Tropical Storm Hilda Weakening in the Central Pacific; Tropical Storm Watch in Effect for Hawaii

August 11,2015

Highlights:

• A tropical storm watch was issued Tuesday morning by the Central Pacific Hurricane Center for the Big Island of Hawaii.
• Formally a hurricane, Hilda was downgraded to a tropical storm with the 2 p.m. Tuesday evening local time advisory.
• Hilda remains a strong tropical storm with maximum sustained winds of 65 mph as of Tuesday afternoon, Hawaii time.
• The center of Hilda is about 265 miles southeast of Hilo, Hawaii, and is moving slowly west.
• Hilda is expected to continue to weaken due to increasing wind shear over the next few days.
• The latest thinking suggests a weakened Hilda may move near or south of the Big Island Thursday, with impacts lingering into Friday. 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

Hawaii Rainfall Forecast

A U.S. Air Force Reserve Hurricane Hunter reconnaissance mission Tuesday confirmed that wind shear is beginning to take a toll on Hilda. Hilda's forward motion has also slowed considerably, and will likely continue into Wednesday.
Hilda may still be a named tropical system as it limps toward Hawaii's Big Island in a much weaker state, either as a tropical depression or a weak tropical storm.
Satellite imagery Tuesday afternoon, Hawaii time, showed convection becoming displaced east of the center of circulation. This indicates a weakening trend, one that should continue into Wednesday.
Already a rather small tropical cyclone, Hilda will feel intensifying shearing winds aloft from a strong subtropical jet stream blanketing the Hawaiian Islands and areas to the east of the island chain. Shearing winds push a tropical cyclone's thunderstorms away from its center, weakening it, as we saw with Guillermo last week. The entrainment of ver dry air will also aid in steady weakening of Hilda. Another factor in the expected weakening of Hilda is its slow motion could result in upwelling of colder sub-surface water.
The latest thinking is this wind shear will leave an increasingly shallow circulation of Hilda to migrate west-northwest toward the Big Island of Hawaii Thursday.
With the rate of weakening still a little uncertain, the Central Pacific Hurricane Center hoisted 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 and southeast-facing shores of the Big Island and Maui, persisting at least into early Wednesday, according to the National Weather Service. Waves of up to 8-12 feet and dangerous rip currents are expected.
POSSIBLE: Hilda will likely bring widespread and locally heavy rain to the Big Island starting as early as Wednesday, perhaps extending into some of the other islands into the weekend. The Big Island may see 6 to 12 inches of rainfall with isolated amounts up to 18 inches and a flash flood watch has been issued for the Big Island beginning Wednesday evening. Some gusty winds are possible too, particularly over higher elevations and exposed coastal locations.
(INTERACTIVE: Hawaii Radar)
All interests in the Hawaiian Islands should continue to monitor the progress of Hilda.
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)