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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 200632

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Fri Jan 19 2018

High pressure across the Eastern Pacific and a trough in the
Central Pacific will lead to a weak ridge north of the Hawaiian
Islands for the weekend. Expect easterly trade winds for most
islands with modified afternoon sea breezes along southern and
western slopes of Kauai and Oahu through Sunday. Unsettled weather
continues over the state next week ahead of a stronger frontal
system moving across the Central Pacific veering light wind
directions towards the southeast on Wednesday. Vog may move
northward into the smaller islands on Wednesday and Thursday.


The large scale picture shows a strong high pressure system
continuing to drift into the East Pacific with a low and cold
front moving across the Central Pacific. The positions of these
two large scale systems will keep a weakened ridge north of the
islands with lighter easterlies from Molokai to the Big Island.
For Kauai and Oahu, a modified easterly trade wind regime will
develop through Sunday with periods of southeasterly winds across
portions of the southern slopes and on-shore winds over western
slopes each afternoon. The forward motion of the cold front will
stall roughly 200 miles northwest of Kauai by Sunday evening.

Satellite analysis this evening continues to show a large band of
unstable cumulus clouds drifting across the islands from east to
west along with a weak upper level trough near Kauai likely enhancing
shower activity through the weekend. The highest chances for
showers will occur along the windward slopes of each island
however some of the more unstable showers will occasionally drift
into leeward areas especially in the early morning hours.

Monday and Tuesday...A high pressure ridge between two low
pressure systems looks stronger in the forecast guidance compared
to 24 hours ago. This stronger ridge will produce a brief increase
in easterly trade wind speeds across all islands. Expect a
continued wet weather pattern for windward areas with the highest
rainfall chances over the Big Island and Maui as unstable clouds
ride in with the easterly trade winds.

Wednesday through Friday...A stronger low pressure system will
move into the Central Pacific on Wednesday with a trailing cold
front approaching the islands from the northwest. Wind directions
will veer towards the southeasterly direction as the front
gets closer with nighttime land breezes and daytime sea breezes
developing. Prefrontal convergence bands will drive precipitation
chances along with daytime heating and convection over island
interior sections producing afternoon clouds and scattered
showers. VOG from the volcano on the Big Island may also be drawn
up with the south to southeasterly wind flow over the smaller
islands on Thursday and Friday.


Easterly trade winds will trend downward tonight and Saturday,
particularly over the western islands, as a trough of low pressure
approaches the state from the west. Clouds and showers will
continue to focus primarily over windward and mauka areas through
the period, with occasional MVFR cigs/vsbys in SHRA expected.

AIRMET Sierra is in effect for mountain obscuration across
windward portions of Kauai, Oahu, Molokai, Maui and the Big
Island. These conditions will likely remain in place through the
overnight hours.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of all islands. Some improvement is
possible over the western end of the state late tonight.


Trade winds have started to weaken as a surface high north of the
state moves east, but winds will remain at small craft advisory
levels through tonight before dropping Saturday. Trade wind swell
generated east of the state will keep seas above 10 feet,
and the Small Craft Advisory has been extended through Saturday
for most zones. Seas are expected to subside below advisory level
Saturday night. The High Surf Advisory (HSA) for east facing
shores has been extended through Sunday.

Surf will remain below advisory levels for the remaining shores
through this weekend. A small northwest swell (330 degrees) is
expected arrive Saturday night, with a reinforcing long- period
west-northwest (300 degrees) swell expected Sunday night. The WNW
swell will peak Monday night and gradually declining through
Wednesday. This second swell may warrant an HSA for exposed west
and north facing shores Monday and Tuesday.


High Surf Advisory until 6 PM HST Sunday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters-
Kaiwi Channel-Maui County Windward Waters-Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Windward Waters-Big Island
Leeward Waters-Big Island Southeast Waters.




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