603 
FXHW60 PHFO 041935
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
935 AM HST Sun Apr 4 2021

.SYNOPSIS...
Breezy trade winds will persist for the next few days as strong
high pressure holds far north of the state. A band of moisture 
will produce active showers along mainly windward portions of 
most islands today, then drift south of the state on Monday as
a drier air mass moves in from the north. Trade winds will 
decrease on Wednesday and remain at moderate strength into next 
weekend, leading to typical trade wind weather.

&&

.DISCUSSION...
Breezy trade winds are in place, and a band of moisture is 
focusing rainfall over the central portion of the island chain 
this morning. A rather strong 1040 mb high parked over 1500 miles 
north of the state is driving the trades, while a nearly 
stationary cutoff low sits about 1300 miles northeast of the 
islands. A band of low-level moisture, the tail end of a 
dissipated front extending from the low, is draped over Oahu and 
Maui County and is creating persistent showers over windward 
areas. Expect this feature to creep southward today and lead to a 
decrease in rainfall on Oahu late this afternoon and an increase 
in showers on the Big Island, where a passing upper-level trough 
may trigger an afternoon thunderstorm over leeward and interior 
slopes. Showers have greatly diminished on Kauai, where a drier 
air mass is moving in from the north.

Trade winds will hold at locally breezy strength during the next
few days as the band of moisture creeps southward. Expect wet
conditions for windward Maui and the Big Island tonight, where the
passing upper-level trough could trigger a few heavy showers along
with winter weather on the high Big Island summits. Reduced 
shower activity is due on Oahu and Kauai. As the moisture band 
sags southward on Monday, showers will linger on windward Big 
Island, while a somewhat dry and stable air mass settles over the 
rest of the island chain. Dew points will fall into the mid to 
upper 50s, and precipitable water will drop below an inch. As a 
result, there will be a cool feel to the breezy trades, and modest
showers will become confined to windward terrain.

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 through Friday. The islands should remain just 
far enough away from the trough to produce a stable trade wind 
flow. Shallow bands of moisture will ride in on the trade winds
and produce showers over mainly windward areas. 

&&

.AVIATION...
Shower coverage has increased across the central islands due to a
band of clouds that has moved into the area overnight. This band
combined with breezy northeast trade winds is focusing most of the
showers over windward and mountain locations, with some making it
into leeward sections. MVFR CIGS/VSBYS can be expected in and
around these passing showers, especially through the morning 
hours. Expect this general trend to continue tonight through 
Monday morning. 

AIRMET SIERRA for mountain obscuration remains in effect for 
windward sections of Maui, Molokai, and Oahu above 2000 ft due to
clouds and showers. 

AIRMET TANGO will continue for moderate turbulence below 9000 ft
for Maui and the Big Island and may need to be expanded to
include the western end of the state later today.

&&

.MARINE...
Large high pressure north of the state is producing a tight 
enough southern pressure gradient down across the local waters to  
maintain moderate to fresh trade winds. A Small Craft Advisory 
(SCA) is now in effect through 6 PM HST Tuesday to account for 
locally strong trades across those typically windier areas around 
Maui and the Big Island. The very subtle strengthening and
southern movement of the northern Pacific high will likely 
produce more areawide fresh trades later today into tonight. 
The combination of this boost in trades and an incoming higher 
northeast swell will result in elevated seas early in the week. 
Thus, to account for stronger trades and resultant rough seas, the
ongoing SCA will likely need to be expanded westward to encompass
more Hawaiian coastal waters. Mid week trades will begin to 
exhibit minor weakening with agitated seas lowering to under SCA 
heights from Thursday into next weekend.

Recent ASCAT satellite passes have confirmed that a surface low 
about 1,500 nautical miles northeast of the islands is producing 
near gale to gale winds along its northern flank. Gales around the
northwest to west periphery of this low as it moves northeast 
away from the area is generating a fetch region of higher swell 
that will be directed toward the state. Thus, the current 4 to 5 
foot, low period north swell will gradually veer more northeast 
and build to greater than 7 feet the next couple days. North and 
east facing shore surf will respond with a steady rise with east 
facing surf likely reaching High Surf Advisory (HSA) thresholds by
late Monday. East coast surf should then hold at HSA levels through
Thursday with a slow decline expected from Friday into the weekend.
A series of Southern Hemispheric gales will push several south 
swells towards the islands through the week. These small, long 
period swells will reach the islands through mid to late week and 
maintain near seasonal average surf along southern coastlines.

&&

.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...Wroe
AVIATION...Gibbs
MARINE...Blood