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Hawaiian Islands Synoptic Discussion and Guidance

595
FXHW60 PHFO 100645
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
845 PM HST Mon Dec 9 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
likely 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 potent 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
over the islands. Although mostly dry conditions remain in place, an
area of enhanced moisture skirting the southeast Big Island waters
accompanying a trough passing to the south will continue to
support isolated to scattered showers in this area overnight into
Tuesday. A few overnight showers will also remain possible across
the waters north of Kauai near the stalled frontal boundary.

The latest short-term (through Wednesday) guidance depicts the weak
ridge over the islands drifting northward as the stalled frontal
boundary just north of Kauai diminishes. This will allow light
trades to fill in over the eastern end of the state Tuesday through
Wednesday. 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 key differences
between the solutions with regard to how far east this upper feature
and front make it into the area. Despite these differences,
increasing moisture and rainfall chances along with gusty trades
remain a possibility by or come Friday as the attendant cold front
moves into the area. Breezy trades along with a drying trend will be
possible over the weekend as high pressure builds north of the state
and the front diminishes.

&&

.AVIATION...
Light winds with overnight land breezes and afternoon sea
breezes will continue. Most of the state will have VFR
conditions. The only exception may be some passing MVFR conditions
on the windward slopes, especially on the Big Island. No AIRMETs
are currently in effect and none are anticipated.

&&

.MARINE...
A northwest swell continues to spread down across the coastal
waters early this evening. The observations from the NDBC buoys,
51001 and 51101, located northwest of Kauai showed this swell was
1 or 2 feet above the Wave Watch III model guidance earlier
today. However, the latest observations from those buoys show the
swell is now close to 9 feet with a wave period of around 14
seconds. The PacIOOS buoy at Waimea Bay indicates the significant
wave height has recently increased to 7 to 8 feet with a wave
period of close to 14 seconds. Surf produced by this swell is
expected to gradually build, so 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 by 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.

The pressure gradient remains disrupted across the main Hawaiian
Islands early this evening due to a weakening east-northeast to
west-southwest oriented frontal boundary that has stalled about
75 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 through mid-week. That
being said, the back to back large northwest swells arriving
during the next few 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 has been showing some uncertainties
recently, 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 will likely bring stronger northeast 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 by Friday.
For the moment, the forecast indicates these strongest winds will
be mainly by 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 for
later this week, since conditions 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



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