Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 222031 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
Issued by National Weather Service SAN FRANCISCO CA
1031 AM HST Tue May 22 2018

Locally breezy trade winds will persist across the state this
week. The low-level trade wind flow will continue to transport
low clouds and brief showers over windward and mauka sections of
the island chain.


.DISCUSSION...The forecast this morning remains on track and no
updates are needed at this time. Please see the previous forecast
discussion below for complete details.


.PREV of 8:28 AM PDT Tuesday...A 1031 mb surface
high is centered near 37N 166W, or more 1100 miles north-
northwest of Lihue. Elsewhere, the leading edge of a band of
broken to overcast low clouds and showers associated with a
weakening surface front is about 440 miles north of Lihue. The
surface high is pushing this front toward the east-southeast at
about 10 mph. The tightening pressure gradient south the high is
producing locally breezy trade winds across the state early this
morning. Evidence for this included an ASCAT pass over the western
and central islands several hours ago, which showed a wide spread
area of 20 to 25 mph winds over the ocean adjacent to the state.

Loops of water vapor imagery indicate the islands are sandwiched
between a weak upper-level ridge to the east and an upper-level
low and trough to the west. The southwesterly flow aloft between
these systems is transporting patches of cirrus rapidly across
most of the smaller islands. The atmosphere remains relatively
stable over the region. Satellite derived precipitable water (PW)
values are near normal across must of the smaller islands from
Oahu to Maui County, while the PW appears to be slightly higher
in the vicinity of Kauai and the Big Island. Satellite imagery
shows broken low clouds along windward sections of most islands.
Radar indicates scattered showers are embedded within the clouds
near the eastern and southeastern Big Island, while isolated
light showers are evident near and upstream of windward Oahu,
Molokai and Maui.

The forecast guidance shows the surface front will continue to
weaken as it shifts toward the east-southeast today. At the same
time, the surface high will build northwest and north of the
state, where it will dominate our weather through the remainder of
the week. As a result, the locally breezy trade winds will likely
persist through the end of this week.

The models show there may be a slight increase in PW starting
Wednesday. This may provide for somewhat wetter conditions across
windward and mauka areas starting around mid-week. However, a
weak mid-tropospheric ridge is forecast to be above the islands
around this same time, so no heavy rainfall is expected.

Note that both the GFS and ECMWF models appear to show remnant
moisture from the front, which was described above, may
eventually be carried toward the state some time this weekend.
This may cause another slight increase in low clouds and showers
over some windward and mauka sections starting late Saturday
or Sunday.

A Special Weather Statement remains posted for the Kau District
on the Big Island due to intermittent volcanic ash emissions at
the Halemaumau / Kilauea crater. At least one of these episodes
occurred shortly before sunset Monday evening, with an ash cloud
detected several thousand feet above the crater. Close monitoring
of the volcano on the Big Island will continue, since scientists
at the Hawaii Volcano Observatory indicate that additional steam-
driven explosions may occur at any time, and with no advance
warning. Ashfall Advisories or Warnings will be issued if


High pressure north of the area will maintain locally strong trade
winds for the next few days. AIRMET TANGO for low level
turbulence is posted over and immediately south through west of
the mountains. Trade winds will help focus scattered showers along
the windward and mountain areas. Expect conditions to remain
widely VFR, with brief periods of MVFR is passing showers.
Conditions are not expected to be widespread enough to warrant the
need for mountain obscurations.

Periodic explosive eruptions at Kilauea/Halemaumau crater
continue to produce periods of volcanic ash, with the plume of
emissions predominately rising to the inversion level, between
8-10 kft and extending to the southwest over Kau district on the
Big Island. MVFR VIS should be expected in this area. Volcanic
ash SIGMET TANGO series remains in place at this time.


High pressure will shift slowly eastward well to the north of the
state through the work week, keeping moderate to breezy trade
winds in place across the coastal waters. The trades may trend
down slightly over the weekend into early next week as high
pressure weakens and shifts northeastward and further away from
the island chain. A Small Craft Advisory (SCA) remains in effect
for the typically windy waters around Maui County and the Big
Island through Friday, and this SCA may need to be extended into
the upcoming weekend. The trades are expected to peak tonight and
Wednesday, so a SCA has been issued for the Kauai and Kaiwi
Channels during this time as well.

Surf is expected to remain below advisory levels across all
shores through early next week.

A new small to moderate south to southwest (200-210 deg) is
expected to begin filling in through the day Wednesday, then hold
through Friday before lowering into the upcoming weekend.

Small northwest swells will continue through the upcoming
weekend, with a slight bump in surf heights expected on

Small to low end moderate surf will remain rough and hold in
place along east facing shores through the upcoming weekend.


Small Craft Advisory until 6 PM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 PM HST
Wednesday for Kauai Channel-Kaiwi Channel.


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