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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 260639

Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Wed Apr 25 2018

Trade winds will begin to decrease late tonight through Thursday
night as high pressure to our north weakens. Low level flow will
shift to southeasterlies late Thursday and Thursday night as a
front approaches from the northwest. This front will bring
increased rainfall to the islands, with showers reaching Kauai
Thursday night and Friday, before moving down the island chain
Friday night through Sunday morning. Showers may linger into the
new work week over the Big Island.


A 1031 mb high far north of the main Hawaiian Islands continues
to drive moderate to locally breezy trade winds across the chain this
evening. Satellite loop shows a band of broken to overcast low
clouds embedded within trade flow approaching the islands from the
east, but skies are generally partly cloudy to clear across most
areas of the islands at this hour. Trade showers will likely pick
up tonight as the cloud band arrives. Rainfall will continue to
favor windward and mauka areas.

Models show trade winds will begin to decrease late tonight
through Thursday night as the high to our north weakens and gets
pushed eastward by a front expected to develop to our northwest.
Local winds will begin to change from weak northeasterly trades
to southeasterlies as this front approaches the islands late
Thursday and Thursday night. The front will reach Kauai early
Friday, spreading increased showers along and ahead of it.
Uncertainty remains concerning the strength of the front as it
moves across the islands Friday into Sunday. Models show colder
temperatures aloft developing by Sunday, likely making this front
a more prolific rain producer as it reaches the Big island late in
the weekend. The probability for heavy rainfall increases from
Friday through Sunday, but it remains too early to tell which
island, if any, is under threat. The frontal boundary may stall
near the Big island, possibly providing that islands with lots of
rainfall into early next week.

We expect drier trade wind flow will return to the smaller islands
after frontal passage, setting up a pattern of windward and mauka
showers, post-frontally, into next week. Moisture from the frontal
boundary near the Big island may enhance these showers from time
to time.


Moderate strength trade winds will begin to weaken on Thursday as
a trough approaches the islands from the northwest. AIRMET Tango
remains in effect overnight for Tempo Moderate Turbulence over and
downwind of mountain areas. This low level turbulence AIRMET will
likely be removed by Thursday morning as trade wind speeds

A band of unstable clouds just east of the Big Island will drift
slowly across the state producing periods of MVFR ceilings and
showers mainly along windward slopes of all islands through
Thursday morning. A few of these showers will drift leeward,
however mostly VFR conditions are forecast along southern and
western slopes of each island.


A surface high far to the NNE will move E and gradually weaken
over the next two days as a low develops about 1000 miles N of
the islands. Fresh to locally strong trade winds will weaken
Thursday, then collapse completely Thursday night as a weak trough
develops over the area. A Small Craft Advisory (SCA) currently
posted for the windier marine zones around Maui and the Big Island
will likely be allowed to expire Thursday morning as the high
weakens and trade winds begin to diminish.

A front associated with the developing low is expected to reach
Kauai sometime Friday, then move down the chain through Saturday
before potentially stalling near the Big Island through the rest
of the weekend. Initially, right behind the front, N winds may
briefly reach SCA criteria in some of the marine zones. The low
will send a mix of swells and seas toward the islands this weekend
into next week, with associated seas rising above 10 feet in most
zones, necessitating a SCA from late Saturday into Monday.

A moderate long-period NNW swell is expected to arrive tomorrow
and peak Friday, with peak surf heights below advisory levels. The
low that develops N of the islands late in the week will remain
in place until early next week. The amount of swell/surf that
arrives in the islands is somewhat uncertain, and will depend on
the evolution of the low and associated fetch. Current indications
are that a fairly significant (but poorly organized) swell will
produce advisory-level surf along exposed N and W facing shores,
with the peak of the swell around Sunday.

Elsewhere, several pulses of relatively small SW to S swell are
expected over the next week or so, with a peak in swell energy
around Sunday. Rough and choppy surf along E facing shores will
diminish Thursday and Friday. The Oahu Surf Discussion (SRDHFO) contains
further details on the sources of the swells.


Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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