FXHW60 PHFO 232021
Area Forecast Discussion
National Weather Service Honolulu HI
1021 AM HST Thu Mar 23 2017
A trade wind regime is here and will continue through at least the
upcoming weekend. Showers riding in with the trades will favor the
windward and mountain areas, especially the night and morning
hours. The strength of the trades will be strengthening Friday to
moderate to locally strong speeds, and this pattern will hold
through at least the early part of next week.
The clouds and showers of an old front have moved up the chain
from the Big Island to as far as Oahu this morning, thus ending
about a week's worth of pleasant dry weather. It has been somewhat
showery across the windward sections of Molokai Maui, and parts of
windward Big Island thus far this morning. On Oahu, there
certainly are more clouds over the Koolau's but not much in the
way of showers. The models does not reduce the frequency of these
trade showers until Friday afternoon. In fact it appears to hint
the showers could be more tonight and Friday morning than this
A layer of high cirrus clouds is adding to the general low level
cloudiness resulting in the appearance of mostly cloudy skies.
The cirrus is part of an upper level disturbance with the back
edge of the field now in the Kauai Channel. It is moving east at
30 mph and at this speed the cirrus should clear Oahu this
afternoon and Maui this evening.
The strength of the trades will be light to moderate overall the
rest of today, favoring local sea breezes for the sheltered areas
of Kauai, Oahu, and Lanai. But due to the high clouds, the
development of the low level afternoon clouds will be restricted
and limited due to the lack of sunshine. What is keeping the
trades light today is a weak surface ridge located 240 miles north
of Kauai. The ridge is being held there by a front located 470
miles north of Kauai. The ridge will strength on Friday as the
front vacates the area. This will allow locally strong trades to
spread to waters W and S of the Big Island by Friday afternoon...
then to waters around Maui County Friday night. A new high forms
NE of the islands late in the weekend that will continue to
feed the islands with locally breezy trades into the first half of
As noted earlier, the frequency of the trade showers will be
down on Saturday, but picks up to more or less the typical amount
starting Saturday night. Despite this uptick, there will be plenty
of sun to go around especially along the leeward beaches. The
somewhat breezy trades will help to cool things down.
Low clouds from an old frontal boundary are embedded with
prevailing easterly flow, providing occasional MVFR ceilings along
windward slopes of Molokai, Maui and the Big Island. VFR
conditions dominate elsewhere. We will monitor the low clouds to
see if AIRMET Sierra for mountain obscuration is needed today, but
for now no AIRMETS are in effect.
A surface high is currently located far east northeast of the state
with a surface ridge trailing west to about 200 nm north of Kauai.
As the high weakens through tonight, so will the ridge. However, a
new high will set up to the northeast and strengthen starting
Friday. Winds are expected to reach Small Craft Advisory (SCA) strength
by Friday afternoon or Friday night for the typical windy areas
around Maui County and the Big Island. Trade winds will strengthen
further this weekend boosting wind speeds to fresh to locally
strong over the coastal waters as the high builds. Rough and
choppy conditions will persist along east facing shores due to
the strengthening trade winds.
A strong low pressure system currently about 2100 miles northwest
of Hawaii will bring a very large west-northwest swell that will
build across the state Saturday and Saturday night. Surf along
most north and west facing shores will peak above warning level
heights. Surf will be the largest on Kauai and Niihau, and
slightly lower over the rest of the islands due to some blockage
caused by Kauai and Niihau. The SCA will need to be expanded this
weekend for the incoming swell and strengthening trade winds.