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Hawaiian Islands Synoptic Discussion and Guidance

824
FXHW60 PHFO 240745 CCA
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
840 PM HST Wed Sep 23 2020

.SYNOPSIS...
Trade winds will continue into the weekend, but will diminish on
Sunday, then remain light for most of next week. An area of enhanced
moisture riding in on the trades will bring a few windward showers
to the smaller islands overnight and early Thursday after already
soaking the Big Island, where showers will be tapering off
overnight. After Thursday, the airmass will be fairly dry and
stable, so just a few showers are expected into next week.

&&

.DISCUSSION...
Our latest Record Event Report (RERHFO) succinctly summarizes
today's weather, with a new daily rainfall record in Hilo (and still
counting), and a daily record high temperature tied in Lihue. An
area of low- and mid-level moisture (remnants of last week's eastern
Pacific TS Karina) riding in on the locally breezy trade wind flow
has been soaking windward Big Island since this morning, with some
rain gages reporting 12 hour rainfall totals over 9 inches. Honolii
stream gage showed a water rise from 2 feet this morning to a peak
near 14 feet this afternoon, representing the highest water levels
in months, with water levels now slowly receding. While some of this
moisture spread to portions of Maui County, Kauai and Oahu saw yet
another hot and mostly dry day.

The forecast anticipates that the moisture slug will gradually move
W and NW over the next 12-36 hours, with a drying trend on the Big
Island beginning shortly, with rainfall likely done by Thursday
morning. Some increase in windward showers is still expected on
Kauai and Oahu overnight and early Thursday, but nothing like the
rainfall seen on the Big Island. As this moisture moves W of all
islands by Thursday evening, a return to our mostly dry trade wind
weather regime is expected for a couple more days.

A surface ridge to the N will support a continuation of locally
breezy trade winds into the weekend, but the longer range outlook
anticipates light winds over the islands beginning Sunday and Monday
persisting for most of next week. This will occur as the remnant of
eastern Pacific TS Lowell passes NE of the islands, and several low
pressure systems (and an associated front) pass N of the islands.
This will press the surface ridge southward over the area, leading
to a very weak pressure gradient. A weakening ESE flow Sunday will
likely give way to light and variable winds by Monday, with island-
scale land and sea breezes driving a daily weather pattern
characterized by clear nights and mornings, with afternoon interior
clouds possibly dropping a few showers. This looks to be a mostly
dry pattern, but forecast models have been showing an area of
moisture developing W of Lowell that could be near the islands
Sunday and Monday, and could potentially fuel an increase in
showers.

&&

.AVIATION...
Passing trough and attendant deep moisture plume will work through
the Islands late tonight through early Thursday morning bringing
moderate to locally heavy rain to windward areas, particularly Big
Island and Maui. Lee areas can expect mainly light showers.
Improving conditions are finally noted at PHTO as wind turns
offshore there. This trend is forecast to continue as moisture
continues lifting northwest eventually reaching Kauai during the
predawn hours. Moderate to locally breezy trades will continue
through the forecast period.

AIRMET Sierra for mountain obscuration is currently in effect for
windward sections of Oahu through Big Island. An eventual expansion
to Kauai is probable.

&&

.MARINE...
A broad 1025 mb surface high lies 720 nm north of Kauai this
evening. It is moving eastward, and should be 1200 nm northeast of
Oahu this coming Friday night. The high dissipates a few hundred
miles off the northern California Monday. The trailing surface ridge
will be moving south in the coming days, being a position 450nm
north of Kauai Saturday night, then stalling a couple of hundred
miles north of Kauai on Sunday. The southward advancement of the
ridge is caused by a cold front that enters the far northern
offshore waters Tuesday evening. The front then falls apart north of
Kauai Wednesday and Thursday. The remnants of once Tropical Cyclone
Lowell will be stalling over the far eastern offshore waters Monday
night.

A Small Craft Advisory (SCA) is in effect for the typically windier
near-shore waters of Maui County and waters south of the Big Island.
The SCA is good to until 6 am HST Friday. With the ridge dropping
south, the trade winds will be trending down to light over the
western part of the island chain. As the front dissipates during the
first half of next week, a light northeast wind flow can be expected
across our island waters.

A north-northwest swell will be making its way southward overnight
and Thursday resulting in moderate surf along the exposed north
facing shores. A gradually lowering is expected through Saturday.
This swell will be propped up by a small reinforcing north swell due
in Thursday evening. A small pulse from the north- northwest is
possible Monday, followed by a larger one Wednesday.

The current diminishing south southwest swell will be reinforced by
a series of small long period swells into the weekend. The first is
from the south-southwest on Friday, followed by a similar magnitude
but shorter period on Saturday. A small south swell is likely as
well on Saturday. These swells will be subsiding through Monday.

Surf along the east facing shores will be lowering to small over the
weekend due to the lighter trades. Next week Monday, we may get a
bit of the swell from the weakening Tropical Storm Lowell, resulting
in an increase in surf along the east facing shores.

&&

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

&&

$$

DISCUSSION...Birchard
AVIATION...JVC
MARINE...Lau

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