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Hawaiian Islands Synoptic Discussion and Guidance

173
FXHW60 PHFO 230639
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Fri Mar 22 2019

.SYNOPSIS...
A ridge of high pressure will continue to generate breezy trade
winds through the early part of next week. The remnants of a
dissipating front may provide a boost in trade showers Monday
and Tuesday.

&&

.DISCUSSION...
A dry and stable trade wind regime continues this evening.
Satellite imagery shows the southern fringe of the frontal
boundary grazing Kauai, while widely disperse clouds with embedded
showers lies upwind of the remaining islands. These clouds will
provide a coverage of isolated to scattered light showers, favoring
the windward and mountain areas early on. But as the evening
progresses, this coverage increases leading to an uptick in trade
showers. These clouds and showers will gradually diminish after
sunrise, resulting in a mostly dry afternoon. This process repeat
itself Saturday night into Sunday, although we may see a little
more trade showers on Sunday. The afternoon soundings from LIH and
ITo, shows a strong inversion at around 7k feet, and do not see
much fluctuation through Saturday night. There is a slight rise on
Sunday to about 10k feet.

Models are in agreement for the boundary to remain a few miles off
Kauai's north shore through the weekend. At times, though, the
southern fringe of the boundary will graze Kauai. The boundary,
which stretches to the California coast, will start breaking down
over the weekend, and the remnants get embedded in the trade wind
flow which will carry them toward the islands. The bulk of these
remnants will move across the islands between Monday and Tuesday
morning. As not above the inversion is expected to rise slightly
as early as Sunday to support these remnants.

An approaching front from the northwest will cause a breakdown in
the trades on Tuesday and Tuesday night, resulting in light and
variable winds area-wide on Wednesday. Light westerly winds
spread over the smaller islands in the afternoon. The front is
expected to reach Kauai Thursday mid-morning, and continue down
the island chain the rest of Thursday and Thursday night. Thus far,
the new GFS and old ECMWF, are in fair agreement. The winds behind
the front will be breezy from the north but shift quickly towards
the northeast by Thursday night as the front dissipates.

&&

.AVIATION...
High pressure north of the state will keep moderate trade winds in
place through Saturday, with low clouds and showers favoring
windward slopes and coasts, primarily during the overnight and
early morning hours. Some MVFR conditions will be possible as
showers move through, with Kauai the most likely target through
the overnight hours.

AIRMET Sierra remains in effect for mountain obscuration across
windward sections of Kauai. This AIRMET will likely remain in
place through much of the night. The AIRMET may need to be
expanded to include windward sections of some of the other islands
later tonight as well.


&&

.MARINE...
Trade winds will shift out of a more easterly direction and
strengthen into the fresh to strong category Saturday through
Monday as high pressure builds north of the state. A combination
of the trades and a new north-northwest swell has led to seas
climbing above the small craft advisory threshold (10 ft) this
afternoon, which is above predicted levels by a couple of feet.
Seas will hold at these levels through Saturday, then gradually
lower through the second half of the weekend into Monday as this
north-northwest swell eases. Trades should begin to weaken next
Tuesday through the midweek time frame as the ridge shifts south
toward the area and another front approaches. Guidance continues
to indicate light and variable winds by Wednesday followed by a
quick shift out of the north Thursday as the boundary makes it
into the area.

Surf along north and west facing shores will hold around advisory
levels through Saturday due to the current north-northwest swell,
then steadily ease late Saturday through Monday. A small
reinforcement out of a more northerly direction is expected Monday
night through Tuesday. A long-period northwest swell will fill in
through the day Wednesday and hold into the second half of the
week. Surf should remain below advisory levels Sunday through
Wednesday for north and west facing shores.

Surf along east facing shores will remain rise over the weekend
due to fresh to strong trades. A downward trend will be possible
by the midweek time frame of next week as the trades weaken.

Surf along south facing shores will remain small each day with
mainly background long period southerly pulses and short-period
southeast energy expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Saturday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Small Craft Advisory until 6 PM HST Sunday for all Hawaiian
waters except leeward waters of Oahu and Maui counties.

&&

$$

H Lau/Jelsema/Gibbs

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