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Temp-1.1 C
RH100 %
WindNE 12 mph
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Hawaiian Islands Synoptic Discussion and Guidance

505
FXHW60 PHFO 250639
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Sun Jan 24 2021

.SYNOPSIS...
A strong high northeast of the area and a trough to the southwest
will combine to produce locally strong southeast winds over the
islands through midweek. Very humid air carried by the southeast
flow will keep the Big Island quite wet over the next couple of
days. The smaller islands will be less rainy, but some of the rain
may reach the smaller islands by mid-week. During the second half
of the week the trough will move away to the west. Winds will
shift out of the east and rainfall will diminish.

&&

.DISCUSSION...
A strong 1039 mb high far northeast of the area and a trough
about 675 miles west of Kauai are combining to produce locally
strong east to southeast winds over the main Hawaiian Islands.
High clouds blanket the area. Radars show widespread showers
moving over the Big Island from the southeast, with only isolated
showers over the smaller islands.

A deep northwest to southeast trough aloft will persist west of
the islands over the next few days. This trough is making the
atmosphere over the islands unstable and helping to pull very
moist air over the islands from the southeast. This will produce
heavy rain over the Big Island with snow over the summits of Mauna
Kea and Mauna Loa. A Flash Flood Watch and a Winter Weather
Advisory are in effect for the Big Island. Less rain will reach
the smaller island, at least through Monday evening. Some of the
moisture may spread over the smaller islands by Tuesday.

The global models show the trough aloft weakening starting Wednesday
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...
Breezy east-southeast winds continue in windward areas tonight and
Monday. Meanwhile winds will remain generally light in leeward
areas tonight, with moderate east-southeast winds developing
during the day. The most unsettled weather is expected over
east and southeast facing sections of the Big Island where
widespread MVFR conditions can be expected, with occasional IFR
cigs/vsbys. Across the smaller islands passing showers will bring
temporary MVFR cigs/vsbys to windward areas, with shower coverage
expected to increase late tonight and continue through the day
Monday. Generally VFR conditions will prevail in leeward areas,
although beginning Monday afternoon, some MVFR cigs/vsbys could
become more common.

AIRMET Sierra remains in effect for mountain obscuration across
northeast through southeast facing sections of Kauai, Oahu, and
the Big Island. These conditions will likely continue through the
night and may expand to include Maui and Molokai.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of all islands. These conditions are
expected to continue through Monday. AIRMET Tango is also in
effect for strong surface winds over some adjacent coastal waters.

&&

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. Large scale pattern
change across the Pacific over the past week featuring a blocking
pattern setting up east of the Date Line will translate to an end
to the active run of large swells that began in the second half
of December and continued through the first half of January. Surf
will respond and drop well below the January average Tuesday
through midweek.

For the long range, guidance shows the developing hurricane-force
low around 2000 nm northwest of Hawaii this evening tracking
northward toward the western Aleutians through Tuesday. 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 into the weekend. Large errors
typically associated with these sources lead to low forecast
confidence.

Surf along south facing shores will trend up Monday night through
midweek, as an out-of-season Tasman source arrives. Nearshore
PacIOOS buoy observations at Samoa reflected this source moving
through a few days 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 PM Tuesday for east facing shores of
the Big Island, Mauai, Molokai, Oahu and Kauai.

Flash Flood Watch through Tuesday afternoon for the Big Island.

Winter Weather Advisory through 6 PM Tuesday for the summits of
Mauna Kea and Mauna Loa on the Big Island.

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

$$
DISCUSSION...Donaldson
AVIATION...Jelsema
MARINE...Gibbs

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