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Hawaiian Islands Synoptic Discussion and Guidance

182
FXHW60 PHFO 211343
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
343 AM HST Thu Jun 21 2018

.SYNOPSIS...
Trade winds will continue to strengthen through tonight as
high pressure builds northwest of the state, becoming breezy on
Friday, and lasting through part of the weekend. The frequency of
the trade showers will also be on the rise. These showers will be
favoring the windward and mountain areas, although the breezy
trades will blow some of these showers into the lee areas of the
smaller islands from time to time.

&&

.DISCUSSION...
After a couple of days of light winds and dry weather, we will be
returning to a more typical trade wind pattern. Trades will be
becoming locally strong by sundown, with further strengthening
to moderate to strong area wide by Friday night. This comes about
as a new surface high strengthens to 1035 MB while passing 1040
miles north of the islands on Saturday and Saturday night.
Typically, with this strength of 1035 MB while passing north of
the islands would warrant a Wind Advisory for notorious locations
such as Lanai, the Kohala area of the Big Island, and perhaps the
central valley of Maui. High res wind models are not indicating
this of yet. Trades may ease off slightly Sunday but this could
premature. Certainly, the trades will scale down winds to locally
strong on Monday.

An upper level low, approaching the islands from the east, will
come close to Maui and the Big Island before scooting off to the
north on Sunday. It will affect the depth of the marine layer by
raising it from the current 6 to 8k feet to 10k feet over the Big
Island and to 8k feet over the smaller islands tonight. The marine
layer deepens further to 14k feet over the Big Island and to 12k
feet over the smaller islands on Friday. It stays this way to
until Sunday where the air mass stabilizes and the marine layer
lowers down to 8 to 10k feet. This condition persists through
at least Monday. Both the ECMWF and GFS continues to show a broad
but light precipitation field upwind of the islands, a sign of
plenty trade showers next week.

&&

.AVIATION...
Mainly dry weather with light background winds will continue
across the state through mid-morning. Isolated MVFR ceilings are
possible along some windward facing slopes in brief showers. Areas
of MVFR visibilities are possible across parts of the Big Island
due to volcanic haze/emissions from Halemaumau/Kilauea crater and
the East Rift Zone. Otherwise, VFR conditions are expected to
prevail. The trade winds will strengthen later today. This may
result in a slight increase in the coverage of low clouds and
trade showers over windward sections, especially later today and
tonight.

No AIRMETs are currently in effect, and none are anticipated
this morning. However, as the low-level trade wind flow
strengthens, we will monitor observations in case we need to
issue AIRMET Tango for mechanical turbulence downwind of the
higher terrain on the islands starting later today or tonight.

&&

.MARINE...
The Wednesday evening ASCAT pass only covered the eastern waters,
but did find areas of 20 kt winds in the Alenuihaha Channel and
south of the Big Island. Trade winds are expected to continue
building today as high pressure strengthens far northeast of the
state, and a Small Craft Advisory (SCA) has been posted through
Sunday for the typically windy areas around the Big Island and
Maui County. Trade winds will strengthen further heading into the
weekend, with SCA possibly needed for some additional windward
zones.

Surf will gradually build along east facing shores as trade winds
build today into the weekend. For all other shores, surf will
remain small with no significant swells anticipated through the
weekend. Overlapping long-period south swells are expected next
week with the first swell expected to arrive around Tuesday. The
forecasted local surf heights will be refined as the initial swell
reaches the buoy near American Samoa during the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory from 6 AM this morning to 6 PM HST Sunday
for Maalaea Bay, the Pailolo and Alenuihaha channels, and waters
south of the Big Island.

&&

$$

Lau/Houston/Jacobson

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

Current Conditions
Temp		7.1 C
RH		20 %
Wind		NE 18 mph
Road		Open (4x4)