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Hawaiian Islands Synoptic Discussion and Guidance

096
FXHW60 PHFO 251341
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
341 AM HST Fri Sep 25 2020

.SYNOPSIS...
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 showers limited to coastal areas at night and during the
morning, and interior locations during the afternoon and evening
hours.

&&

.DISCUSSION...
Currently at the surface, a 1023 mb high is centered around 900
miles north-northeast of Honolulu, and is driving moderate to
locally breezy trades across the island chain early this morning.
Meanwhile, Tropical Storm Lowell remains far to the east of the
state 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 partly cloudy
skies in most leeward areas. Radar imagery shows scattered
showers moving into windward areas, with a few of these showers
spilling into leeward areas. Main short term concerns revolve
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 today. 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 day.

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. 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 and locations near the coast during the night and morning
hours, with a few daytime heating and sea breeze driven showers
possible in leeward areas each afternoon. Tuesday through
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.

&&

.AVIATION...
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.

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 through today.

&&

.MARINE...
Fresh to strong easterly trades will continue through tonight,
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 has been
extended through tonight for the windier waters around Maui
County and the Big Island. For the upcoming week, guidance shows
the light winds continuing each day in response to a weakness
developing in the ridge as a series of fronts pass to the north.

Surf along north facing shores will remain up today due to the
first sizable swell of the season arriving out of the north
yesterday. The swell will begin to ease through the weekend. The
active pattern across the northern Pacific is expected 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 and slowly fills in
today. 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
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 into the first
week October.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...JVC
MARINE...Gibbs

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