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Hawaiian Islands Synoptic Discussion and Guidance

945
FXHW60 PHFO 221341
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
341 AM HST Wed Jan 22 2020

.SYNOPSIS...
A cold front north of the island chain and another front approaching
the islands from the northwest will keep the Hawaiian Islands in
a dry and light wind weather pattern through Friday. The slowing
forward motion on the northwestern cold front will cause this
frontal boundary to stall out near Kauai on Friday and Saturday,
bringing increasing clouds and showers to the Garden Isle.
Trade winds will also return to the region on Friday spreading
isolated to scattered showers to windward and mountain areas of
all islands through Sunday morning. 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 Monday.

&&

.DISCUSSION...
Most islands this morning are showing clear skies with few
exceptions along southeastern slopes of Maui and the southeast to
east slopes of the Big Island. A weak front roughly 400 miles
north of Kauai will continue to drift eastward away from the
state. The state remains under the southwest edge of the
subtropical ridge keeping the islands in a light and variable
wind pattern. When the large scale winds diminish, surface
heating and cooling differences between islands and ocean produce
overnight land and daytime sea breezes.

Today 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. Trade winds will bleed through the front on Friday
with a brief northerly direction quickly veering towards the east
by Saturday morning. Isolated to scattered windward and mauka
showers will return with the trade winds through Sunday morning.

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
southeast direction over the western half of the state by Sunday
pushing the clouds and showers towards the northwest away from
Kauai. 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. The southeastern slopes of the
Big Island will benefit from enhanced showers during this period
with a light background southeast wind flow.

&&

.AVIATION...
VFR conditions will prevail with light and variable winds.
Outside of some afternoon clouds associated with the sea breezes,
mostly clear conditions are expected. The best chance for any
showers will be over windward (eastern end of the state) and
interior areas in the afternoon.

No AIRMETS are in effect and none are anticipated.

&&

.MARINE...
The main marine hazard the next couple of days will be related to
a pair of large NW swells (and associated seas/surf), with the
first swell in the process of arriving. Long-period forerunners
(21-27 seconds) of this swell have been steadily building at buoys
51001 and 51101 to the NW of Kauai through the night, and are
showing up at the nearshore buoys as well. Although surf along
most N and W facing shores may start off on the smaller side this
morning (with long lulls between sets), surf will steadily
increase through the day, and peak at dangerously high levels
tonight into Thursday before gradually lowering later Thursday
into Friday. A High Surf Warning and a Small Craft Advisory remain
posted 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).
The second large NW swell is expected to arrive Friday night and
peak at warning level heights Saturday, then gradually diminish
Sunday and Monday.

A surface ridge will be near 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, pushed S by a high building 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.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 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-Kona-Kohala.

Small Craft Advisory until 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.

&&

$$

DISCUSSION...Bohlin
AVIATION...Gibbs
MARINE...Birchard

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