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Hawaiian Islands Synoptic Discussion and Guidance

077
FXHW60 PHFO 101339
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
339 AM HST Tue Dec 10 2019

.SYNOPSIS...
Light easterly winds will persist through Wednesday, which will
allow the land and sea breeze regime to continue for most areas.
Outside of a few clouds and showers setting up over interior areas
each afternoon due to sea breezes, mostly dry and stable conditions
are anticipated through this time. A modest increase in moisture is
expected through the second half of the week as an upper disturbance
and cold front approach and move into the area. Shower coverage will
trend up through this time with breezy northeast to east trade winds
returning. A more typical trade wind pattern appears likely for
the upcoming weekend as drier air fills in.

&&

.DISCUSSION...
Water vapor imagery showed a ridge extending eastward across the
region and a shortwave trough far northeast of the state driving
a cold front toward the west coast. The surface analysis showed
the tail-end of this frontal boundary stretched out from east to
west north of Kauai and a weak ridge extending west-southwestward
near the islands. Although mostly dry conditions remain in place,
an area of enhanced moisture skirting the southern Big Island
waters accompanying a trough passing to the south will continue to
support isolated to scattered showers in this area through the
early morning hours today.

The latest short-term (through Wednesday) guidance depicts the weak
ridge near the islands drifting northward as the stalled frontal
boundary just north of Kauai diminishes. This will allow light
trades to hold over the eastern end of the state. The land and
sea breeze regime, however, remains likely over the western end
due to the proximity of this ridge axis. The mostly dry and stable
regime should continue with the best chance for a few clouds and
showers remaining over interior areas each afternoon due to the
sea breezes and peak heating.

For the extended period (Thursday through the weekend), guidance
depicts lowering upper heights associated with a strong short wave
trough approaching and moving into the area Wednesday night through
Friday. Although confidence is beginning to increase as the guidance
comes into better agreement, there remains some small differences
between the solutions with regard to how far east this upper
feature and its attendant cold front make it into the area.
Despite these differences, increasing moisture and rainfall
chances along with gusty trades remain a possibility Thursday
through Friday. Breezy trades along with a drying trend is
expected over the weekend as high pressure builds north of the
state and the front diminishes.

&&

.AVIATION...
Light winds will remain in the forecast through this period.
Expect afternoon sea breezes and overnight land breezes. Most of
the state will have VFR conditions. The only exception is the
southeast coast of the Big Island, which has clouds and showers
this morning which is causing some mountain obscuration.

AIRMET Sierra for mountain obscuration is in effect for the south
coast of the Big Island from PHTO to South Cape.

&&

.MARINE...
The current large northwest swell continues to spread down across
exposed coastal waters early this morning. The latest observations
from the NDBC buoys, 51001 and 51101, located northwest of Kauai
indicate the swell is 9 to 10 feet with a wave period of 13 to 14
seconds. The PacIOOS buoy at Waimea Bay shows the significant
wave height about 8 feet with a wave period of around 14 seconds.
Therefore, a High Surf Advisory (HSA) remains in effect for most
north and west facing shores of the smaller islands.

A significantly larger northwest swell is expected to arrive
Wednesday. This swell will likely cause surf to reach the High
Surf Warning criteria along most north and west facing shores of
the smaller islands starting Wednesday night. This large swell,
which is expected to persist into Friday, may also cause surf to
reach the HSA criteria along west facing shores of the Big Island
starting late Wednesday night or Thursday. The northwest swell is
expected to gradually subside from Friday night through Saturday.
A new short period north-northeast swell and strengthening trade
winds are expected to produce increasing choppy surf along most
east facing shores from Friday into Saturday.

The pressure gradient remains disrupted across the main Hawaiian
Islands early this morning due to a weak east-northeast to west-
southwest oriented frontal boundary located about 125 nm north of
Kauai. As a result, the trade winds across the coastal waters
remain light to locally moderate. This relatively weak background
trade wind flow is expected to persist through Wednesday. We do
not anticipate winds will reach the Small Craft Advisory (SCA)
criteria over any waters during the next couple of days. That
being said, the back to back large northwest swells arriving
during the next couple of days will likely cause seas to build.
This will result in SCA conditions due to elevated seas starting
early Wednesday morning, and continuing into this weekend.

The forecast guidance, which continues to show uncertainties in
the details about the winds and weather across the state later
this week, appears to indicate a new surface high will build
northwest of the islands by Thursday, and move east to a position
north of the state later this week. This is expected to push a
new front down toward the state from the northwest. The arrival of
this front may bring stronger trade winds from Thursday night
into Friday. As a result, in addition to the SCA for large seas,
winds may reach the SCA criteria over some waters starting
Thursday night. For the moment, the forecast indicates these
strongest winds will be mainly over the typically windier waters
adjacent to the islands of Maui County and the Big Island, and
possibly in the vicinity of Kauai. Mariners should monitor future
forecasts, since conditions later this week are subject to change
due to these forecast uncertainties.

&&

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

&&

$$

DISCUSSION...Gibbs
AVIATION...Chevalier
MARINE...Houston

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