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Hawaiian Islands Synoptic Discussion and Guidance

101
FXHW60 PHFO 270633
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
833 PM HST Sat Sep 26 2020

.SYNOPSIS...
High pressure northeast of the state will keep moderate trades in
place through Sunday with mainly windward showers and the
occasional leeward spillover. A band of showers will bring some
wet weather to windward areas and a few more showers to leeward
locales Sunday night into Monday morning. A drier airmass will
settle in Monday afternoon, with the trades trending downward to
light levels by Tuesday. A land and sea breeze pattern is then
expected Wednesday through late next week, with a few showers
affecting areas near the coast at night and over the island
interiors each day.

&&

.DISCUSSION...
Currently at the surface, a 1027 mb high is centered around 1600
miles northeast of Honolulu, while a trough of low pressure is
located 700 miles southeast of Hilo. Meanwhile the remnant low of
former tropical cyclone Lowell is roughly 1100 miles east of the
state. The local pressure gradient continues to drive moderate
trade winds across the island chain this evening. Infrared
satellite imagery shows mostly clear to partly cloudy skies across
most of the state, with a few pockets of more extensive cloud
cover. Radar imagery shows a few showers moving into windward
slopes and coasts, with generally rain free weather in leeward
locales. Main short term concerns revolve around trade wind trends
and rain chances the next few days.

High pressure northeast of the state will keep moderate trades in
place through Sunday. The trades are then expected to ease Monday
through Tuesday as the gradient relaxes due to a cold front
approaching from the northwest and the remnant low of former
tropical cyclone Lowell approaching from the east. The trades
will likely be light enough for some overnight land breezes and
daytime sea breezes each day, with better chances for Tuesday when
the trades will be the lightest. A weak ridge of high pressure
will set up over the central and western islands Wednesday through
Friday, keeping light winds in place with widespread sea breezes
expected during the day and land breezes at night. High pressure
is then expected to build in from the east and northeast next
weekend, allowing the trades to return.

As for weather details, fairly typical trade wind conditions will
persist through Sunday, with scattered windward showers and the
occasional leeward spill over. A band of moisture is then expected
to move into the islands late Sunday afternoon or Sunday evening,
and linger through Monday morning. This band is well defined in
satellite imagery around 300 miles east of the Big Island with
MIMIC total precipitable water imagery showing PW values of 1.5 to
1.8 inches. As a result, we should see a wet trade wind pattern
become established Sunday night into Monday morning, with showers
reaching leeward areas more regularly as well. The airmass appears
to dry out pretty well by Monday afternoon, although there will
likely be enough lingering moisture for a few showers in both
windward and leeward areas. Fairly dry weather should then persist
state wide Monday night through Wednesday night, with a few
isolated showers possible near the coast each night and over the
island interiors each afternoon/early evening. Some deeper
moisture appears to move in from the south and southeast late in
the week, and this may increase shower coverage, but the land and
sea breeze pattern is expected to hold in place. A more typical
trade wind shower pattern should then take hold late next weekend.

&&

.AVIATION...
A weakening high pressure ridge north of the islands will bring
lighter winds to the forecast on Sunday. Expect increasing land
and sea breezes along SW to NW sections of each island with
afternoon interior cloud ceilings and brief periods of showers.

No AIRMETs in effect and none are forecast.

&&

.MARINE...
Trades winds will trend down into next week. Localized land and
sea breezes will develop and continue for the next several days as
the ridge to the north weakens.

Surf along north facing shores will slowly lower through early
next week. The next swell is expected Wednesday and peak Thursday,
with surf nearing advisory level, while another arrives Friday.

Surf along south facing shores will remain up through the weekend
as a long-period south-southwest swell fills in, with heights
nearing advisory level (8 ft faces) Sunday as it peaks. A gradual
downward trend is expected into early next week. For the long
range, the active trend is expected to continue.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Morrison

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