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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 240646

Area Forecast Discussion
National Weather Service Honolulu HI
846 PM HST Sat Jan 23 2021

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 Sunday night through Wednesday as a
low aloft deepens northwest of the area.


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 to overcast 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 Sunday.

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 surface winds to veer south of east and bring very
moist air towards the islands. Increased rainfall is likely
starting Sunday night, 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.


A strong high pressure system far north of the islands will keep
breezy to locally windy east to southeast winds through Monday.
Scattered to numerous rain showers remain in the forecast 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 northeast through southeast 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.


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 remain up overnight,
then steadily lower Sunday through Monday. Heights should dip
below advisory levels for all exposed north and west facing shores
by Sunday night. 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 Sunday night 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 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.


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

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

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

Wind Advisory until 6 AM Sunday for Kahoolawe and parts of
Molokai, Maui and the Big Island of Hawaii

Small Craft Advisory until 6 PM HST Tuesday for all Hawaiian
waters except the Alenuihaha Channel.

Small Craft Advisory until 6 PM HST Sunday for the Alenuihaha

Gale Watch from Sunday evening through Tuesday afternoon for
Alenuihaha Channel.



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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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