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Hawaiian Islands Synoptic Discussion and Guidance

969
FXHW60 PHFO 141949
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
949 AM HST Thu Nov 14 2019

.SYNOPSIS...
Light winds will continue today as a front approaches Hawaii from
the northwest. Increasing showers are expected over Kauai and the
Big Island on Friday, and 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 through
tonight with daytime sea breezes and gentle nighttime land
breezes with afternoon clouds and clearing skies tonight. 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 morning bringing clouds, showers, and a jump
in humidity.

The front is expected to weaken and stall 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,
Molokai and Lanai will stave off the majority of showers through
Saturday morning before shower coverage increases later in the
day.

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...
Another day of light winds across the island chain today will
allow for sea breeze development later this afternoon. The
atmosphere is still fairly stable, so only anticipate isol -SHRA
development with any interior cloud build ups this afternoon.
Overall, clouds will continue to favor windward areas, as they
are this morning. Winds are expected to shift more southerly ahead
of an approaching front by Friday.

No AIRMETs are expected.

&&

.MARINE...
A front approaching from the northwest will shift winds to a more
southerly direction today, 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 appears to be on track with guidance
based on the recent observations from buoys 51001 and 51101. A High
Surf Advisory (HSA) goes into effect at noon today with rising surf
through the afternoon. Niihau and Kauai will see advisory surf by
early afternoon along north and west facing shores, while the north
shores of Oahu, Molokai, and Maui should reach it later this afternoon.
The HSA is in effect through Friday afternoon as the swell gradually
declines. 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 expected to reach SCA 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 from noon today to 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.


&&

$$


DISCUSSION...Morrison
AVIATION...Dye
MARINE...TS

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



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