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Hawaiian Islands Synoptic Discussion and Guidance

644
FXHW60 PHFO 280620
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
820 PM HST Mon Jan 27 2020

.SYNOPSIS...
Rather benign weather conditions will persist over the next few
days as a ridge sits just north of Kauai. Light winds will persist
over Kauai and Oahu while southeast trade winds are expected over
Maui County and the Big Island. An approaching front will push the
ridge over the state Wednesday night and Thursday bringing light
winds across the islands. The front will likely bring wet and unsettled
weather to portions of the state Friday and early Saturday as trades
rebuild.

&&

.DISCUSSION...
High pressure centered off the California coast is extending a
surface ridge just north of Kauai. The placement of the ridge close
to the islands has veered our trade winds from the east southeast.
Moderate trades will remain into Wednesday over the Big Island
and Maui County but Kauai and Oahu will see light winds as the Big
Island wind shadow shifts over those islands. Expect afternoon
seabreezes and nighttime land breezes over Kauai and Oahu with
very limited afternoon clouds and showers over interior sections.
The rest of the state will maintain limited windward showers.

An upper level ridge is keeping a cap on any convection over the
islands, and hence the limited showers forecasted over the next
few days. The upper ridge will move aside later this week as an
upper trough approaches Thursday bringing instability edging in
from the northwest Thursday night and Friday. A surface front
associated with the upper trough will push the surface ridge over
the state by Thursday, cutting off island trades and allowing
widespread afternoon sea breezes and nighttime downslope flow.

Wet and unsettled weather is expected as the instability increases
and the front brings enhanced moisture to the area Thursday night
and Friday. The front is forecast to stall somewhere between
Kauai and Oahu Friday afternoon while high pressure behind the
front reinvigorates our northeast trade winds. As the frontal
moisture, instability, and island terrain interact there may be a
possibility for heavy showers especially over the western end of
the state. The greatest potential for wet and unsettled
conditions will be over windward areas, especially near the
stalled front.

Uncertainty on shower coverage lingers heading into the weekend,
while it appears that moderate trade winds will prevail. The
latest runs of the GFS and ECMWF wash out the front and lift it
northward, perhaps leaving some shallow moisture behind to fuel
wet trades on Saturday. Both models suggest mid level ridging
will build overhead on Sunday as trade winds hold, favoring a
typical pattern of windward rainfall.

&&

.AVIATION...
Expect a VFR evening that will last well into Tuesday as a stable
and rather dry air mass lies over the area. A surface ridge is
currently 120 nm N of Kauai, and will continue to drift S through
Tuesday. This will cause our winds to veer southeast across the
smaller islands, leading to a daytime sea breeze and nighttime
land breeze pattern. The Big Island will remain under a softening
trade wind flow. With daytime heating, look for some clouds to
form over the interior and lee areas. Showers will be kept to a
minimal due to a strong low level inversion base at 6 to 7k feet.
A light east wind component will still be present across the
windward waters, bringing in a few stray light showers into the
windward areas of the larger islands through Tuesday. There are no
AIRMETs in effect and none is expected through at least 16z.

&&

.MARINE...
A ridge of high pressure north of the area will slowly move south
as a front passes north of the state. Winds are expected to be
light and variable across Kauai and Oahu, and moderate to locally
fresh out of the east to southeast over Maui County and the Big
Island through most of the week. A shift the in weather pattern
is possible Thursday night into Friday as a stronger front
approaches from the northwest.

The current northwest swell continues to gradually decrease, but
will hold surf at low-end advisory levels across exposed north
and west facing shores of smaller islands through tonight.
Therefore, the High Surf Advisory for north as west facing shores
of the smaller islands has been extended through tonight.

The Small Craft Advisory (SCA) has been scaled back as winds and
swell eased over parts of Maui. However, the SCA remains in effect
for the Alenuihaha Channel and other waters around the Big Island
through tonight. The SCA will likely be dropped Tuesday morning
as winds and swell continue to decline.

A progressive weather pattern will keep surf elevated along north
and west facing shores. The next northwest swell is expected to
arrive Tuesday night and peak at solid advisory levels by
Wednesday morning. A larger, potentially warning level, northwest
to north-northwest swell is expected towards the end of the week.

Surf along exposed east facing shores will remain rather small
through the period due to the lack of persistent trades locally
and upstream across the eastern Pacific. An increase in surf is
possible over the weekend as trades are expected to return.

Surf along south facing shores will remain small through the
week. However, background south swells should be sufficient to
keep things from going flat.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Tuesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Small Craft Advisory until 6 AM HST Tuesday for Alenuihaha
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$


DISCUSSION...Morrison
AVIATION...Lau
MARINE...Kinel

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



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