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Hawaiian Islands Synoptic Discussion and Guidance

647
FXHW60 PHFO 210628
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
828 PM HST Mon May 20 2019

.SYNOPSIS...
An approaching cold front will continue to break down the high
pressure ridge north of the islands. Wind speeds will remain light
east to southeasterly through Friday allowing local scale land and
sea breezes to form over each island. Cloud ceilings and scattered
showers will favor afternoon and early evening hours through the
week. Trade winds are forecast to return just in time for the
Memorial Day weekend and last through the first half of next week.

&&

.DISCUSSION...
The GOES satellite imagery this evening shows broken to overcast
clouds over each island in a typical light wind sea breeze
weather pattern. Light southeasterly winds caused downsloping air
to warm up surface temperatures over the Kahului Airport today setting
a new daily record high temperature of 93 degrees for this station.
This breaks the old daily record of 91 degrees set on this day in
1978. Scattered rainfall reports over the past 6 hours across the
state show anywhere from 0.01 to 0.66 inches of rain has fallen
at several locations. Clouds and shower activity will diminish
later tonight over the islands as more stable down slope land
breezes develop.

In the big picture, a 1030 MB surface high in the northeast
Pacific has drifted further away from the Hawaiian Islands as a
surface cold front moves into the region from the northwest. The
forward motion on this front will stall out just north of Kauai.
The position of this stalled cold front will keep the islands on
the edge of the subtropical ridge axis, with weaker surface
pressure gradients leading to a continued light east to southeast
wind pattern. Local scale surface cooling and heating of the
islands, relative to the ocean temperatures, will cause overnight
land and daytime sea breezes to form.

This sea breeze circulation will produce hot and humid weather
conditions over the Hawaiian Islands each day with cooler
temperatures overnight through Friday. Clouds and scattered
showers will form along convergence areas over mountains, island
interior sections, and along sea breeze and southeasterly wind
convergence zones. Wind directions will become more erratic than
usual in the afternoon hours due to shower activity and movement
of sea breeze to light southeasterly wind convergence boundaries.

High pressure builds back in over the state and the cold front
lifts north allowing moderate easterly trade winds to return just
in time for the Memorial Day weekend. This trade wind weather
pattern will likely continue through the first half of next week.
Expect slightly cooler and less humid conditions with clouds and
scattered showers favoring windward and mountain areas of all
islands.

&&

.AVIATION...
With a 1030 MB surface high far to the northeast of Hawaii and a
front approaching from the northwest, light southeasterly or even
light variable flow will persist across the islands tonight. ISOL
-SHRA will be possible along the windward locations, though a
brief shower could pass over the leeward side. Sea breezes are
expected to develop at almost all TAF sites aft 21Z Tuesday.

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
Tuesday night, 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 Tuesday. Winds over the
coastal waters will strengthen and shift out of the east starting
Tuesday night 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...Bohlin
AVIATION...JT
MARINE...Donaldson

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



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