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Hawaiian Islands Synoptic Discussion and Guidance

951
FXHW60 PHFO 211932
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
932 AM HST Tue May 21 2019

.SYNOPSIS...
A front to our northwest will continue to break down the ridge
to our north through this week. Winds will remain light east to
southeasterlies through Friday, allowing for development of local
land and sea breezes. Cloud cover and rainfall will favor mauka
areas during afternoon and early evening hours, with clearing
expected each night. Trade winds are forecast to return Memorial
Day weekend and last through the first half of next week.

&&

.DISCUSSION...
A front far northwest of the main Hawaiian Islands marks the
western edge of the subtropical ridge north of the chain. The
orientation of the front and ridge is such that winds continue to
blow from the east southeast across local waters. Wind speeds are
light enough, especially across the smaller islands, to allow
development of local land and sea breezes. Satellite loop shows
scattered low clouds to clear skies across the islands this
morning, except for southeast facing Maui and Big Island shores
and slopes, where broken low clouds are noted. Radar shows these
cloudier areas have scattered moderate showers. Cloud and shower
coverage will likely increase across interior sections of the
islands within the next few hours.

Models show little or no pattern change across the islands
through the rest of the work week as we remain stuck between high
pressure to our northeast and a front to our west and northwest.
The front will continue to move toward the islands today, but then
stall far enough away to spare us any direct effects. The upper
trough supporting the front will also remain far enough west of
the islands to prevent significant destabilization. Models show
the ridge will build back westward this weekend as troughing to
our west lifts northward. This will allow winds to back a bit to
east northeasterlies once again, restoring trade flow and the
associated pattern of windward and mauka showers. This restored
trade flow should then last into next week.

So, expect a mild convective pattern of afternoon and evening
clouds and showers across mauka areas, with nighttime clearing,
through Friday. Showers may become moderate at times. Expect a
shift back to a trade wind pattern this weekend into next week,
with clouds and showers shifting to windward and mauka areas.

&&

.AVIATION...
Expect weak southeasterly background flow to persist today as
Hawaii remains sandwiched between a cell of high pressure to the
northeast and an approaching front from the northwest. This
pattern will allow for a diurnally driven sea breeze/land breeze
regime. By afternoon, the highest concentration of clouds and
showers will likely reside over the interior and along mauka
slopes.

No AIRMETs are currently in effect.

&&

.MARINE...
A front approaching from the northwest will stall just northwest of
the offshore waters. A broad northeast to southwest surface ridge has
shifted south near Kauai ahead of the front. Through tonight, fresh
easterly trade winds will persist south of the Big Island of Hawaii
and through the Alenuihaha Channel, with fresh southeast winds over
Big Island and Maui County windward waters. The southeast flow will
be partly blocked by the Big Island and Maui, so winds around islands
from Lanai to Kauai will be gentle and variable. A high northeast of
the area will strengthen and move southeast starting tonight. Winds
over the coastal waters will strengthen and shift out of the east and
could reach Small Craft Advisory speeds around the Big Island and Maui
through the middle of the week.

Small to moderate surf will continue along south facing shores through
Friday. A fairly large swell from southeast of New Zealand is forecast
to reach the coastal waters Saturday. This swell may produce surf near
the advisory level along south facing shores through Memorial Day.
Surf along north facing shores will be quite small through Thursday.
A moderate northwest swell arriving Friday will peak Saturday then
slowly subside through Memorial Day. Surf along east facing shores
will remain small through the middle of the week. As winds strengthen
northeast of the area during the second half of the week, surf will
build along east facing shores.

&&

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

&&

$$

DISCUSSION...Powell
AVIATION...M Ballard
MARINE...Foster

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



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