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Temp5.4 C
RH37 %
WindSW 15 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 211344

Area Forecast Discussion
National Weather Service Honolulu HI
344 AM HST Mon May 21 2018

A high far northeast of the islands will move east and weaken as a
front approaches from the north. A new high will build north of
the area through the second half of the week.


As of 3 AM, surface observations and satellite loops show locally
breezy easterly trade winds blowing across the islands. An area of
showery low clouds that has been moving over the islands has
partly broken up and is moving away to the west.

A 1025 mb surface high about 1150 miles northeast of the islands
near 31N 141W will weaken and move east as a front moves toward
the islands from the north. The front will stall north of the
islands and a strong new high will build north of the islands
through the second half of the week.

Water vapor satellite loops show a low aloft about 750 miles west
of Kauai with a ridge aloft over the islands. The ridge will
keep the atmosphere relatively stable so showers will be focused
over mainly windward areas, and excessive rainfall is not
expected. Lower leeward areas will remain mostly dry except for
afternoon showers over the lee slopes of the Big Island and east
Maui. Southwest flow aloft between the low and ridge will carry
some high, cirrus, clouds over the islands.

Close monitoring of Kilauea on the Big Island continues, as
Hawaii Volcano Observatory personnel indicate that additional
steam-driven explosions are possible at any time. Ashfall
Advisories or Warnings will be issued if necessary.


The strong and gusty low-level trade wind flow persists across
the island chain early this morning. In addition, relatively
stable atmospheric conditions prevail across the region, with a
low-level trade wind inversion around 7000 ft. Low-level
mechanical turbulence will continue to the lee of the higher
terrain on all of the islands. As a result, AIRMET TANGO remains
in effect, and will likely continue through tonight.

The northern fringe of a band of moisture is affecting Kauai,
while clouds and showers appear to be decreasing from Oahu to the
Big Island. Brief periods of MVFR ceilings and reduced visibility
in showers are mainly across windward slopes. Cloud cover does
not appear to be extensive enough to warrant an AIRMET for
mountain obscuration on Kauai at the moment.

Brief explosive eruptions at Kilauea/Halemaumau crater continue
to produce periods of volcanic ash with a plume of emissions
extending to the west and southwest over the Kau District. There
was a brief ash plume just before 100 AM HST, while a more
continuous ash plume became evident starting around 200 AM HST.
Volcanic ash SIGMET TANGO remains in place at this time. Experts
from the Hawaii Volcano Observatory continue to emphasize that
additional explosive episodes may occur at any time, and with no
advanced warning.


Fresh to strong trades associated with high pressure north of the
waters are forecast to hold through the week. A small craft
advisory for the typically windier channel waters east of Molokai,
south of the Big Island and Maalaea Bay remains in effect through
Tuesday. Seas will remain rough over exposed waters, especially
south of the Big Island and the channels.

The long-period south-southwest (190-200 deg) swell that filled
in and peaked over the weekend will continue to ease through
Tuesday. Another similar pulse out of the south-southwest
(200-210 deg) is expected to begin filling in through the day
Wednesday, then hold through Friday before lowering into the
upcoming weekend.

Up north, surf will continue to focus at select reefs exposed to
trade wind energy wrapping in through the first half of the week.
A small north-northwest (320 deg) pulse is expected to fill in
Wednesday, which will support small surf along north and west
facing shores.

Surf along east facing shores will remain rough as fresh to
strong trades hold locally and upstream south of high pressure. A
gradual downward trend is possible through Wednesday as a front
passes to the north and the large area of upstream fresh breezes

For the long range, guidance continues to depict plenty of energy
setting up within Hawaii's swell window from the Tasman Sea over
the next few days. ECMWF and GFS show back-to-back gale- to
storm-force lows nosing northward through the Tasman focusing
energy down the 200-220 deg directional band Monday through
Wednesday (peak seas reaching the 30-40 ft range). WAVEWATCH III
depicts two main long-period pulses from these sources out of 220
deg filling in at Pago Pago with the first source arriving
Thursday night into Friday (5/25) and the second pulse arriving
Saturday night into Sunday (5/27). If this scenario evolves as
predicted, these sources should make it to the local Hawaiian
waters toward the end of the month (around 5/29).


Small Craft Advisory until 6 PM Tuesday for Maalaea Bay, the
Pailolo and Alenuihaha channels and waters south of the Big



Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office