Current Conditions
Temp1.3 C
RH13 %
WindNNE 21 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 140639

Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Fri Dec 13 2019

Surface high pressure passing north of the islands will keep
breezy to locally windy trades blowing tonight. The trades will
ease some over the weekend as the high moves away. Rainfall will
favor windward and mauka locales, with a few showers briefly
reaching leeward spots. The jet stream will bring increasing high
clouds tonight. A dry, stable airmass will take control of our
weather this weekend and the first half of next week, with few
showers and considerable sunshine. The next high passing far to
the north will return locally windy trades Monday and Tuesday.


A surface trough is moving away to the west of the islands.
Meanwhile, high pressure passing to our north is bringing locally
windy conditions to the islands this evening. This high will
continue moving to the east over the next couple of days, so
trade winds will trend down just a bit over the weekend.

An upper trough to the west of the islands has destabilized our
airmass over the last few days and enhanced trade wind showers.
This trough will weaken as we head into the weekend, leading to
fewer and less intense trade showers. High level moisture riding
in on the subtropical jet will continue to bring high clouds at
times, and these will increase tonight into Saturday before
pushing away later Saturday as the jet shifts to the southeast of
the islands.

Trade wind showers will decrease further early next week as stable
conditions and drier air move in. A strong upper high centered
just to our north will make for generally nice pre-holiday weather
with lots of sunshine and relatively few showers. Rainfall
will be light and should favor windward and mauka sections during
the nights and mornings. Showers will likely increase again
starting Thursday as mid-level heights fall near the islands and
cooler temperatures aloft start to make the airmass a little more
unstable again.

The surface high to the north will continue eastward into the
weekend, so trade wind speeds on Saturday will be a notch below
today's levels. Winds will even be a notch lower on Sunday as a
col between two highs passes north of the islands. The next
surface high will pass well north of the islands on Monday,
returning breezy to locally windy trade wind speeds for Monday
and Tuesday. The winds should ease Wednesday, but may increase
again on Thursday as the next in the series of highs passing to
the north approaches our longitude.


A large area of showers is still present southwest of Kauai. Some
of this moisture will affect Kauai with lower ceilings and
visibility into the overnight hours. AIRMET Sierra is currently in
effect for Oahu and Kauai for mountain obscuration. The rest of
the island chain is experiencing periods of mountain obscuration
but nothing consistent as of now. As we progress into Saturday, we
will be in a more traditional northeast tradewind pattern with a
majority of the rain showers affecting the windward slopes. Winds
may be strong enough in some areas to push a few showers into
leeward sections.

Winds will continue to be moderate. AIRMET Tango is currently in
effect for low level turbulence below 9,000ft but we may be able
to cancel that AIRMET as the winds weaken overnight. But with
moderate winds forecast for Saturday, the AIRMET may need to be
activated again.


The current northwest swell will continue to subside. Surf
tonight will hold just below the advisory level along north and
west facing shores, then lower to moderate levels Saturday. A
short period north-northeast swell will keep surf elevated along
exposed east facing shores, mainly on Kauai and Oahu, through
Saturday but should stay just below advisory levels.

A reinforcing large northwest swell is forecast to build late
Sunday into Monday and peak Monday night, possibly near warning
levels. This swell should slowly decline Tuesday into Wednesday,
but will likely be reinforced by another large northwest swell
Wednesday night into Thursday.

High pressure north of the state will keep moderate to locally
strong trade winds over the area through Saturday. The combination
of the current northwest swell and the strong trade winds is
resulting in a Small Craft Advisory (SCA) for all coastal waters,
except the Maui leeward waters. Winds will begin to trend down
slightly, likely resulting in small craft winds being confined to
the typically windier zones around Maui County and the Big Island
Saturday. Seas will also lower as the northwest swell subsides.

A slight decline in trade winds and period without Small Craft
conditions is possible Sunday as the current high moves east and
a new high builds north. Trade winds should return to moderate to
locally strong speeds by early next week and seas will build
again with the reinforcing northwest swell.


Small Craft Advisory until 6 AM HST Saturday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha
Channel-Big Island Windward Waters-Big Island Leeward Waters-Big
Island Southeast Waters.




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