748
FXHW60 PHFO 141320
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
320 AM HST Tue Oct 14 2025

.SYNOPSIS...
An upper level low pressure system continues to glide south
towards the Hawaiian Islands this morning. This unstable system
will combine forces with low level cloud bands and produce
extended periods of wet weather across the islands from Wednesday
into the weekend. Moderate to breezy trade winds during this time
period will help to lift showers up over windward island mountains
where the highest rainfall amounts are expected. Showers will
likely become locally heavy in terrain favored locations with
potential for thunderstorm activity from Wednesday night through
Friday afternoon. Forecast projections for this weekend show the
unstable upper low drifting westward away from the state with
improving weather trends in the forecast.

&&

.DISCUSSION...
The latest water vapor satellite imagery this morning shows a low
pressure system with two embedded low centers roughly 300 to 500
miles north of the Hawaiian Islands. Looking into the infrared
satellite channel we see an area of numerous thunderstorms
continues to fire surrounding the northern-most low center. This
unstable cold core upper low will continue to drift south towards
the islands over the next two days. Roughly 500 miles east of the
Big Island a long unsettled cloud band is riding towards the
islands on the easterly trade winds. This cloud band is the
convergent remnants of an old Eastern Pacific cold front. Based on
the latest short range forecast model guidance, the upper low and
remnant low level convergent cloud band will combine forces
bringing periods of wet weather to the entire island chain
from Wednesday night through Friday.

The short range forecast calls for increasing shower trends as the
unstable upper low approaches the islands from the north.
Currently only a few bands of showers are moving through the
islands in a hybrid light to moderate easterly trade wind and
leeward land and sea breeze pattern. Morning 2 AM HST (12Z) upper
air balloon soundings from Hilo and Lihue show subsidence
temperature inversion heights in the 7,000 to 9,000 foot range
respectively. Although the Lihue sounding is likely elevated a bit
due to a passing shower band at the time of observation. The large
scale downward motions (subsidence) will weaken as the low center drifts
into a position near Kauai and Oahu by Wednesday afternoon,
lifting the subsidence temperature inversion in the process.

By Wednesday morning expect inversion heights around the 8,000
foot level or higher across the state, leading to periods of
increasing wet weather trends. At this point, much will depend
upon how the smaller scale (mesoscale) weather pattern interacts
with the larger upper low. Low level convergence bands and trade
winds lifting clouds up over windward island mountains will be the
primary drivers for any locally heavy rainfall. While any location
may see periods of locally heavy rain, windward areas and
mountains will be favored especially near the upper low center, or
under any developing thunderstorms. Storm total rainfall estimates
for this entire two to three day event may exceed 2 inches in
terrain favored locations. For the moment it remains too early to
pin down any islands for flooding, however a Flood Watch may be
needed at some point as this event unfolds. Any flooding potential
may tend to favor the western half of the state as these islands
will be closer to the unstable upper low center.

Thunderstorm activity will favor the western half of the state
including the islands of Niihau, Kauai, and Oahu from Wednesday
night through Friday. The Big Island may also see isolated
thunderstorms from late Thursday morning to early evening as the
low level convergent band drifts through the island.

Starting from Friday night to Saturday we begin to see another
change in the weather with decreasing shower trends as the upper
low begins to drift westward away from the Hawaiian Islands.
Expect decreasing local area showers this weekend in a moderate to
locally breezy easterly trade wind pattern, with some lingering
passing showers favoring the windward and mountain areas during
the overnight to early morning hours.

Long range weather models are hinting at yet another unstable
upper low drifting towards the Hawaiian Islands early next week,
potentially triggering yet another round of wet weather across the
island chain. Stay tuned.

&&

.AVIATION...
A hybrid moderate easterly trade wind and afternoon sea breeze
pattern will continue today. Onshore sea breezes will favor
terrain sheltered leeward areas. Shower trends will increase by
early Wednesday morning. Brief periods of MVFR conditions are
possible in passing showers mainly over windward and mountain
areas today.

No AIRMETs in effect.

&&

.MARINE...
A trough north of the state will continue to disrupt trades
through the rest of today, supporting gentle to fresh easterly winds
prevailing across the coastal waters. This trough is forecast to
fade while high pressure builds far to the northeast later this
evening. As this occurs, expect trade winds to build back into the
region through the rest of the week. Meanwhile, a an upper level
low will move over the islands beginning around Tuesday night.
This low is already producing thunderstorms near the offshore
waters, which will continue through Friday. As the low comes
closer to the islands it will destabilize the atmosphere enough to
support isolated thunderstorm chances across the coastal waters
from Tuesday night through Friday.

Latest buoy observations show that a moderate, long period
northwest (320 degree) swell is currently moving through the
islands. This swell is originating from a deep low pressure system
that produced storm force winds in the far NW Pacific during the
second half of last week, then continued to aim gale force winds
at Hawaii through early Sunday as it weakened. Additionally, the
quick-moving remnants of Post-Tropical Cyclone Halong also
produced a closer, very brief fetch aimed at Hawaii from a
slightly more WNW (300 degree) direction late last week. As a
result, the current northwest swell is expected to peak this
morning, then begin its decline later this afternoon. A High Surf
Advisory remains in effect for north and west facing shores of
Niihau, Kauai, Oahu, and Molokai, and for north facing shores of
Maui through this evening. A Small Craft Advisory (SCA) is in
effect for all Kauai Waters, Kauai Channel, Oahu Windward Waters,
Kaiwi Channel, Maui County Windward Waters and Big Island Windward
Waters due to seas around 10 feet. With winds increasing tonight
especially across the windier waters and channels a SCA will go
in effect this evening around Maui Co. and the Big Island.

Small, medium period south swell energy will continue through the
week with minor pulses of longer periods arriving tonight and
again on Saturday. East shores will remain small due to light
winds, but should begin to increase beginning Wednesday as trades
strengthen.

&&

.FIRE WEATHER...
Wind speeds and afternoon minimum humidity levels will likely
remain below critical fire weather thresholds this week. Unstable
conditions will produce increasing wet weather trends across the
state from Wednesday through Friday. Locally heavy rain and
thunderstorms are possible during this time period. Temperature
inversion heights near Maui and the Big Island will range from the
6,000 to 7,000 feet elevation range today.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for north and west
facing shores of Niihau, Kauai, Oahu, Molokai, and north facing
shores of Maui.

Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County
Windward Waters-Big Island Windward Waters.

Small Craft Advisory from 6 PM this evening to 6 PM HST
Wednesday for Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big
Island Leeward Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Bohlin
MARINE...Walsh
FIRE WEATHER...Bohlin