Thursday, December 4, 2014

Hagupit's (Ruby's) Storm Surge Much Different than Haiyan's (Yolanda's)

As Super Typhoon Hagupit (Ruby) approaches the Philippines, we wonder about the storm surge potential. This is especially fresh in everyone's mind, as Super Typhoon Haiyan (Yolanda) generated a massive storm surge along the coasts of Samar and Leyte just 13 months ago. Surge heights near Tacloban exceeded 6.1 m (20 ft), and thousands of people died.

Hagupit's storm surge will be different than Haiyan's for several reasons. Fortunately, Hagupit appears to be weakening as it approaches the Archipelago. The latest forecast from the Joint Typhoon Warning Center suggests the center of Hagupit's circulation will skirt the northern coast of Samar as a category-3 typhoon, with sustained winds of 100 kts (115 mph, 195 kmh). Although such winds are powerful enough to cause considerable damage, they are a far cry from Haiyan's maximum sustained winds of 195 mph, which was the most powerful wind speed at landfall of any tropical cyclone in history.

Super Typhoon Hagupit is forecast to track near the northern coast of Samar as a category-3 cyclone.
Image: Weather Underground

Nonetheless, satellite loops from approximately 0300-0600 UTC Dec 05 show Hagupit coming out of an eyewall replacement cycle. On the back end, a large eye, and likely a large wind field, emerge. Large wind fields generate higher surges than smaller wind fields, and may help a category-3 typhoon generate a surge more typical of a category-4 or 5 system. So we must remain vigilant about potential surge.

Hagupit developed a large eye and likely a large wind field following an eyewall replacement cycle. The large wind field is of concern because it will help enhance storm surge levels. Image: NOAA

Hagupit is also taking a different track than Haiyan, which should change the storm surge pattern considerably. Both PAGASA and the Joint Typhoon Warning Center forecast Hagupit to track near the northern coast of Samar. This will mean that the highest surge potential will be for areas of northern Samar and southern Luzon, while areas of southern Samar and Leyte, which observed catastrophic surge in Haiyan, should observe relatively low surges.

Surge snapshot map for Typhoon Hagupit shows high, medium and low surge potential as Hagupit's eye skirts the northern coast of Samar. Note the offshore winds, and low surge, near Tacloban.

The map above provides a potential surge "snapshot" as Hagupit's eye tracks near the northern coast of Samar. In this scenario, offshore winds along the east coast of Leyte, near Tacloban, would minimize surge levels, and even produce negative surges in some areas. It should be noted that this surge map is a possible scenario that could happen at one point in time. This map does NOT indicate predicted surge heights for the duration of the storm.

Also keep in mind that it is better to consider a range of landfall locations instead of just zeroing in on one forecast path. Therefore, this map should just be used to provide a concept of surge potential. Also note that storm surge heights are very localized. Therefore, although this map paints coastal regions in the same color, in the real world, surge levels will vary considerably across short distances.

In November, 2013, Super Typhoon Haiyan tracked just south of Tacloban, Leyte. This track enabled destructive winds to blow into San Pedro and San Pablo Bay, pushing a massive surge towards Tacloban. This image was originally posted in the blog on Nov 12, 2013. Satellite Image: CIMSS/ SSEC/ Univ of Wisconsin- Madison.

Although it is likely that storm surge levels will be low near Tacloban and other areas devastated by Super Typhoon Haiyan last year, some areas in northern Samar and southern Luzon will likely observe higher surge levels than during Haiyan, because of Hagupit's different storm track.

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