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Hawaiian Islands Synoptic Discussion and Guidance

026
FXHW60 PHFO 270647
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
847 PM HST Tue Jan 26 2021

.SYNOPSIS...
High pressure north of the state will keep breezy to locally windy
trades in place through Friday, with a slight decrease in the
trade wind speeds expected over the weekend. Showers will favor
windward and mauka areas with a few showers reaching leeward
areas from time to time. An old front may bring an increase in
showers to the islands on Friday however. The trades will diminish
and shift southerly early next week in advance of a cold front,
with a more widespread rainfall event possible Tuesday into the
middle of next week if the front moves into the area.

&&

.DISCUSSION...
Currently at the surface, a 1032 mb high is centered around 1375
miles north of Honolulu, while a trough of low pressure is located
around 450 miles west-southwest of Kauai. The resulting gradient
is producing moderate to breezy trades across the island chain
this evening. Infrared satellite imagery shows mostly cloudy to
overcast conditions across the state. Meanwhile, radar imagery
shows most shower activity has shifted off to the south and west
of the islands, with only a few trade showers moving into windward
Big Island, and approaching windward sections of the smaller
islands from the east. Main short term focus revolves around the
strengthening trade winds during the next couple days.

High pressure will build slowly southward and closer to the
islands tonight through Thursday, before becoming absorbed into a
new re-enforcing high building southward far to the north of the
state Friday through the weekend. The gradient will remain rather
tight across the islands through the period, particularly
Wednesday through Friday, with breezy to locally windy conditions
can be expected, peaking on Thursday. The trades will only slowly
ease over the weekend, diminishing into the moderate to locally
breezy range on Sunday. The trades are forecast to diminish into
the light to moderate range Monday, then become disrupted and
shift southerly next Tuesday in advance of a cold front.

As for the remaining details, drier and more stable conditions
have overspread the islands this evening, with the deeper
moisture and low level convergent focus having shifted south of
the state. As a result, we should see more typical trade wind
weather prevail tonight, with this pattern generally holding
through the weekend. Showers will favor windward and mauka areas
and a few showers will reach leeward areas from time to time. We
could see an increase in shower coverage on Friday however,
particularly over windward areas, as both the GFS and ECMWF show
some old frontal remnants sliding southward through the islands.
This would likely send a few more showers into leeward communities
as well. Showers should continue to favor windward and mauka
areas on Monday, but as the trades ease, some leeward shower
development will be possible in the afternoon hours as a result of
localized sea breeze development. An approaching cold front could
bring more widespread rainfall to the islands Tuesday into
Wednesday of next week.

Some lingering high clouds are expected tonight and Wednesday,
although they should be thinning out with time. This should bring
more sunshine to the islands on Wednesday then has been see in the
past couple days, with the high clouds expected to have vacated
the area by Thursday.

&&

.AVIATION...
Breezy trades and prevailing VFR will continue tonight.
Increasing moisture tonight will bring increasing windward
showers and possible MVFR conditions.

AIRMET Sierra for tempo mountain obscuration is in effect for
Windward Oahu.

AIRMET Tango remains in effect for low-level mechanical turb and
strong surface winds.

&&

.MARINE...
A strong high pressure system far north of the state will keep
fresh to locally strong easterly trade winds in place through
Friday. A slight decrease in trade winds into the moderate to
breezy range will develop by this weekend, as a weaker high
passes by far north of the island chain. These strong easterly
winds will continue to produce elevated seas and large surf along
exposed east facing shores through Friday.

A Small Craft Advisory (SCA) remains in effect for all Hawaiian
waters through late Wednesday afternoon. This SCA will likely be
extended into Friday for most windward waters and channels. A High
Surf Advisory (HSA) also remains in effect for exposed east
facing shores through early Thursday morning. This HSA may also be
extended into Friday due to continued strong trade winds. The
easterly trade wind swell will gradually decrease from Friday into
the weekend as the high pressure system north of the islands
weakens.

A new moderate, long-period northwest swell will likely spread
across the area late Friday, and continue through Saturday night.
This northwest swell will slowly lower from Sunday into the first
half of next week. A small, short-period north-northeast swell is
also expected to arrive in the islands Thursday, and will likely
continue through the weekend. This overlapping north-northeast
swell should diminish by Monday night.

Several small swells from the northwest to north will move into
the islands early next week.

A small out-of-season swell from the Tasman Sea may cause a
slight increase in surf along south facing shores on Wednesday.
Otherwise, expect small surf along south facing shores from
Thursday through the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Thursday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-Big Island North and East.

Small Craft Advisory until 6 PM HST Wednesday for all Hawaiian
waters-

&&

$$

DISCUSSION...Jelsema
AVIATION...Morrison
MARINE...Bohlin/Houston

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



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