868
FXHW60 PHFO 211324
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
324 AM HST Sat Sep 21 2024

.SYNOPSIS...
Breezy easterly trades will continue today, then briefly ease
into the light to moderate range Sunday through Monday as the
surface ridge weakens to the north. Showers will primarily favor
windward and mauka locations, though a few could reach leeward and
interior areas as an upper disturbance drifts southward over the
region Sunday. Breezy trades and more stable conditions are
expected to return Tuesday, persisting into the latter half of
next week.

&&

.DISCUSSION...
Guidance remains in good agreement, showing breezy easterly
trades holding today as the surface ridge stays far north of the
state. Satellite imagery shows a decent amount of low cloud
coverage over and upstream of the islands (likely due to a weak
mid-level trough in the area), which has enhanced windward and
mauka shower coverage this morning. Rainfall totals over the past
six hours (ending at 2 AM HST) have generally stayed below a tenth
of an inch, with the highest accumulations near three-quarters of
an inch. Expect this trend to persist through the morning hours
today, with mostly light showers passing through.

A slight pattern shift is expected tonight through Monday as
upper heights lower in response to a broad, weak upper low
drifting southward into the area, while the surface ridge weakens.
Lighter trades, combined with a weakened subsidence inversion,
could lead to increased windward showers, with some either
spilling over into leeward and interior areas or forming over
leeward areas due to localized sea breezes in the afternoon. The
best chance for thunderstorms will remain north of the state where
the better mid-level lapse rates and deeper moisture are
expected.

Upper heights will rise Monday night through midweek as the upper
low fills and lifts away. This, combined with the surface ridge
rebuilding to the north, will support a return to a more stable
and breezy trade wind pattern. Although confidence is low this far
out, some models suggest an area deeper tropical moisture
associated with a disturbance passing far south of the state
clipping portions of the area late next week. Expect a wetter
trade wind pattern for these areas if this were to materialize.

&&

.AVIATION...
An upper level low is anticipated to move over the state during
the weekend time frame. Enhanced SHRA are expected mostly over
windward and mauka locations, with limited spillover to leeward
areas. MVFR conds can be expected with isolated IFR conds in
heavier prolonged SHRA. VFR should generally prevail outside of
SHRA activity.

No AIRMETs are in effect. AIRMET Sierra may be needed for mtn
obsc for windward locations during the overnight hours through
this weekend.

&&

.MARINE...
A high pressure ridge far north of Hawaii will keep fresh to
strong easterly trade winds in the forecast today. A cold frontal
system passing through the far northern Central Pacific basin
will weaken the ridge from Sunday through Monday, briefly easing
trade wind speeds into the gentle to moderate range. A Small Craft
Advisory (SCA) remains in effect for the windier waters around
Maui County and the Big Island. This SCA will likely diminish on
Sunday and Monday as the trade winds briefly weaken. Fresh to
strong trade winds should return by Tuesday of next week and the
trades may approach SCA thresholds for our typical windier waters
over the same eastern islands.

Surf along north and west facing shores will trend up beginning
next Tuesday in response to a gale racing eastward across the
Date Line near the Aleutians this weekend, then into the Gulf of
Alaska early next week. Guidance shows this northwest swell
peaking locally Tuesday night into Wednesday out of the northwest,
then fading Thursday into Friday as it shifts out of a more
north-northwest direction.

Surf along south facing shores will remain small through the
weekend, with mainly a mix of background long-period southwest and
short-period southeast swells moving through. An upward trend is
possible Tuesday through midweek due to a slightly larger
southwest swell arriving.

Surf along east facing shores will remain small and choppy today,
then decrease Sunday into next week as the trades lower locally
and upstream across the eastern Pacific.

&&

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

&&
$$

DISCUSSION...Gibbs
AVIATION...Walsh
MARINE...Bohlin