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Hawaiian Islands Synoptic Discussion and Guidance

971
FXHW60 PHFO 050635
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
835 PM HST Fri Dec 4 2020

.SYNOPSIS...
A high pressure system northeast of the Hawaiian Islands will
continue to move eastward away from the state as a cold front
moves into the region from the west. Trade winds will diminish as
the front approaches the islands from the northwest with winds
becoming more variable through the weekend. Higher pressure will
build back in north of the state with moderate trade winds and
passing showers forecast from Monday to Tuesday.

&&

.DISCUSSION...
The satellite imagery this evening shows cloud bands from an old
frontal boundary drifting over the western half of the state.
Enhanced passing showers will continue over Oahu and Kauai through
the early morning hours. Another cold front roughly 1100 miles
northwest of Kauai will approach the state over the next few days,
decreasing wind speeds across the region.

Short range forecast guidance shows the islands on the edge of the
high pressure ridge axis through the weekend. Weaker large scale
pressure gradients under this ridge will weaken wind speeds
towards a light and variable wind pattern through the weekend. A
background large scale light southeasterly flow will develop.
In the absence of strong large scale winds we expect local scale
light and variable land and sea breezes to form over the islands
through Sunday. Onshore sea breezes each day will lead to clouds
building up over mountains and island interior sections with a few
showers in the afternoon to early evening hours.

On Monday, the front stalls north of the islands and another
migratory high moves into the Central Pacific basin from the west.
High pressure building north of the islands will lead to a return
to moderate trade winds from late Monday afternoon through
Tuesday, becoming locally breezy by Wednesday.

In the longer range forecast on Wednesday, light to moderate
southeasterly winds will develop across the region ahead of the
next approaching cold front northwest of the state. By Thursday
we see increasing trends in passing showers due to an upper level
trough moving across the state from west to east in a moderate
southeasterly weather pattern. Weather models are in fair
agreement with this progressive weather pattern. The timing on
these wind and weather conditions may change a bit as the 6 to 7
day forecast period gets shorter.

&&

.AVIATION...
High pressure northeast of the state will weaken and shift further
east tonight and Saturday as a cold front approaches from the
northwest. As a result, the trades will diminish tonight and early
Saturday, with sea breezes developing in many areas Saturday
afternoon. Clouds and showers will favor windward areas,
particularly windward Kauai tonight. Much drier weather is
expected on Sunday, with a few isolated showers possible over the
island interiors.

AIRMET Sierra remains in effect for mountain obscuration across
windward sections of Kauai. This AIRMET will likely remain in
place for most of the night.

AIRMET TANGO remains in effect for moderate low-level turbulence
over and downwind of terrain for all islands. Conditions should
improve by daybreak Saturday.

&&

.MARINE...
High pressure moving west to east north of the state will maintain
moderate to locally fresh trades through tonight. A Small Craft
Advisory (SCA) is in effect through Saturday morning for those
typically winder waters around Maui and Big Island. Trades will
drop off over the weekend, especially over the western end of the
state. A SCA may be needed later this weekend for building seas in
association with the next incoming large, long period northwest
swell.

The large northwest swell that peaked Wednesday will continue
to fade. Wave modeling is depicting another northwest swell
moving down the island chain quickly on Saturday that may bring
surf along northern-exposed shores near High Surf Advisory
levels. Long period forerunners from a larger, longer period
northwest swell is anticipated to fill in early Sunday which may
drive surf heights along north and west-facing shores to well
above High Surf Warning levels. This northwest swell will diminish
enough to drop surf to below Surf Advisory levels around Wednesday.

Relatively longer duration Surf Advisory to Warning-level surf
will exacerbate ongoing coastal erosion issues...especially with
higher than predicted water levels. The best chance for water to
reach vulnerable coastal properties and roadways will be around
and after daybreak on Monday when the daily high tide could
coincide with the peak of this early week large northwest swell.

Surf along east-facing shores will remain small over the weekend,
then trend up slowly next week as the trades fill in locally and
upstream of the islands. Surf along south-facing shores will
remain small with a mix of small, short period southeast and
background south southwest energy moving up from the Southern
Hemisphere.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Blood

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



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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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