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Hawaiian Islands Synoptic Discussion and Guidance

992
FXHW60 PHFO 241351
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
351 AM HST Sun Jan 24 2021

.SYNOPSIS...
Strong high pressure northeast of the area will maintain locally
strong trade winds over the islands through much of the coming
week. Windward areas will remain rather rainy. Rainfall will
likely become more widespread tonight through Wednesday as a low
aloft deepens northwest of the area.

&&

.DISCUSSION...
A strong 1041 mb high far north of the area is producing locally
strong northeast to east trade winds over the main Hawaiian
Islands. Windward areas have broken low clouds, while leeward
areas have partly cloudy skies. Radars show numerous showers
moving over windward areas with some of these showers spreading
leeward on the smaller islands. These conditions will continue
through the day today

The global models show a northwest to southeast trough aloft
deepening west of the islands over the next few days. This trough
will destabilize the atmosphere over the islands. The trough will
also cause the low level flow to veer out of the southeast and
bring very moist air towards the islands. Increased rainfall is
likely starting tonight, especially over the Big Island of
Hawaii. The global models disagree on how much of the moisture
will move over the islands, so there is quite a bit of uncertainty
in how widespread and heavy the rainfall will be.

The global models show the trough aloft lifting northwest starting
Wednesday night as a ridge aloft builds over the area from the
north. The atmosphere over the islands will become more stable and
the moist air will move away to the west. That should bring a
decrease in rainfall for the second half of the week, with showers
focused over mainly windward areas.

&&

.AVIATION...
A strong high pressure system far north of the islands will
maintain locally windy east to southeast winds through Monday.
Scattered to numerous rain showers remain in the forecast today
with the highest shower coverage developing over mountains and
along east facing slopes. Expect brief periods of MVFR conditions
in showers along the eastern slopes of most islands, extending
into the southeastern slopes of the Big Island.

AIRMET SIERRA remains in effect for Tempo Mountain Obscuration above
2500 feet along north through east sections of Oahu and Kauai due
to MVFR ceilings in showers.

AIRMET TANGO remains in effect for Moderate Turbulence below 8000
feet over and immediately southwest through northwest of all
island mountains.

&&

.MARINE...
Strong east-southeast winds associated with high pressure will
continue over the Hawaiian waters through the upcoming week. Rough
conditions with seas at and above the Small Craft Advisory level
of 10 ft will be the result across exposed waters.

Surf along east facing shores will remain rough due to the
aforementioned strong onshore winds locally and upstream across
the eastern Pacific driving large seas through the area. The
advisory in place will continue through much of the week.

Surf along north and west facing shores will steadily lower
through Monday as the northwest swell eases. Heights should dip
below advisory levels for all exposed north and west facing shores
by tonight. Large scale pattern change across the Pacific
featuring a blocking pattern setting up east of the Date Line will
translate to an end to the active period that began in the second
half of December. Surf will respond and drop well below the
January average Monday through midweek (near flat levels except
for areas exposed to trade wind energy).

For the long range, guidance shows the current storm developing
off the coast of Japan tracking northward tonight through
Wednesday across the far western Pacific to the western
Aleutians/Bering Sea. Wave models reflect this and show small,
long-period energy arriving through the day Thursday out of the
west-northwest, then trending up out of the northwest Friday and
Friday night. Large errors typically associated with these sources
lead to low forecast confidence.

Surf along south facing shores will trend up Tuesday through
midweek, as an out-of-season Tasman source arrives. Nearshore
PacIOOS buoy at Samoa reflected this source moving through 24 to
48 hrs ago, which increases confidence locally. Surf should drop
and return to normal levels through the second half of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Weather Watch until late Tuesday night for the summits of
Mauna Loa and Mauna Kea on the Big Island of Hawaii.

High Surf Advisory until 6 PM today for north and west facing
shores of Niihau, Kauai, Oahu and Molokai and north facing shores
of Maui.

High Surf Advisory until 6 PM Sunday for west facing shores of
the Big Island.

High Surf Advisory until 6 PM Monday for east facing shores of
Kauai, Oahu, Molokai, Maui and the Big Island of Hawaii.

Small Craft Advisory until 6 PM Wednesday for all Hawaiian
waters.

&&

$$
DISCUSSION...Donaldson
AVIATION...Bohlin
MARINE...Gibbs

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