Prepared by: Chip Guard, Warning Coordination Meteorologist, National Weather Service Forecast Office Guam Dr.Mark Lander, PEAC Scientist, Water and Environmental Research Institute, University of Guam
This is the coordinated National Weather Service (NWS) Weather Forecast Office (WFO) Guam and University of Guam (UOG) Water and Environmental Research Institute of the Western Pacific (WERI) assessment for tropical storm and typhoon activity for the island of Guam for the remainder of 2019. This assessment is based on independent Pacific-wide typhoon predictions, internal forecast schemes for Micronesia activity, the current and predicted states of the El Niño—Southern Oscillation (ENSO) climate pattern, and the historical tropical cyclone activity associated with past ENSO states. It is prepared for the Government of Guam and Guam citizens and visitors. These predictions could change over the next few months.
Background: For Micronesia, there is a relatively predictable relationship between tropical cyclone activity and the state of ENSO. In the tropics, tropical cyclones generally move from east or southeast to west or northwest. Thus, if a tropical storm or typhoon develops southeast or east of Guam, it will often track toward the island. If a tropical storm or typhoon develops west or north of Guam, it will usually move away from the island. When an El Niño occurs, tropical storms and typhoons begin to develop earlier in the year and farther to the east toward eastern Micronesia. They tend to move toward the west, west-northwest or northwest, often toward Guam. When a La Niña event occurs, the storms tend to develop later in the year and west of or near the Mariana Islands. In this case, they usually move west before significantly intensifying. During El Niño events, the chance of Guam getting a direct hit triples when compared to the chance during non-El Nino periods. During an ENSO-neutral state, which is the transition state between El Niño and La Niña, the chance of getting a direct hit by a tropical storm or a typhoon is much better than during La Niña, but not as good as during El Niño. In general, the odds of Guam getting a typhoon are about 1 in 5 or about once every 5 or 6 years. In El Niño years, the odds are 1 in 3 or about once every 3 years, while in La Niña the odds drop to 1 in 10 or about once every 10 years.
Options: So where are we now? We are now in the waning stages of a weak El Niño. What do we expect for the future? Most climate forecast models suggest that we will remain in an El Niño state, either weak or moderate, until the end of the year. While back-to-back El Niño events are not common, we have had one as recently as 2014 and 2015. We have had a rather robust post-El Niño drought and only feeble early season tropical cyclone development. Super Typhoon Wutip was an extension of the 2018 season rather than the beginning of the 2019 season.
Outlook: So what does this mean for Guam?
1. Our predictions are similar to those for 2018. We expect more tropical cyclone activity than we had in 2016 and 2017, about the same activity as in 2018 for Guam (Typhoon Mangkhut, Severe Tropical Storm Maria), but not quite as busy as 2018 for the CNMI (Super Typhoon Yutu, Typhoon Mangkhut, Typhoon Jebi).
2. For the remainder of the year, there is a 50-60% chance (1 event every 1.5-2 years) of getting a strong tropical storm (sustained winds 50-73 mph) and a 25-30% chance (1 event every 3-4 years) of getting a Category 1 typhoon (sustained winds 74-95 mph). The chance of getting a Category 2 typhoon (sustained winds 96-110 mph) is about 15-18% (1 event in 6-7 years), while the chance of getting a Category 3 typhoon (sustained winds 111-129 mph) is around 8-10% (1 event every 10-13 years). Chances of getting a Category 4 typhoon (sustained winds 130-155 mph) is around 3-4% (1 event every 25-35 years) and finally, the chances of getting a Category 5 (sustained winds 156-195 mph) is less than 1-2% (1 event every 50-100 years). These percentages will likely need to be fine-tuned as the season evolves. Tropical cyclone activity for Guam could begin a little late, toward mid-summer, but keep in mind that the weather patterns can change quite rapidly. Remember, we are in the only basin that can get a typhoon any month of the year. There is no meteorological reason why Super Typhoon Yutu could not have passed over Guam instead of Saipan and Tinian. Thus, the odds of getting a severe tropical storm or a Category 1 or 2 typhoon is actually pretty good with the expected weather pattern.
3. A move toward a moderate El Niño will likely decrease the sea level heights, exposing the reefs to more direct sun during low tides and increasing chances for coral bleaching.
4. The below normal rainfall we are experiencing will transition to above normal rainfall by August. Normal to above normal rainfall will likely persist through the end of the year.
Keep in mind that these predictions can change as the year progresses.