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Hawaiian Islands Synoptic Discussion and Guidance

806
FXHW60 PHFO 301330
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Mon Nov 30 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 during the early morning hours today.
Cooler and drier trends will spread from west to east starting
today. 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...
This mornings water vapor satellite imagery shows an upper level
low passing through the eastern half of the state. This low will
produce enough instability for a few thunderstorms over the Big
Island and the eastern half of Maui County today. Local radar
imagery shows scattered to numerous showers drifting into the
windward slopes of each island on the waning trade winds. Drier
conditions will spread from west to east across the state later
today. Unstable conditions will linger over the Big Island this
afternoon, as the upper low drifts southeastward, for scattered
showers and possible thunderstorms.

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 through Thursday. Local scale land
and sea breezes will develop over each island as the large scale
winds diminish. 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 later
today allowing drier and more stable air to move in across the
region. Medium range model solutions are in fair agreement at this
point with several days of light winds along with cooler
temperatures and lower rainfall trends through Thursday.

&&

.AVIATION...
High pressure north of the state will weaken today, allowing the
trades to diminish, with sea breezes developing in many areas this
afternoon. Showers will favor windward and mauka areas through the
early morning hours, with a few daytime heating showers possible
over the island interiors this afternoon.

AIRMET Sierra remains in effect for mountain obscuration over
windward sections of Kauai and Oahu. Conditions should improve
in the next few hours.

AIRMET Tango remains in effect for moderate upper level turbulence
between FL200 and FL300. Conditions should improve by daybreak.

&&

.MARINE...
Moderate to fresh northeast trade winds will gradually shift out
of the east and relax through the day as a cold front passes to
north. This will result in a land and sea breeze regime statewide
later today 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 build later today due to a
mix of lingering north-northeast swell and a new north-northwest
swell arriving. This new swell should peak by tonight, 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) overnight 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 drop through the first half of
the 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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