580 
FXHW60 PHFO 040611
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
811 PM HST Sat Apr 3 2021

.SYNOPSIS...
High pressure building far north of the state will gradually
strengthen the trade winds. An upper level trough moving over the
islands will bring some high clouds and enhance the trade wind
showers through the remainder of the weekend, particularly for
Maui and the Big Island. Trade winds will decrease on Wednesday 
and remain at moderate strength into next weekend, leading to 
typical trade wind weather.

&&

.DISCUSSION...
No changes to the forecast are expected this evening. Showers over
the windward slopes of Haleakala this afternoon caused several
streams to rise, but most are on a downward trend this evening as
the showers appear to be tapering off. Additional showers are
expected over the windward sides of the islands overnight and
tomorrow. 

The showers are the remnants of a front, and are being carried in
on the trade wind flow. The high to the north is expected to 
slowly move to the east through Sunday, and will boost the trade 
winds over the region. Meanwhile an surface low far northeast of 
the state is expected to linger into the middle of the week. 

An upper level trough moving over the islands will provide some
instability which is expected to enhance the trade wind showers.
Additionally the trade wind inversion is lifting, which combined
with the stronger winds will carry some of the showers to leeward
areas. The upper level trough is also bringing some high clouds to
the area. Enough instability may be introduced that a thunderstorm
or two could be triggered over the Big Island Sunday afternoon or
evening. With clouds and showers reaching higher in the
atmosphere, its also possible for some winter weather to fall over
the Big Island summits above 12,000 ft.

The trough is expected to exit the islands Sunday night, and a
drier airmass moving in from the north. This should result in a
downward trend in shower activity for the start of the new work
week. Expect breezy trades to linger into at least Tuesday as 
high pressure holds to the north. Showers should once again become
focused along windward slopes with mainly dry conditions leeward,
aside from typical afternoon showers on the Kona slopes.

Trade winds will likely drop a notch on Wednesday and remain at 
moderate strength into next weekend. The GFS and ECMWF show a slow
moving mid to upper level trough just east of the state and a 
ridge to the west. The islands should remain just far enough away 
from the trough to produce a stable trade wind flow and typical 
trade wind weather. However, there is some uncertainty at this 
time.

&&

.AVIATION...
Prevailing VFR is noted at all TAF sites with the exception of
PHTO which is experiencing prevailing MVFR and intermittent IFR 
within an area of steady rain that is also impacting Windward
Maui. With signs of offshore flow now developing, the expectation
is for an improving trend over Windward Big Island during the 
next few hours. As moderate trades strengthen to breezy during the
forecast period, they will usher in a greater coverage of low 
clouds and showers. Prevailing MVFR will become increasingly 
likely over the windward sections of Oahu through Big Island from 
midday Sunday into Sunday night.

AIRMET Sierra remains in effect for Windward Maui and Big Island.

AIRMET Tango for lee turbulence may be needed as early as Sunday 
morning.

&&

.MARINE...
High pressure north of the state will keep fresh to strong trade
winds in place through Wednesday. The trades will ease into the
moderate to fresh range late next week as the high north of the 
islands weakens. A Small Craft Advisory (SCA) remains in effect
for the typically windy waters around Maui and the Big Island 
through 6 AM Monday. The SCA will likely need to be extended in
time and expanded to include include most - if not all - marine 
zones Sunday afternoon through Tuesday as the trades peak. 

The current north swell will build during the next couple days 
and gradually shift around to a more northeast direction. Surf
along north and east facing shores will follow suit with a rise
during the next couple days, with surf likely reaching High Surf
Advisory (HSA) levels along east facing shores by Monday 
afternoon, then holding above advisory thresholds through 
Thursday, with a slow lowering Friday through next weekend. A
series of small south swells will move through the islands during
the next 7 days, keeping south shore surf near the seasonal
average for early spring.

&&

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


&&

$$


DISCUSSION...M Ballard
AVIATION...JVC
MARINE...Jelsema