446 
FXHW60 PHFO 081357
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
357 AM HST Mon Apr 8 2019

.SYNOPSIS... 
High pressure will strengthen as it builds northeastward and away
from the island chain through the first half of the work week,
strengthening the trades across the state. Moderate trades today
and tonight, will strengthen to breezy levels by Tuesday, and 
windy levels by Wednesday. Fairly typical trade wind weather is 
expected through mid week, with showers favoring windward and 
mauka areas, and a stray shower reaching leeward areas from time 
to time. Although breezy to windy conditions will continue 
Thursday through the weekend, big changes to the weather pattern 
are expected across the state. An upper level low will move over 
the eastern islands, bringing unsettled weather to the entire 
island chain late in the week into next weekend. The potential for
heavy rainfall, thunderstorms and flash flooding will be highest 
across the Big Island and Maui County although unsettled and wet 
trade wind weather will likely affect Oahu and Kauai as well.

&&

.DISCUSSION...
Currently at the surface, the remnant moisture associated with a
dissipated front remains in the vicinity of Kauai early this 
morning, while a 1027 mb high is centered around 1050 miles
north-northeast of Honolulu. With the dissipation of the front
overnight, the high has become the dominant weather feature
influencing the winds across the island chain, with light to 
moderate trades now prevailing statewide. Infrared satellite 
imagery shows clear to partly cloudy skies across Oahu, most of 
Maui County and leeward Big Island, with mostly cloudy conditions
over Kauai and windward Big Island. Radar imagery shows scattered
to numerous showers over Kauai, scattered showers over windward
sections of the Big Island, with mainly isolated windward showers
across the rest of the state. Main forecast concerns revolve 
around the potential for heavy rainfall, flash flooding, and 
thunderstorms later this week.

Today through Wednesday, 
High pressure will strengthen as it shifts northeastward over the
next couple of days, reaching a strength of 1042 mb around 1600 
miles northeast of Honolulu by Tuesday night and Wednesday. 
Generally moderate trade winds are expected to prevail today and 
tonight, with the trades then steadily strengthening Tuesday 
through Wednesday. Breezy conditions are expected by Tuesday 
afternoon and Tuesday night, with windy conditions expected for 
Wednesday. Fairly typical trade wind weather is expected through 
the period, with showers favoring windward and mauka areas 
particularly during the overnight and early morning hours, along 
with the occasional leeward spillover shower. There will likely 
be an increase in shower activity Tuesday night and Wednesday 
however, particularly over the eastern end of the state, as an 
upper level low closes off in the vicinity of 150W and sends some 
deeper moisture westward towards the eastern islands. 

Wednesday night through Sunday,
The pattern becomes much more interesting during the long term 
period, with both the GFS and ECMWF advertising an unsettled 
stretch of weather across the island chain. Breezy to windy 
conditions are expected to continue through the period, as strong
high pressure northeast of the state shifts back westward and 
merges with a new high to the north of the islands over the 
weekend. The trades are expected to peak in the Thursday through
Saturday time frame, and could reach advisory levels across
portions of the island chain. In addition to the strong winds, 
model solutions continue to show the closed upper level low 
retrograding westward and into the eastern islands by Thursday 
night or Friday, then lingering in this vicinity through the 
weekend. Precipitable water values aren't particularly high, but 
they do increase into the 1.5 to 1.8 inch range late in the week 
through the weekend over the eastern end of the state, well above 
normal for this time of year. This increase in moisture combined 
with the cold temperatures aloft with H5 temperatures dropping to
-12 to -14C, and orographic lift provided by the strong trade 
winds will likely bring periods of heavy rainfall, the chance for 
thunderstorms, and the potential for flash flooding to the island 
chain. At this time the eastern islands, particularly Maui and the
Big Island, appear to have the highest probabilities of seeing 
the most significant weather impacts. This will continue to be 
monitored closely as the event draws closer, and a Flash Flood 
Watch may eventually be needed for portions of the state later 
this week.
 
In addition, accumulating snow will be possible at the higher 
elevations of the Big Island Thursday through at least Saturday, 
and a Winter Storm Watch may eventually be needed later this week.

&&

.AVIATION...
Moisture from a dissipated front will continue to provide clouds 
and showers to Kauai and vicinity today, with tempo MVFR ceilings 
and VIS conditions.

A line of clouds just east of the area is expected to reach the 
rest of the islands later today. As trade winds will slowly build,
much of the incoming clouds will affect mainly windward slopes, 
bringing brief MVFR ceiling and VIS conditions. Otherwise VFR 
conditions are expected to prevail for rest of area.

AIRMET for mountain obscuration will continue at least through 
this morning for Kauai. With the increasing trade winds, AIRMET
for turbulence may be needed tonight for areas just over and lee
of the mountains.

&&

.MARINE...
Moderate trades will increase to fresh speeds today as high 
pressure strengthens far northeast of the state. By Tuesday 
afternoon, the high is expected to strengthen to a 1040 mb high 
far northeast of the state and will result in winds increasing to 
fresh to strong levels across the typically windier areas around 
Maui and the Big Island. As the pressure gradient tightens 
Wednesday into Thursday, winds should increase to strong speeds 
across all Hawaiian Waters and will be approaching gale force over
the windier areas around Maui and the Big Island. 

Also, unsettled weather is expected starting Thursday due to an 
upper level low moving into area waters. The low will bring an 
increase of showers and the increasing threat of thunderstorms 
Thursday and Friday. Depending on the track of the low, unsettled 
weather could linger through the weekend. 

A new moderate northwest swell will bring moderate surf to north
and west facing shores of the smaller islands today and will 
gradually decrease tonight through the middle of the week. Small 
reinforcing west-northwest swells will keep small surf rolling in
towards the latter half of the week along north and west facing 
shores. As the trades increase to strong speeds, rough surf will 
be building along east facing shores. Surf is expected to reach 
advisory levels along east facing shores by around Thursday of 
this week and stay elevated through the weekend. Small surf is 
expected for south facing shores through next week with only 
background swells expected.

Currently there are no marine warnings in effect, but Small Craft
Advisory (SCA) level winds are expected to return by Tuesday
afternoon across the windier areas of Maui and the Big Island. The
SCA areas will likely expand to all Hawaiian Waters sometime 
Wednesday or Thursday due to increasing trades and seas building
above the SCA threshold. Gale warnings will also be possible for
the latter half of this week, especially over the Alenuihaha 
Channel.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Jelsema
AVIATION...Hui
MARINE...Kino