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Hawaiian Islands Synoptic Discussion and Guidance

073
FXHW60 PHFO 221330
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Sun Apr 22 2018

.SYNOPSIS...
A shroud of high clouds will most of the main Hawaiian Islands
through today. Trade showers will decrease, leading to drier
weather across windward and mauka areas. Another area of low
level moisture will increase trade showers Monday night into
Tuesday. Trade winds will become locally windy Monday into
Tuesday, then slowly decline later in the week. A cold front may
push down the chain with clouds and showers by next weekend.

&&

.DISCUSSION...
Trade wind weather will continue for the next several days as a
strong surface high passes far north of the islands through early
next week. Low clouds and showers will favor windward and mauka
areas. This high will strengthen trade winds to locally windy
levels across the area Monday through Tuesday. Wind speeds will
gradually decrease after that, with trade flow likely becoming
disrupted Friday as a surface low passes to the north. By Friday
night and Saturday, models show a late-season frontal boundary
passing down the chain. We expect weakening and veering winds late
Thursday into Friday, with stronger northeast winds moving in
Saturday after the frontal boundary passes.

Thick middle and high clouds, associated with the subtropical
jet, continue to cover most of the main Hawaiian Islands this
morning. Models keep this area around through today. Radar shows
shower activity has decreased overnight across local waters, with
the bulk of remaining activity across windward waters and windward
slopes. Another trade wind shower area may affect windward Maui
and the Big Island late Monday night or Tuesday. Rainfall timing,
placement and intensity on Friday will depend on how much trade
flow is disrupted. In general, expect an increase in showers
Friday night into Saturday associated with the frontal boundary.

&&

.AVIATION...
A high pressure ridge north of the islands will continue to
produce locally strong trade winds across the islands. An upper
level jet stream will slowly weaken, slowly decreasing moderate
turbulence over the islands this morning. A stable atmosphere will
keep mostly VFR conditions in effect for most areas with periods
of MVFR ceilings and scattered showers expected along windward
slopes of each islands.

AIRMET Sierra is in effect for tempo mountain obscuration over
windward sections all islands. Conditions are expected to remain
in place this morning for most areas.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of mountain ranges for all islands due to the
moderate to breezy trade winds. Expect these conditions to
continue through at least Sunday.

AIRMET Tango in remains in effect for tempo moderate clear air
turbulence associated with upper level jet stream. Turbulence
aloft will decrease through the morning hours.

&&

.MARINE...
Fresh to strong trades are forecast to hold through much of the
upcoming week as high pressure remains established to the north.
The small craft advisory currently in place for most marine areas
from Oahu to the Big Island will hold through tonight. The
advisory for the typically windier locations from Maui to the Big
Island has been extended through Tuesday. Other marine zones may
become added Monday through Tuesday as high pressure strengthens
and a moderate northwest swell builds down the island chain
boosting seas up to around the 10 ft mark.

Rough surf is forecast to persist through the week along east facing
shores due to fresh to strong trades holding in place locally and
just upstream of the area. The best chance for advisory-level surf
will come Tuesday through Thursday as the trades increase locally
and expand farther upstream in response to 1030-1035 mb high
building to the north. Although confidence remains low this far out
in the forecast, there could be a break in the trades by Thursday/Friday
as a front approaches and moves into the area.

A new northwest (320 deg) swell is expected to begin slowly
filling in by this evening, peak Monday, then slowly ease through
midweek. This source is from a potent system that reached storm-
force category Friday as it crossed the date line just south of
the Aleutians between 40N and 50N (around 2000 miles northwest of
the state). Surf is forecast to peak on Monday at heights below
advisory levels along north and west facing shores.

A reinforcing moderate northwest (330 deg) swell is forecast to fill
in Thursday, peak Thursday night into Friday, then ease into the
weekend. This reinforcement will be from a developing storm that is
currently near the Kamchatka Peninsula and forecast to track east
while skirting the Aleutian Islands Sunday through Wednesday.
Surf associated with this feature should remain below advisory
levels along north and west facing shores.

A reinforcing southwest (220 deg) swell is forecast to fill in
Monday and hold through midweek due to recent activity across the
southern Pacific/Tasman Sea. Small to moderate surf will result
along south facing shores each day.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Oahu
Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maui
County Leeward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.

&&

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

Small Craft Advisory until 6 AM HST Monday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters-Maui County
Leeward Waters.

&&

$$

DISCUSSION...Powell
AVIATION...Bohlin
MARINE...Gibbs

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



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