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Hawaiian Islands Synoptic Discussion and Guidance

278
FXHW60 PHFO 270621
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
821 PM HST Sun Jan 26 2020

.SYNOPSIS...
Moderate trade winds will continue through early Monday, with
a few light showers favoring windward areas. Winds will shift to
southeasterlies Monday through Thursday as a series of fronts
approach from the northwest, with land and sea breezes developing
across the smaller islands. Rather dry conditions will prevail
through much of the work week. A strong front will move into the
islands Friday through the weekend, potentially bringing wet and
unsettled weather.

&&

.DISCUSSION...
Currently at the surface, a 1030 mb high is centered well east-
northeast of the state, with the ridge axis extending westward to
a location around 275 miles north of Kauai. The resulting
gradient is producing moderate trade winds across the island chain
this evening. Infrared satellite imagery shows clear to partly
cloudy skies in place across most of the state, with a few
pockets of enhanced cloud cover over windward locales. Meanwhile,
radar imagery shows very little shower activity across the state.
Main short term concerns revolve around rain chances and wind
trends over the next few days.

The surface ridge will remain north of the island chain through
Tuesday, before sliding southward and over the state Wednesday and
Thursday. The first in a series of cold fronts will approach from
the northwest through the first half of the work week, with a
second and stronger cold front moving into the western islands
Friday, before stalling over the weekend. Moderate trade winds
are expected to prevail through Monday morning, with the strongest
trades across the eastern islands which will be further away from
the ridge axis. Boundary layer winds begin to shift around to the
southeast Monday afternoon, and will hold in this general
direction through much of the work week. This will keep moderate
east to southeast winds in place across the eastern end of the
state, while most of the smaller islands see light and variable
winds featuring daytime sea breezes and overnight land breezes.
Strong trade winds could return from northwest to southeast down
the island chain late Friday into next weekend as a strong high
builds in behind the second front.

Rather dry weather will prevail across the islands through late
in the work week, as precipitable water values hold generally at
or below 1 inch and deep layer ridging persists over the region.
A few light showers will be confined primarily to windward areas
tonight and Monday morning. By Monday afternoon, we should begin
to see a few cloud buildups and interior showers develop as sea
breezes become more common. This pattern featuring a few cloud
buildups and showers over the island interiors will then hold in
place through Thursday, although some windward showers will
continue across the eastern islands in areas favored under east-
southeast to southeast boundary layer flow. We will likely begin
to see some jet cirrus begin to spread across the islands on
Wednesday as well. It appears the high clouds will be thin
initially, then thicken up a bit Wednesday night into the
weekend. More unsettled weather appears to be heading this way
beginning Thursday night or Friday, and holding into the weekend
as the stronger of the two fronts enters the islands. Although the
models differ on the specifics, it appears that some wet weather
is in store for at least portions of the state, particularly
windward locales Friday through next Sunday.

&&

.AVIATION...
Expect a VFR evening area-wide, while under a weakening trade
wind flow. Widely scattered areas of low clouds littered the
islands windward waters. As they get carried onshore by the
trades, it will squeeze out a few light showers/sprinkles over
the windward and mountain areas, and cause brief periods of MVFR
ceilings. Do not expect any vis restrictions with these showers
as the air mass is stable, with a strong inversion at 7 to 8k
feet. As noted, the moderate trades will be weakening into Monday
as the surface ridge currently 230 nm N of Kauai, drops south to
about 100 nm by Monday evening. In doing so, the trades give way
to daytime sea breezes, felt especially over the interior and lee
areas of all islands. With the addition of daytime heating, we
will find some isolated, spotty afternoon clouds over these areas.
But the stable airmass will contain and limit any afternoon
convective showers to isolated at best. No AIRMETs are expected
through 16z Monday.

&&

.MARINE...
A ridge of high pressure north of the area will maintain moderate
to locally strong trade winds across most coastal waters through
Monday. Starting Monday night, winds are expected to decrease and
veer to the southeast as a front passes north of the state. Winds
are expected to be light and variable across Kauai and Oahu and
moderate to locally fresh over Maui County and the Big Island
through most of the week. A shift in weather pattern is possible
Thursday into Friday as a front approaches from the northwest.

The current northwest swell continues to remain elevated, but is
gradually decreasing throughout the state. A High Surf Advisory
is in effect for exposed north and west facing shores of the
smaller islands through Monday afternoon and west facing shores of
the Big Island through tonight.

A Small Craft Advisory (SCA) remains in effect for all Hawaiian
waters except Maalaea Bay through tonight due mainly to large seas
from the northwest swell. The SCA will be scaled back Monday as
seas continue to lower from the northwest.

Surf will remain elevated along north and west facing shores as a
progressive weather pattern across the northern Pacific keeps a
series of moderate to large northwest swells moving through the
islands, at least through the middle to latter part of the week.
The next northwest swell is expected to arrive Tuesday night.
Latest model guidance is suggesting the swell to peak at solid
advisory levels sometime Wednesday.

Surf along exposed east facing shores will remain rather small
through the period due to the lack of persistent trades locally
and upstream across the eastern Pacific.

Surf along south facing shores will remain small through the
week. However, background south swells expected through Tuesday
should be sufficient to keep things from going flat.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Surf Advisory until 6 AM HST Monday for Kona-Kohala.

Small Craft Advisory until 6 AM HST Monday for all Hawaiian
waters except Maalaea Bay-

&&

$$

DISCUSSION...Jelsema
AVIATION...Lau
MARINE...Kino

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



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