEMPO groups beyond 8 to 12 hours. Limit time length of PROB and TEMPO, i.e. ≤ 2 hours.Limit time length of PROB and TEMPO, i.e. ≤ 2 hours. Eliminate forecast of thunderstorms beyond 12 hours.Eliminate forecast of thunderstorms beyond 12 hours. Use VCTS or CB in lieu of TEMPO and PROB groups.Use VCTS or CB in lieu of TEMPO and PROB groups. Keep in mind climatology, i.e. time of day, month.Keep in mind climatology, i.e. time of day, month. Only use TEMPO during categorical events.Only use TEMPO during categorical events. Key on Key on organizedorganized versus unorganized convection. versus unorganized convection.

AcknowledgmentsAcknowledgments

Dan Baumgardt, Science and Dan Baumgardt, Science and Operations Officer, NWS La Crosse, WI Operations Officer, NWS La Crosse, WI for much appreciated assistance in for much appreciated assistance in data assimilation and critique of this data assimilation and critique of this presentation.presentation.

Forecasting Thunderstorms in Forecasting Thunderstorms in Terminal Aerodrome Terminal Aerodrome

Forecasts (TAFs)Forecasts (TAFs)

Some new insightsSome new insights

Steven ThompsonSteven Thompson

National Weather Service (NWS)National Weather Service (NWS)

La Crosse, WILa Crosse, WI