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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 231945
Area Forecast Discussion
National Weather Service Honolulu HI
945 AM HST Mon Nov 23 2020
Breezy trade winds will continue into Tuesday the diminish a bit
as we head into the holiday weekend. Moisture embedded in the
trades will provide for passing showers mainly for windward and
mauka areas. An upper trough will move across the areas tonight
through Wednesday allowing for the the possibility for a few
thunderstorms. A drier and more stable airmass will filter in over
the first half of the holiday weekend. A weak frontal band may
move across the area late Friday or Saturday accompanied by a
period of showers. A cool and dry airmass will follow in the
High pressure is centered far to the north northeast of the area.
This is resulting in rather breezy trade winds across the area.
An east to west showery band of low clouds is embedded within this
trade wind flow. This has been providing for numerous showers
over windward and mauka areas of Maui and the Big Island with
scattered to locally numerous showers falling over windward and
mauka areas of the rest of the state. The trade winds are
sufficiently strong enough to blow a few brief showers over to
leeward areas of the smaller islands. Water vapor imagery shows an
upper level trough is centered a few hundred miles to the west of
the state. A broad swath of high cirrus clouds will continue
streaming over the area from time to time due to the troughs
The surface high is forecast to weaken a bit over the next couple
of days resulting in slightly lighter trade winds. Meanwhile, the
upper trough to our west will slowly move eastward over the next
couple of days and amplify as it does so. Instability associated
with the upper low along with sufficient low level moisture, will
be the ingredients for increased chances for showers and possible
thunderstorms. The highest probabilities for thunderstorms will
be near and just east of the upper trough axis. As the trough axis
moves east, the chance of thunderstorms will shift from west to
east with time.
As we head into the first part of the thanksgiving weekend, a
drier and more stable airmass is expected to be in place. Trade
winds will continue in the light to locally moderate range.
Passing light showers will be possible over some windward and
mauka areas mainly during the overnight through mid morning hours.
High pressure building to the northwest will allow trade winds to
gradually increase heading into the weekend. The building trades
may push a dissipating front over the islands late Friday or
Saturday, leading to a period of increased in showers primarily
over northeast facing slopes. A cooler and drier airmass will
follow in the fronts wake Saturday night and Sunday.
An east to west oriented band of moisture will remain focused on
the state, bringing periods of MVFR conditions in low clouds and
showers through much of the next 24 hours, especially for windward
areas from Oahu to the Big Island. Some improvement is possible
this afternoon before conditions deteriorate again late this
evening. Gusty trade winds will continue for the next 24 hours,
with a very gradual weakening trend through Tuesday.
AIRMET Sierra for mountain obscuration remains in effect for
north through east sections of Oahu through Big Island.
AIRMET Tango for low-level mechanical turbulence remains in
effect over and downwind of terrain for the entire area.
Strong high pressure positioned northeast of the chain will
maintain fresh to locally strong easterly trade winds over all
Hawaiian waters today. Surface high pressure will generally remain
over the northeastern Pacific through the week with only a subtle
weakening of the southerly pressure gradient back across the
islands. Thus, trades will gradually weaken to more moderate
magnitudes through mid to late week. Winds through the locally
windier island channels and bays will stay strong through
Tuesday. A late week drop off to fresh speeds are forecast across
the more eastern channels and bays surrounding Maui and Hawaii
Counties. Recently strong trade flow in tandem with a background
northwest swell has created churned up seas that have achieved and
maintained 10-plus sea heights in both the near and offshore
waters. A Small Craft Advisory (SCA) is in effect for most waters
around Oahu and the Big Island (except for the leeward Maui
waters) through tonight to account for both these stronger trades
and resultant rough seas. As winds begin to slowly subside through
Wednesday, expect the SCA to be trimmed back to just the Pailolo
and Alenuihaha Channels, Maalaea Bay and the Big Island's leeward
and windward waters.
This past week's northwest swell has peaked and will be dampening
out through tomorrow morning. This downward trend in swell will
have surf along north and west-facing shores declining through
the middle of the week. A low, moderate period northwest swell
will arrive on Wednesday with another slightly larger, moderate
period northwest swell due to reach the islands this weekend.
These swells will provide a boost to north and west-facing
shoreline surf through the Thanksgiving weekend. Strong trade
flow will maintain advisory surf along east-facing shorelines
through today. Although trades will weaken through mid week, they
may stay up enough tonight into Tuesday to warrant an extension of
the ongoing High Surf Advisory for eastern exposed shores. Eastern
wind wave action may wrap around the southeastern tips of the
islands and provide a minor boost to southern shore surf. While
there is still some longer period swell energy being reported at
the American Samoa buoy this morning, the resulting background
swells reaching the islands in the next few days will only produce
small south-facing shoreline surf.
High Surf Advisory until 6 PM HST this evening for Kauai
Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-
Windward Haleakala-South Big Island-Big Island North and East.
Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.
Small Craft Advisory until 6 PM HST this evening for Kaiwi
Channel-Maui County Windward Waters-Big Island Windward Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office