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Hawaiian Islands Synoptic Discussion and Guidance

405
FXHW60 PHFO 260633
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
833 PM HST Mon Mar 25 2019

.SYNOPSIS...
A weakening high pressure system north of the islands will track
southeastward as trade wind speeds decreasing on Tuesday. A cold
frontal system approaching the Hawaiian Islands from the
northwest on Tuesday will break down the ridge north of the state
setting up light winds lasting through the weekend. Clouds and
showers from this front will move into Kauai late Wednesday night
with unsettled weather slowly spreading southeast down the rest of
the island chain from Thursday into Friday. Expect lingering unsettled
showers over the windward sections of Maui and the Big Island this
weekend.

&&

.DISCUSSION...
This evenings GOES satellite imagery shows a large band of clouds
and showers lingering over and north of Kauai. Periods of
unstable showers will continue to move in along windward and
mountain slopes of all islands during the overnight period with
higher precipitation trends favoring the island of Kauai.

In the big picture, a weakening 1025 MB surface high about 700
miles north of the Hawaiian Islands will continue to track towards
the southeast through Tuesday. Decreasing surface pressure
gradients will cause trade wind speeds to become light starting
tomorrow, allowing modified sea breezes to develop over the
leeward slopes of each island by afternoon. Minimal shower
activity remains in the forecast grids from Tuesday through
Wednesday. By Tuesday night offshore land breezes will form with
onshore sea breezes with clouds forming over mountain and interior
sections of each island on Wednesday.

A cold front will bring clouds and elevated showers to Kauai by
Wednesday night. Clouds and numerous showers along the frontal
band will slowly spread southeastward to the rest of the island
chain on Thursday and Friday. The diminishing frontal band will
stall over the eastern half of the state on Friday with elevated
showers mainly affecting windward and mountain slopes of Maui and
the Big Island. Cooler northerly winds will develop after frontal
passage and island temperatures will run a few degrees colder
through Friday.

On Saturday and Sunday, a weak high pressure system will track
across the Central Pacific keeping light winds across the island
chain. Some unsettled showers associated with the diminished cold
front will favor windward slopes of each island through Sunday.

By next week Monday and Tuesday, another shallow cold front moves
into the region from the northwest and light winds will continue
across the region. Additional clouds and scattered showers along
this next frontal band will move into the northwestern islands on
Tuesday morning. Stay tuned for changes in the precipitation
timing part of the forecast for next week, as a higher degree of
uncertainty remains with the long range model guidance with this
next frontal passage.

&&

.AVIATION...
High pressure north of Hawaii will maintain a trade wind flow,
but as the high moves slowly to the east through Tuesday, winds
will gradually weaken. Lingering moisture over Kauai will maintain
clouds and showers island wise tonight, with the other islands
seeing some trade wind showers over windward slopes. As winds
weaken on Tuesday, expect to see some clouds develop over interior
and leeward areas mainly during the afternoon hours.

AIRMET Sierra continues for mountain obscuration on Kauai.
Anticipating this to continue throughout the night, with VFR
conditions expected elsewhere.

&&

.MARINE...
The trade winds have started its weakening trend, and this trend
will continue through Tuesday. The winds will finally become light
and variable early Wednesday, then change to westerly in the
afternoon as a front continue its approach to the islands. The
front is expected to reach Kauai Thursday afternoon. The slow
moving front will dissipate across the eastern islands between
Thursday and Saturday. A moderate to fresh northerly wind flow
fills in immediately behind the front, then veer to more typical
trade winds over the weekend as a weak high pressure system passes
north of the islands. Another front will be approaching the
islands from the west late in the weekend, where it will be
altering the wind flow around the islands once again.

We currently have a Small Craft Advisory (SCA) for the typically
windy near-shore waters around Maui County and waters south of the
Big Island to 6 AM HST Tuesday. We expect this SCA to run its
course with no extension.

The north-northwest swell from this weekend continues to decline.
There is small reinforcement from the north tonight through
Wednesday. Then we have series of moderate to large swells for the
second half of the week. First, a large northwest swell with a 6
foot swell and a 17 second period arrives Wednesday and Wednesday
night. A north- northwest (340 degrees), 7 foot swell, 13 second
period, follows in Thursday night through Friday. Then, a more
west swell of 300 degrees, 6 feet and 15 seconds, comes in
Saturday and Saturday night. The Wednesday swell has the potential
of reaching advisory level surf for at least the north facing
shores. But we will be monitoring closely for all three of them.

With the dying trades, surf along east facing shores will be
trending down as well during the next 24 to 48 hours.

A couple of long period swell from the south will maintain small
surf along the south facing shores throughout the period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...M Ballard
MARINE...Lau

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



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