Lecture 24 Flash Floods in Hawaii

Hilo Flood November 2000

Oahu Flood November 1995

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Previous lecture: Weather Hazards in HawaiiMidlatitude cyclones - cold fronts

! flooding

! high winds

! waves and swell

Kona lows and upper-level

troughs

! flooding

! high winds

! waves and swell

Extra strong Hawaiian highs

! high trade winds

! waves and swell

Tropical cyclones

!winds

! waves and swell

! storm surge

! flooding

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Flash Floods in Hawaii

! The Flash Flood Problem

! Flood Ingredients• Weather Patterns

• Terrain effects

• Soil moisture

! Past Events

! Detection and Forecasting Tools

! NWS Advisories

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The Flash Flood Problem in Hawaii

! 322 events in 40-year period

! 8.3 per year

! Causes most of the direct wx-related fatalities

! November is the worst month for storms (1-2 flash floods)

! June has the best weather

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Factors that Contribute to Flood Challenge

1. Small watersheds: Steep slopes and shallow soils make

the time between peak rain and peak discharge small, as

little as 15 minutes. Half of the State is within 5 miles of the

shore, therefore lead time for a response time is very short.

2. Terrain influences: The typical Hawaiian flood occurs when

a convective cloud becomes anchored to mountains.

3. Precipitation processes: Clouds may be deep

cumulonimbus or "shallow" warm rain clouds. Scale of

convective clouds is relatively small.

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Ingredients for Heavy Rain! Moisture – lots of water vapor

" Large amounts of low level moisture results in unstable air and more rainfall

" Large amounts of mid-level moisture helps preserve parcel buoyancy

! Large upward motion" Low level convergence & upper

level divergence needed for heavy rainfall.

! Slow storm motion (i.e. long duration)" Increases total rainfall over

basin Enhanced IR image during peak rainfall period of the Nov. 2000 Big Island flood event.

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Rainfall vs Water

Vapor

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Hurricanes Possess Lots of Water Vapor

Hurricane Georges, September 1998

Blue line ~ Water vapor in columnRed line – Sea level pressure

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Four Heavy Rain Patterns:Provide Moisture and Upward Motion

1. Cold front passage

2. Kona low• Slow moving

• Ample upward motion

• Advects moisture from deep tropics

3. Upper-level low or trough • over tropical system or its

remnants is very dangerous. e.g., Nov 2000 Flood

4. Tropical Cyclone• tropical storm

• hurricane

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Kona Lows

Cause more flash floods in Hawaiithan any other storm system

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Kona Low

A non-frontal subtropical cyclone that initially forms aloft.

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Mesoscale or Island Scale Features

! Terrain forcing" Ideal lifting mechanism

for prolonged heavy rains" Rain maxima often over

slopes exposed to low level flow

! Lee-side convergence zones (a.k.a. “plumes”)" Enhanced low level

convergence." Southeasterly or

southerly flow can cause “plume” to drift over downstream islands

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Hana Flood

Big Island Plume and Terrain Anchoring

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Hana Flood

Big Island Plume and Terrain Anchoring

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Rainfall Distribution: New Year’s Storm

24-hour rainfall in inches.

Distribution determined by

the terrain.

Contours for a 100-yr

rainfall event with overlay

of 20” rainfall contour.

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Contributing Factors! Soil moisture content from previous rains

! Previous Rainfall: Rate, duration, pattern

! Soil type, depth and stratification: determines infiltration rate & flow type

Paauau Stream overtopping Belt Highway at Pahala during Nov. 2000 Big Island flood event.

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Contributing Factors! Land use affects

basin response" Agriculture" Natural forest

" Urbanized

! Basin slope & size" Small, steep: < 1 hr

response time

! Channel condition" Debris dams?

Aerial view of Kaneohe, Oahu. Note encroachment of urban areas within small, steep-walled basins.

Geographic Aspects: Development of flash floods occurs in valleys and on unstable hillsides. Islands with narrow coastal transportation corridors can have large sections isolated by debris flows. This occurred on Oahu on New Years Eve, 1987 when all transport to East Oahu was severed.

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Small Basins mean Fast Response

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Notable Events! November 2000

" SE and E Big Island" $70 Mil." 37 inches/24-hrs (22 in./6-

hr)

! Dec 1991 Anahola Flood

" East Kauai" $5 Mil. & 4 fatalities

! Dec 1987/Jan 1988 “New Years Flood”

" East Oahu" 22 inches/24-hrs" $34 Mil.

! Nov 1996 “Election Day Storm”

" S. & Cen. Oahu" 14 inches/12-hrs" $13 Mil.

Bridge washout on Komohana St. in Hilo the day after the Nov. 2000 Big Island flood event.

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Hilo Flood 1-2 November 2000

7170

7174

71707130

7016

7014

7000

Glenwood

Mountain

View

Waiakea

Uka

Piihonua Hilo Airport

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30

25

20

701325

20 20

20

7170

7174

71707130

7016

7014

Glenwood

Hilo Airport

7016

Mountain

View

Waiakea

Uka

Piihonua Hilo Airport

25

30

25

20

25

20 20

207000

7170

7174

71707130

7014

7013

24.7

26.9

29.1

29.8

26.6

24-hour rainfall (solid)

and 100-year flood

rainfall (dashed).

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Hilo Flood 1-2 November

2000

! 155.2 ! 155.15 ! 155.1 ! 155.05 ! 15519.65

19.7

19.75

19.8

19.85

7170

7174

7170

7130

7016

7014Waiakea

Uka

Piihonua Hilo Airport

2525

2020

7013

25

7170

7174

7170

7130

7016

7014

Hilo Airport7016

Waiakea

Uka

Piihonua Hilo Airport

25

7170

7174

7170

7130

7014

7013

24.7

26.9

29.1

24.7

26.9

29.1

3 miles

24-hr rainfall (solid)

and 100-yr flood

rainfall (dashed).

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Hilo Flood 1-2 November

2000

Figure 1 Hourly rainfall rate (inches per hour)

for Hilo for the period 1 to 2 November 2000.

0

2

4

6

Ho

url

y R

ain

fall (

in/h

r) Hilo, HI

11/1/00 11/2/00

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Oahu Flood 4 November 1996

Election Day Flood 1996

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Analysis for 1200 UTC 25 January 1996 of 250 mb streamlines and isotachs (every 5 m/s).

Upper Level Divergence of Winds

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Low Level Convergence of Winds

Analysis for 1200 UTC 25 January 1996 of sea-level pressure (mb).

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January 1996

Flood Case

Display of radar reflectivity (dBZ) of storms located

over O'ahu at 1258 UTC 25 January 1996. (a) PPI and

(b) RHI located along black line in a). White numbers in

a) denote locations of LARC rain gages.

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Storm Total Rainfall

Analysis for 1200 UTC 25 January 1996 of sea-level pressure (mb).

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Oahu New Year’s Eve Flood

Contours for a 100-yr rainfall event

with overlay of 20” rainfall contour

from the New Year"s Eve Storm.

Kahena Street, Hahaione Valley

on January 2, 1988. Flood waters

gouged a 10–20 ft channel in the

roadway. (Photo courtesy of T.

Giambelluca).

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Detection & Forecasting Tools

" Balloon and Aircraft Soundings

" Satellite Imagery and products

" Radar Imagery and products

" Rain gage data" Stream gage data" Numerical models" Spotters" Experience

Manoa Valley flood Halloween 2004

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Soundings

Lihue sounding just prior to heavy rains from Dec. 1991 Anahola flood event

Balloon and Aircraft soundings provide

information on moisture content and stability of air.

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Satellite Products

" Visible: Hi-res views of circulations and low level boundaries

" IR provides cloud top temperature

" WV: Useful for tracking upper-level motion

• Jets, cyclonic circulations, short waves

Water vapor image just prior to onset of Nov. 2000 Big Island flood event

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Satellite Products

A growing number of derived products! GOES products

" Total Water Vapor

" Stability

" Winds

" Low tracking

! Polar orbiter products" Water vapor

" Rate rate " Surface winds 850 mb low tracking product

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Additional Forecast Tools

! Radar data" Short term forecast tool" Look for signs of heavy rain initiation

• Pre-existing lines of showers or outflow boundaries can intensify over mountainous terrain

! Surface data" Watch for changes in wind speed/direction, dew

point, and 24-hr pressure trend

! Experience" Years of looking at numerical and observational

data improves skill in most cases

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Flash Flood Detection – Rain Gages

! Automated system provides alarm for intense rain.

! “Ground truth” on actual rainfall…but" Wind bias

" Intense rainfall low bias

" Spatial coverage limitations

! Provides some lead time

! 2 to 4 inches per hour significant

Automated rain gage with phone telemetry at Kokee radar station.

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Anahola Flood 13-14 December 1991

Hourly rainfall from gage at Anahola during the 1991 Anahola flood.

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Doppler Radar

Doppler radar measures

rainfall rates, wind speeds.

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Radar Data & Derived Products

! Provides good spatial and temporal resolution" 1 deg x 1 km" Update every 5 to 6 minutes

! Excellent for cell motion! Cells with reflectivity cores >= 50

dBZ bear close monitoring" Slow moving?" terrain anchored?

! Cross-sections for vertical structure

! Rainfall estimates" 1-hr, Storm total, User defined" Bias problems…validate w/gages

! Vert. Integrated Liquid (VIL)" >=25 kg/m2 signficant

Storm total rainfall from Molokai WSR-88D covering December 1999 flood event.

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Basin Areal-Mean Estimated Rainfall

! Derived from reflectivity data

! Radar “bins” assigned to stream basins" Basins delineated using digital

elevation model data

! Reflectivity converted to rain intensity

! Intensity values for each volume scan averaged over all bins in basin

! Basin accumulations determined

! Basin rainfall data compared to stream data to determine flash flood threshold rainfall

3-hr basin accumulation from January 1996 flood event

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Stream Gages! Real-time capability on

growing number of sites

! 1- to 4-hour routine transmission interval

! Emergency broadcast capability available if flood threshold known

! Excellent “ground truth” of flood conditions

! Limited lead time for operations

! Data used for rainfall threshold calibration (AMBER)

USGS stream gage at Kahana Stream

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Other Tools

• Spotters" “Eyes and ears” of NWS in the field

" Trained volunteers

" Law enforcement and emergency management officials

• Experience" Years of working flood events improves skill in

most cases

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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.

Weather Modeling

Challenges in modeling heavy precipitation in Hawaii

1. Convective clouds are relatively small.

2. Terrain impacts occur on small scale.

3. Lack of data over the ocean.

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Numerical Models

! Global models" Guidance up to 16 days" Identify weather patterns and

ingredients favorable for heavy rains

" Precipitation forecast is poor, but can be useful in qualitative sense

! Regional models" Guidance up to 3 days" Provides better details on spatial

distribution" Rainfall forecasts are better than

global models, but still relatively poor and used mainly in qualitative sense.

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NWS Flash Flood Advisories

1. Flood Potential Outlook (36 hr in advance)

• event is possible within 36 hr

2. Flood Watch (36 hr in advance)

• event is likely within 36 hr

3. Flash Flood Warning (updated every 3 hr)

• threat to life and property is imminent or occurring

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Questions?

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