009
FXHW60 PHFO 041317
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
317 AM HST Mon Aug 4 2025

.SYNOPSIS...
The high pressure ridge north of the islands will weaken over the
next several days, decreasing trade wind speeds especially as the
remnants of former Tropical Cyclone Gil drifts westward, passing
just north of the Hawaiian Islands on Wednesday and Thursday. Warm
and very humid conditions with a slight increase in island shower
activity will accompany Gil`s passage.

&&

.DISCUSSION...
The satellite picture this morning shows a large band of stable
stratocumulus clouds moving from east to west into all islands.
A strong subsidence temperature inversion aloft will tend to
decrease cloud cover and shower activity later this morning. A
large rotating band of clouds near 20N 140W, associated with the
remnants of former Tropical Cyclone Gil, will continue to drift
westward over the next few days. This unstable and very humid air
mass will pass just north of the Hawaiian Islands from Wednesday
through Thursday, disrupting the trade wind flow and producing
very warm and humid weather across the state. Clouds and shower
will be enhanced a bit during this time period. However, much of
these shower impacts will depend on the track and how close this
system gets to the Hawaiian Islands.

The breezy weather pattern will change today through Tuesday as
the high pressure ridge north of the state weakens. Moderate to
locally breezy trade winds today will noticeably decrease in
strength into the light to moderate range over the next 24 to 36
hours. A high pressure ridge aloft will bring down enough
subsidence (downward moving air) to keep shower activity to
minimum levels through Tuesday. Expect very warm and humid
conditions from Tuesday night through Thursday as the remnants of
former Tropical Cyclone Gil continue to weaken as it approaches
the islands from the east. This remnant tropical system will pass
just north of the Hawaiian Islands, cutting off trade winds and
increasing island shower trends especially over leeward areas.
The latest global weather models take the strongest part of this
system far enough north to limit significant shower activity.

The challenge with this rainfall forecast are directly tied to
changes in the forcing mechanism for rainfall production. As the
trade winds diminish the more typical windward mountain lifting
pattern will switch into a more hybrid light trade wind sea breeze
island heating driven pattern. In this hybrid sea breeze pattern
island interior and typically drier leeward sections will see more
afternoon cloud and shower development. High boundary layer
moisture levels with surface dew point temperatures in the low 70s
and temperature inversion heights ranging from 8,000 to 9,000
feet will aid in convective shower development from Wednesday
through Thursday. Any of these developing slow moving showers
could produce brief locally heavy shower activity in some leeward
areas.

Drier conditions return briefly with moderate trade winds on
Friday and Saturday. Long range global models currently show
another weakening tropical system, currently Tropical Depression
Eight-E, will pass north of the islands from Sunday through
Monday, once again disrupting trade winds and producing warm and
very humid weather. Stay tuned for weather forecast updates as the
track of this next approaching tropical system will likely change
over time.

&&

.AVIATION...
Moderate to locally breezy trade winds will continue today before
decreasing into the light to moderate range by Tuesday afternoon.
Windward cloud cover and brief passing showers will diminish
later this morning as more stable trends move over the Hawaii
Region.

AIRMET Tango remains in effect for moderate low level turbulence
over and immediately south through west of island mountains. This
AIRMET will likely be cancelled later today as wind speeds aloft
decrease.

&&

.MARINE...
High pressure north of the state will weaken and drift eastward
through Tuesday. Trade wind speeds will gradually lower through
the middle of the week as the remnants of former Tropical Cyclone
Gil passes to the northeast of the state, then strengthen out of
the east southeast late Thursday through the end of the week. The
Small Craft Advisory (SCA) has been extended for the windier
waters and channels around Maui County and the Big Island through
tonight.

A moderate, long period south swell has begun to slowly fill in
overnight and is expected to peak below High Surf Advisory (HSA)
criteria this evening into Tuesday before declining Wednesday.
Another swell is expected from a low formed south of New Zealand
at the end of July, that measured seas of 35 to 40 feet aimed
towards Hawaii. This swell is expected to build on Thursday and
hold through the end of the week which could get near HSA criteria
at its peak.

Short period trade wind swell will keep east facing shores elevated
and choppy today due to strong trade winds. By late this evening into
Tuesday, a moderate, medium to long period east swell is expected
to fill in from the energy from former Tropical Cyclone Gil.

A small, medium period northwest swell originating from Typhoon
Krosa in the western Pacific will bring a small bump up in surf
along north facing shores through today.  Another bump is
expected as a small, medium to long period northwest swell looks
to fill in Thursday and Friday.

&&

.FIRE WEATHER...
Humidity levels will show increasing trends with gradually
decreasing winds from today through Tuesday. The trade wind
inversion heights will drop to around 5000 to 6000 feet with very
low relative humidity continuing near the inversion level along
the upper elevations of Haleakala on Maui and the mid to upper
slopes of the Big Island. Elevated fire weather concerns will
continue in these areas through Tuesday afternoon. Humidity levels
will increase dramatically from Tuesday night through Thursday as
the remnants of former Tropical Cyclone Gil drifting westward,
passes just north of the Hawaiian Islands.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Tuesday for typical windier
waters and channels around Maui and the Big Island.

&&

$$

DISCUSSION...Bohlin
AVIATION...Bohlin
MARINE...Shigesato
FIRE WEATHER...Bohlin