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Temp0.4 C
RH5 %
WindSSW 18 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

025
FXHW60 PHFO 261355
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
355 AM HST Tue Mar 26 2019

.SYNOPSIS...
A weakening high pressure system north of the islands will track
southeastward as trade wind speeds decrease today. A cold front
approaching the Hawaiian Islands from the northwest will break
down the ridge north of the state setting up mostly 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 mornings radar imagery shows scattered showers drifting
across the region in a light trade wind flow pattern. GOES
Satellite continues to indicate a large band of unstable clouds
developing along a low level trough north of Kauai. More stable
stratocumulus clouds are depicted northeast of the state. A cold
front approaching the islands from the northwest is located
roughly 900 miles northwest of Kauai as of 4 AM HST. The upper air
12Z sounding from Lihue this morning shows an elevated subsidence
inversion around 10,000 feet likely associated with the low level
instability.

A weakening 1023 MB surface high remains north of the islands
drifting southeastward over the next 24 hours. Decreasing surface
pressure gradients will cause trade wind speeds to decrease
through the day, allowing modified sea breezes to develop over
the leeward slopes of each island by afternoon. Minimal shower
activity remains in the forecast grids for most areas through
Wednesday. The high pressure ridge north of Hawaii will diminish
this afternoon resulting in offshore land breezes tonight and
onshore sea breezes 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 into 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 Maui and the Big Island
through the weekend.

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. 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...
Weaker winds are expected today as a high pressure system to the
north moves to the east. Clouds and showers lingering over Kauai
in the predawn hours should begin to thin after daybreak. Limited
clouds and showers will remain focused over windward areas, but
with the light winds, anticipate some clouds developing over
interior and leeward areas mainly during the afternoon hours.

AIRMET Sierra continues for mountain obscuration on Kauai, and
will likely linger into the first part of today. VFR conditions
expected elsewhere.

&&

.MARINE...
Moderate to fresh easterly trades will trend down into the light
to moderate range by this afternoon, then shift out of the east-
southeast tonight into Wednesday as a front approaches the region.
This will allow a land and sea breeze pattern to become
established near the coasts late tonight through Wednesday night.
The background flow will shift from the east-southeast Wednesday
to the northwest Wednesday night. Light to moderate northerly
winds will quickly fill in behind the front through the day
Thursday and continue through Friday. The progressive pattern will
continue over the weekend as another front approaches.

A small northerly swell has filled in and will keep the surf up
along exposed north facing shores today. As this source fades
through the day Wednesday, a new long-period northwest swell is
expected to build down the island chain with a peak likely
happening late Wednesday through Wednesday night. This source will
hold into Friday, then fade into the weekend. A more north-
northwest swell is expected to fill in Friday and may near
advisory levels at its peak by Friday night. A third reinforcement
out of the northwest is expected to build down the island chain
through the day Saturday and continue into early next week.

Surf along south facing shores should come up slightly Wednesday
night and linger into Friday as a new long period south swell
arrives. Spectral density plots from the Samoa buoy confirm this
source moved through a couple of days ago, so the timing for the
arrival by Thursday morning is expected locally.

Surf along east facing shores will remain small with the light
winds expected locally and upstream into the upcoming weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

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

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