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Hawaiian Islands Synoptic Discussion and Guidance

607
FXHW60 PHFO 300642
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
842 PM HST Sun Nov 29 2020

.SYNOPSIS...
A weakening high pressure ridge north of the islands will keep
light and variable winds with local scale land and sea breezes in
the forecast through much of the work week. An upper level
disturbance passing through the eastern half of the state will
keep enhanced showers in the forecast through early Monday
morning. Cooler and drier trends with only isolated to scattered
showers are possible each afternoon over island interior sections,
with clearing at night through Thursday. Moderate to breezy trade
winds return by Friday.

&&

.DISCUSSION...
Local radar imagery shows scattered to numerous showers drifting
into each island on the waning trade winds. An upper level trough
moving through the eastern half of the state will keep enhanced
showers in the forecast through Monday morning with drier
conditions moving into the region by Monday afternoon. Isolated
thunderstorms are possible over the eastern islands of Maui and
the Big Island of Hawaii through Monday. The shower activity
continues to move in quickly from the northeast tonight decreasing
chances for significant rainfall accumulations.

Meanwhile, the high pressure ridge north of the islands will
continue to weaken as a cold front moves across the Central
Pacific basin. Trade winds will diminish into the light and
variable range for most areas from Monday through Thursday. Local
scale land and sea breezes will develop over each island. Onshore
sea breezes will build clouds and a few showers over island
interior sections each afternoon. Land breezes, caused by
overnight cooling over the land, will clear out clouds and
showers during the overnight to early morning hours.

By Friday morning, a high pressure system builds in north of the
islands causing a return in the trade wind flow. Expect moderate
to breezy trades from Friday afternoon into Saturday with more
typical windward and mountain shower activity.

By Sunday another cold front approaching the Hawaiian Islands
from the northwest will weaken the high pressure ridge yet again,
keeping the islands in a fairly dry east to southeast wind regime.

Global weather models are in good agreement in the short term
forecast with the upper trough moving east of the state on Monday
allowing drier and more stable air to move in across the region.
Longer range model solutions are in fair agreement at this point
with several days of light winds with cooler temperatures through
the work week.

&&

.AVIATION...
High pressure north of the state will weaken through the night,
allowing the trades to diminish, and land breezes to develop by
daybreak Monday. Light winds will continue through the day Monday,
with sea breezes developing in many areas. Showers will favor
windward and mauka areas overnight, with a few showers reaching
leeward areas as well. Shower coverage is expected to notably
decrease toward daybreak Monday. Drier conditions should ensue
statewide on Monday, with a few daytime heating showers possible
over the island interiors.

AIRMET Sierra remains in effect for mountain obscuration over
windward sections of Kauai, Oahu, Maui, Molokai and the Big
Island. Conditions should improve toward daybreak Monday.

AIRMET Tango remains in effect for moderate upper level turbulence
between FL180 and FL270. Conditions should improve by daybreak
Monday.

&&

.MARINE...
Moderate to fresh northeast trade winds will gradually shift out
of the east and relax Monday as a cold front passes to north. This
will result in a land and sea breeze regime statewide Monday
through midweek as the progressive pattern featuring a series of
fronts passing to the north continues. Fresh trades could briefly
return late in the week (Friday/Saturday) as a ridge noses
eastward behind one of the fronts. Despite the light winds, the
largest swell of the season is expected to build down the island
chain Tuesday and peak Tuesday night through midweek - driving
seas well above the 10 ft Small Craft Advisory threshold for
waters exposed to northwest swells.

Surf along north facing shores will remain up into Monday due to
a mix of lingering north-northeast swell and a new north-
northwest swell arriving. This new swell will peak Monday, then
gradually ease into Tuesday.

The largest swell of the season is expected to build down the
island chain through the day Tuesday with up to 25 second
forerunners. Guidance has trended up with this event over the past
few days and now depicts a peak Tuesday night into Wednesday with
surf likely reaching the giant (40 ft faces or greater) category
along exposed shores. Latest satellite data reflect this potential
and showed a decent sized area of hurricane force winds (up to 65
kt) focused at the islands within the 310 to 320 degree
directional band Saturday. Recent SOFAR drifter buoys near the
fetch are recording wave heights (40-45 ft) this evening that are
mostly in line with predicted levels.

Impacts will include significant beach erosion with water
potentially sweeping across areas of the beaches that typically
remain dry Tuesday night through the day Wednesday. The best chance
for water making it onto the vulnerable coastal roadways and
property will be after midnight through daybreak Wednesday as the
peak of the swell and daily peak tide coincide.

Looking ahead into next weekend, another significant event
appears possible based on the latest model cycles - that are
depicting a more compact, hurricane-force low developing and
lifting northeastward across the central Pacific Friday through
Saturday driving seas to 50 ft. WAVEWATCH III and ensemble wave
data reflect this and depict another significant swell arriving in
the islands Sunday night into early next week. Forecast
confidence this far out remains low and changes are likely in the
coming days as conditions evolve.

Surf along east facing shores will hold today, then drop quickly
early this week as the trades diminish.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Gibbs

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



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