Current Conditions
Temp0.3 C
RH26 %
WindNNE 21 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

397
FXHW60 PHFO 230631
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
831 PM HST Sun Apr 22 2018

.SYNOPSIS...
High pressure passing north of the islands on Monday will give us
a bit of an uptick in trade wind speeds. Areas of moisture riding
in on the trades will give 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 night through Saturday, followed by
drier, cool northerly winds.

&&

.DISCUSSION...
Quiet weather prevails across the Aloha State on this Sunday
evening, with widely scattered light showers mainly over windward
areas. Rain gauges show only scattered light amounts of less than
a quarter inch over windward Kauai during the past six hours,
while little or no rain has fallen elsewhere around the state.
Scattered high clouds continue to stream across the eastern half
of the state, while scattered to broken low clouds are present
over windward areas. Trade winds continue at fresh to locally
strong levels over the islands, due to a 1029 mb high centered
about 950 miles NW of Kauai. Aloft, a weak upper trough located
several hundred miles WSW of the state continues to bring the high
clouds up over the eastern islands, while an upper ridge located
well to our west and extending over and north of the state is
providing a stable environment. The 00Z soundings reflect the dry
and stable atmosphere in place, with PW values around one inch and
typical inversion heights around 6000 feet. MIMIC-TPW satellite
imagery shows a similarly dry airmass over and upstream of the
state, except to the NE of Kauai where an area of slightly higher
PW values of 1.2 to 1.3 inches is present.

Overnight, expect scattered windward showers to increase somewhat
late tonight, although rainfall amounts should remain mostly light
due to the stable atmosphere in place. Most leeward areas will
remain dry. 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 Monday 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 sweeps a weak
cold front quickly through the state Friday into Saturday, while
the last two GFS runs are much less progressive aloft and stall
the surface front over the western half of the state by Saturday,
with deep tropical moisture being drawn northward over 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 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 VFR conditions through Monday afternoon with periods of
scattered showers along eastern 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 will continue to diminish over the next 12 hours. A
disturbance in the trade winds will move in along north and east
slopes of all islands by Monday night with increasing trends in
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 at least Monday
afternoon.

&&

.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 Monday. The SCA will expand to
all coastal waters Monday night 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 Monday night 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 overnight, then peak
through the day Monday. The swell will slowly ease 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 on Monday/Monday night 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
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 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 Monday 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

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