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Hawaiian Islands Synoptic Discussion and Guidance

348
FXHW60 PHFO 190621
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
821 PM HST Sat May 18 2019

.SYNOPSIS...
A high pressure system northeast of the Hawaiian Islands will
keep trade winds in the forecast with scattered overnight and
early morning showers through Sunday. A cold front approaching
from the northwest will break down the ridge north of the islands
as trade winds transition to light and variable from Monday into
Tuesday. A light easterly background wind flow will allow land and
sea breezes to develop over leeward areas of each island from
Tuesday through Friday. Clouds and isolated to scattered showers
during this period will favor windward mountain and interior
sections of each island in the afternoon and early evening hours.

&&

.DISCUSSION...
A broad high pressure ridge 500 miles north of the islands will
keep moderate trade winds in the forecast through Sunday night with
scattered showers favoring windward and mountain areas in the
overnight and early morning hours. On Monday the large scale
weather pattern changes as a cold front approaches the islands
from the northwest breaking down the ridge. Decreasing surface
pressure gradients over the islands on Monday will cause surface
winds to weaken and veer towards the east southeast. Two upper level
disturbances passing over Kauai may produce elevated showers from
Kauai to Oahu on Monday and Tuesday.

These lighter wind speeds will continue through the work week
with a light easterly wind flow interacting with land and sea
breezes forming along leeward slopes of each island. Weather
conditions will feel hot and humid next week due to the light wind
pattern and high sun angle this time of year. Most islands will
see clouds building over island interior sections with isolated to
scattered showers trending towards the afternoon and early evening
hours. Scatterd showers will develop along sea breeze and light
trade wind convergence zones. Wind directions near these
leeward convergence zones will become more erratic and variable in
the afternoon hours as light background winds fight with sea
breezes for dominance.

Extended outlook weather model solutions suggest we may return to
moderate trade winds by next weekend. However, this far out in
the forecast cycle our confidence remains only fair that these
trade wind speeds will be strong enough to overcome an established
land and sea breeze cycle along leeward slopes. Much will depend
upon the strength of the high pressure ridge north of the islands.

&&

.AVIATION...
The locally strong low-level trade wind flow will persist through
Sunday afternoon. Low clouds and showers transported by the
trades will continue to affect windward facing slopes through
Sunday. Expect brief periods of MVFR conditions to occur over
north through east facing sections due to lower ceilings and
reduced visibilities, especially from late tonight through Sunday
morning. Otherwise, expect VFR conditions to prevail.

No AIRMETs are currently in effect. We will monitor satellite
imagery overnight in case AIRMET Sierra might be required for
mountain obscuration over some windward areas. In addition, based
on the VAD wind profiles across the state, we do not see a need
for AIRMET Tango for low-level mechanical turbulence leeward of
the higher terrain.

&&

.MARINE...
High pressure northeast of the state will maintain moderate to
locally strong trade winds through the weekend. 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 early
next week 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 next week, while high
pressure strengthens northeast of the island chain. This will
bring a slight increase in the east to east-southeasterly winds
Tuesday night through Friday.

A series of small southwest and south swells will produce small
to moderate surf along south facing shores through the middle of
next week. A low pressure system currently developing southeast
of New Zealand is expected to strengthen tonight into Sunday and
will likely produce a long-period south-southwest swell for next
weekend. If this storm materializes as expected, surf may
approach the High Surf Advisory criteria Memorial Day weekend.

A small northwest swell will fill in tonight, peak on Sunday,
then gradually lower into early next week. Another small to
moderate sized northwest swell is possible late next week into
next weekend. Small, choppy surf along east facing shores may get
a slight boost on Sunday due to the stronger trade winds. Another
increase around mid-week is possible due to a developing fetch of
moderate trades east of the islands.

&&

.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...Houston
MARINE...Jelsema

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



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