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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 181950

Area Forecast Discussion
National Weather Service Honolulu HI
950 AM HST Sat Jan 18 2020

Persistent high pressure northwest of the main Hawaiian Islands
will drive strong and gusty trade winds across local waters
through Sunday. Stable and dry conditions will keep showers
confined to windward slopes of most islands. Trades will weaken
early next week and will likely be disrupted Wednesday through


Strong high pressure northwest of the main Hawaiian islands
continues to drive strong and gusty trade winds across local
waters this morning. Overnight soundings show our airmass is quite
dry and stable, with less than 0.8 inches of PW and positive LI
values derived from the Lihue sounding. Remnant moisture from an
old frontal boundary keeps the airmass above the Big Island a bit
more wet and unstable. Satellite loop shows broken to overcast
stable low clouds, embedded within trade flow, covering windward
and mauka areas of the smaller islands. Skies are clearer near the
the Big Island, where broken low clouds cover windward lower
slopes. Radar shows isolated to scattered light showers over and
upwind from the islands of Maui County.

The current weather pattern will persist through the weekend.
Surface high pressure will settle north of the state and mid level
ridging will maintain stable conditions with a strong low level
inversion. For the rest of the weekend, expect modest windward
showers, at best, across the islands. With dew points hovering in
the upper 50s to lower 60s in the breezy trades, it will feel
cooler than a week ago.

Trades will gradually decline Monday night and Tuesday as the
surface high moves off to the east. Stable trade wind weather will
persist, with light showers confined to windward slopes and
afternoon sea breezes possible by Tuesday. Trades will likely be
disrupted for the second half of the work week as an approaching
front pushes the surface ridge over the islands. A stable land and
sea breeze pattern is expected. Models show the front stalling
near or just north of Kauai Friday night and Saturday as trades
gradually rebuild.


A stratocumulus cloud field over Oahu, Kauai and points north is
mingling with more showery low cumulus over windward coastal
waters of Maui County. The stratocumulus base is between 3 and 4
thousand feet so an AIRMET for mountain obscuration has not been
issued for the smaller islands, however, a AIRMET may be needed
for windward Maui County if the aforementioned low cumulus area
pushes into the windward slopes. A mountain obscuration AIRMET is
in effect for the slopes along the windward Big Island. This
AIRMET may be lifted at the noon issuance as those low clouds are
expected to partially disperse by midday. An AIRMET for low level
turbulence will remain with strong NNE to NE winds persisting
through Sunday.


Surface high pressure centered far northwest of area is producing
strong northeasterly winds across the coastal waters. These
strong winds are causing rough, elevated seas. A Small Craft
Advisory is in effect for all Hawaiian waters through Sunday
afternoon due to a combination of the strong winds and elevated
seas. The surface high will move slowly east to a position north
of the islands by Sunday. As a result, the winds are forecast to
gradually shift out of a more easterly direction from later this
weekend into Monday. The high will then move far northeast of the
area Monday night. A surface ridge trailing from this high is
forecast to then shift south over the state Tuesday and Wednesday.
This will cause the background flow to become lighter, and shift
out of the east-southeast by mid-week.

Moderate rough surf is occurring along north facing shores of
most of the islands due to the strong northeast winds and a
short-period north swell. This short-period north swell is
expected to gradually decrease through the day and into tonight.
A longer period north-northwest swell is expected to arrive early
Sunday. This swell may cause surf to reach the HSA thresholds
along most north and west facing shores of the smaller islands
from Sunday afternoon into Monday. This swell will gradually lower
from late Monday through Tuesday.

A large low pressure system is forecast to race eastward from
near Japan to the International Date Line into early next week.
The captured fetch associated with this system is expected to
send a large northwest swell toward the islands late Tuesday
night and Wednesday. This may cause surf to approach the High
Surf Warning criteria along some north and west facing shores of
the smaller islands from late Wednesday into Thursday. In
addition, surf may also approach the HSA criteria along west
facing shores of the Big Island during this event.

Elsewhere, expect moderate, choppy surf along most east facing
shores into early next week. Small south-southwest swells will
maintain background surf along south facing shores through the
middle of next week.


Small Craft Advisory until 6 PM HST Sunday for all Hawaiian




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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