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Hawaiian Islands Synoptic Discussion and Guidance

957
FXHW60 PHFO 252001
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1001 AM HST Mon Sep 25 2023

.SYNOPSIS...
Moderate to locally breezy trade winds will remain in the
forecast through the week. Passing showers will tend to favor
windward and mauka locations, especially during the overnight
through early morning hours each day. A slight increase in
windward showers will be possible from Friday into the weekend.

&&

.DISCUSSION...
Latest surface analysis reveals that a cold front, which extends
from a strong low pressure system currently located off the U.S.
Pacific Northwest, stretches into the northern Central Pacific
north of the islands. Meanwhile, high pressure is located far
north-northwest of the islands with a ridge axis extending to the
east, north of the main Hawaiian Islands but south of the front.
Although the ridge is not particularly strong, it's proximity to
the state is maintaining moderate to locally breezy easterly trade
winds.

Morning satellite and radar imagery shows isolated to scattered
clouds and showers embedded within the trades moving into windward
areas, particularly over the eastern end of the state. Fewer
clouds are noted upstream of the smaller islands where showers
are more isolated. Early morning 00z soundings from Lihue and Hilo
showed strong trade wind inversions between 7,000 and 9,000 feet
with precipitable water values near 1.20-1.30 inches, which is on
the lower side of the normal range for late September but still
fairly typical.

Expect decreasing windward shower trends through the rest of the
morning, becoming isolated this afternoon. In addition, cloud
buildups and a few showers will also be possible over the Kona
slopes of the Big Island from late this morning through early
evening.

The forecast remains fairly consistent from day to day through
Thursday with only slight changes in cloud and shower trends in
the forecast, mainly over windward and mountain areas. The high
pressure north of the islands will move eastward and become re-
established to the far northeast of the state by the end of the
work week, keeping moderate to locally breezy trade winds in the
place. Additional moisture associated with the remnants of former
Tropical Depression 14E may bring an increase in trade wind
showers by Friday. The increase in shower activity could continue
into the weekend as cloud bands, associated with the former cold
front that will be stalled north of the state for much of the work
week, are captured by the trade winds and move into the islands
from the east by Saturday.

Beyond next weekend, model guidance shows a low pressure system
and an associated cold front moving into the Central Pacific from
the west. Ahead of this feature, which is forecast to stall
northwest of the islands, winds will veer more southeasterly
across the island chain. A ridge aloft looks to lower inversion
heights and increase subsidence, decreasing shower activity
throughout the first half of next week. However, changes will be
possible as models come into better agreement over the next few
days.

&&

.AVIATION...

Moderate to locally breezy trade winds will remain in place
through the next few days. Scattered showers will mainly affect
windward and mountain locations with isolated showers elsewhere.
There may be brief periods of MVFR ceilings and visibility
associated with any heavier activity, but not expecting anything
widespread. Most of shower activity will also favor the night time
and early morning hours.

AIRMET TANGO remains in effect for tempo moderate turbulence below
9000 feet for areas south through west of mountainous terrain,
though trades could weaken slightly later today.

&&

.MARINE...
Fresh to strong easterly trades will persist through most of the
week, which supports the Small Craft Advisory (SCA) continuing
for the windy waters around Maui County and the Big Island. This
advisory may need to be expanded to other waters by Wednesday as
the ridge strengthens north of the state. Guidance shows the
trades backing down slightly Friday through the weekend as a
weakness forms in the ridge.

Surf along south facing shores will slowly trend down on Tuesday,
as the current long-period south-southwest swell declines. Surf
will drop below average around Wednesday, then remain small
through Friday. Guidance does depict an upward trend due to a
small southeast swell arriving over the weekend.

Surf along north facing shores will trend up slightly Tuesday,
then remain up through much of this week due a series of north to
north-northeast swells. A small north (350-360 degrees) swell
should arrive around Tuesday, followed by a modest north-
northeast (020 degrees) swell from Thursday through Friday. For
the extended, guidance is hinting at similar activity for north
and west exposures next week beginning around Tuesday, but from
swell energy out of a more typical northwest direction.

Surf along east facing shores will remain small and rough into
Tuesday, then potentially trending up around midweek as the
trades pick up.

&&

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

&&

$$

DISCUSSION...Vaughan
AVIATION...Tsamous
MARINE...Kino

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