High Wind and Heavy Rain in Hawaii, May 3 to 6, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
334 AM HST Fri May 3 2019

.SYNOPSIS...
An out of season cold front will approach the state from the
northwest today, then advance down the island chain Saturday through
early Sunday. Winds will shift out of the south-southwest through
the day, then strengthen late tonight into Saturday as the front
moves into the area. Instability associated with an upper trough
accompanying the front will translate to increasing rainfall
chances. Periods of heavy showers, gusty winds and even a few
thunderstorms will be possible along and ahead of the front. Drier
and cooler air along with northwest winds will fill in behind the
front Saturday night into early next week.

&&

.DISCUSSION...
Recently animated water vapor imagery showed an upper low drifting
southeastward several hundred miles northwest of the state. A cold
front was analyzed around 400 miles northwest of Kauai. Remnant
moisture associated with the tail-end of an old boundary located
over the islands has been fueling showers overnight. Peak rainfall
accumulations ranged from 1 to 2 inches from 6 pm through
midnight over parts of Kauai, Maui and the Big Island. Most of the
shower coverage after midnight has shifted off the coasts due to
the winds becoming light and veering out of the east-southeast
(giving way to land breezes). The exception has been over windward
Maui and portions of the Koolaus, where steady showers have
continued.

Short-term guidance depicts this lingering moisture axis over the
state beginning to lift northward ahead of the approaching front
today. Winds will veer out of the south through the day, then to the
south-southwest late this afternoon through tonight. Daytime heating
along with increasing instability and plenty of moisture will be
enough to trigger afternoon showers with some potentially becoming
heavy.

The front will reach Kauai early Saturday, then bring unsettled
weather down the island chain through Saturday night. We expect the
front will be near Kauai in the predawn hours on Saturday, reach
Oahu midday, move to Maui County Saturday afternoon and evening,
then possibly advance to the Big Island by predawn hours on Sunday.
During this time, a potent upper low will drop southward into the
upper trough and approach the state from the north. A jet stream
along the southern flank of the upper low should move over the
islands and produce forcing that will generate heavy showers along
the front. The eventual position and strength of the compact upper
low will be critical, but at this time, the greatest amount of
forcing and coldest air aloft look to remain just northeast of the
state. However, a line of heavy showers or thunderstorms along the
front could be capable of producing strong winds. Another item of
note is the potential for gusty downsloping southwest to west winds
along the front. It does not appear that a wind advisory will be
needed, though localized wind gusts of around 40 mph are a strong
possibility on Saturday, especially on Oahu and parts of Maui
County. At higher elevations, the high summits on the Big Island
will almost certainly experience warning level winds, while advisory
level conditions are expected on summit of Haleakala.

On Sunday, the front will likely weaken near the Big Island as the
compact upper level low passes north of the state. Latest guidance,
however, depicts a secondary band of moisture moving through that
could produce another round of showers and gusty northwest winds.

Stable and rather cool conditions are expected to fill in on the
northwesterly flow later Sunday and Monday. The latest guidance
drops dew points into the upper 50s, which will provide rather
cool conditions for early May. As the upper level low weakens and
lifts off to the northeast, stable and rather dry northerly winds
will likely lead to dry conditions through the middle of next
week.

&&

.AVIATION...
Moisture from an old front has been feeding the showers around
the islands and coastal waters, with most of the landfalling
precipitation affecting Maui and the Big Island this morning.
Much of the rain has been lingering because of increasing
instability from an upper air disturbance and surface front
advancing toward the state from the northwest. VFR will
predominate today, but brief to TEMPO MVFR conditions in lower
ceilings and showers will occur over the individual isles with
greater frequency as the day progresses.

As the surface and upper level features get closer, winds will
veer toward the south through the day.

An AIRMET for mountain obscuration is currently in effect for Maui
and the Big Island.

&&

.MARINE...
A somewhat unseasonable weather pattern will unfold through the
weekend, as an approaching deep-layer low drives a cold front down
the island chain. The front will bring the potential for heavy
showers, a slight chance of thunderstorms, and locally gusty
winds. In addition, sustained SW to NW winds may briefly reach
Small Craft Advisory criteria as the front passes.

Light surface winds are in the process of veering to the S this
morning, due to the gradient between a ridge extending toward the
islands from the NE, and the approaching cold front, still about
350 nm NW of Kauai. Winds will veer to the SW and increase as the
front draws closer tonight through Saturday. The front is
expected to pass Kauai early Saturday, Oahu Saturday afternoon,
Maui county Saturday evening, and the Big Island Saturday night
and Sunday. A wind shift to the NW is expected after the front
passes, but as the parent low lingers just N of the islands on
Monday, moderate winds are forecast to shift to the W. Light and
variable winds Tuesday may become light to moderate trade winds
Wednesday and Thursday as the low dissipates.

The current small NW swell is expected to persist through Saturday.
The approaching low will send a moderate sized short-period NW
swell toward the islands Saturday night through Monday, with surf
expected to remain below advisory levels as the swell peaks Sunday
night. A small long-period NW swell is possible around the middle
of next week, as is a small short-period NE swell. Relatively
small S and SW swells will arrive in the islands through the
middle of next week, with an uptick in swell energy expected today
into the weekend, and then again early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from this evening through late Sunday night for
Big Island Summits.

&&

DISCUSSION...Gibbs
AVIATION...Kinel
MARINE...Birchard