The Moisture-Stability Index (MSI) and The Moisture-Stability Index (MSI) and its Application to Flash Flood its Application to Flash Flood

Forecasting in the Hawaiian IslandsForecasting in the Hawaiian Islands

Kevin KodamaKevin Kodama

and and

Robert BallardRobert Ballard

NOAA/NWS HonoluluNOAA/NWS Honolulu

OutlineOutline

Flash flooding in HawaiiFlash flooding in Hawaii

Ingredients for flash flooding and heavy rainfallIngredients for flash flooding and heavy rainfall

Moisture-Stability Index (MSI)Moisture-Stability Index (MSI)

MethodologyMethodology

ResultsResults

Summary – Application to OperationsSummary – Application to Operations

Flash Flooding in HawaiiFlash Flooding in Hawaii

One of the main natural One of the main natural hazards in the Hawaiian hazards in the Hawaiian IslandsIslandsResponsible for most of Responsible for most of the direct weather-related the direct weather-related deaths in HIdeaths in HIFeb-Mar 2006, StatewideFeb-Mar 2006, Statewide– 7 deaths, 28 events7 deaths, 28 events

Oct 2004, ManoaOct 2004, Manoa– 8.7”/5-hrs, $100 Mil8.7”/5-hrs, $100 Mil

Nov 2000, Big IsNov 2000, Big Is– 37”/24-hrs (22”/6-hrs)37”/24-hrs (22”/6-hrs)– $70 Mil$70 Mil

Flash Flood Warnings and WatchesFlash Flood Warnings and Watches

Flash flood warnings: Issued by NWS when flooding Flash flood warnings: Issued by NWS when flooding poses a threat to life and property and occurs within 6-poses a threat to life and property and occurs within 6-hrs of cause.hrs of cause.– Hawaii: Often less than 1-hr and sometimes less than 30-Hawaii: Often less than 1-hr and sometimes less than 30-

minutes of heavy rain onsetminutes of heavy rain onset– Little to no time for preparation, mainly reactionLittle to no time for preparation, mainly reaction

Flash flood watches: Issued by NWS when there is a Flash flood watches: Issued by NWS when there is a possibility of flash flooding with 48-hrs.possibility of flash flooding with 48-hrs.– More uncertainty but sufficient time for meaningful preparationsMore uncertainty but sufficient time for meaningful preparations– Preposition equipmentPreposition equipment– ““Lean forward” posture by emergency managementLean forward” posture by emergency management– Challenge to improve both accuracy and lead timeChallenge to improve both accuracy and lead time

Ingredients for Flash FloodingIngredients for Flash Flooding

In most cases, Hawaii flash flooding due to heavy rainfallIn most cases, Hawaii flash flooding due to heavy rainfall– Rare cases: dam failureRare cases: dam failure– No cases: ice jamsNo cases: ice jams

Thus, ingredients needed are...Thus, ingredients needed are...– Rainfall of sufficient intensity and durationRainfall of sufficient intensity and duration– Necessary hydrologic factors (antecedent moisture, basin Necessary hydrologic factors (antecedent moisture, basin

characteristics, etc.)characteristics, etc.)

Ingredients for Heavy RainfallIngredients for Heavy Rainfall

Moisture, always present, but...Moisture, always present, but...– Deep moist layer is betterDeep moist layer is better– Greater low level moisture more Greater low level moisture more

unstableunstable

Upward MotionUpward Motion– Provided by airmass instability Provided by airmass instability – Also assisted by orographic forcingAlso assisted by orographic forcing

High Precipitation EfficiencyHigh Precipitation Efficiency– More water vapor converted to More water vapor converted to

precipitationprecipitation

The Moisture-Stability Index (MSI)The Moisture-Stability Index (MSI)

Developed by Bob Ballard, HFO Science & Ops OfficerDeveloped by Bob Ballard, HFO Science & Ops OfficerForecasting by anomalies Forecasting by anomalies

Uses standardized anomalies (N) of 500 and 700 hPa temperatures Uses standardized anomalies (N) of 500 and 700 hPa temperatures and precipitable waterand precipitable water– 500 hPa: Diagnose instability aloft500 hPa: Diagnose instability aloft– 700 hPa: Diagnose temp near trade wind inversion level700 hPa: Diagnose temp near trade wind inversion level– PW: Diagnose moisture availabilityPW: Diagnose moisture availability

Anomalies combined to produce MSIAnomalies combined to produce MSIMSIMSI = = NNpwpw - - NN500500 - - NN700700

– Negative values of 500 and 700 mb anomalies contribute positively Negative values of 500 and 700 mb anomalies contribute positively – i.e., i.e., Cooler than normal temps increase MSICooler than normal temps increase MSI

MSI ScaleMSI ScaleNear normal: -1.5 to +1.5Near normal: -1.5 to +1.5

Positive MSI scalePositive MSI scale– Slightly wetter than normal: +1.6 to +2.5Slightly wetter than normal: +1.6 to +2.5– Enhanced showers: +2.6 to +3.5Enhanced showers: +2.6 to +3.5– Towering Cu: +3.6 to +4.5Towering Cu: +3.6 to +4.5– Thunderstorms: +4.6 to +6.5Thunderstorms: +4.6 to +6.5– ““Wow!”: >+6.5Wow!”: >+6.5

Negative MSI scaleNegative MSI scale– Slightly drier: -1.6 to -2.5Slightly drier: -1.6 to -2.5– Dry and stable: -2.6 to -4.5Dry and stable: -2.6 to -4.5– ““HI Visitor Bureau Wx!”: <= -4.6HI Visitor Bureau Wx!”: <= -4.6

Study MethodologyStudy Methodology

Is MSI correlated with rainfall intensity?Is MSI correlated with rainfall intensity?

Compare maximum rain rates from automated Compare maximum rain rates from automated gage network with MSI from nearest sounding.gage network with MSI from nearest sounding.– Lihue, Kauai sounding: 36 gages on Kauai & OahuLihue, Kauai sounding: 36 gages on Kauai & Oahu– Hilo, Hawaii sounding: 28 gages on Maui & Big IslandHilo, Hawaii sounding: 28 gages on Maui & Big Island– Molokai & Lanai not includedMolokai & Lanai not included

Only 3 gagesOnly 3 gages

Which sounding?Which sounding?

Rain GagesRain Gages

Study Methodology cont.Study Methodology cont.

Rain rates obtained from gage alarms sent to Rain rates obtained from gage alarms sent to WFOWFO– 15-minute sampling period15-minute sampling period– Converted to in/hr rateConverted to in/hr rate

Maximum rain rate from all alarms on island Maximum rain rate from all alarms on island compared to MSI from closest soundingcompared to MSI from closest sounding

Alarms from 06Z to 18Z vs. 12Z sounding MSIAlarms from 06Z to 18Z vs. 12Z sounding MSI

Alarms from 18Z to 06Z vs. 00Z sounding MSIAlarms from 18Z to 06Z vs. 00Z sounding MSI

474 alarm vs. MSI data points474 alarm vs. MSI data points

MSI vs Rainfall IntensityMSI vs Rainfall IntensityMSI vs. IntensityKauai/Oahu/Maui/Hawaii

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-5.00 -4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

MSI

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MSI vs Rainfall IntensityMSI vs Rainfall Intensity

Most intense rates have high MSIMost intense rates have high MSI– Max rate >=3”/hr, then MSI >= +3.0 (22 of 28 cases)Max rate >=3”/hr, then MSI >= +3.0 (22 of 28 cases)

MSI < -1.0 then all cases <3”/hr max rateMSI < -1.0 then all cases <3”/hr max rate

Many cases of high MSI but low rainMany cases of high MSI but low rain– Intense rain core missed gage?Intense rain core missed gage?

SummarySummary

Looked at utility of Moisture-Stability Index (MSI) for Looked at utility of Moisture-Stability Index (MSI) for flash flood operations in Hawaiiflash flood operations in HawaiiMSI assesses heavy rainfall potential by analyzing MSI assesses heavy rainfall potential by analyzing anomalies of 700 & 500 mb temperature and total anomalies of 700 & 500 mb temperature and total precipitable waterprecipitable waterStudy compared MSI with the max rain rate data from an Study compared MSI with the max rain rate data from an automated rain gage network automated rain gage network Moisture-Stability Index is related to max rain intensityMoisture-Stability Index is related to max rain intensity– Max rate >=3”/hr, then MSI >= +3.0 (22 of 28 cases)Max rate >=3”/hr, then MSI >= +3.0 (22 of 28 cases) – MSI < -1.0 then all cases < 3”/hr (and fewer alarms)MSI < -1.0 then all cases < 3”/hr (and fewer alarms)

Application to operationsApplication to operations– Calculate MSI from model fields for longer range guidanceCalculate MSI from model fields for longer range guidance– Incorporate into Gridded Forecast Editor (GFE)Incorporate into Gridded Forecast Editor (GFE)

MSI as GFE GuidanceMSI as GFE Guidance

MSI calculated in GFE out to 10 days for GFS, ECMWF, NOGAPS MSI calculated in GFE out to 10 days for GFS, ECMWF, NOGAPS modelsmodels

Forecasters can use MSI fields for improved situational awareness of FF Forecasters can use MSI fields for improved situational awareness of FF potential and use it as guidance for flash flood watchespotential and use it as guidance for flash flood watches

Further StudyFurther Study

Incorporate radar data into analysisIncorporate radar data into analysis– Improved spatial representationImproved spatial representation

Verification statistics of model-derived MSIVerification statistics of model-derived MSI– Any useful bias adjustments?Any useful bias adjustments?

Questions?Questions?