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Hawaiian Islands Synoptic Discussion and Guidance

314
FXHW60 PHFO 160155
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
355 PM HST Sun Dec 15 2019

.SYNOPSIS...
Trade winds will strengthen tonight and Monday, with breezy
conditions expected into Tuesday. The trades will trend down
Wednesday into Thursday, before increasing to locally windy levels
late Thursday into next weekend. The trades will continue to deliver
low clouds and showers to windward areas with a few showers
occasionally spreading leeward.

&&

.DISCUSSION...
Trade winds (at times quite strong) will prevail through the
upcoming week, supported by a pair of high pressure cells passing
far N of the islands, with the central pressure of the second high
expected to be anomalously high. Winds may turn SE next weekend as
low pressure develops and strengthens W of the islands, with this
low potentially impacting island weather in just over a week from
now.

Moderate trade winds will increase overnight and Monday as high
pressure currently NNW of the islands passes to our N, and will
remain breezy until a weakness in the ridge passes to the N around
Wednesday. An even stronger high is expected to pass N of the
islands late Thursday into Saturday, likely bringing a period of
locally strong trade winds heading into next weekend. Central
pressure of this high is expected to be an unseasonably high 1034 mb
as it passes, and model guidance indicates very strong trade winds
at the end of the week, with open ocean wind speeds of 30-35 mph
near the islands.

As low pressure deepens W of the area next weekend, the gradient
between the gradually weakening high and the developing low may
bring strong SE winds to the state. Confidence is somewhat
diminished as to the details, but model guidance has been fairly
consistent indicating a somewhat energetic weather pattern in about
a week from now as the low nears the islands.

In the meantime, expect passing windward showers and moderate trade
wind until trade winds increase later tonight and Monday. A weak mid-
level shortwave is expected to pass from N to S over the area Monday
and Monday night. The combined effects of increased orographic lift
supplied by the increased trades, the passing shortwave, and
moisture associated with a frontal remnant caught in the trade flow
will likely lead to an increase in trade wind showers, and a
slightly wetter regime than previously thought.

A mid-level ridge will build over the islands Tuesday and Wednesday,
supporting a stable island atmosphere, but the trade winds should
still be able to muster a few windward showers. With the strong cap
in place, the already strong trade winds may become especially gusty
Monday night and Tuesday, especially near and downwind of terrain.
The mid-level ridge will weaken Thursday and Friday, and increasing
low level convergence associated with the new high building N of the
area will likely fuel an increase in trade showers, with another
frontal remnant potentially introducing moisture into the trade flow
as well.

&&

.AVIATION...
Strengthening trade winds expected tonight as a high pressure system
north of Hawaii settles to the northeast of the area.The trades will
focus incoming low clouds and showers over windward slopes resulting
in brief mountain obscurations this evening which could increase in
coverage late tonight. However, no AIRMETs are in effect at this
time. AIRMET Tango will likely be issued later tonight or Monday
morning for leeward mountain turbulence due to the strengthening
trades statewide.

&&

.MARINE...
A 1026 mb high pressure building north of the state tonight will
bring an increase in trade winds and will result in strong trades
through the first half of the week. Winds may reach gale force
Monday night into Tuesday across the Alenuihaha Channel and waters
south of the Big Island. Therefore, a Gale Watch is in effect for
these waters. Winds may slightly decrease Wednesday night into
Thursday, but a new 1035 mb high building north of state will lead
to stronger trades towards the end of the week. Gale force winds are
likely for the typical windy areas around Maui County and the Big
Island Friday into Saturday. Winds will likely remain strong through
the next weekend.

Forerunners from a new northwest swell will begin to fill in late
tonight and the swell is expected to steadily build through the day
Monday. Surf will likely reach high end advisory levels Monday night
as the swell peaks. Another advisory level northwest swell is
expected to steadily build Wednesday night. The combination of
strong trade winds and large northwest swells will bring widespread
Small Craft Advisory conditions through most of the upcoming week,
with Gale warnings possible Monday night through Tuesday and once
again Friday into Saturday.

With the expected boost in trades throughout the new work week, surf
along east facing shores will be on the rise. Surf will likely reach
the High Surf Advisory (HSA) criteria of 8 feet for east facing
shores later in the week with the stronger surge of trade winds. It
should be noted that waters and ocean conditions will be very rough
and choppy through most of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Gale Watch from Monday evening through Tuesday afternoon for
Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast
Waters.

Small Craft Advisory until 6 PM HST Monday for Alenuihaha Channel-
Big Island Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory from 6 AM Monday to 7 PM HST Wednesday for
Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-
Maui County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters.

Small Craft Advisory until 7 PM HST Wednesday for Maalaea Bay-
Pailolo Channel.

&&

$$

DISCUSSION...Birchard
AVIATION...Foster
MARINE...Kino

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