Tropical Storm Hilda Weakens, Flash Flood Watch Remains for Hawaii

August 12,2015

Highlights:

• Tropical Storm Hilda has slowed to a crawl about 250 miles southeast of Hilo, Hawaii.
• A tropical storm watch remains in effect for the Big Island of Hawaii (Hawaii County).
• A flash flood watch is also in effect for the Big Island.
• Hilda continues to weaken thanks to strong wind shear.
• Crippled by this wind shear, what may eventually be left of Hilda's center is expected to pass south of the Big Island Thursday, as a tropical depression or remnant low. 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?

Latest Information and Infrared Satellite

Hilda Watches/Warnings

Projected Path

A strong subtropical jet stream is proving to be the ultimate guardian of the Hawaiian Islands in the case of Hilda.
The strong west-southwest winds aloft are pushing convection away from the center of Hilda. For a majority of the time since Tuesday, the center of circulation was completely exposed.
A U.S. Air Force Reserve Hurricane Hunter reconnaissance mission Tuesday found Hilda's wind field was becoming increasingly lopsided, with its strongest winds in the eastern semicircle. As of Wednesday local time, Hilda barely hangs onto tropical storm strength.
Hilda's forward motion has also slowed to a crawl. Another factor in the weakening of Hilda is its slow motion, resulting in upwelling of colder sub-surface water.
The latest thinking is this wind shear will eventually fracture Hilda's circulation, leave an increasingly shallow circulation to migrate westward south of the Big Island of Hawaii Thursday, as a tropical depression or as only a remnant low. Most computer model guidance shows weakening below tropical storm strength by late Wednesday or early Thursday local time.
That said,  the Central Pacific Hurricane Center has continued a tropical storm watch for Hawaii County -- the Big Island -- meaning tropical storm-force winds are possible within the next 48 hours.
For now, here are what impacts we're expecting, based on the current forecast:
LIKELY: High surf will continue on east-facing shores of the Big Island and Maui, persisting at least into early Thursday, according to the National Weather Service. Waves of up to 12-18 feet and dangerous rip currents are expected.
POSSIBLE: A flash flood watch remains in effect for the Big Island. Despite Hilda's weakening, the Big Island may see periods of locally heavy rainfall from Wednesday evening through Friday or 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)