691
FXHW60 PHFO 060144
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
344 PM HST Sat Oct 5 2024

.SYNOPSIS...
Gentle to moderate trade winds will continue for the rest of the
weekend, bringing a few showers to windward areas. Additionally,
afternoon sea breezes will bring clouds and showers to leeward and
interior areas. A low aloft will destabilize the island
atmosphere, supporting the potential for some locally heavy
showers and a slight chance of thunderstorms, first for Kauai and
Oahu (including surrounding waters) tonight, then gradually
spreading to the remainder of the state by Sunday. Stronger trade
winds and increasingly stable conditions will return next week.

&&

.DISCUSSION...
Prevailing background winds across the islands have been in the
gentle to moderate range today as a result of a weakened Northeast
Pacific High far north of the state. These gentle to moderate
background trade winds winds will remain in place throughout the
rest of the weekend. Meanwhile, local satellite and radar imagery
shows that low-level convergence has resulted in afternoon sea
breeze induced showers across select leeward areas.

Some tweaks have been made to rain and thunder chances through
the rest of the weekend based on latest National Blend of Models,
ECMWF, and GFS guidance with this afternoon`s forecast updates,
but the overall forecast philosophy remains unchanged. For the
rest of the weekend, a low aloft will shift southward, eventually
settling over the central islands on Sunday. This low will
destabilize the atmosphere and increase vorticity aloft as 500mb
temperatures cool to around -11C. As inversion heights increase,
expect taller cloud development and heavier showers. Additionally,
the increasing instability will support slight chances of
thunderstorms that will include Kauai and Oahu tonight, then
spreading to the remaining islands on Sunday, including the slopes
of the Big Island Sunday afternoon.

Given the relatively light background flow, afternoon sea breezes
are expected along select leeward areas again on Sunday. The
heavier showers will be most likely over leeward areas during the
afternoon and evening periods, and over windward areas during the
overnight and morning hours. The caveat to the forecast, however,
is that precipitable water values are expected to be near the 1.2
to 1.5 inch range, which is very close to normal for early
October. Therefore, although there should be sufficient moisture
for a few heavier showers and perhaps isolated thunderstorms, the
amount of moisture available is not significantly higher than what
is to be expected for this time of year.

Models still show trade winds gradually increasing as early as
late Sunday into early next week as a new high pressure to the
distant northwest builds eastward. Stability will also increase as
the low aloft weakens and moves away to the northeast. Some
instability may linger through Monday, but generally, increasingly
stable trade wind conditions should return by Tuesday and persist
through much of next week. With the return of a more typical
trade wind pattern, expect clouds and showers to favor primarily
windward and mauka areas.

&&

.AVIATION...
Light to moderate trade winds will continue this weekend, with
afternoon sea breezes developing over terrain-sheltered leeward
areas. A developing upper-level low over the islands will increase
low cloud and shower coverage through at least Sunday, favoring
windward areas overnight and leeward/inland areas in the afternoons.
VFR conditions are expected to prevail, with periods of MVFR and
isolated IFR conditions possible in the heavier showers. There is
also a slight chance of thunderstorms over the eastern half of the
state tonight, then state-wide through the weekend, due to increasing
instability from the low aloft.

No AIRMETs are currently in effect. AIRMET Sierra may be needed
tonight if low clouds and showers become widespread enough to
obscure island terrain.

&&

.MARINE...
The ridge to the northeast of the islands has weakened further,
leaving gentle to locally moderate trades across the region. An
upper level low has moved into the offshore waters to the
northeast of the islands, and is about 170 nm northeast of Kauai
this afternoon. The global models show this upper level feature
strengthening as is sinks southward in the next 24 hours. Although
no thunderstorms have yet been observed, the chance for
thunderstorms remains in the forecast for the offshore and coastal
waters near the upper low through tomorrow afternoon. By tomorrow
night, the low is forecast to lift back to the north.

A surface ridge looks to start forming to the north of the islands
tomorrow, which will help the trades to begin to become re-
established across the region. By Monday morning, Small Craft
Advisory (SCA) level winds are possible in the typical windier
coastal waters near Maui County and the Big Island. Once SCA winds
return, we can expect those to continue through at least mid-week.

A pair of overlapping small, short period north swells should
maintain small surf across north facing shores through the
remainder of the weekend. A short lived small, short to medium
period northeast swell is expected through tomorrow, which will
also boost surf along east facing shores. There is the potential
for a moderate long period north-northwest swell as early as
tomorrow, which would peak Monday below advisory levels for north
facing shores, with the swell lingering into the middle of the
new week.

A moderate, long period south swell hasn`t shown on the near shore
buoys yet, and the latest guidance now indicates it should arrive
later this evening. Expect this to then fill in tonight, peaking
perhaps a little later tomorrow, hold into Monday, and then
gradually declining through the middle of next week. This swell
may boost surf along south facing shores to near or slightly below
advisory levels during its peak.


&&

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

&&

$$

DISCUSSION...Vaughan
AVIATION...TS
MARINE...M Ballard