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Hawaiian Islands Synoptic Discussion and Guidance

015
FXHW60 PHFO 261359
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
359 AM HST Sun Jan 26 2020

.SYNOPSIS...
Dry weather and moderate trade winds will continue through Monday
morning, with a few light showers favoring windward areas. The
winds will shift southeasterly Monday afternoon through Thursday
as a series of cold fronts approach from the northwest, with
daytime sea breezes and overnight land breezes becoming common
across most of the smaller islands. Rather dry conditions will
prevail through much of the work week, although a few light
showers will continue to trickle into windward sections of the
eastern islands, and a few cloud buildups and showers will be
possible across the island interiors each afternoon. A strong cold
front will move into the islands Friday into next weekend,
potentially bringing a wet and unsettled weather pattern to
portions of the state.

&&

.DISCUSSION...
Currently at the surface, a 1026 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 early this morning, although some of the more sheltered
leeward areas have cooled enough to allow for land breeze
development. Infrared satellite imagery shows clear to partly
cloudy skies in place, with a few pockets of enhanced cloud cover
over windward slopes. Radar imagery shows isolated showers moving
into windward areas from Molokai eastward to the Big Island, with
scattered showers moving into windward sections of Oahu and Kauai
along with some leeward spillover. 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 the
second and stronger cold front moving into the western islands
Friday, then stalling somewhere across the island chain next
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
cold front that is forecast to stall across the area.

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 low/mid level ridging persists over the
region. A few light showers will be confined primarily to windward
areas today through 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's difficult to know how thick
this will be at the moment, but it does appear to thicken up a bit
Wednesday night and Thursday. More unsettled weather appears to
be heading this way beginning Thursday night or Friday, and
holding into next 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 and Saturday.

&&

.AVIATION...
A ridge of high pressure to the north of the islands will persist
through twenty-four hours and beyond. Moderate trade winds will
continue to carry bands of low topped cumulus and stratocumulus
towards north and east facing slopes and coasts. Isolated MVFR
CIG and VIS are to be expected in passing showers. Otherwise VFR
conditions will prevail all areas.

&&

.MARINE...
A ridge of high pressure north of the area will maintain moderate
to locally strong trade winds across most coastal waters through
Monday. Beyond that time frame, a couple of fronts approaching the
area from the northwest through the week will weaken the winds
and turn them toward the east and southeast.

The current northwest swell remains elevated this morning, and a
High Surf Warning remains in effect for exposed north and west
facing shores of the smaller islands through 6 pm this evening
and a High Surf Advisory for west facing shores of the Big Island
through 6 am Monday. Latest buoy readings from NOAA buoy 51101
and 51001 have started to show a gradual decrease over the past
few hours, but continue to show swell heights of between 11 feet
to 13 feet with an average period of about 16 seconds early this
morning. With an average travel time from these buoy to Oahu of
around 11 hours, this would suggest a gradual downward trend this
afternoon and possibly dropping to advisory levels by late in the
day. We will continue to monitor the buoys and the High Surf
Warning currently in effect through late this afternoon should
either get dropped to an advisory for tonight or get extended.

A Small Craft Advisory is in effect for all Hawaiian waters
except Maalaea Bay through today due to a combination of winds
and large seas from the northwest swell. Some marine zones will
likely hold on to SCA level seas into Monday.

Surf will remain elevated along north and west facing shores as a
progressive weather pattern across the northern Pacific keeps a
series of large northwest swells moving through the islands, at
least through the middle to latter part of next week. The next
northwest swell is expected to arrive Tuesday night from a large
fetch of gales currently blowing in the northwest Pacific. Latest
model guidance is suggesting the swell to peak at high end
advisory levels sometime on 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 next week.
A couple background south swells expected over the weekend and
again on Tuesday should be enough to keep things from going flat.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST this evening 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 PM HST this evening for all
Hawaiian waters except Maalaea Bay-

&&

$$

DISCUSSION...Jelsema
AVIATION...Bedal
MARINE...Kino

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



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