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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 250639

Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Thu Sep 24 2020

Fairly typical trade wind weather is expected through Saturday,
with showers favoring windward and mauka areas and a stray shower
spilling leeward from time to time. The trades will become light
Sunday through Monday, with a few showers continuing to affect
windward areas and a shower or two developing over leeward areas
during the afternoon hours each day. A land breeze and sea breeze
pattern is expected to take hold Tuesday through late next week,
with limited to coastal areas at night and during the morning,
and interior locations during the afternoon and evening hours.


Currently at the surface, a 1025 mb high is centered around 900
miles north-northeast of Honolulu, and is driving moderate to
locally breezy trades across the island chain this evening.
Meanwhile, Tropical Storm Lowell remains far to the east of the
state, around 1700 miles east of Hilo, over the cooler east
Pacific waters. Closer to home, infrared satellite imagery shows
scattered to broken low clouds pushing into windward areas with
the trades, with mostly clear skies in leeward areas, with the
exception of leeward Big Island where some lingering daytime
heating driven cloud cover remains. Radar imagery shows scattered
showers moving into windward areas, as well as increasing trade
showers upstream of the state. Leeward areas remain generally dry,
although a few diminishing showers continue to hang on over
leeward Big Island. Main short term concern revolves around trade
wind trends and rain chances during the next few days.

High pressure north of the islands will keep moderate to breezy
trades in place through Friday. The trades will then ease over the
weekend as the high north of the state slides into the far east
Pacific and a trough of low pressure develops north of the
islands. A further easing of the trades is expected next week, as
the remnant low of tropical cyclone Lowell approaches from the
east, and a cold front approaches from the west. This is expected
to drop trades into the light range Monday through late next week,
with a sea breeze/land breeze pattern likely in leeward areas each

As for the day to day weather details, fairly typical trade wind
conditions will prevail through Saturday. Showers will favor
windward and mauka areas, particularly during the night and early
morning hours, with a stray shower reaching leeward areas from
time to time. Shower coverage is expected to be the greatest
tonight, before becoming more sparse Friday into the weekend. The
forecast becomes a bit more uncertain Sunday and Monday, as the
ECMWF shows a band of moisture ahead of the remnants of Lowell
pushing into the islands, while the GFS keeps this band of
moisture northeast of the state. With the lightened wind regime
anticipated, we should see showers limited to windward areas
during the night and morning hours, with a few daytime heating and
sea breeze driven showers possible in leeward areas. Tuesday
through next Thursday, the trades will be largely cut off, and
this should result in a fairly dry land breeze/sea breeze driven
shower pattern. During this time, showers should favor coastal
areas overnight and during the morning hours, and interior
sections of the islands during the afternoon and evening.


Locally breezy trades will remain relatively dry through the
forecast period. Prevailing VFR is forecast with only
isolated/brief instances of MVFR expected within heavier windward
and mauka showers, especially Maui and Big Island.

AIRMET Tango is in effect for tempo low-level turbulence over and
south through west of the mountains of all islands. Conditions
will likely continue well into Friday.


Fresh to strong easterly trades will continue through Friday, then
gradually trend down over the weekend, potentially enough for a
localized land and sea breeze pattern to set up near the coasts
beginning as early as Sunday. The Small Craft Advisory currently in
place through the night will likely need to be extended through
the day Friday for the windier waters around Maui County and the
Big Island. Guidance shows the light winds holding through the
first half of next week in response to a weakness developing in
the ridge as a series of fronts pass to the north.

Surf along north facing shores peaked today due to the first
sizable swell of the season arriving out of the north. The latest
observations at the offshore buoys suggest this swell has peaked.
Expect plenty of energy holding into Friday, before dropping off
quickly through the weekend. The active pattern across the
northern Pacific is forecast to continue over the weekend through
early next week with back-to-back systems diving southeastward
across the Date Line from the western Aleutians. If this scenario
materializes, the first swell will arrive Wednesday and peak
Thursday with the second one arriving Friday. The first of the two
could approach the advisory level by Thursday.

Surf along south facing shores will remain up over the weekend as a
new, long-period south-southwest swell arrives Friday from recent
activity down south. Heights will near the advisory level (8 ft
faces) by Saturday as it peaks. A gradual downward trend is expected
Sunday into early next week. For the long range, the active trend is
expected to continue down south around New Zealand within Hawaii's
swell window. The latest guidance depicts a broad, storm-force
low bottoming out at 929 mb south-southeast of New Zealand late
this weekend. A large area of gale- to storm-force winds setting
up within our swell window will drive seas into the 35 to 40 ft
range. If this evolves as predicted, expect a large south swell by
the end of the first week in October.

Surf along along east facing shores will remain small through the
upcoming weekend. A small easterly swell from tropical cyclone
Lowell in the eastern Pacific can't be ruled out early next week,
especially for the eastern end of the state.

Water levels will gradually trend down each day through the last
week of the month and into October.


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




Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office