Favorable Conditions for the Development of a Heavy

Favorable Conditions for the Development of a Heavy Rainfall Event during the 2006 Wet Period Yi-Leng Chen and Chuan-Chi Tu University of Hawaii at Manoa Department of Meteorology August 19, 2008 1

Outline l Introduction i. the 2006 wet period ii. literature review iii. scientific objectives l Data & Methods l Preliminary Results i. moisture contribution to Hawaiian heavy rainfall episodes ii. favorable conditions for the Kahala Mall Flooding l Summary & Conclusions l Future work 2

Introduction l National Weather Service (NWS): 43 -day heavy rainfall period in Hawaii from Feb. 19 to Apr. 2, 2006 as the” wet period”. l 111 Flash Flood Warnings were issued on 26 days (climatologically, only 2~3 flash flood events during the same period) Kahala Mall Flood on March 31, 2006(HST) Source : NWS report (Nash et. Al, 2006) (http: //www. prh. noaa. gov/hnl/). ophoto Source: KHNL ("KHNL NBC 8") online news; http: //khnl. com/Global/story. asp? S=4863583&nav=menu 55_14_3_3 3

19 February 2006 to 2 April 2006 Ø 111 Flash Flood Warnings were issued on 26 days through the period. Ø 88 Special Marine Warnings (for waterspouts and/or strong thunderstorms over the water within 40 miles of land that are capable of producing winds greater than 40 mph or large hail). Ø 11 Severe Thunderstorm Warnings. Ø 5 Winter Weather Advisories (means snowfall of 2 to 5 inches is likely in the next 24 hours). Ø 3 Severe Thunderstorm Watches (means severe thunderstorms with winds above 58 mph and/or large hail are possible within 6 hours) on Feb 19, March 24, March 30. Ø 2 Winter Storm Watches (means snowfall of 6 inches or more is possible in the next 36 hours). Ø 2 High Wind Warnings (means sustained winds above 40 mph and/or gusts above 60 mph) for the upper summits of Mauna Kea and Mauna Loa. Strong winds are a fairly common event on the summits, especially during the winter. Ø 4 1 Winter Storm Warning. 1 Tornado Warning (Nash, Rydell and Kodama, 2006).

Record Breaking Event Location Record Type New Record Date Mt. Waialeale, Kauai, HI All-time wettest month on record Mar, 2006 Punaluu, Oahu, HI All-time wettest month on record New Record Old Record Date 93. 71 inches 90. 07 inches Apr, 1971; normal is 34. 70 inches Mar, 2006 37. 55 inches 36. 70 inches Jan, 1922; normal is 9. 62 inches Lihue, Kauai, HI All-time wettest month on record Mar, 2006 35. 95 inches 22. 91 inches Dec, 1968; normal is 3. 58 inches Mt. Waialeale, Kauai, HI Wettest March on record Mar, 2006 93. 71 inches 81. 95 inches Mar, 1951; normal is 34. 70 inches Punlauu, Oahu, HI Wettest March on record Mar, 2006 37. 55 inches 35. 21 inches Mar, 1920; normal is 9. 62 inches Lihue, Kauai, HI Wettest March on record Mar, 2006 35. 95 inches 14. 54 inches Mar, 1951; normal is 3. 58 inches Pahala, Hawaii, HI Wettest March on record Mar, 2006 30. 14 inches 28. 93 inches Mar, 1980; normal is 6. 30 inches Moloaa, Kauai, HI Wettest March on record Mar, 2006 23. 84 inches 21. 97 inches Mar, 1951; normal is 6. 20 inches Waimanalo, Oahu, HI Wettest March on record Mar, 2006 22. 29 inches 19. 43 inches Mar, 1963; normal is 3. 62 inches 5 http: //www. ncdc. noaa. gov

Polarward extending middle or high cloud bands from tropical to extratropical regions (Mc. Guirk et al. , 1987; 1990) l Jayawardena (2007) : Favorable climatological conditions for more frequent tropical plumes associated with tropical moist air over the Northern Central Pacific during the 2006 wet period: (1)cold SST anomalies in the Central and Eastern equatorial Pacific (2)weaker than normal local Hadley circulation (3)negative PNA (Pacific North American) pattern (4)an anomalous semi-permanent low pressure system west of Hawaii 6 Source: Jayawardena (2007, Fig. 3)

SSM/I Precipitable Water From 19 Feb- 02 April 2006: • Precipitable water is 10 -20 mm above the normal values over the Hawaiian Islands. • Anomalous low precipitable water over the Central and Eastern Pacific implying weaker than normal ITCZ; anomalous high precipitable water over the maritime continent and SPCZ suggesting stronger than normal Walker circulation. These are 7 typical La Niña conditions.

Literature review of wintertime heavy rainfall over Hawaii Synoptic View: l Hawaiian heavy rainfall events are linked to one of the four synoptic disturbances in cool seasons (Oct ~ April): (1)Kona storm (2)upper level trough (3)mid-latitude cold front (4)tropical disturbance (Blumenstock and Price, 1967; Schroeder, 1977; Kodama and Barnes, 1997). l Case studies of Hawaiian heavy rainfall associated with Kona storms. - A squall line event with bow echo produces heavy rainfall in Kauai Island (Businger et al. , 1998). - The low-level southeasterly wind contributes to heavy rainfall at the southeast flank of Mauna Loa volcano (Kodama and Barnes, 1997). Climatological View: l The negative PNA pattern is favorable for more Kona days and wintertime rainfall in that year (Chu et al. , 1993). 8

Data & Method l NCEP reanalysis data l SSMI Atmos. precipitable water (PW) data l TRMM derived rain rate data l Area Avg. PW calculation l EW, NS cross sections 9

Time series of area avg. NCEP (1∘X 1∘) reanalysis PW (kg/m^2) heavy rainfall episodes that Flash Flood Watches were in effective (accroding to NWS) 1 2 3 4 5 Climatology value ~28. 85 10

Periods with high PW over Hawaii are corresponding to periods with sortherly winds at low latitudes and Hawaii. NCEP Time-latitude 850 mb meridional wind[m/s] & PW[kg/m^2] (at 158 W) heavy rainfall episodes that Flash Flood Watches were in effective Red box: Hawaii region lat 1 2 3 4 5 time 1 2 3 4 5 11

Low pressure system to the west of Hawaii corresponds to the pronounce southerly wind over Hawaii especially during the second half period. NCEP Time-longitude 850 mb geopotential height[gpm]& meridional wind[m/s] (at 21 N) heavy rainfall episodes that Flash Flood Watches were in effective Red box: Hawaii region time Kahala Mall Flood 5 5 4 4 3 3 2 2 1 1 lon 12

Kahala Mall Flooding (around March 31 22 Z UTC) 3/31 12 Z to 4/1 00 Z 12 hr TRMM accumulated rainfall (mm) Oahu NE-SW orientated rain bands across Oahu. Kahala Mall 13

250 mb wind/ HGT(contour) & theta/ HGT(contour) & Q-G frontogenesis/ 300 -350 mb PV at 3/30 12 Z 14

GOES IR 3/31 00 Z ~ 3/31 21 Z (flooding occurrence) 15

3/31 12: 19 HST reflectivity(d. BZ) 12: 23 HST vertically integrated liquid water(Kg/M**2) 16

Kahala Mall Flooding (Mar. 31, 2006) 250/850 mb HGT (3/26 ~ 3/31 12 Z) Hawaii 17

Kahala Mall Flooding (Mar. 31, 2006) SSM/I PW 3/26~3/31 (moisture tongue across Hawaii) High thetae band across Hawaii corresponds to the moisture tongue from tropics to Hawaii (showing convective instability over Hawaii) 18

Upper-level conditions: 250 mb wind/ HGT(contour) & theta/ HGT(contour) & Q-G frontogenesis/ 300 -350 mb PV at 3/28 12 Z Hawaii 19

Upper-level conditions: 250 mb wind/ HGT(contour) & theta/ HGT(contour) & Q-G frontogenesis/ 300 -350 mb PV at 3/29 12 Z Hawaii 20

Upper-level conditions: 250 mb wind/ HGT(contour) & theta/ HGT(contour) & Q-G frontogenesis/ 300 -350 mb PV at 3/31 00 Z Oahu 21

Upper-level conditions: 250 mb wind/ HGT(contour) & theta/ HGT(contour) & Q-G frontogenesis/ 300 -350 mb PV at 4/1 00 Z Hawaii 22

Vertical cross section of PV (contour), zonal wind & vertical motion along 21 N at 3/31 00 Z(22 hr before flood) PV advection enhances downstream (Hawaii) upward motion Red box: Hawaii region 23

Summary and Conclusions l For the 5 heavy rainfall episodes, the southerly wind component east of a low pressure system brings in the tropical moisture over the Hawaiian Islands. l The following is the evolution of the development and dissipation of Kona Low for the Kahala Mall Flood case: 1. When the eastward-moving mid-latitude trough extends southward and connects with the subtropical trough, northerly flows behind the mid-latitude trough bring the cold air to the subtropics. It leads to upper -level frontogenesis and tropopause folding in the subtropics. 2. The upper-level subtropical trough deepens in response to baroclinic forcing with closed geopotential contours (e. g. , Kona Low) 3. As the mid-latitude trough continues to move eastward and separated from the subtropical trough, the Kona low is cut off from the westerly current with a ridge axis to the north. 4. Then, the Kona low weakens when there is no cold air advection from the north. 24

Summary and Conclusions l The following large-scale conditions are favorable for the development of the Kahala Mall Flooding (around 3/31 22 Z UTC) : 1. [Tropical contribution] At low levels, the southerly wind component at the 850 -h. Pa level ahead of the low strengthened as the Kona low deepened. The southerly wind component brings in moisture to Hawaii from the south. The high thetae band extending from the tropics to Hawaii with convective instability over Hawaii. 2. [Mid-latitude contribution] During the development of the Kona low west of Hawaii, PV advection aloft enhances the upward motion over the Hawaii region. The upper-level baroclinic forcing provides the lifting needed for the release of instability. 25

Thank you 26

Tropical Weather Discussion These messages are issued 4 times daily to describe significant synoptic weather features in the tropics. Plain language is used in these discussions. l Tropical Weather Outlook This outlook normally covers the tropical and subtropical waters, discussing the weather conditions, emphasizing any disturbed and suspicious areas which may become favorable for tropical cyclone development within the next day to two. In the Atlantic, the outlook is transmitted daily at 0530, 1130, 1730, and 2230 Eastern local time. In the eastern Pacific, it is transmitted daily at 0100, 0700, 1300, and 1900 Eastern local time. For the Central Pacific, transmission times are 1000 and 2200 UTC. Existing tropical and subtropical cyclones are mentioned, as are depressions not threatening land. Given for each system are its location, size, intensity, and movement. For the first 24 hours of a depression or tropical cyclone, the outlook includes a statement identifying the AFOS and World Meteorological Organization (WMO) headers for the advisory on it. l 27

Hurricanes (winds > 64 kt ) Hurricane Warning A warning that sustained winds 64 kt (74 mph or 119 kph) or higher associated with a hurricane are expected in a specified coastal area in 24 hours or less. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force. l Hurricane Watch An announcement of specific coastal areas that a hurricane or an incipient hurricane condition poses a possible threat, generally within 36 hours l 28

Tropical Storm (34 -64 knots) Tropical Storm Warning A warning for sustained surface winds, associated with a tropical cyclone, within the range of 34 to 63 knots (39 to 73 mph), expected in a specified coastal area within 24 hours. l Tropical Storm Watch An announcement that a tropical storm poses or tropical storm conditions pose a threat to coastal areas generally within 36 hours. A tropical storm watch should normally not be issued if the system is forecast to attain hurricane strength. l 29

Tropical Storm Summary l Written after subtropical and names tropical cyclones have moved inland advisories have been discontinued. These advisories will be terminated when the threat of flash flooding has ended or when the remnants of these storms can no longer be distinguished from other synoptic features capable of producing flash floods. They will be initiated when and if flash flood watches are posted in the United States because of an approaching system. Storm summaries will continue to be numbered in sequence with tropical cyclone advisories and will reference the former storm's name in the text. Summaries will be issued at 0100, 0700, 1300, and 1900 Eastern Daylight Time (EDT). The only exception will be the first one in the series may be issued at a nonscheduled time. 30

l Hurricane Local Statement A public release prepared by local National Weather Service offices in or near a threatened area giving specific details for its county/parish warning area on (1) weather conditions (2) evacuation decisions made by local officials (3) other precautions necessary to protect life and property. 31

l Coastal Flooding is the inundation of land areas adjacent to bodies of salt water connected to the Atlantic Ocean, Pacific Ocean, or Gulf of Mexico, caused by sea waters over and above normal tidal action. This flooding may impact the immediate oceanfront, gulfs, bays, back bays, sounds, and tidal portions of river mouths and inland tidal waterways. 32

l Coastal Flood Advisory Minor flooding is possible (i. e. , over and above normal high tide levels). l Coastal Flood Watch Flooding with significant impacts is possible. l Coastal Flood Warning Flooding that will pose a serious threat to life and property is occurring, imminent or highly likely. 33

High Seas Forecast (HSF) Marine forecasts for the major oceans of the world. In this context, major gulfs or seas are included within these forecast areas. l High Surf Large waves breaking on or near the shore resulting from swells spawned by a distant storm. l High Surf Advisory A High Surf Advisory is issued when breaking wave action poses a threat to life and property within the surf zone. High surf criteria vary by region. l High Surf Warning A High Surf Warning is issued when breaking wave action results in an especially heightened threat to life and property within the surf zone. High surf criteria vary by region. l 34

Small Craft Advisory (SCA) An advisory issued by coastal Weather Forecast Offices for areas included in the Coastal Waters Forecast or Nearshore Marine Forecast (NSH) products. Hawaii, Samoa - Sustained winds 25 knots or greater and seas 10 feet or greater. l Small Craft Advisory for Hazardous Seas (SCAHS) - An advisory issued by coastal Weather Forecast Offices. This advisory may be issued when wind speeds are lower than small craft advisory criteria, yet waves or seas are potentially hazardous due to wave period, steepness, or swell direction. Thresholds governing the issuance of Small Craft Advisories for Hazardous Seas are specific to geographic areas. Hawaii, Samoa - Seas 10 feet or greater. l 35

Small Craft Advisory for Rough Bar (SCARB): An advisory issued by NWS for specialized areas near harbor or river entrances known as bars. Waves in or near such bars may be especially hazardous to mariners due to the interaction of swell, tidal or river currents in relatively shallow water. Thresholds governing the issuance of Small Craft Advisories for Rough Bar are specific to local geographic areas, and are based upon parameters such as wave steepness, wind speed and direction, and local bathymetry. l Small Craft Advisory for Winds (SCAW): This advisory may be issued when wave heights are lower than small craft advisory criteria, yet wind speeds are potentially hazardous. Thresholds governing the issuance of small craft advisories are specific to geographic areas. Pacific (HI, Guam, etc) - Sustained winds 25 knots or greater; except in Guam where it is sustained winds of 22 to 33 knots. l 36

l Marine Weather Statement A National Weather Service product to provide mariners with details on significant or potentially hazardous conditions not otherwise covered in existing marine warnings and forecasts. Marine weather statements are also used to supplement special marine warnings. 37

Urban and Small Stream Flooding of small streams, streets, and low-lying areas, such as railroad underpasses and urban storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening nor is it significantly damaging to property. l Urban Flash Flood Guidance A specific type of flash flood guidance which estimates the average amount of rain needed over an urban area during a specified period of time to initiate flooding on small, ungaged streams in the urban area. l Urban Flooding of streets, underpasses, low lying areas, or storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening. l 38

l Urban and Small Stream Flood Advisory This advisory alerts the public to flooding which is generally only an inconvenience (not life-threatening) to those living in the affected area. Issued when heavy rain will cause flooding of streets and low-lying places in urban areas. Also used if small rural or urban streams are expected to reach or exceed bankfull. Some damage to homes or roads could occur. 39

l Flash Flood A rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (e. g. , intense rainfall, dam failure, ice jam). However, the actual time threshold may vary in different parts of the country. Ongoing flooding can intensify to flash flooding in cases where intense rainfall results in a rapid surge of rising flood waters. 40

l Flood Categories Terms categorize the severity of flood impacts in the corresponding river/stream reach at each forecast point. *Minor Flooding* - minimal or no property damage, but possibly some public threat. *Moderate Flooding* - some inundation of structures and roads near stream. Some evacuations of people and/or transfer of property to higher elevations. *Major Flooding* - extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations. *Record Flooding* - flooding which equals or exceeds the highest stage or discharge at a given site during the period of record keeping. 41

Flash Flood Watch Issued to indicate current or developing hydrologic conditions that are favorable for flash flooding in and close to the watch area, but the occurrence is neither certain or imminent. l Flash Flood Warning Issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely. l Flash Flood Statement (FFS) In hydrologic terms, a statement by the NWS which provides follow-up information on flash flood watches and warnings. l 42

l Wind Advisory Sustained winds 25 to 39 mph and/or gusts to 57 mph. Issuance is normally site specific. However, winds of this magnitude occurring over an area that frequently experiences such winds. l High Wind Sustained wind speeds of 40 mph or greater lasting for 1 hour or longer, or winds of 58 mph or greater for any duration. 43

High Wind Advisory This product is issued when high wind speeds may pose a hazard. The criteria for this advisory varies from state to state. In Michigan, the criteria is sustained nonconvective winds greater than or equal to 30 mph lasting for one hour or longer, or winds greater than or equal to 45 mph for any duration. l High Wind Watch This product is issued when there is the potential of high wind speeds developing that may pose a hazard or is life threatening. In Michigan, the criteria is the potential for sustained winds greater than or equal to 40 mph and/or gusts greater than or equal to 58 mph. l High Wind Warning This product is issued when high wind speeds may pose a hazard or is life threatening. In Michigan, the criteria is sustained winds greater than or equal to 40 mph lasting for one hour or longer, or winds greater than or equal to 58 mph for any duration. l 44

Extreme Wind Warning (EWW) Extreme Wind Warning inform the public of the need to take immediate shelter in an interior portion of a well-built structure due to the onset of extreme tropical cyclone winds. An EWW for extreme tropical cyclone winds should be issued when both of the following criteria are met: a. Tropical cyclone is a category 3 or greater. b. Sustained tropical cyclone surface winds of 100 knots (115 mph) or greater are occurring or are expected to occur in a WFO’s county warning area within one hour. l Hurricane Force Wind Warning A warning for sustained winds, or frequent gusts, of 64 knots (74 mph) or greater, either predicted or occurring, and not directly associated with a tropical cyclone. l 45

l l l l Severe Local Storm Watch Severe Thunderstorm Warning Severe Thunderstorm Watch Severe Weather Potential Statement Severe Weather Probability Severe Weather Statement Storm Warning Tornado Watch Winter Storm Warning Winter Storm Watch Winter Weather Advisory 46

250 mb wind/ HGT(contour) & theta/ HGT(contour) & Q-G frontogenesis/ 300 -350 mb PV at 3/30 12 Z 47

GOES IR 3/31 00 Z ~ 3/31 21 Z (flooding occurrence) 48

3/31 12: 19 HST reflectivity(d. BZ) 12: 23 HST vertically integrated liquid water(Kg/M**2) 49

About TRMM 3 B 42 l 3 B 42 is a level 3 (gridded) product with input from multiple sensors using TMI (TRMM Microwave Imager), SSM/I (Special Sensor Microwave/Imager), AMSR-E (Advanced Microwave Scanning Radiometer ), AMSU-B (Advanced Microwave Sounding Units -AMSUs) and PR (Precipitation Radar). l The combined instrument rain calibration algorithm (3 B-42) uses an optimal combination of 2 B-31, 2 A-12, SSMI, AMSR and AMSU precipitation estimates (referred to as HQ), to adjust IR estimates from geostationary IR observations. l http: //disc. sci. gsfc. nasa. gov/datapool/TRMM_DP/01_Data_Pro ducts/02_Gridded/index. html 50