Hurricane (Typhoon) — Guam, Guam

2023-05-23 to 2023-05-25 · Guam, Guam

Event narrative

On May 17th a circulation that had been over the area for a few days was classified as Invest 97W by JTWC. 97W was located to the west-southwest of Chuuk at that time. Scatterometer data showed a west-wind burst south of the circulation with winds up to 30 knots. On the morning of May 18th, 97W was located near 6N147E. At this time strong convection was located to the southeast of the center with cloud-top temperatures down to -83C. These temperatures correspond to cloud tops up to 56k feet. The circulation continued to organize over the next few days. Scatterometer satellite imagery shows a well-defined circulation center surrounded by 20 to 25-knot winds by the 20th of May.

On the afternoon of May 20th, the circulation was classified as Tropical Depression 02W. The tropical depression was located near 5.6N 149.0E or about 235 miles west-southwest of Chuuk with maximum sustained winds of 30 mph. 02W continued to intensify and by the morning of the 21st of May it became Tropical Storm Mawar. Mawar was located near 5.9N 149.2E at about 185 miles west-southwest of Chuuk.

Winds were up to 40 mph. Mawar continues to intensify and by the morning of the 22nd of May, it became a typhoon. At that time Typhoon Mawar was located near 8.3N 148.1E, or about 260 miles west-northwest of Chuuk with maximum sustained winds of 80 mph.

Early on the morning of May 23rd, the eye entered radar range of the radar. The radar indicated a circular eye with a diameter between 20 to 25 miles. Radar at that time may have been seeing the highest portion of the eye. Because of the Stadium Effect, the earlier figure may be too large. As Mawar approached Guam the size of the eye measured by the radar decreased by mid-afternoon to around 12 miles.

The eye became elliptical at times during the afternoon. As Mawar got closer to the

radar it could calculate wind speeds in the system. By late afternoon the Doppler radar indicated winds of 140 mph at 20000 feet.

Mawar continued to intensify as it moved towards the Marianas and by the evening of May 23rd it was located near 12.1N 146.3E or about 140 miles southeast of Guam. By that time Mawar had winds up to 155 mph. In the Northwest Pacific, a typhoon with winds of 150 mph or higher is classified as a super typhoon.

By the morning of the 24th of May, Mawar was located only 70 miles southeast of Guam. During the morning of the 24th radar and satellite imagery detected the eye was deteriorating slightly. It was determined that Mawar was undergoing an eyewall replacement. During an eyewall replacement cycle(ERC) winds near the eye

usually decrease briefly. This also happened to Mawar and later that morning it was estimated to have decreased from 155 mph to 140 mph.

For the nearly 30 hours that the center of Mawar had been within range of the Guam radar, Mawar's motion displayed the typical trochoidal bobble that TCs often make. Mawar's prolonged northward bobble on Wednesday, 24 May spared Guam's low-lying southeast coastline from potentially significant coastal inundation but also brought the heaviest rains and strongest winds of the southern eyewall over much of Guam.

Mawar's closest point of approach to Guam was between 08Z-10Z 24 May, as the center of Mawar's eye passed just north of Andersen AFB and Ritidian. The southern eye may likely have come on shore, based on some accounts from AAFB.

Between 05Z and 18Z of 24 May (3 pm 24 May and 4 am 25 May), much of Guam felt the brunt of the southern eyewall. Destructive northwest winds became typhoon-force west winds soon after sunset, then increased further while turning southwest before midnight (24 May/14Z).

Ground assessments by NWS staff and unmanned aerial drone assessments by the University of Guam Drone Corps show that most damage came from the west-northwest (early winds, as Mawar's center approached north Guam) to the southwest (after the closest point of approach, as Mawar's center moved away from Guam).

While many trees snapped from winds of these directions, stronger trees were snapped or blown down by west and southwest winds. TC intensity guidance pointed to an onset of rapid intensification around 12Z following the completion of the ERC.

A review of the Guam International Airport observations (PGUM) shows that the duration of tropical storm force winds, and greater (39+ mph), was nearly 14 hours'approximately from 03Z to 17Z 24 May (1 PM 24 May to 3 AM 25 May).

The highest winds recorded was on an RSOS at WFO Guam with maximum sustained winds of 91 mph with gusts to 132 mph at 2154 CHst on the 24th. The ASOS at the Guam International Airport recored sustained winds of 71 mph wihth gusts to 105 mph at 1954 Chst on the 24th. The lowest SLP of 970.9 mb was from the ASOS at the Guam International Airport at 2154 Chst on the 24th. WFO Guam report the minimum

SLP at 966.5 mb at 2200 Chst on the 24th.

During its slow passage over Guam, the southern eyewall of Mawar brought long-duration torrential rains with much of Guam receiving around 2 feet of rain in the 72 hours of 23-25 May. At the Guam International Airport (WFO Guam), 24.71 inches of rain was recorded during a 24-hour period from 24 May/00Z to 25 May/00Z (10 am Wednesday 5/24 to 10 am Thursday 5/25 ChST), with the bulk of this rain occurring in a 16-hour period from 04Z to 20Z 24 May (2 pm 24 May to 6 am 25 May ChST).

A USGS rain gauge in Dededo (northern limestone plateau of Guam) recorded a rainfall rate of 6.92 inches per hour from 1145Z to 1245Z (945 p.m. to 1045 p.m. ChST) Wednesday evening, 24 May, as the southern eyewall pivoted overhead. This rain gage also recorded the highest storm total of 28.12 inches.

While Mawar led to remarkable rainfall across Guam, it was not unprecedented: 33 inches of rain was measured during the slow passage of Typhoon Pamela in 1976. The JTWC 1976 annual report also notes that 27 inches of rain occurred during a 24-hour period during Pamela's passage. Although there is no rainfall data during the passage of Karen (1962), historical records suggest that Mawar's 24-hour and storm total rainfall are possibly the second-greatest on record for Guam (daily data from 1945 to present).

Once to the west of Guam, Mawar steadily intensified to a peak satellite-based estimated intensity of 185 mph at 26May/03Z as noted in JTWC's Bulletin #25.

The center of Mawar passed very close or directly over the PacIOOS wave buoy located roughly 3 miles off of northern Guam's Ritidian Point. The buoy measured a significant wave height of 28.97 feet at 0930Z, 24 May, with a maximum wave height of 47 feet. Between 0930Z and 1130Z, the Ritidian buoy showed a steady drop in significant

wave heights as the center of Mawar passed nearby. At 1130Z, the buoy reported a significant wave height minimum of 17.13 feet before the seas steadily rose. Sometime soon after 1430Z, the buoy stopped reporting as it broke from its mooring. Mawar's slow westward crawl, combined with its rapid re-intensification, led to an abrupt shift and rise in west-southwest swell.

An increase in WSW swell energy led to significant coastal erosion along the western side of Guam, particularly northern Guam. The most dramatic coastal effects were noted along the northwestern Guam coastline north of Tanguisson to the western side of Ritidian Point. This is where high water marks were observed up to nearly

15 feet above the normal water level along with an erosional step 3-5 feet tall in some locations. Guam's coastlines saw extensive erosion along all shores with salt water running up well above normal water levels due to the combination of storm surge, ocean surface wind stress, and wave action. These shore-line effects were most noted along Guam's northwest coastline, where erosion left 'steps' in the beach along the tree

line 3-5 feet tall and pushed debris as high as 10-14 feet above the normal water level while powerful winds stripped jungles and cliff sides of their lush vegetation.

Guam, having experienced the totality of the southern eyewall for a significant portion of the overnight hours Wednesday night into Thursday morning (24-25 May ChST), saw extensive, and in some locations, devastating damage. Nearly 100% of Guam was left without power, water and most cellular communications. One mandatory evacuation was issued for folks in low-lying coastal areas of southeast Guam with the concern of potentially devastating coastal inundation as the (then) Super Typhoon Mawar was forecast to pass just south of Guam.

Impacts ranged from moderate in the south to devastating in the north, closest to the

eye of Mawar, where the full brunt of the category 4 typhoon winds was felt. While

concrete structures received little damage, significant impacts were noted to wooden

and tin structures, particularly in northern Guam. Defoliation of trees was near 100% in

parts of central and northern Guam, except for areas that had some topographical shielding from the winds.

Some structures, including high-rise hotels, exhibited damage to facades. Across

northern Guam, numerous wood and tin structures were destroyed. Houses with concrete walls but wood or tin roofing saw extensive or complete roof failure.

While concrete power poles held firm, several wooden power poles were snapped or tilted. Throughout the island, and particularly in central and northern Guam, numerous power and communications lines were also downed.

Numerous roads were impassable due to large downed trees or other wind-blown debris, while a number of low-lying roads and those with poor drainage were impassable due to severe rain-related floodwaters.

Guam's coastlines saw extensive erosion along all shores with salt water running up well above normal water levels due to the combination of storm surge, ocean surface wind stress, and wave action. These shore-line effects were most noted along Guam's northwest coastline, where erosion left 'steps' in the beach along the tree line 3-5 feet tall and pushed debris as high as 10-14 feet above the normal water level while powerful winds stripped jungles and cliff sides of their lush vegetation.

The island's last mature specimen of the critically endangered Serianthes nelsonii tree, also known as the hyun lgu in CHamoru, was ravaged by Typhoon Mawar. While the trunk of the tree is still rooted in the ground, the main boughs of the tree were snapped off and all that remains is a leafless wood pillar, according to a report from Marine Corps Base Camp Blaz, where the tree is located. It remains alive, but recovery

is uncertain, the reports stated. The tree, mature since the 1960s, would have survived Supertyphoon Karen in 1962, as well as typhoons Pamela, Omar, Paka and Pongsona.

Two S. nelsonii survived the storm at the University of Guam. These trees were germinated in June 2020 using seeds collected from the mature tree on Camp Blaz. About 3,000 seeds remain in storage, according to Mario Martinez, a NAVFAC Marianas natural resource specialist. With numerous pests affecting the species, they require a large amount of attention to thrive.

Although there have been no reports of injuries or deaths directly resulting from Mawar, 2 males drowned in separate swimming incidents: one having swam out to an island on the reef's edge of central west Guam as Mawar approached and was subsequently pulled into the powerful currents; and a 2nd having entered the waters near a SW Guam marina within hours of Mawar having moved to the west of Guam and subsequently was pulled out to sea in the strong currents.

Wider weather episode

Mawar developed over the Northwest Pacific Ocean in the second half of May. It moved northwestward from near Chuuk to eventually cross the Mariana Islands near Guam and Rota.

Source: NOAA Storm Events Database, event_id 1120097. Narrative written by the NWS forecast office that issued the report.