Lecture 23 Weather Hazards

and Forecasting in Hawaii

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Previous Lecture:

Hazards Associated with Midlatitude Cyclones• Heavy Snow

– Blizzard Conditions, Snow, Sleet and Freezing Rain, Aircraft Icing, Riming, Snow Avalanches• Heavy Rain

– Flooding, Flash Floods, Mud Slides• High Winds

– Nor’easter, Blizzards, Turbulence• Large Ocean Swell and Waves

– High Surf, Storm Surge• Severe Thunderstorms

– Tornados, Large Hail, High Winds, Lightning

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• Weather Hazards in Hawaii

• Weather Forecasting• Motivation• Collect observations• Run weather prediction

models• Construct Forecasts• Issue advisories,

watches and warnings

Lecture 23 Weather Hazards

and Forecasting in Hawaii

Manoa Valley flood Halloween 2004

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Weather Hazards in Hawaii• Heavy Rains and Flash Flooding• High Winds• Large Ocean Swell and Surf• Storm Surge• Blizzards at High Altitude (Mauna Kea,

Mauna Loa and Haleakala)• Severe Thunderstorms

– Water spouts, Lightning, Flooding, High Winds, Tornados (rare).

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Weather Hazards in Hawaii

From 1970 to 1990, the National Weather Service (NWS) reported an average of six flash floods per year and numerous high wind events (> 20 m s-1), causing casualties (20 in all) and severe property damage. The January 1980 flood on Maui resulted in > $50 million damage in one event. The New Years Eve Flood of 1987 caused more than $35 million damage on Oahu.

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Weather Patterns in Hawaii

There are four large-scale weather patterns thatproduce hazardous weather in Hawaii.1. Midlatitude Cyclones

Cold frontal passage, shear lines2. Kona Lows3. Strong Surface High Pressure4. Tropical Cyclones

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Weather

Patterns in

Hawaii

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Weather Hazards in Hawaii1. Midlatitude cyclones - cold fronts

• flooding• high winds• waves and swell

2. Kona lows• flooding• high winds• waves and swell

3. Extra strong Hawaiian highs• high trade winds• waves and swell

4. Tropical cyclones • winds • waves and swell• storm surge• flooding

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Kona Low

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A Question of Size

1980 Winter Storm vs. Hurricane Iniki, 2 PM HST on September 12, 1992

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Monthly Distribution of Kona Lows

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Kona Low

• Kona lows all have an upper-level low aloft.

• The upper-level low is cold.

• New convection forms near and east of upper level low center.

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Kona Low• The surface low moves

southward as it deepens.

• The surface low moves westward as it dissipates.

• Thunderstorms form on the east side of the surface low.

• The strongest winds are on the north side and in rainbands.

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Rainbands in Kona Lows

This water vapor image shows moist air associated with squall line (line of thunderstorms).

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Squall line Passage and High Winds

Squall line at left is bowed by strong NW winds. Wind speeds of 90 mph were observed in Lihue when the squall line passed.

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Hurricane Force Trade Winds

Research Vessel Holo Holo sank in the Alenuihaha Channel under high wind conditions, December 1978.

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Hurricane Force Trade Winds

Hurricane force trade winds recorded by anemometer in Waikaloa, Hawaii, December 1978.

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Strong Hawaiian High

Sea-level pressure analysis for 13 December 1978.

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Histogram showing monthly distribution of 33 strong trade wind events at Lihue, Kauai.

SEASONAL DISTRIBUTION OF EVENTS

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Weather Forecasting

To save lives and protect property.

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• The U.S. arguably has the world’s worst weather: hurricanes, tornados, large hail, blizzards, droughts, heat waves….etc.

• Hazardous weather is the reason the National Weather Service (NWS) was founded.

• The mission of the NWS is to reduce the loss of life and the loss of property associated with weather related hazards, and to mitigate the economic impact of disruptive weather.

• Weather forecasters look at current state of the weather and forecast maps and add their personal experience to come up with a forecast.

Weather Forecasting

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NWS Watches and Warnings

The NWS issues advisories to warn the public.• Winter Storm Watch

– Heavy snow or a blizzard is possible, but the exact timing, location, or occurrence of the storm is still uncertain. A watch means to get prepared for a storm.

• Winter Storm Warning– A life-threatening storm is likely with 6 inches of snow or more in

12 hours or less; or 8 inches or more in 24 hours,– Heavy ice accumulations that cause extremely dangerous

conditions and significant damage,– Strong winds, and/or– Wind chills indices -40F or colder

• Blizzard Warning– A storm with winds 35 mph or greater AND significant snow or

blowing snow with visibilities less than 1/4 mile.

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OBSERVATIONS• Surface observations - surface pressure, wind, temp.,

and dew point, current weather• Doppler radar - rainfall rate, winds• Satellite imagery - clouds, moisture, winds• Radiosonde and aircraft soundings - vertical profile of

wind, temperature, and dew point

NUMERICAL WEATHER MODELS• High speed computers• High speed communication

Tools for Weather Forecasting

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Satellite Observations

Satellite observations provide information on cloud distribution and winds. Key for data sparse Hawaii.

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Radar Observations

Four radars provide rain rates and wind data for near vicinity of Hawaii. Important for nowcasting of thunderstorms and flooding.

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Local Numerical Weather Prediction

A high resolution model of the atmosphere can simulate the impact of the Big Island on the wind field.

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1. Input all available observations.2. Interpolate data to points on an even grid.3. Apply laws of physics, including parameterization of

surface and cloud processes too small for the model to directly include - integrate equations forward in time.

4. Output resulting forecast as contoured maps for interpretation.

Four Steps in Weather Modeling

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Use of Satellite Observations

Satellite Data over the ocean are interpolated to a grid.

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Designing a

nested grid

over Hawaii

Input all observationsonto a gridand apply laws of motion.

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Computer Models of the Atmosphere

High resolution models use a series of nested grids to include impact of mountains on weather in Hawaii.

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Challenges in Weather Modeling

1. Observations are often lacking, especially over the oceans, or there are mistakes in reporting.

2. The grid scale of the model is not fine enough to fully capture the physics of clouds or terrain. A faster computer helps.

3. There are difficulties with the equations of motion at the boundary of the air and the surface. Equations become nonlinear.

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Impact of Model Grid Spacing

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Questions?

Kona Low Simulation

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