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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 220630

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Thu Feb 21 2019

Generally light winds are expected across much of the area for the
next couple days. Fairly dry conditions will also remain over the
western half of the state. A surface trough lingering near the
Big Island will enhance clouds and showers over Maui and the Big
Island into the weekend. High pressure systems moving in from the
northwest may strengthen the winds early next week.


Not much change to the ongoing forecast philosophy this evening,
with a mostly dry weather pattern currently holding. Radar is
depicting a few echoes over Maui county, falling from the layered
mid cloud deck streaming over the area from the SW. Very little of
this precipitation appears to be reaching the ground. Otherwise,
nearly stationary showers over waters about 50-70 nm SE of the Big
Island are associated with a lingering surface trough. A broken to
overcast deck of clouds based near 8000 feet covers area from Oahu
to the Big Island, and any forecast update would attempt to
anticipate its overnight evolution. Latest guidance indicates this
cloud deck will shift E of Oahu later tonight, and remain in place
over Maui county and the Big Island. A Wind Advisory for the Big
Island summits is set to expire at midnight, but winds have yet to
diminish. A forecast update will be issued later this evening; to
increase cloud cover over the E end of the island chain, and to
extend/expire the Wind Advisory for the summits.


Water vapor imagery shows an upper level ridge just east of the
state with a shortwave trough passing by to our north. At the
surface, a trough is located just east of the Big Island and a
weak low is a few hundred miles west-northwest of Kauai, with a
very weak pressure gradient between them across the islands. The
00z sounding from Lihue shows a solid inversion just below 6kft
and a precipitable water of 0.80 inches (which is about a
standard deviation below normal for this time of year). The Hilo
sounding showed a weaker low-level inversion and a more
pronounced one near 13kft, and a PW of 1.46 inches (which is about
a standard deviation above normal). MIMIC total precipitable
water imagery captured this gradient across the islands, with
higher values above 2 inches just offshore south and east of the
Big Island that have made only slight progress westward during
the day.

With the light gradient in place across the state, onshore sea
breezes were pretty widespread today. Cloud buildups have mainly
affected interior and northeast sections of the islands, with
little in the way of rainfall from them. The afternoon clouds are
most noticeable across Kauai/Oahu away from the more widespread
mid-level clouds farther east. The sea breezes will give way to
offshore land breezes this evening, leading to some clearing
overnight, at least across the western islands. Skies will remain
at least partly cloudy with some showers in the east, where there
is greater moisture closer to the surface trough.

The surface trough near the Big Island will drift westward slowly
through Friday, spreading additional low-level moisture across
the Big Island and eventually Maui County as well. Rainfall
chances are greatest on Friday, then start to taper off Saturday
into Sunday. With the wind shifting more southeast near the Big
Island, highest rainfall amounts will be focused across windward
and southeast sections. An upper level trough moving through the
area may enhance lift, and with the marginally unstable airmass
and abundant moisture we could see embedded heavy rain showers.
However, the upper trough and surface feature will be weakening
with time and don't appear to provide a strong focus for
convection. At this point we're not expecting widespread flash
flooding and therefore do not anticipate a flash flood watch,
although localized heavy rain is possible.

Over the weekend, the surface trough will weaken and shift
eastward as a new high builds in from the northwest of the state.
Deeper moisture and some showers will linger longest across the
Big Island, but conditions there should start to stabilize by
Sunday night/Monday. This first high will be somewhat brief,
dissipating early next week as a front passes by far north of the
state. Another high will build in behind the weakening front,
bringing an increase to north/northeast winds and cooler/drier
conditions by the middle of next week.


Mid-level clouds continue to stream across the islands from Oahu
to the Big Island early this evening. The low-level flow will
remain weak through Friday. A surface trough near the Big Island
is forecast to track slowly westward. Abundant tropical moisture
is also in the vicinity of the Big Island. This is causing low
clouds and showers to increase over the windward Big Island
tonight. These low clouds and showers are then expected to spread
over portions of Maui on Friday. AIRMET Sierra for mountain
obscuration due to these low clouds and showers is in effect early
this evening for the windward Big Island. This AIRMET may need to
be extended later tonight.


Light northeast winds will continue across most Hawaiian waters
into the upcoming weekend, then shift out of the north and
increase into the light to moderate category late Saturday through
Sunday. Near the coasts, overnight and early morning land breezes
followed by late morning and afternoon sea breezes will continue
through Saturday. The increase out of the north to northeast
direction late Saturday through Sunday may be strong enough to
hold off the sea breezes from developing Sunday. The exception
will be around the Big Island due to a trough of low pressure that
is forecast to linger in the area into the weekend. Moderate to
fresh breezes out of the southeast direction will hold across the
windward and southeast Big Island waters through Friday. The
trough will begin to shift east over the weekend as high pressure
builds to the northwest and a front passes to the north.

A cold front will approach and move into the area from the north
Monday night through midweek, which could bring fresh to strong
north to northeast winds across the waters Tuesday through

Despite the light winds in place, seas are slow to lower due to the
large easterly swell that has been in place. This swell has peaked
and will continue to lower through the weekend.

Surf along east facing shores will respond and gradually ease into
the weekend as the swell lowers. The high surf advisory has been

After a long stretch of large surf and northerly winds for our north
facing shores through the first half of February, surf will near a
minimum for the month through the weekend. Storms over the far
northwest Pacific have been quickly lifting north-northeast from
Japan to the Bering Sea, ultimately limiting our traditional setup
for northwest swell sources.

An upward trend is expected next week in response to a potential
powerful hurricane-force low projected to develop off the coast of
Japan Saturday, then race northeastward toward the Aleutians/Date
line Sunday. WAVEWATCH III and the ECMWF-Wave solutions depict a
large long-period northwest (320 deg) swell evolving from this
source moving through the Hawaiian waters Tuesday through midweek.
If this materializes, surf could near warning levels for exposed
north and west facing shores Wednesday through Thursday.
Additionally, strong northerly winds accompanying a cold front
moving down the island chain will result in rough conditions with
small craft advisory winds and seas.


Wind Advisory until midnight HST tonight for Big Island Summits.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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