Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 231319
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
319 AM HST Mon Apr 23 2018

.SYNOPSIS...
High pressure passing north of the islands today will provide
breezy to locally strong trade winds over the state through
Tuesday. Areas of moisture embedded in on the trades will bring
increased clouds and showers to some windward and mauka areas at
times this week. Trade wind speeds will gradually decline during
the latter half of the week as low pressure develops to our north.
A cold front may move down the island chain late Friday through
Saturday, followed by drier and cooler northerly winds.

&&

.DISCUSSION...
Quiet weather prevails across the Aloha State on this early Monday
morning, with scattered light showers mainly over windward areas.
Rain gauges show only light amounts of less than a quarter inch
over windward sections of the islands during the past six hours.
Scattered to broken high clouds continue to stream across the Big
Island, while scattered to broken low clouds are present over
windward areas of all islands. Trade winds continue at fresh to
locally strong levels over the islands, due to a 1030 mb high
centered about 1000 miles north of Kauai. Aloft, a weak upper
trough located several hundred miles WSW of the state continues to
bring the high clouds up over the Big Island, while an upper
ridge located well to our west and extending over and north of the
state is providing a stable environment. The 12Z soundings
reflect the dry and stable atmosphere in place, with PW values
around 1.15 inches, and typical inversion heights between 6000 and
7000 feet. MIMIC-TPW satellite imagery shows a similarly dry
airmass over and upstream of the state, except for slightly
higher PW values of 1.2 to 1.3 inches brushing by over and to the
northwest of Kauai.

Only subtle changes are expected through the next few days, as a
breezy trade wind weather pattern continues, and patches of
increased moisture embedded in the trades move through the islands
at times. Trade wind speeds should peak today into Tuesday as the
surface high passes north and NNE of the state, with windy
conditions in some areas. A few areas may approach wind advisory
levels during this time. The high will then stall well to the NE
of the islands by midweek, with trade wind speeds relaxing back to
locally breezy levels. Showers will remain focused over windward
areas, with a few showers spilling into leeward sections at times.

Forecast uncertainty increases for the later part of the week into
the weekend. Models show yet another large mid/upper level trough
and surface low developing northwest or north of the state
beginning late Thursday into the weekend, but there remain major
differences in the details. The latest ECMWF run continue to sweep
a rather weak cold front quickly through the state Friday into
Saturday, while the last three GFS runs are much less progressive
aloft and stall the surface front somewhere over the state by
Saturday, with deep tropical moisture being drawn northward over
at least some of the islands. The latest GFS ensemble mean and
many of the ensemble members have trended toward a less
progressive solution aloft, with the surface front also stalling
somewhere over the state. For now, we expect trade winds to die
out Thursday night and Friday as the front approaches, and will
continue to depict a clean frontal passage through the state by
Saturday with cooler and dry northerly flow behind the front.
However, the forecast for the late week into the weekend bears
watch during the next several days.

&&

.AVIATION...
Expect mostly VFR conditions through the afternoon hours with
periods of scattered showers along north and east slopes of all
islands. Moderate strength trade winds will continue to produce
low level mechanical turbulence over and downstream of mountains.
High level cirrus clouds across the eastern islands will continue
to diminish over the next 12 hours. A disturbance in the trade
winds will move in along the windward slopes of all islands later
tonight with increasing low cloud ceilings and shower activity.

AIRMET Tango remains in effect for Tempo Moderate Turbulence over
mountains and over south through west sections of all islands.
This AIRMET will likely continue through Tuesday.

&&

.MARINE...
Fresh to strong trades are forecast to hold through much of the
this week as high pressure remains north of the area. A Small
Craft Advisory (SCA) is currently in place for most marine areas
from Oahu to the Big Island through the day today. The SCA will
expand to all coastal waters tonight due to winds and seas as
high pressure passes to the north and a new northwest swell fills
in.

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 tonight through Wednesday morning
as the wind speeds peak. There could be a break in the trades by
Thursday/Friday as a front approaches and moves into the area.

The aforementioned northwest swell has begun to fill in and will
continue to build down the island chain this morning, then peak
through the day today. The swell will slowly ease tonight through
midweek. This source is from a potent system that reached storm-
force category last 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 today 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 storm-force low that
is currently centered near the western Aleutian Islands tracking
toward the east-northeast through Tuesday. 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
today 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 AM HST Wednesday 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.

Small Craft Advisory from 6 PM this evening to 6 AM HST
Wednesday for Kauai Northwest Waters-Kauai Windward Waters-Kauai
Leeward Waters-Kauai Channel-Oahu Windward Waters-Big Island
Windward Waters.

&&

$$

DISCUSSION...Jacobson
AVIATION...Bohlin
MARINE...Gibbs


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