945
FXHW60 PHFO 171415
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
415 AM HST Thu Jul 17 2025

.SYNOPSIS...
A near static and quiet weather pattern will undergo change the
next few days as rain chances increase going into and through the
weekend. The western half of the state will likely experience a
more wet weekend while light to locally breezy regional trade
winds remain in place through the end of the week. Today`s
showers will focus along windward exposures, upslope mauka and
within higher elevations. Stable and drier weather is expected to
return early next week.

&&

.DISCUSSION (CORRECTED AT 415 AM)...
The current synoptic summertime pattern remains entrenched over
the Hawaiian Islands. This being of high pressure ridging at all
levels positioned north and northeast of the state. Light, random
showers passing within the trade belt has wettened many windward
locales with brief light rain episodes. The windward slopes of Big
Island`s Hamakua, Hilo and Puna regions have received the most 24
hour rainfall of typically a quarter of an inch or less. The
remainder of the state has picked up no more than a few hundredths
to a tenth of an inch over east-facing regions while many leeward
areas have remained bone dry. A surge of higher mid to low level
moisture passing through today will thicken up clouds and increase
primarily windward and upslope mauka/high elevation shower
activity. The caveat will be along more wind-protected Big Island
Kona slopes whereas the enhanced afternoon sea breeze will
increase upslope clouds and areal rain chances. The large scale
downstream pressure gradient from a 1035 mb high centered near
45N145W has been tight enough to produce statewide interior light
to breezy trades. Downslope or channeled winds have been locally
strong over such areas as the central Maui Valley (Kealaloloa
Ridge) and North Kohala`s northernmost tip (Upolu Point) where
winds of 25 to 30 mph sustained with gusts to over 40 mph have
been commonplace the past several days. Little change in the
strength and position of the surface high will guarantee another
few days of this wind behavior.

Geopotential heights will begin to gradually fall from the
northwest as a weak mid to upper low or troughing develops west
of the islands the next few days. Height falls of around 400
meters and increased 500 mb vorticity, along with a surge of
unseasonably higher moisture (1.6 inch pwats) advecting in from
the east, will decrease stability and boost rain chances mainly
north of 21N...better focusing the higher precipitation over Oahu
and Kauai from late Friday through early Sunday. Deterministic
guidance is forecasting deepening moisture where yesterday`s 4k ft
resident boundary layer depth (inversion heights) should double
to around 8k ft by Saturday. The majority of the GFS and EC
ensemble guidance QPF members depict a distinct swath of higher
rain paralleling near 20N latitude. While confidence is moderately
high that more western islands will experience a more wet (than
dry) weekend, the chance of thunderstorms and confidence of
occurrence remains low. NWP guidance does not cool upper level
temperatures enough (around minus 7 deg C at 500 mb) to provide a
strong enough instability signal to place thunder in Kauai or
Oahu`s weekend forecast. The proximity of the low across the far
northern maritime will likely be the impetus to isolated offshore
storms this weekend. As of now, the highest confidence lays in
higher rain probabilities N of 20N with periods of heavy rain from
as early as tomorrow (Friday) night through as late as early
Monday. The timing of the heaviest rain will depend upon the
evolution of the upper low in accordance with the greatest
moisture advection.

A more stable summer-like trade wind pattern is expected to return
by late Monday. As greater moisture exits stage left on Monday, a
drier air mass in its wake will envelope the eastern half of the
state. The upper level low will move further away, northeast of
the region...weakening into a trough as ridging expands in from
the south.

&&

.AVIATION...
Moderate to locally breezy trades through tomorrow. Low cigs and
SHRA should mainly impact windward and mauka locations, as well as
the South Kona slopes of the Big Island. Trades will ease slightly
Friday and Saturday, and SHRA activity may increase as the
atmosphere becomes less stable.

AIRMET Sierra is in effect for mtn obsc for windward sections of
all islands. Conds should improve in the morning hours.

AIRMET Tango is in effect for low level turb blw 070 downwind of
island terrain.

&&

.MARINE...
High pressure will remain far northeast of the state through the
rest of the week. Fresh to locally strong trades will persist
through the rest of today, then drop slightly as a weak low level
trough develops in the easterly flow and moves across the state
tonight into the weekend. A Small Craft Advisory (SCA) remains in
effect for the typical windy areas around Maui County and the Big
Island through 6AM Friday.

A fading medium period south swell combined with a fading south-
southeast swell will maintain below summer average surf on the
south facing shores through today. A slight bump in surf is
expected this weekend as a new long-period southwest swell
arrives from the Tasman Sea, along with a new small short-period
south-southeast swell. The Tasman swell is expected to peak on
Saturday, with surf heights topping near the summer averages
before gradually declining into early next week.

Surf along east facing shores will continue to remain rough and
choppy due to the fresh to locally strong trade winds and a small
short-period northeast swell that is expected to fill in this
weekend. Surf along north facing shores will remain nearly flat
through the weekend.

Minor coastal flooding along the shoreline and in low-lying coastal
areas will be possible around the daily high tide starting Monday
of next week due to peak monthly tides and higher than predicted
water levels at select locations.

&&

.FIRE WEATHER...
Fire weather concerns will lower the next few days as a more wet
pattern develops across the state. Trade winds will remain light
to locally breezy through the day before subtly weakening this
weekend. A higher frequency of showers is expected from late
tonight into Friday and this will hold through the weekend. Drier,
more stable weather is expected to return early next week. This
may increase the fire weather threat over more leeward areas that
do not receive the benefit of significant weekend rains.

&&

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

&&

$$


DISCUSSION...Blood
AVIATION...Walsh
MARINE...Walsh
FIRE WEATHER...Blood