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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 220636

Area Forecast Discussion
National Weather Service Honolulu HI
836 PM HST Mon May 21 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.


A 1025 mb surface high is centered near 31N 140W, or about 1250
miles east-northeast of Honolulu. A surface ridge extends west
from this high through a point less than 800 miles north of Hilo.
The relatively tight pressure gradient south and southwest of
these features is maintaining locally breezy trade winds across
the state early this evening. Elsewhere, the leading edge of a
band of overcast low clouds and showers associated with a weak
surface front is about 550 miles north of Lihue. A 1031 mb
surface high northwest of this front is pushing it toward the
east-southeast at 10 to 15 mph.

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 varying amounts of cirrus clouds
rapidly across most of the islands. The atmosphere remains
relatively stable over the region. Satellite derived precipitable
water (PW) values are near or below normal across must of the
smaller islands, while the PW is slightly higher near and upstream
of the Big Island. Satellite imagery shows broken low clouds
along many windward areas of most islands, with mainly isolated
showers evident in radar reflectivity data at this time. An area
of low clouds and isolated showers upstream of the Big Island may
move over windward sections of the Big Island later tonight.
Otherwise, expect mostly dry conditions from tonight through

The forecast guidance shows the high east-northeast of the
islands will weaken during the next 24 hours, while the surface
front will continue to weaken as it shifts toward the east-
southeast. This will allow the new high to 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 the 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 later this week. However, a weak mid-
tropospheric ridge is forecast to be above the islands, and 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 Saturday night and 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. Two of these episodes occurred
around 600 PM and 800 PM HST this 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. Conditions around the islands should remain
predominately VFR, with scattered showers focusing along windward
and mountain areas. Any passing showers could introduce brief
periods of MVFR CIGS/VIS, however, conditions are not expected to
be widespread enough to warrant the need for mountain

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 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 Wednesday. This SCA will likely need to be extended
through the remainder of the work week, and possibly into the
upcoming weekend. The peak of the trades will occur Tuesday
through Wednesday, and the SCA may need to be expanded to include
some other marine areas during this time.

The current south southwest swell will decline tonight and
Tuesday. A new small to moderate 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. Surf along
south facing shores is expected to remain below advisory levels
through the week.

Small northwest swell will continue through the remainder of the
work week and into the upcoming weekend. A slight bump in surf
heights is expected on Wednesday however.

Small to low end moderate surf will remain rough and hold in
place along east facing shores through the week and into the
upcoming weekend. Surf heights are expected to remain below
advisory levels.


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




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