Haiku, Hawaii Weather Forecast Discussion

718
FXHW60 PHFO 220312 CCA
AFDHFO
Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
512 PM HST Sun Sep 21 2025
.SYNOPSIS...
Breezy trades with occasional showers through Monday. This is in
response to an upper low northeast of the state pulling in a more
moisture rich air mass from the east. A surface trough far
northeast of the islands will weaken surface ridging to the north
of the region. This will result in lighter trade flow through the
week. Showers will mainly focus along windward upslope mauka the
next couple of days. The deepening trough northeast of the area
will increase statewide rain chances during the middle of the
week.
&&
.DISCUSSION...
A typical breezy late September day here in the Hawaiian Isles
with the occasional passing light shower. The impetus to this
more recent unsettled weather pattern has been an upper level low
meandering northeast of the state. Many windward and higher
elevation locales have remained partly to mostly overcast today
with most picking up just a few hundredths of an inch with
isolated half inch within the usual interior Kauai wet spots.
Occasional thin bands of higher moisture trapped within
established trades have and will continue to skate through,
quickly dropping a couple of hundredths of an inch during their
western passage. Radar has noticeably quieted down since this
morning and, while many windward areas and ridge tops will remain
mostly cloudy and experience those hit or miss brief showers, an
overall drier trend is anticipated through Monday. Trades have
eased just enough to allow the local breezes to kick in over more
wind sheltered regions as leeward Big Island and west Maui.
After a couple of days of similar somewhat unsettled partly
cloudy weather, a deepening Central Pacific trough will increase
rain chances Wednesday. A mid to upper level low will develop
along the trough axis Wednesday and dive further south toward
Hawaii. This scenario will weaken trade flow and direct a higher
channel of moisture toward the western end of the islands during
the second half of the week. While the low is not expected to get
much closer than five or six hundred miles northeast of the island
chain, its associated trough could drape over the state. This
may result in cloudier days with more frequent showers in closing
out the week. The nature or movement of this trough axis will
determine if next weekend will lean towards more wet or not.
Western ridging is expected to strengthen and lift the trough
northeast early next week. Surface high pressure does not appear
to re-establish itself by Monday as weak lower level troughing
lingers into next week. This means that, while trades will return
along with scattered showers from continued bands of higher
easterly moisture, they will remain light enough to allow small
scale breezes to remain dominant across many leeward or non east-
facing communities.
&&
.AVIATION...
High pressure north of the state continues to maintain locally
breezy easterly trade winds this afternoon. However, this high
will weaken tonight through the first half of the week as a new
high becomes established farther north. This will allow trades to
ease a bit from this evening onward. Showers will continue to
favor north through east sections of the islands throughout the
forecast period, though VFR flight conditions are expected to
prevail at most locations.
AIRMET Tango remains in effect for moderate low-level turbulence
downwind of island terrain this afternoon. Based on latest trends
and model guidance, however, expect that this AIRMET will be
cancelled later today as trades ease.
&&
.MARINE...
Locally strong trades have already eased below the Small Craft
Advisory threshold per this afternoon`s ASCAT pass. Moderate to
locally fresh trades prevail through Wednesday, then developing
low pressure NE of the islands further weakens winds late this
week.
Existing SSW swell declines through Monday followed by a smaller
SSW swell expected to peak on Wednesday. A trough NE of Hawaii
will generate a short period fresh NNE swell that will reach the
islands on Thursday and peak on Friday bringing moderate surf to
exposed shorelines. Evolution of this trough and its close
proximity to the islands brings increased uncertainty to the surf
forecast for N and E facing shores early next week. For now, E
facing shores will experience surf falling below the seasonal
average during the near term as winds decline through mid-week
followed by a bump from the aforementioned NNE swell during the
latter half of this week.
&&
.FIRE WEATHER...
An upper level low within a relatively moist resident air mass
will maintain a conditionally unstable regional atmosphere.
Overnight scattered windward and higher terrain showers with an
occasional cell making it over the ridge tops into leeward. There
will be enough early day sun along leeward slopes to create
localized sea breezes that will aid in building overcast and
initiating more frequent afternoon showers. A trough digging down
from the northeast will regulate light to moderate trades into
mid week. So, higher humidity and light winds maintain a lower
threat for developing fire weather conditions the next several
days. Inversion heights will hover around 3,000 ft at sunrise and
lift to around 5 to 6,000 ft during the afternoon.
&&
.HFO WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
DISCUSSION...Blood
AVIATION...Vaughan
MARINE...JVC
FIRE WEATHER...Blood

NWS HFO Office Area Forecast Discussion