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Hawaiian Islands Synoptic Discussion and Guidance

621
FXHW60 PHFO 200634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Sat Oct 19 2019

.SYNOPSIS...
Lighter trade winds are expected Sunday, with stronger trades
moving back in Monday and Tuesday. Clouds and showers will favor
windward and mauka areas.

&&

.DISCUSSION...
Have updated the windward Oahu portion of the forecast to increase
showers for tonight based on radar. Also added in more clouds
mostly over Maui County and the Big Island to reflect the high
clouds observed on satellite.

A high pressure system far to the northeast of the islands has an
associate ridge to our north. This has been bringing easterly
trade winds to the region which are expected to continue most of
tonight. A front is expected to pass north of the area over the
next day or so, which will help to push the ridge closer to the
islands. This will result in some lighter, southeasterly winds,
particularly over Kauai and Oahu.

As the front weakens Monday and into Tuesday, the ridge should
strengthen and lift northward, allowing easterly trades to return
with a boost in speed. Later in the week, there could be another
decrease in the trades as another front approaches.

Throughout most of the forecast period, showers will be carried
in on the trade wind flow, so expect the bulk of the rainfall to
be over windward areas. As we experience lighter southeast flow
over the next day or so, we could see showers over interior and
leeward areas.

Satellite derived precipitable water (PW) shows 2+ inches of PW to
the south of the Big Island today. The models are in good
agreement with the highest concentration of this moisture staying
to the west of the islands as it moves to the northwest over the
next day or so. Kauai and Oahu could see 1.75 inches of PW
tomorrow night, so will be taking a closer look at the PoPs and
associated fields for that time period by the morning package.

&&

.AVIATION...
Moderate easterly trade winds will continue through the evening
hours, then shift out of a more east-southeast direction late
tonight through Sunday. Clouds and showers will favor windward and
mountain locations, with the best coverage likely occurring
through the overnight and early morning hours.

Although no AIRMETS are currently in effect, AIRMET Sierra can't
be ruled out for mountain obscuration if the clouds and showers
over the Koolaus persist and become more widespread overnight.
Otherwise, VFR conditions should prevail, with the exception being
for brief periods of MVFR ceilings and visibilities in and around
passing showers.

&&

.MARINE...
A surface ridge extends southwestward from a 1027 mb high
centered about 1000 nm northeast of the islands, with the ridge
about 475 nm north of Kauai. This is supporting locally strong
easterly trade winds in areas where winds are where most
accelerated by island terrain, with moderate to fresh winds
prevailing. A front passing north of the area will push the
ridge southward the next 24 hours or so, leading to diminishing
winds statewide that will be veering toward the southeast near
Kauai. A Small Craft Advisory (SCA) remains posted through the
night, and will likely be allowed to expire in the morning. As the
front weakens Monday into Tuesday, the ridge will strengthen and
move north, allowing locally strong easterly trade winds to
redevelop, likely requiring a re-issuance of the SCA for windier
zones. Another front approaching late in the week may once again
lead to diminishing winds.

Surf is expected to remain below advisory levels along all shores
through the upcoming week. A small north-northwest swell will
diminish further Sunday into Monday, with a new northwest swell is
possible Thursday into next weekend. Near shore buoys detect a
new long-period south swell moving in this evening, with this
swell expected to linger into Monday before diminishing. Trade
winds will push short-period wind waves onto east facing shores
for at least the next couple of days, trending down later in the
week.

See the Collaborative Nearshore Swell and Wind Forecast (SRDHFO)
for more details on surf and swell sources.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.


&&

$$


DISCUSSION...M Ballard
AVIATION...Gibbs
MARINE...Birchard

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



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