Hawai'i Ho'ohekili Skywarn Weather Spotter Newsletter National Weather Service, Honolulu, HI Dry Season Edition, 2018 Issued — May 2018 Spotter Newsletter Volume 17 La Nina gone, but what’s next?? During April 2018, the tropical Pacific returned to ENSO- neutral, as indicated by mostly near-to- below average sea surface temperatures (SSTs) along the equator (See Figure). The latest weekly Niño indices were near zero in all regions. Atmospheric indictors related to La Niña also continued to fade. While convection remained suppressed near and east of the Date Line, rainfall near Indonesia was also below average during the month. Low -level winds were near average over most of the tropical Pacific Ocean, and upper-level winds were anomalous westerly over the eastern Pacific. Overall, the ocean and atmosphere system reflected a return to ENSO-neutral. Making ENSO forecasts in spring is especially complicated. It’s a time of transition, and small changes in conditions can have large effects down the road. This month’s ENSO forecast finds it most likely that neutral conditions will last through the summer and into early fall. What about next winter?? The forecast possibility of El Niño nears 50% by the winter, as many of the computer models are trending to warmer tropical sea surface conditions in the later months of 2018. In the historical record (dating back to 1950) we’ve had 7 two-year La Niña events. These events have been followed by El Niño twice: 1972 and 2009. The probability of remaining in neutral conditions is about 40%, something that has happened three times in the record: 1956, 1985, and 2012. Less likely is a return to La Niña conditions—that scenario is given about a 10% chance. One thing we can be certain of is that forecasters are looking forward to next month, when the models are increasingly getting past the spring predictability barrier and are more reliable. (Text from the ENSO Diagnostic Discussion issued by CPC, and the ENSO blog from NOAA.) Record Rainfall and Flash Flooding - Kevin Kodama, Service Hydrologist April is the last month of the wet season for the Hawai- ian Islands. Usually, heavy rain event frequency drops off and rainfall totals decrease. Not this year. The most significant events of the entire wet season, both in terms of hydrologic magnitude and impacts to the pub- lic, occurred in mid-April 2018. The events were set up by a large scale environment consisting of an upper lev- el low pressure system which tapped into enhanced moisture embedded within the low level trade winds. These conditions produced torrential rainfall and severe flooding over portions of Kauai and Oahu from late Fri- day, April 13 through Sunday, April 15, 2018. The rain- fall on Oahu came from a mesoscale convective system (MCS) that was moving westward toward Oahu in the trades. Prior to making landfall, the MCS appears to have merged with a leeside convective cloud line streaming off of northwest Molokai and intensified. The area of in- tense rainfall with rates greater than 5 inches per hour moved across east Oahu between 7:00 PM and 9:00 PM HST on April 13. Although the affected area was localized, the intensity of the rainfall produced tremendous amounts of run- off which produced substantial damage in Waimanalo along the windward slopes of the Koolau Range, and from Aina Haina to Hawaii Kai along the leeward slopes. The MCS dissipated south of Oahu shortly after moving offshore. The Kauai event began shortly before midnight on Saturday, April 14 when the initial thunderstorm cells moved over northern side of the island. Several waves of thunderstorms through the morning hours resulted in the closure of Kuhio Highway at Haena and at the Hanalei Bridge. The Saturday morning heavy rainfall was not sustained since the cells moved across the area instead of anchoring in one spot. By 8 AM HST, the highway at the Hanalei Bridge reo- pened as conditions settled down. However, shortly after noon the first of three episodes of intense rainfall initiated over north Kauai. Unlike most heavy rain events which affect the island, this round of rainfall was centered along the lower elevations and coastal sections instead of over the upper slopes. Around this time several landslides between Waikoko and Wainiha closed Kuhio Highway. As rainfall continued through the afternoon, severe flooding washed out the Wainiha Powerhouse Road and destroyed two unoccupied homes. The first intense rainfall episode ended at around 7 PM HST as rainfall eased over north Kauai. The second intense rainfall episode started around midnight on April 15 with thunderstorms firing up over the north- facing slopes of Kauai. Rainfall rates greater than 5 inches per hour were recorded during the peak of this episode. With heavy rainfall covering most of the northern drainage basins, streams and rivers responded dramatically. Flash flooding in Hanalei River put portions of Kuhio Highway near Hanalei Bridge under 5 to 8 feet of water. Large flood waves in Wainiha River and Kalihiwai Stream inundated several properties as well. At around sunrise, the thunder- storm centroid shifted off the northern drainages and moved over northeast Kauai. While this helped lower water levels in the north part of the island, the shift resulted in damaging flash floods in Anahola Stream and Kapaa Stream, and flooding in other smaller drainages. Thunderstorms rapidly weakened then dissipated around 8 AM HST to end the second episode. By 10 AM HST, it became evident that the event was not over as rain bands developed east of Kauai and moved over the island. The intensification of these rain bands marked the start of the third rainfall episode, and these thunder- storms became anchored once again over the northern slopes of the island. With the drainage basins already saturat- ed, the resulting flood wave in Hanalei River was very large and put Kuhio Highway under deep water once again. Around 1 PM HST, the thunderstorms shifted to the northeastern portion of the island. Over the next several hours, the storm centroid shifted slowly southwestward putting intense rainfall of over 4 inches per hour over eastern then southeastern Kauai. Significant flooding occurred in the Koloa and Kalaheo areas, with homes off of Wailaau Road sustaining considerable damage. The last thunderstorms of the third rainfall episode dissipated around sunset. Rainfall totals on Kauai were record-breaking. The highest 24-hour total from the real-time sites was 28.15 inches at Hanalei from 2:00 AM HST April 14 to 2:00 AM HST April 15. The data logger failed at the Hanalei site during the event so the total would have been higher. An observer from the Community Collaborative Rain, Hail, and Snow Net- work (CoCoRaHS) in lower Wainiha Valley reported 36.49 inches during the period from 7:45 AM HST April 14 through 7:00 AM HST April 15. A couple of days after the event, data from an automated rain gage in Waipa about 1 mile west of Hanalei were retrieved and analyzed. The results showed that this site recorded 49.69 inches during the 24-hour period ending at 12:45 PM HST April 15. This total, if certified, breaks the current U.S. 24-hour record of 43 inches at Alvin, TX on July 25-26, 1979, and the State of Hawaii record of 38 inches at Kilauea (Kauai) on January 24-25, 1956. The National Climatic Extremes Committee is in the process of validating this claim. It was fortunate and amazing that there were no deaths or reports of serious injuries. The Hawaii Emergency Management Agency reported that 532 homes were affected by flooding on Kauai and Oahu. Damage estimates were already close to $20 million just for public properties. Estimates of damage to private and other properties were not yet available. What does NWS Honolulu do during a volcanic eruption? 1. Immediately issue a Volcanic Ash SIGMET. A SIGMET, or Significant Meteorological Infor- mation, is a weather advisory that contains meteorological information concerning the safety of all aircraft. 2. Immediately issue an Ashfall Advisory or Warning for the potentially affected areas. 3. Issue a Marine Ashfall Advisory/Warning if ash expected over the Coastal Waters. 4. If ash is expected at any airports, amend the Airport Forecast to include volcanic ash. 5. Coordinate with the Volcanic Ash Advisory Center (VAAC) in Washington DC on subse- quent advisory/warning products. For volcano updates see: https://volcanoes.usgs.gov/volcanoes/kilauea/status.html Plumes from Halemaumau Crater and the Lower East Rift near Leilani Estates 2017 vs Past Hurricane Totals 2017 Track Review Central Pacific Tropical Cyclones By Year (1970 - 2017) 18 16 16 East Pacific 14 Central Pacific 12 11 11 10 Average Per Year: 4.6 10 9 8 8 7 7 7 7 7 6 6 6 6 5 5 5 5 5 5 4 4 4 4 4 4 4 4 3 3 3 3 3 3 2 2 2 2 2 2 2 2 140W 1 1 1 1 1 0 0 0 0 1970 1972 1974 1976 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 On the Web: NOAA National Weather Service Honolulu HI: http://www.weather.gov/hawaii/ FEMA Flood Preparedness Information: https://www.ready.gov/floods Hawaii Emergency Management Agency: http://dod.hawaii.gov/hiema/ U.S. Drought Monitor: http://droughtmonitor.unl.edu/ Edward “Chevy” Chevalier presents at the latest Skywarn Spotter Training class at NWS Honolulu. This class was attended by Amateur Radio Operators (Ham) from Oahu. .