Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 250135

Area Forecast Discussion
National Weather Service Honolulu HI
335 PM HST Sun Sep 24 2017

Trade winds will gradually ease through tomorrow as a surface
ridge to the north erodes and a weak surface trough approaches
from the east. Mainly windward clouds with passing showers will
prevail through Tuesday, though leeward Big Island slopes will see
afternoon and evening clouds and a few showers. A brief increase
in shower activity is expected around Wednesday. Gentle to locally
moderate trade winds will likely persist through Friday.


The islands remain under a somewhat stable, decreasing, and
moderate trade wind flow. A surface ridge about 500 miles north of
Kauai is driving the trade winds, while mid level ridging is
producing an inversion between 6,000 and 7,500 ft. This inversion
remains slightly weakened by an upper level trough stretching
across the region, which allowed a few briefly heavy showers to
flare up along the Kona slopes of the Big Island last night.
Otherwise, below normal moisture content in the trade wind flow
continues to generate modest windward rainfall of around a quarter
of an inch or less per 24 hours.

A gradual decline in trade winds will occur through tomorrow,
while a similar rainfall pattern persists. The surface ridge will
move to within 300 miles north of Kauai by Monday, and a weak
surface trough currently 375 miles east and northeast of the
state will drift closer. As a result, trade winds will decline,
with afternoon sea breezes overpowering the trades across some
leeward areas. Modest rainfall will continue to be focused across
windward slopes, but afternoon clouds and possibly a brief shower
will occur across leeward terrain, especially on the Big Island.

Some increase in shower activity is expected midweek. The surface
trough currently 450 miles east of the state will dissipate,
leaving a diffuse area of enhanced low level moisture in the trade
wind flow. This moisture is expected to pass over the islands on
Wednesday and provide a brief increase in mainly windward
rainfall. A mid level ridge will be building overhead during this
time, reducing chances for any heavy showers.

A gentle to moderate trade wind flow and a typical pattern of
windward rainfall is expected to persist through the remainder of
the work week, with a change possible next weekend. The GFS and
ECMWF are in fairly good agreement that a front will approach the
state on Saturday, causing winds to shift out of the southeast.
Models diverge on Sunday, as the GFS shows winds trending more
easterly while the ECMWF suggests a southerly wind pattern. Due to
low confidence, the forecast for Sunday is a blended solution
featuring persistent southeast winds. Heavy and/or widespread
rainfall do not appear to be likely at this time.


High pressure northeast of the state will keep a light to locally
moderate trade wind flow in place through Monday, with clouds and
showers favoring windward and mauka areas. Light sea breezes and
inland showers over interior and leeward Big Island this afternoon
and persist will both dissipate in the late evening. Brief MVFR
CIG/VIS is possible in passing showers, otherwise VFR conditions
will prevail all areas.

No AIRMETs are in effect and none are anticipated.


Trade wind speeds will remain below the Small Craft Advisory
threshold through the forecast period.

Surf along south facing shores will remain small through the
middle of this week, with mainly background southeast and
southwest swell energy expected. A couple small, long-period
pulses out of the southwest from the Tasman Sea could give a slight
boost to south shore surf over the next few days.

A small northwest swell will subside through today, before
another small reinforcement from that direction fills in beginning
tonight, lasting through the middle of the week. Neither of these
swells are expected to provide more than a slight bump up in surf
for north and west facing shores through the forecast period.





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