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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 250133

Area Forecast Discussion
National Weather Service Honolulu HI
333 PM HST SAT SEP 24 2016

Moderate trade winds will gradually strengthen on Sunday and
Monday as high pressure builds far north of the state. A typical
trade wind weather pattern will dominate through at least Friday,
focusing showers along windward slopes of all islands along with
afternoon clouds and showers over the Kona slopes of the Big
Island. However, a brief increase in shower activity is expected
on Monday.


An increasingly stable trade wind pattern settled over the island
chain today as deeper moisture moved away to the southwest.
Rainfall has been meager and confined to windward slopes. The
exception is on the Big Island, where lingering moisture led to
extensive interior and leeward cloud cover, though rainfall was
limited. The afternoon soundings showed an building inversion
between 5500 and 6000 ft and precipitable water values dropping
to near normal levels.

Trade winds will be gradually increasing on Sunday and Monday.
The pressure gradient over the islands remains somewhat weak due
to the presence of a surface trough several hundred miles
northwest of Kauai and a cold front 600 miles to the northwest.
The front will stall and begin to slowly dissipate on Sunday,
allowing strong high pressure passing to the north near 40N to
exert a greater influence. As a result, trade winds will be on the
rise on Sunday and will become locally breezy on Monday.

Typical trade wind weather is due on Sunday, confining modest
showers to windward slopes. High pressure aloft will build as the
air mass continues to dry out, but a weak mid level trough,
currently located about 500 miles east of the state, will pass
overhead on the trade wind flow on Monday, leading to an increase
in mainly windward rainfall. The GFS is depicting more moisture
associated this feature than the ECMWF, and the forecast
represents a blend of these models.

Locally breezy trade winds and a typical shower pattern focusing
clouds and showers over windward areas will prevail from Tuesday
through at least Friday. Heading toward next weekend, the GFS and
ECMWF are hinting at tropical moisture moving in from the
southeast, though the models have not been consistent in the
evolution of this feature.


Trade winds in the light to moderate range will continue into
Sunday then strengthen late Sunday afternoon and night. Expect
windward sections of the islands to see some MVFR ceilings at
times as the trades sweep low clouds and low level moisture into
the islands from the east. Soundings indicate cloud tops mostly in
the 09k to 12k feet range with some possibly as high as 13K-16K
feet. Overall, VFR conditions to be prevalent in most areas.


Moderate to fresh trade winds will prevail through tonight
followed by an increase in winds Sunday and Monday as high
pressure builds to the north of the state. A Small Craft Advisory
(SCA) will likely be posted for the typically windy waters around
the Big Island and Maui on Sunday, and the SCA will likely remain
in place through much of the week.

Surf will generally be on the small side through the week. A
series of mainly small pulses of south-southwest swell will
continue. A very small northwest swell will peak tonight and
Sunday, then fade as it shifts more northerly on Monday and
Tuesday. Easterly trade winds will maintain moderate short period
surf along east facing shores.





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