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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 220616

Area Forecast Discussion
National Weather Service Honolulu HI
816 PM HST Tue Jan 21 2020

A cold front approaching the islands from the northwest will keep
the Hawaiian Islands in a dry and light wind weather pattern
through Friday. Slowing forward motion on this cold front will
cause the frontal boundary to stall out over Kauai and the Kauai
Channel on Friday and Saturday bringing increasing clouds and
showers to the Garden Isle. However, east to east-southeast trade
winds will return to the region bringing isolated to scattered
showers to windward and mountain areas of all islands through
Monday. Another cold front moving through the Central Pacific
basin, north of the state, will bring several days of a dry light
wind weather pattern starting next week Tuesday.


This evening satellite imagery shows very stable conditions around
the islands with the Big Island plume developing clouds and
showers over the waters just south of the smaller islands.
Elsewhere the evening offshore mountain and land breezes are
clearing out the cloud cover over each island. The state remains
under the southwest edge of the subtropical ridge axis keeping the
islands in a light and variable wind pattern. When large scale
winds diminish, surface heating and cooling differences between
islands and ocean product overnight land and daytime sea breezes.

Tuesday through Friday, expect continued land and sea breezes for
most islands with only isolated shower activity possible each day
favoring the afternoon hours. Clouds will develop over island
mountain and interior sections each day due to converging sea
breezes, then clear out each night as diverging land breezes take
over. A cold front will stall out over Kauai on Friday with
increasing clouds and showers forecast over the Garden Isle into
the weekend.

On Saturday and Sunday, showers along the weakening frontal band
linger over Kauai and the Kauai Channel on Saturday as easterly
trade winds return to the state. Wind directions will veer to the
east-southeast direction over the western half of the state by
Sunday pushing the clouds and showers towards the northwest away
from Kauai. We continue to forecast isolated to scattered showers
over windward and mauka areas of all islands after the trade
winds return. This slight increase in trade wind shower activity
will only last a few days.

By Monday, the southwest edge of the subtropical ridge will move
back over the islands as we transition back to several days of a
dry land and sea breeze pattern from next week Tuesday onward. The
southeastern slopes of the Big Island will benefit from enhanced
showers during this period with a light background southeast wind


VFR conditions will prevail with light and variable winds.
Overnight clearing will be followed by afternoon clouds and
a the potential for a few showers, mainly over interior areas as
sea breezes setup.

No AIRMETS are in effect and none are anticipated.


The main marine hazard the next couple of days will be related to
a pair of large northwest swells (and associated seas), with the
first already prompting a High Surf Warning and a Small Craft
Advisory for exposed shorelines/marine zones. See the Coastal
Hazard (CFWHFO) and Marine Weather Messages (MWWHFO) for
additional details, as well as the Oahu Surf Discussion (SRDHFO).
Long- period forerunners (21-24 seconds) of this swell are
already being detected at buoys 51001 and 51101 to the NW of
Kauai, and surf is expected to build through the day Wednesday,
and peak at dangerously high levels most areas Wednesday night
before gradually lowering Thursday into Friday. Timing for the
peak of the swell will be a little later for the Big Island, with
the expectation that W facing shores will be added to the High
Surf Warning with the early morning forecast update. The second
large NW swell is expected to arrive Friday night and peak at
warning level heights Saturday, then gradually diminish Sunday and

A surface ridge will remain over the islands into Friday, keeping
winds generally light and variable, but favoring a southerly
direction near Kauai, and an easterly direction near the Big
Island. A weak front will move into the area from the N Friday
into Saturday as a high builds NW and N of the islands. This high
will support light to moderate trade winds over the weekend as
the front stalls and dissipates near Kauai and Oahu.


High Surf Warning from 6 AM Wednesday to 6 PM HST Thursday for
Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North
Shore-Oahu Koolau-Olomana-Molokai-Maui Windward West-Maui
Central Valley-Windward Haleakala.

Small Craft Advisory from 6 AM Wednesday to 6 PM HST Thursday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-
Kaiwi Channel-Maui County Windward Waters-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