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RH14 %
WindW 9 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 202032 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
1032 AM HST Wed Oct 20 2021


A dry and stable airmass overhead will bring mostly sunny skies
today. The remnants of an old front will reach Kauai later
tonight, and may also spread to Oahu Thursday or Friday, bringing
an increase in clouds and some showers. Trades will start to
increase Thursday and become breezy by the weekend. Drier and more
stable air will also return this weekend.


Visible satellite across the Hawaiian Islands shows mostly sunny
skies this morning. Two weak cold front lie north of the island
chain, one about 200 miles north of Honolulu and the other about
700 miles north. High pressure behind these features is keeping
gentle trade winds over the islands, reduced somewhat due to the
fronts. Some areas protected from the trades may switch to
seas breezes today. Soundings from Hilo and Lihue at 12Z show a
low, stable inversion of 5000-5500 ft over the state. Sea level
dewpoints also relatively dry, with most readings in the upper 50s
and lower 60s. Expect the pleasant weather to continue through the
rest of the day.

Clouds and some showers associated with the closest front will
move south and reach Kauai late tonight. This moisture will
continue to become more diffuse as the front dissipates, perhaps
spreading a few showers to Oahu Thursday and Thursday night. Not
surprisingly, some mixed signals exist in the models as to
whether the band will actually hold together near Oahu, or whether
we'll just see more of an increase in trade wind showers for all
islands starting Thursday night instead. Either way, the trend
will be a bit wetter over the next 36 hours. Wet weather will
continue across Kauai on Friday (and possibly Oahu) as weak upper
troughing and the moisture from frontal remnants linger near the
northern end of the state. The surface high north of the front
will dig towards the south allowing winds to gradually ramp up to
locally breezy levels by Thursday, and breezy to locally windy on
Friday and Saturday.

Over the weekend, mid-level ridging builds in and subsidence takes
back over, leading to more sunshine and fewer showers on
Saturday. Although the airmass will remain quite stable into
Sunday and Sunday night, a very minor increase in trade showers is
expected associated with a patch of moisture coming in on the
trades. Depending on the timing of this area with respect to the
diurnal cloud cycle, it might not be very noticeable. Another high
will move in from the W far to the NW of the islands, keeping
breezy to locally windy trades blowing through the weekend.


Gentle trade winds are expected today across the state ahead of a
weakening frontal boundary that is currently north of Kauai and pushing
southward towards the main Hawaiian Islands. Therefore, showers
and areas of low clouds will be quite sparse today, though the
light background flow may allow for a few light isolated showers
along north through east facing sections of the islands and
adjacent waters. Due to the weak background trade wind flow, sea
breezes could develop in sheltered areas, though the stability and
dry air overhead should preclude any convective showers across
interior sections or leeward areas today.

As the remnants of the front reach Kauai this evening, showers
and cloud cover are expected to increase. Although no AIRMETs are
currently in effect, this could lead to mountain obscuration
becoming a concern along Kauai's windward slopes by the end of
today. These clouds and showers will then spread east and may reach
Oahu by early tomorrow morning. Strengthening trade winds will
return tomorrow through Friday, which could warrant an AIRMET
Tango for moderate low-level turbulence.


A surface ridge just N of Kauai extends from a high centered to
the distant NW of the islands, thereby supporting light to locally
moderate NE trade winds. A weak front about 150 nm N of Kauai
lies just N of the ridge, and is moving slowly S. As the front
continues to move S into Thursday, and a new high builds N of the
area, the ridge near Kauai will dissipate in favor a new, stronger
ridge much farther N. End result will be a gradual increase in NE
trade wind flow on Thursday, shifting to the E at moderate to
locally strong speeds by Friday. A Small Craft Advisory will be
required for the windier zones around Maui and the Big Island as
this unfolds. The ridge will remain sufficiently strong to
maintain this trade wind flow into next week.

The weak front is expected to stall and dissipate near the
islands, bringing a slight increase in showers, mainly over
windward waters. However, there remains uncertainty as to whether
this modest moisture increase will extend as far S as the Big
Island before dissipating over the weekend.

NNW swell will be on the increase into tonight, with buoy
observations indicating a new, relatively long-period (14-16
second) swell building over the past several hours. Observed wave
heights/periods are near forecast guidance, increasing confidence
that peak surf heights (expected tonight and early Thursday) along
exposed N and W facing shores will be below High Surf Advisory
level thresholds. As this swell diminishes into Friday, a new N
swell is expected to build - potentially to advisory levels along
N facing shores. Confidence is not high as to how large this
swell will get as the islands will lie on the edge of a fetch
associated with a rapidly-developing and powerful low moving into
the Gulf of Alaska the next couple of days. Better chances for
larger swells on the E end of the island chain. This swell brings
the potential for moderate harbor surges in both Kahului and Hilo
harbors. A low near the Aleutians will generate a small to
moderate NW swell for early next week. A slight increase in long-
period S swell is expected the next day or two, with increased
trades over the weekend supporting a slight boost in short-period
wind waves along E facing shores.






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