135 
FXHW60 PHFO 041313
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
313 AM HST Sun Apr 4 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 today, particularly for Maui and the Big Island. Trade 
winds will decrease on Wednesday and remain at moderate strength 
into next weekend, leading to more typical trade wind weather.

&&

.DISCUSSION...
Some modification to the grids with the morning package, but the
overall forecast philosophy remains the same. Have lowered the 
PoPs for some areas on Kauai and Oahu today, in line with some of
the higher resolution model guidance. Have boosted QPF values for
windward areas, particularly Maui and the Big Island, in light of
rainfall totals overnight, and what a blend of the models shows 
through tomorrow night. Have also tapped down wind speeds just a
bit during the middle to end of the week in line with a trend in 
the models which show winds won't be quite as strong as previously
forecast, but still expecting moderate trades to end the week.

High pressure to the north will maintain the trade wind flow. The
nearly stationary surface low to the northeast will remain in
place for the next few days. As the high slowly moves to the east
and strengthens, the resultant pressure gradient over the islands
is expected to tighten which will cause a bit of an uptick in our
winds. Frontal remnants riding in on the trades have been 
reaching the windward sides of the islands with the main focus 
overnight being over Maui, with some making it to Oahu. 

An upper level trough axis located roughly northeast to southwest
over the western end of the state overnight will continue to have
impacts on our weather into tonight. High clouds are streaming
over the islands from the southwest and west on the east side of
the trough. The trough is also bringing colder upper level
temperatures to the islands. Both Lihue and Hilo overnight
soundings had 500 mb temperatures near -12 C (below normal). 
Precipitable water (PW) at Lihue was around 1 inch (near normal 
for April), while at Hilo it was 1.43 inches (a little above 
normal). The trough is expected to be east of the islands by 
tomorrow morning. There is the chance for a thunderstorm or two to
pop on the Big Island this afternoon/evening, and as mentioned 
above, rainfall totals for windward areas in particular are likely
to be higher than normal trade winds. Along with all of this, 
there is the possibility that any precipitation that reaches the 
Big Island summits will fall in the form of freezing rain and/or 
snow. 
 
Lower PW values are expected to spread across the islands from 
the north behind this upper level trough, and 500 mb temperatures
are expected to return to near normal, both of which will help to
usher in a drier and more stable airmass. 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...
Strengthening trades have prompted issuance of AIRMET Tango for
lee turbulence for Big Island through Maui. There is potential for
additional expansion during the forecast period especially as the
trade inversion strengthens from north to south. In the near 
term, MVFR still prevails over Windward Big Island and Maui where 
AIRMET Sierra remain in effect. As trades usher in a greater 
coverage of low clouds and showers this afternoon into this 
evening, MVFR will become increasingly likely for Molokai through 
Kauai. 

AIRMET Sierra for mountain obscuration is in effect for Windward 
Big Island and Maui.

AIRMET Tango for lee turbulence is in effect for Big Island
and Maui.

&&

.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 in the week as the high north of the
islands weakens. A Small Craft Advisory (SCA) is now in effect 
for the typically windy waters around Maui and the Big Island 
through 6 PM Tuesday. The SCA will likely need to be expanded to 
include most - if not all - marine zones tonight 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 steady
rise, with surf likely reaching High Surf Advisory (HSA)
thresholds along east facing shores by late Monday, then holding 
at advisory levels through Thursday. This swell will then slowly
lower 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 PM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big 
Island Southeast Waters.


&&

$$


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