Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 261957

Area Forecast Discussion
National Weather Service Honolulu HI
957 AM HST Tue Sep 26 2017

A slow increase in the trades is expected over the next couple
days as a surface ridge to our north strengthens. The trades will
peak on Thursday at locally breezy levels. Some increase in trade
wind showers is expected tonight through Wednesday night as a
band of moisture associated with a weak trough pushes across the
islands. Our winds will shift around to the southeast by the
weekend as a cold front approaches from the northwest. This will
allow for daytime sea breezes and overnight land breezes in most
areas, along with bringing some vog northward and into the smaller


Currently at the surface, a 1023 mb high is centered around 1900
miles northeast of Honolulu, with a ridge axis extending
southwestward to a location around 250 miles north of Kauai.
Meanwhile, a westward moving surface trough is located around 175
miles east of Hilo this morning. Visible satellite shows an area
of clouds associated with this trough slowly tracking westward
towards the state. Over the islands, mostly sunny skies prevail
over leeward sections, with partly cloudy skies and isolated
showers windward. Upper level ridging persists over the area with
soundings showing midlevel inversions around 7000 feet at Lihue
and Hilo this morning.

Clouds and showers associated with the approaching trough will
reach the Big Island late tonight, and spread westward over the
smaller islands Wednesday and Wednesday night. The trough will be
weakening as it reaches Hawaii, with the showers becoming more
diffuse through the day Wednesday. Rainfall amounts will be light
to moderate and focus over the windward sections of the islands.

The surface ridge north of Kauai will lift further away to the
north over the next couple of days, resulting in a gradual
strengthening of the trade winds beginning Wednesday night. Light
to moderate trade winds are expected through Wednesday, with the
trades then increasing to moderate and locally breezy levels
by Thursday. The trade winds will continue to funnel in occasional
passing showers over the islands on Thursday as the surface trough
slides west of the area.

The trades will weaken again on Friday into the weekend as a cold
front approaches from the northwest. Winds will also shift
southeasterly over the weekend, likely allowing for overnight
land breezes and daytime sea breezes across the island chain.
Clouds and showers will be most common over leeward and interior
sections of the islands during the afternoon and evening hours and
near the coast during the overnight and early morning hours.
Additionally, some volcanic haze will likely work northward into
the smaller islands Saturday through early next week.

In respect to direct impacts to Hawaii from the approaching
front, there remains a significant amount of uncertainty with
respect to the overall synoptic pattern evolution over the weekend
into early next week, with quite a bit of wavering in the model
solutions from run to run as well as significant differences between
the GFS and ECMWF. The big question is how far east the cold
front manages to move, with 12Z versions of both models stalling
the system just west of Kauai around Sunday before backing it
westward as the trades fight back in from the east. The current
forecast reflects this solution, but confidence is low given
previous model interaction with the islands. Stay tuned as we get
closer to the event.


An overall dry and stable airmass will continue to limit shower
activity across all islands. A light background flow will allow
for afternoon seas breezes that will bring some clouds to the
interior of the islands, with clearing expected again tonight. VFR
conditions are expected to prevail, with no AIRMETs anticipated.


Trades are expected to strengthen Wednesday night and reach Small
Craft Advisory criteria Thursday and Thursday night for some of
the waters around Maui County and south of the Big Island, before
quickly lowering again Friday. Over the weekend, the trades will
be replaced by a light SE to S wind flow, as a front nears, but
stalls just outside the offshore waters northwest of Kauai.

A weakening low level trough, embedded in the trade wind flow just
east of the Big Island, will pass through the main Hawaiian islands
Wednesday, adding a boost to the trade showers.

All surf is expected to remain below advisory levels through the
forecast period. Surf along south facing shores will remain small
through the week with mainly a mix of background southeast and
southwest swells. A small bump from the NW is expected tonight,
gradually fading through Friday. A moderate size NW swell is
slated to arrive early next week, with forerunners showing up
Sunday afternoon. Surf along east facing shores will remain
relatively small through the period, even with the brief boost in
the trades between Wednesday night and Friday.





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