National Weather Service Forecast Office Guam and the University of Guam Water and Environmental Research Institute Typhoon Soudelor Wind Assessment for Saipan

Typhoon Soudelor passed directly over the island of Saipan (15.2° N : 145.8° E) with a ferocity that stunned island residents. The level of damage, especially to vegetation, wooden homes, and utility poles was severe. The storm had a very unique structure when it hit Saipan late on the night of 02 August 2015: its core was of midget dimensions, with the calm region of the eye only 4 miles across at ground level. In fact, the entire core of the typhoon – eye and eyewall convective ring – tracked within the 12 miles of separation between the north and south bounds of the island. Soudelor's “pin-hole” eye on satellite imagery at its time of passage over Saipan was a spectacular example of this relatively rare phenomenon. The landfall intensity of Soudelor was much higher than anticipated in the last forecasts that most island residents probably heard before the near-midnight rampage of the storm. The forecast intensity shortfall (real and perceived), the severity of the damage (enough to fuel a widespread belief of island residents that tornadoes had accompanied the typhoon), and the very unusual structure of the typhoon prompted a quick request and authorization for a meteorological assessment. On behalf of the Pacific Region Headquarters of the National Weather Service, and working around limited flights to the island, the complete loss of base-load power and municipal water service, and the general post-typhoon chaos, the meteorological assessment team members Charles P. “Chip” Guard and Mark A. Lander arrived on Saipan one week after the typhoon event to conduct a thorough damage assessment.

After reanalyzing more than a hundred original damage pictures obtained on-site by the assessment team, assessing some additional damage information from subsequent visits, and conducting a careful analysis of other factors relating to typhoon intensity, such as the measurements of the minimum central pressure and the characteristics of Soudelor's eye on radar and satellite imagery, the team estimated Soudelor's equivalent over-water intensity to be 115-knot (~130 mph) sustained wind, which is the lower threshold of a Category 4 typhoon. The typical peak gust associated with a tropical cyclone of this intensity is 140 knots (~165 mph). The treefall pattern across Saipan was surprisingly coherent, and nicely delimited the path of the small typhoon across the mid-section of the island. The “First Wind” was dominant at most locations, with the “Second Wind” having a lesser signal in most areas. This was likely the result of the great extent of treefalls in the “First Wind”. The presence of some trees in close proximity felled in opposite directions was thought by some to be evidence of tornadoes, but most, if not all, of the treefall pattern is consistent with the large-scale swirling cyclonic flow of the typhoon itself. Particularly after carefully studying the treefall pattern and the patterns of other damages, the assessment team felt that the large-scale swirling wind of the typhoon with a sustained wind and peak gust of a single magnitude (e.g., 115 knots G 140 knots) would be an appropriate metric from which one could account for all the observed effects of the typhoon. The patches of heaviest damage are thus viewed as areas where, for reasons of complex terrain and exposure, the peak over water gusts of 140 knots were experienced in full force and for an extended period.

What is contained this presentation is the team's meteorological assessment for Typhoon Soudelor in Saipan, including a determination of the landfall intensity and of the factors contributing to the severe level of damage.