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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 191955
Area Forecast Discussion
National Weather Service Honolulu HI
955 AM HST Tue Oct 19 2021
A ridge of high pressure approaching the islands will make for
gentle trades the next couple of days. A dry and stable airmass
will give lots of sunshine and relatively few showers to the Aloha
State. The remnants of an old front will reach Kauai Thursday, and
might also spread to Oahu Thursday night or Friday, bringing an
increase in clouds and some showers. Trades will start to
increase Thursday and become breezy over the weekend. A dry and
stable airmass will also return this weekend with fine weather
No surprises in store for this morning, with no changes to the
forecast, and few changes anticipated with the afternoon package.
The short term forecast remains relative dry, with weakening
winds. Expect fewer showers than normal through Wednesday with the
dry and stable airmass overhead. Tonight, dewpoints are expected
to drop the upper 50s to lower 60s, but start to rebound tomorrow
Few clouds and showers are over and upstream of the islands, so
don't expect much in the way of cloud coverage today or tomorrow.
Satellite shows the cloud band associated with a cold front north
of the islands to be along 30N. It is this front that is pushing
the ridge over the islands, leading to the weakening of our trade
wind flow. Some areas could see land and sea breezes today, but
more likely tomorrow, as the winds weaken. Even as the winds
weaken, expect a background trade wind pattern.
The front is expected to fall apart as it sinks south and the
parent low lifts to the north. Remnants of the front will likely
impact the northern islands Thursday and Friday, around the same
time as an upper level trough moves over head. The frontal
remnants will boost the low level moisture, and the upper level
feature will help to to provide some support for the possibility
of some enhancement to the showers. That all being said, overall
rainfall amounts are still expected to be fairly modest.
Beyond Friday, expect a new high to build in, allowing breezy
trades to return with another round of drier and more stable
A ridge of high pressure continues to move towards the island
chain from the north today, easing trade wind speeds across the
state. Shower activity will be limited to north through east
sections of the islands and adjacent waters throughout the
forecast period. However, even this shower activity will be mostly
isolated in nature given the increasing stability overhead.
Although brief periods of transient low clouds are possible, VFR
flight conditions are expected to prevail throughout the next 24
hours and beyond.
There are currently no AIRMETs in effect, and none are anticipated
today or tonight.
Moderate trades will be gradually trending down to generally light
to gentle breezes the next couple of days as a series of North
Pacific cold fronts weaken the ridge north of the islands. These
fronts will cross the north offshore waters Wednesday and Thursday
and are expected to dissipate before reaching the state. High
pressure will re-establish itself north of the islands by Friday
and this will strengthen weekend trades back into the moderate to
locally strong category.
A moderate north-northwest swell will slowly fill in tonight and
steadily build throughout the day Wednesday. This swell will
likely peak Wednesday night and gradually decline on Thursday. A
reinforcing north swell is expected to fill in Thursday night and
peak on Friday. Both of these swells could get near the High Surf
Advisory threshold during its peak. With the swells being out of
the northerly direction, harbor surges are expected over the
Kahului and Hilo harbors especially on Friday due to an increase
in swell height (6 to 8 feet) and a decrease in swell period
(12-13 seconds). Swell direction will also be directly out of the
north (010 degrees).
Surf along south facing shores should see a small increase as a new
south-southwest swell fills in further this afternoon and Wednesday.
Surf along east facing shores could see some wrap from the pair of
northerly swells over the next several days. Otherwise, trade wind
swell will be on the increase Friday into the weekend as the trades
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office