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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 250138

Area Forecast Discussion
National Weather Service Honolulu HI
338 PM HST Fri Mar 24 2017

Trades winds will prevail through the week, with a slight
increase into the locally breezy range expected during the
next couple of days. Trade winds will focus clouds and showers
along windward slopes, especially at night and in the morning. An
area of will clear the islands tonight, leading to a decrease in


A surface ridge stalled about 250 miles north of Kauai is driving
moderate easterly trade winds across the state. An area of
diffuse low level moisture, the remnants of a long ago dissipated
front, has fueled rainfall of generally an inch or less of
rainfall along windward slopes from the Big Island to Oahu today.
The main impact over leeward areas has been additional low clouds
and dew points as high as the lower 70s degrees F, which has led
to humid afternoon conditions. This moisture is slowly decreasing
over the eastern end of the state, where the afternoon Hilo
sounding showed a significant drop in precipitable water to March
normal levels. As an upper level trough pushes south of the Big
Island, the Hilo sounding also showed signs of a redeveloping
inversion near 5300 ft.

Easterly trade winds will push low level moisture west of the
island chain tonight. Showers will be largely confined to
windward slopes of Kauai, and to a lesser degree, Oahu, with
minimal rainfall expected elsewhere. A ridge will build in aloft
as the upper level trough continues to move south of the Big
Island, causing the inversion to drop below 5000 ft and as low as
3000 ft. Dew points will drop back in to the mid 60s, which will
bring some relief from today's humid conditions.

A stable, moderate to locally breezy trade wind flow will prevail
during the weekend. With no other areas of significant low level
moisture seen in the short term, expect limited shower activity to
be confined to windward slopes and rather dry conditions leeward
through at least Monday.

In the extended, trade winds will hold through next week with
a slight decrease in speed possible by Tuesday. The next area of
enhanced moisture could lead to an uptick in windward rainfall on
Tuesday or Wednesday, as the upper level ridge is eroded by a
trough skirting by to the north.


Clouds and a few showers left over from an old frontal boundary
remain hung up over the islands this afternoon, and will likely
clear by morning. The trade winds are expected to strengthen on
Saturday which will aid in pushing this moisture west of the
state. For now, AIRMET Sierra is in effect for tempo mountain
obscurations over windward sections of all islands with isolated
mountain obscurations occurring elsewhere. This AIRMET may be
dropped for some or all areas tonight as drier air is expected to
filter in.

As trade winds strengthen Saturday, AIRMET tango may be issued for
moderate turbulence leeward of the mountains.


A large incoming west-northwest swell is forecast to start
arriving at the far northwest buoys (51001/51101) tonight and
near the islands Saturday. Surf near Kauai is expected to reach
warning levels (25 ft north/20 ft west) by Saturday afternoon
before peaking Saturday night. Warning level surf should continue
through the day Sunday, before gradually subsiding into the next
week. The more westerly direction of the swell will allow for
Kauai to create shadowing for the other smaller islands. The
forecast will hold surf for those exposed north and west facing
shores just under the warning level. This will be monitored as
observations start coming in from the northwest buoys, a slightly
larger or more northwest swell could trigger the warning levels.
Surf along west facing shores of the Big Island is also forecast
to reach advisory (8 ft), to possible warning (12 ft) level surf
late Saturday into Monday. No advisories or warnings have been
posted at this time. There is a slight discrepancy in timing
between the Wave Watch III and the ECMWF wave models. The ECMWF
shows the rise and peak about 12 hours behind the Wave Watch III.
The current forecast will continue to follow the Wave Watch III,
but we will wait to issue any advisories or warnings until we
start to see the initial fore runner energy arrive at the
northwest buoys.

A Small Craft Advisory (SCA) is posted for the typically windier
waters around the Big Island and Maui County. High pressure
strengthening northeast of the state will provide locally breezy
conditions. The SCA will also be expanded to cover the remaining
waters by late Saturday morning to account for seas above 10 ft
from the large west-northwest swell and wind waves.

Another large west-northwest swell is forecast for next weekend.
Small long period south swells will continue through this weekend
and into next week, with a slightly larger long period south swell
forecast for Wednesday night into Friday.


Small Craft Advisory until 6 PM HST Sunday 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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