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Hawaiian Islands Synoptic Discussion and Guidance

909
FXHW60 PHFO 210630
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Sun Sep 20 2020

.SYNOPSIS...
A ridge of high pressure north of the state will keep moderate
trade winds in place through the week, with only minor
fluctuations in strength. Fairly typical trade wind weather will
persist, with showers favoring windward and mauka areas,
particularly during the night and early morning hours, and a stray
shower reaching leeward areas from time to time.

&&

.DISCUSSION...
Currently at the surface, a 1023 mb high is centered around 1100
miles northeast of Honolulu, while an elongated area of low
pressure is located around 550 miles south-southeast of Hilo. The
gradient between these features is producing moderate trade winds
across the island chain this evening. Infrared satellite imagery
shows a mix of low and high clouds producing partly cloudy
conditions in most areas, with a few locales seeing a bit more
cloud cover. Radar imagery shows isolated to scattered showers
moving into windward areas. The shower coverage is highest over
Kauai, where a few showers are drifting into leeward areas as
well. Main short term focus revolves around rain chances and trade
wind trends.

A ridge of high pressure north of the islands and a fairly active
Intertropical Convergence Zone (ITCZ) to the south of the state,
will maintain fairly steady moderate trade winds through the week,
with only minor fluctuations in strength. We could see the trades
ease a bit next weekend however, as both the GFS and ECWMF are
advertising a front making some progress southward towards the
island chain.

Day to day weather trends will be difficult to pin down with too
much specificity given the lack of any real substantial moisture
sources upstream of the state. Overall, expect a fairly typical or
perhaps slightly drier than normal trade wind weather pattern to
persist through the next week. This should result in a
continuation of scattered windward/mauka showers, particularly
during the night and early morning hours, with a stray shower
reaching leeward communities from time to time. There could be a
bit of an increase in shower coverage across portions of the
island chain Wednesday and Thursday however, as some of the
remnant moisture associated with the former east Pacific tropical
cyclone Karina slides through. No significant rainfall is expected
through the next 7 days.

&&

.AVIATION...
The high pressure ridge north of the islands will weaken over the
next few days, producing moderate easterly trade winds across the
region. Periods of showers will continue to fall across windward
and mountain areas with isolated showers along leeward areas.
Expect brief periods of MVFR conditions in SHRA mainly in the
overnight to early morning hours along windward and mountain
slopes. Tempo mountain obscurations are possible through the early
morning hours mainly along north and east sections of Kauai and
Oahu.

No AIRMETs in effect. AIRMETs for Tempo Mountain Obscuration are
possible later tonight through Monday morning.

&&

.MARINE...
Moderate to locally strong trades will hold through tonight, then
weaken slightly to moderate speeds as the ridge far north of the
islands weakens. A stronger area of high pressure will move into
range north of the islands about Tuesday night to Wednesday time
frame, leading to a boost in the trades to locally strong levels
again. This new high will continue moving east through the rest
of the week while maintaining locally strong trades.

A Small Craft Advisory (SCA) remains in effect through late
tonight for the typically windy areas around Maui County and
waters south of the Big Island. Trades will then be near
borderline advisory levels to about Tuesday night/Wednesday,
followed by the locally strong trades for the rest of the week,
at which point a new SCA is likely for the typically wind waters
of Maui County and waters south of the Big Island.

A north swell is running at around 2 feet this evening, pretty
much in line with WAVEWATCH III. A new long period north-northwest
swell is due in Tuesday night, peak Wednesday night and Thursday
at around 6 feet and 15 seconds, followed by a gradual decline
through Friday. This translates to surf just below advisory
levels.

The short period trade wind surf along east facing shores will
remain small, even more so with the weakening trades in the next
24 hours. Surf along east facing shores will increase slightly
during the second half of the week with the anticipated boost in
the trades. The arrival of the long period north-northwest swell
may add another foot resulting in moderate surf during the second
half of the week.

The current south-southwest swell has peaked, but a small
southwest swell is due in Monday, leading to near summer time
average of 3 to 5 feet during the first half of the week. Another
small south-southwest swell is possible Friday into Saturday.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...H Lau

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