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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 211934

Area Forecast Discussion
National Weather Service Honolulu HI
934 AM HST Thu Mar 21 2019

Trade winds will continue into next week, even as a cold front
approaches the islands from the northwest. The relatively dry and
stable airmass over the islands today will be replaced with
increasing clouds and showers as a cold front approaches from the
northwest this weekend.


Have made some tweaks to the short term PoPs and associated fields
to align better with the satellite and radar observations this
morning. Have also made some modifications for Monday night into
Tuesday to better reflect the current model depictions for that
time period.

A northeast to southwest cloud band was found over the Kauai
Channel and Oahu during the pre-dawn hours, but the band has been
breaking up since daybreak. The overnight analysis showed the
approaching cold front about 150 miles northwest of Kauai. This
has pushed the surface ridge over the islands leading to the
lighter winds persisting. The high is expected to move to the east
later today and tonight, which will allow the pressure gradient
over the islands to strengthen ahead of the front, which is not
expected to reach the islands.

Low level moisture will increase Friday and into the weekend ahead
of the front, and will bring increasing clouds and showers for the
weekend. Will likely nudge the trade winds back up a notch or two
for this weekend, given the latest model trends. There is good
model agreement with a high pressure system currently far
northwest of the islands moving to the east over the next couple
of days, before settling north of the islands and strengthening
Saturday night. By Monday the front starts to fall apart, with the
high continuing to drive trade winds into the region. The moisture
from the front is expected to get caught up in the trade wind
flow, maintaining more clouds and showers for the start of the new
work week.


Weak trade wind flow over the islands today will gradually
strengthen over the next twenty-four hours. Scattered to broken
high based stratocumulus is spread out across most of the state
this morning but will become focused along windward slopes and
coasts towards evening. Sea breeze driven Cumulus are expected
to develop over island interiors this afternoon then taper off
after sunset. There will be an increased chance of rain over the
smaller islands late tonight as an approaching cold front stalls
to the northwest of Kauai.

Isolated moderate turbulence to the lee of mountains is possible
in response to the strengthening trade winds. However, conditions
are not expected to be widespread enough to warrant an AIRMET.
Isolated MVFR ceilings and visibilities are to be expected in
passing showers, otherwise VFR conditions will prevail all areas.


Trades will gradually increase through Friday. Surface high
pressure, currently centered about 950 nm northwest of Kauai, will
settle north of the state on Friday as a front dissipates about
75 nm north of Kauai. Trades should reach the moderate to fresh
strength later today, then potentially build to fresh to strong
levels tonight and Friday. A further uptick in the trades is
likely late Saturday and Sunday as stronger high pressure passes
to the north. The strongest winds are expected over the typical
windier waters around Maui County and the Big Island, where a
Small Craft Advisory (SCA) may be needed later today or tonight.
In addition, seas may reach border line SCA levels of 10 ft for
additional waters on Friday as a new north-northwest swell
builds. Confidence in SCA conditions increases for the weekend
since stronger trades are expected. Trades will ease early next

Surf along north and west facing shores will hold at or above
March normal through the weekend. The current northwest swell
will produce surf just below advisory levels into Friday. A pulse
of north-northwest swell will build late Friday and may push
north shore surf to the advisory level Friday night and Saturday.
North shore surf will drop below March average late Sunday. For
next week, a small north swell expected Monday night will keep the
surf up for locations that favor this direction through the day
Tuesday. A moderate long-period northwest swell due Wednesday
through Thursday may produce near advisory level surf by

As trade winds build, rough surf along east facing shores will
gradually increase, with wind swell reaching around the March
average of 6 to 7 ft at 7 seconds late Saturday through Monday.
Only trace background southerly swell will occur.






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
Send comments to: Ryan Lyman