Remnants of Hilda Continue to Impact Hawaii

August 13,2015

Highlights:

• Hilda has weakened to a post-tropical cyclone as it drifts west-southwest, centered about 235 miles south of Hilo, Hawaii.
• A flash flood watch remains in effect for the Big Island through Saturday morning.
• See below for more information on what impacts are likely or possible in Hawaii.
• Hilda reached its maximum intensity as a Category 4 hurricane on the Saffir-Simpson Hurricane Wind Scale with top sustained winds of 140 mph late Saturday.

(MORE: Expert Analysis | Hurricane Central)

Hawaiian Concern?

A strong subtropical jet stream has been the ultimate guardian of the Hawaiian Islands in the case of Hilda.
The strong west-southwest winds aloft pushed convection away from the center of Hilda. Since Tuesday, the center of circulation was completely exposed, as it slowly weakened.
Through the weakening process, Hilda's wind field became increasingly lopsided, with its strongest winds in the northern semicircle.
Wind shear fractured Hilda's circulation, leaving an increasingly shallow circulation to migrate southwest of the Big Island of Hawaii.
For now, here are what impacts we're expecting, based on the current forecast:
POSSIBLE: A flash flood watch remains in effect for the Big Island. Despite Hilda becoming a remnant low, portions of the Big Island will see periods of locally heavy rainfall through early Saturday, particularly on east-facing and southeast-facing slopes. Flooding outside normal flood zones is possible.
(INTERACTIVE: Hawaii Radar)
Virtually every system approaching Hawaii from the east since 1950 tracking at least as far north as the latitude of the Big Island of Hawaii eventually weakened to a tropical storm or depression by the time it reached the islands.
(FORECASTS: Honolulu | Hilo | Kona Coast | Maui)
We discussed the reasoning behind this in a piece written in August 2014.
Last August, Iselle became only the second tropical storm to landfall on the Big Island dating to 1950, after strengthening to a Category 4 hurricane.
Incidentally, hurricanes Julio and Ana also passed near the Hawaiian Islands in 2014. Ana was one of only four hurricanes since 1950 to pass within 150 nautical miles of Honolulu, dumping locally heavy rain and generating high surf.

Dating to 1950, the probability of a named storm tracking within 100 miles of Hawaii is greatest in El Nino years, less in La Nina years.

Hurricane specialist Michael Lowry says 17 tropical cyclones of at least tropical storm intensity have tracked within 100 nautical miles of Hawaii dating to 1950. Three of those -- Tropical Storm Flossie (2013), Tropical Storm Iselle (2014) and Hurricane Ana (2014) -- have done so since 2013.
This may not be the season's last named storm to gain Hawaii's attention.
NOAA's 2015 central Pacific hurricane season outlook cited El Nino's tendency for reduced wind shear and more storm tracks coming from the eastern Pacific as reasons to expect an active season in the central Pacific Basin.
Lowry says dating to 1950, there is a 13 percent increase in the chance of a named storm to track within 100 miles of the Hawaii islands during an El Nino year than a neutral year.
MORE: Hurricanes By the Numbers (PHOTOS)