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Hawaiian Islands Synoptic Discussion and Guidance

035
FXHW60 PHFO 071355
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
355 AM HST Sat Dec 7 2019

.SYNOPSIS...
A drier more stable airmass is moving over the islands with light
winds through the weekend. A weak front will stall near Kauai
early next week and high pressure will pass by far north of the
area. Light to moderate trades can be expected early next week.
Another front will approach the area around Thursday with
increased shower activity.

&&

.DISCUSSION...
Several changes/tweaks have been made to the forecast this
morning. Cleared out sky and lowered pops for this morning based
on overnight satellite and latest model guidance. Also made more
minor modifications to the PoPs and associated grids for Sunday
night through Tuesday or so. Winds were generally lowered for
Sunday through Wednesday based on the last few model runs
indicating that winds won't be as strong behind the front expected
to stall near Kauai early next week.

With the shortwave off to the east, and high pressure far to the
northeast, the pressure gradient over the islands is weakening.
With high pressure to the northeast, we can expect trade winds
today, however as the high moves to the east, further weakening
the pressure gradient will result in lighter winds. The lighter
winds will bring land and sea breezes. A stable airmass overhead
will minimize rainfall totals, which will be concentrated to the
afternoon and evening hours.

The southern end of a weakening front will approach Kauai Sunday
night and Monday, which will bring an uptick in showers mostly to
the Garden Isle. High pressure passing to the north will maintain
a trade wind flow, so expect some lingering moisture from the
front to ride in on the trades early next week. As the high moves
to the northeast, look for winds to weaken again Tuesday and
Wednesday.

The models continue to show a stronger front approaching the
islands Thursday and Friday. Some model differences remain, but
expect increased shower activity with the front, and strong
northerly winds for a brief time behind the front, before trades
build back in.

&&

.AVIATION...
Northeast tradewinds are expected today with a weakening of the
wind speeds. This should lead to a daytime sea breeze-nighttime
land breeze scenario for most areas. Any showers we have through
this forecast period should be confined to the windward slopes.
Since we are only expecting relatively brief MVFR conditions on
the windward slopes and the winds will be weaker, we are not
anticipating any AIRMETs through this forecast period.

&&

.MARINE...
Expect trade winds to diminish today, and remain light E-SE through
the weekend, as an E-W oriented surface ridge moves S over the
island chain. The ridge will drift N early next week as a
weakening front stalls near Kauai, bringing light to moderate
trade winds until mid-week. A stronger high building NW of the
area is expected to bring strong NE winds Wednesday into Thursday.
Winds are expected to remain below Small Craft Advisory (SCA)
criteria until then, but combined seas are expected to reach 10 to
11 feet in many zones through tonight, and a SCA has been posted.
Otherwise, seas will likely reach SCA criteria in many zones
Tuesday and Wednesday as large NW swells arrive.

A long-period NNE swell is expected to build today, peak tonight,
and gradually diminish Sunday and Monday. A High Surf Warning has
been issued for E facing shores (where criteria are lower than N
facing shores), and a High Surf Advisory is now in effect for N
facing shores. The NE swell energy will diminish early next week,
but overlapping swells from the NNW/NW will likely bring high
surf to N and W facing shores for much of next week, with warning
level surf anticipated Wednesday/Thursday.

See the Oahu Surf Discussion (SRDHFO) for details on the sources
of the swells arriving in island waters.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Sunday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-Big Island North and East.

High Surf Advisory until 6 AM HST Sunday for Niihau-Oahu North
Shore-Molokai Leeward-Maui Central Valley.

Small Craft Advisory until 6 AM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters-
Kaiwi Channel-Maui County Windward Waters-Alenuihaha Channel-Big
Island Windward Waters-Big Island Southeast Waters.


&&

$$


DISCUSSION...M Ballard
AVIATION...Chevalier
MARINE...Birchard

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



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