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Hawaiian Islands Synoptic Discussion and Guidance

370
FXHW60 PHFO 200553
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
753 PM HST Sun May 19 2019

.SYNOPSIS...
Trade winds will continue tonight and then winds will become
light southeasterlies on Monday as a cold front moves in from the
northwest. The front will stall north of the islands with light
and variable winds in the forecast through Friday. Clouds and
scattered showers will favor windward mountains and interior
sections of each island in the afternoon and early evening time
periods. Moderate trade winds are forecast to return by next
weekend.

&&

.DISCUSSION...
Satellite imagery this evening shows a band of subtropical jet
cirrus clouds moving across the state from the southwest toward
the northeast. A broad band of stable low clouds is just upstream
of Oahu and Kauai. An overcast layer of clouds is forming along
the leeward Kona slopes of the Big Island.

Otherwise a 1025 MB high center lingers far northeast of the
Hawaiian Islands this evening with a ridge axis just north of the
state. A cold front approaching the region from the northwest will
break down the ridge north of the islands on Monday. Decreasing
pressure gradients will cause a transition to light and variable
winds. A weak upper level disturbance will also pass through the
western islands on Monday, producing a slight enhancement in
clouds and showers mainly over Kauai.

Expect light southeasterly winds on Monday allowing sea breezes
to redevelop along sheltered leeward slopes. A light easterly
back ground wind flow from Tuesday through Friday will allow a
stronger land and sea breeze pattern to dominate island weather
conditions. Clouds and isolated to scattered showers will form
along windward mountain ranges, interior sections, and sea breeze
convergence boundaries; favoring afternoon and early evening
hours. Medium range weather model solutions are in good agreement
with keeping this light and variable wind pattern in the forecast
through next week Friday.

Trade winds are expected to return just in time for the Memorial
Day weekend as the stalled front lifts north allowing the ridge
to slowly build back in over the islands. Subtle differences
between the American (GFS) model and the European (ECMWF) model
remain keeping our confidence for returning trades in the fair
category this far out. We continue to lean towards the GFS model
and climatology for the long range wind forecast. Clouds and
showers will then favor more typical windward and mauka areas.

&&

.AVIATION...
Expect a mainly VFR evening across the main Hawaiian Islands area.
A ridge of high pressure located 430 nm N of Kauai will be moving
S throughout tonight and into Monday, weakening the trade winds
and even cause a veering to the SE over the western island through
Monday.

An area of low clouds and showers continues to move through Kauai,
causing a bit of mountain obscuration to the N and E facing slopes
and interior mountains as well. This batch of showers will be
thinning out through 10z, and likely over by 16z. Thus AIRMET
Sierra for Kauai will likely persist through 10z and ending by 16z.

Relative sparse low cloud coverage upwind of the remaining islands
as of 05z. Do expect a slight increase in low cloud coverage as the
night progresses, along with some scattered showers. They will be
affecting mainly the windward and mountain areas, bring brief
periods of MVFR ceiling and vis, not enough though, for AIRMET
Sierra. Aside from the trade showers, a late night shower may pop
up along the Kona coast. Cloud tops mainly at 7k feet.

The dying trades have weakened enough where low level AIRMET
Tango was dropped earlier. There is also a layer of cirrus clouds
based at 30k feet. The cirrus will remain over the area tonight
and thinning out Monday. The cirrus is part of a jet stream
cutting through the islands. Tempo moderate turbulence can be
expected between fl200 and fl250, where AIRMET Tango will be
implemented soon.

&&

.MARINE...
High pressure northeast of the state will keep moderate to
locally strong trade winds in place tonight. As a result, a Small
Craft Advisory (SCA) remains in effect for the typically windy
waters around Maui and the Big Island through 6 AM Monday. The
winds will ease and shift around to the east-southeast Monday
through Tuesday, as the high moves northeast and a cold front
approaches from the northwest. The front will then stall northwest
of the area during the middle and latter part of the week, while
high pressure strengthens northeast of the island chain. This
will bring an increase in the east to east-southeasterly winds
Wednesday through Friday.

A series of small southwest and south swells will produce small
to moderate surf along south facing shores through the middle of
the week. Currently, we have small background southwest swell
from the Tasman Sea. A reinforcing long-period south-southwest
swell is expected to build Tuesday into Wednesday. Further out, a
larger long-period south-southwest swell is expected to arrive
during the Memorial Day weekend, with surf possibly reaching
advisory levels along south facing shores.

The current small northwest swell will continue to decline
through Monday. North shore surf will remain small through
Thursday, with a new small to moderate sized northwest swell
possible Friday into the Memorial Day weekend. Surf along east
facing shores will remain small through the middle of the week. A
developing fetch of moderate trades upstream of the islands could
bring a boost to east shore surf Thursday through the Memorial
Day weekend.

&&

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

&&

$$

DISCUSSION...Bohlin
AVIATION...Lau
MARINE...Jelsema

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