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Area forecast discussion
National Weather Service Honolulu HI
334 PM HST Thu Nov 14 2019

Synopsis...
light winds will continue tonight as a front approaches Hawaii
from the northwest. Increasing showers are expected over Kauai and
The Big Island on Friday, and then Oahu and Maui County Saturday.
Unsettled weather will remain Sunday and into next week as the
trade winds return.

&&

Discussion...
a front approaching from the northwest has pushed a surface ridge
into position just north of the state, effectively cutting off
our normal island trade winds. Light winds will remain into Friday
east of Kauai with daytime sea breezes and gentle nighttime land
breezes with afternoon clouds. Kauai will start seeing some
effects from the front Friday as winds shift out of the south and
clouds and showers increase throughout the day. On the eastern
side of the state, a surface trough containing deep tropical
moisture will breach The Big Island windward coast Friday bringing
increasing clouds, showers, and a jump in humidity throughout the
day.

The front is expected to weaken and dissipate over Kauai Saturday
and Sunday while the surface trough and enhanced moisture slowly
move up the island chain to Maui County. As the front weakens and
washes out, trade winds will increase from the northeast over
Kauai and Oahu, and from the east southeast over Maui County and
The Big Island. Expect partly to mostly cloudy skies and showers
to accompany the front and surface trough with heavier showers
expected for The Big Island due to the enhanced moisture. Oahu and
Maui County islands will stave off the majority of showers
through Saturday morning before shower coverage increases later in
the afternoon.

Sunday, along with our washed out frontal boundary and surface
trough of enhanced moisture, a third player will enter the game.
An upper level low is forecast to drop south over the islands
bringing cold temperatures to the middle and upper levels. This
will destabilize the atmosphere and allow for heavier showers and
the potential for thunderstorms. This upper low is expected to
linger over the islands through midweek before finally exiting
north from whence it came. The location and motion of the low will
be a deciding factor for where heavy rain and thunderstorms
occur. Historically, models have a difficult time forecasting
these features as the larger scale steering flow is commonly very
weak. The current forecast is broadbrushed to account for this
uncertainty.

At the surface, moderate trade winds will remain from Monday
through next week as a transient high pressure moves from west to
east far north of the islands. Monday through Wednesday, showers
will be enhanced by the upper low and a wet trade wind weather
regime is expected. Depending on the strength of the upper low,
there may chance for some thunderstorms mixed in with the enhanced
trade wind showers.

&&

Aviation...
winds have started to veer out of the south on Kauai this
afternoon ahead of an approaching front. Clouds have built up
across interior sections of all islands, but showers have remained
non-existent thanks to a stable atmosphere. Land breezes will
clear these clouds out during the overnight hours.

Windward areas of The Big Island will see an increase in clouds
and shower activity Friday as an area of moisture moves in from
the east. Kauai will also begin to see increasing clouds and
showers Friday associated with the aforementioned front.

No airmets are currently in effect or forecast.

&&

Marine...
a front approaching from the northwest will shift winds to a more
southerly direction tonight, then weaken some Friday before northeast
winds start to develop Saturday. Easterly winds will strengthen
Sunday and could approach Small Craft Advisory (sca) levels through
Monday over the typically windy waters around Maui County and The
Big Island.

The new north-northwest swell is running 1 to 3 feet higher than
the wave watch iii guidance, according to recent observations from
buoys 51001, 51101, and 51000. Therefore, have adjusted the swell
and surf forecasts up accordingly. A high surf advisory (hsa) is
now in effect for north facing shores of all islands. Surf will
continue to build this evening, peak tonight and lower, but remain
at advisory level, through Friday. Due to the swell coming in a
larger than expected, seas are now forecast to reach Small Craft
Advisory (sca) level through Friday as well. Therefore, a Small Craft Advisory has
been issued for all windward zones, as well as portions of the
Kauai and kaiwi channels.

A larger northwest swell is expected this weekend and may produce
low-end warning level surf during the peak Saturday night. Seas
generated by this swell are again expected to reach Small Craft Advisory levels
over exposed waters.

A series of small southerly swells are expected through the weekend.
Surf will remain small along east facing shores due to the light
winds over and upstream of the state, but will likely rise early
next week as high pressure builds in to our north and trades
return.

&&

Hfo watches/warnings/advisories...
high surf advisory until 6 PM HST Friday for Niihau-Kauai
windward-Kauai leeward-Oahu North Shore-Oahu koolau-Olomana-
Molokai-Maui windward west-Maui Central Valley-windward
Haleakala-Big Island north and east.

Small Craft Advisory until 6 PM HST Friday for Kauai northwest
waters-Kauai windward waters-Kauai Channel-Oahu windward waters-
Kaiwi Channel-Maui County windward waters-Big Island windward
waters.

&&

$$

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