631
FXHW60 PHFO 220111
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
311 PM HST Sat Sep 21 2024

.SYNOPSIS...
Moderate to locally breezy trade winds will ease for Sunday and
Monday as the surface ridge to the north briefly weakens.
Rainfall will favor windward and mauka locations, but a few
showers, some heavy, may develop across leeward areas as a
disturbance aloft moves overhead. Breezy trades and more stable
conditions are expected to return on Tuesday.

&&

.DISCUSSION...
A moderate to locally breezy trade wind flow will begin to ease
and destabilize tonight. A surface ridge sitting nearly 1,000
miles to the north is driving the trades, which have stubbornly
remained breezy today. The winds should begin to gradually fall
off this evening as the ridge starts to weaken as drift southward.
An upper-level low centered a couple hundred miles north of Kauai
has so far had minimal effect on local weather aside from some
thin high clouds, but as the low aloft digs southward tonight,
the inversion will lift and weaken. As a result, mainly windward
showers should become a bit heavier than the past few days, and a
narrow and diffuse band of moisture in the trades will likely
bring higher shower coverage from Maui to Oahu overnight.

Trade winds will drop a notch, and showers will likely be heavier
Sunday and Monday. The surface ridge to the north will erode,
causing trades to decline to moderate strength. Meanwhile, the
upper-level low will stall over the western end of the island
chain, causing 500 mb temps to drop to around -9C, steepening the
lapse rate aloft, and significantly weakening the low-level
inversion. The GFS and ECMWF show a lack of organized low-level
moisture in the trade wind flow, which suggests that a general
increase in shower activity is unlikely. However, showers that do
develop could be locally heavy.

A stronger and more stable trade wind flow is expected by midweek.
The locally breezy trades will be driven by a tighter pressure
gradient between the strengthening low-level ridge to the north
and a pair of tropical disturbances passing several hundred miles
south of the islands. The upper-level low fill and drift away to
the west, and a ridge aloft building in from the east will bring
stability. The GFS and ECMWF differ on timing but show a pocket
of moisture in the building trades passing over the islands late
Monday and Tuesday. This will likely be too late for enhanced
rainfall and is expected to produce typical windward showers.
Through the rest of the work week, we will monitor areas of
tropical moisture that could clip the Big Island, but expect
typical windward rainfall for most islands.

&&

.AVIATION...
Moderate to locally breezy trades will weaken slightly heading
into tomorrow and isolated to scattered showers will favor
windward and mountain areas. With temperature inversion heights
expected to remain elevated and weaken heading into Sunday as an
upper level low approaches from the north, expect brief moderate
to heavy downpours in showers with some spillover into leeward
areas. MVFR conditions can be expected in heavier showers with
isolated IFR conditions possible. VFR should prevail outside of
shower activity.

AIRMET Tango is in effect for tempo moderate turbulence between
12,000 and 40,000 feet this afternoon as the upper level low
approaches from the north. This AIRMET will likely be needed
through Sunday morning.

AIRMET Sierra may be needed for mountain obscuration for windward
locations during the overnight hours through this weekend,
depending on if the shower coverage becomes extensive enough to
warrant it.

&&

.MARINE...
Fresh trade winds will ease slightly tonight and Sunday as a
ridge to the north of the islands weakens. Winds have already
dropped below Small Craft Advisory (SCA) thresholds and are
expected to remain below these levels through Monday. The trades
should strengthen back to moderate and locally strong levels
Monday night and hold steady in this range through the remainder
of the upcoming work week.

Surf along north and west facing shores will remain tiny through
Monday. A moderate, long-period northwest swell will arrive on
Tuesday and push surf along north and west facing shores well
above seasonal average, though below High Surf Advisory levels,
Tuesday night and Wednesday. This swell will decline as it shifts
out of the north-northwest on Thursday, then fade on Friday.

Surf along south facing shores will remain just below seasonal
average over the next couple of days as an inconsistent southwest
swell moves through. A reinforcing southwest swell is expected late
Monday into Wednesday. With trade winds on a decline, east shore
surf will be well below seasonal average during the next few days.
An increase in trade winds will produce larger east shore surf
near seasonal average by Tuesday night or Wednesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Wroe
AVIATION...Farris
MARINE...Wroe