149 
FXHW60 PHFO 040126
AFDHFO

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

.SYNOPSIS...
Moderate trade winds will gradually strengthen through Sunday as 
high pressure builds far north of the state. An upper level trough
passing overhead will produce some high clouds and will enhance 
rainfall in the trade wind flow this weekend, especially over the 
eastern half of the island chain late tonight and Sunday. Trade 
winds will decrease on Wednesday and remain at moderate strength 
into next weekend, leading to typical trade wind weather.

&&

.DISCUSSION...
Trade winds are gradually strengthening as showers have become
more active today. High pressure centered about 1,500 miles north
of Kauai will build and creep eastward through tomorrow, leading 
to a further increase in local trades. The trade winds are more 
northeast than usual, thanks to the presence of a slow-moving 
cutoff low sitting about 1,200 miles northeast of the state. A 
shallow band of moisture, the degraded tail end of a front 
associated with the low, is caught up in the trade wind flow and 
is providing moisture for the slightly active shower pattern. An 
upper level trough passing overhead will slowly destabilize the 
atmosphere, enhancing windward rainfall and allowing showers to 
periodically pass over leeward areas through Sunday. The upper 
level trough will generate thin high clouds into tonight and may 
even trigger an isolated thunderstorm over portions of the Big 
Island on Sunday, along with some winter weather on the high 
summits above 12,000 ft.

A drier and increasingly stable air mass will set up over the
state on Monday, leading to a decrease in shower activity. Trade 
winds will remain rather breezy through at least Tuesday as high 
pressure holds to the north, and a building ridge aloft will lead 
to stable conditions along as precipitable water decreases. This 
should translate into modest showers being focused along windward 
slopes and 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...
VFR conditions prevail at all TAF sites with brief periods of 
MVFR conditions occurring at PHTO. Moisture from an old front will
interact with the island terrain overnight with extended periods 
of MVFR conditions possible especially for PHTO and PHLI. 

No AIRMETs are currently in effect. An upper level trough passing
just northeast of the state along with increasing moisture from 
the old front, is expected to enhance windward rainfall tonight 
and Sunday morning leading to AIRMET Sierra for mountain 
obscuration. Strengthening trade winds tonight and Sunday may 
require the issuance of AIRMET Tango for low level turbulence.

&&

.MARINE...
Large high pressure north of the state is creating a tight enough
southern pressure gradient down across the local waters to produce
moderate to fresh northeast trade winds. Locally strong trades 
will warrant a Small Craft Advisory (SCA) for the Pailolo and 
Alenuihaha Channels, Maalaea Bay and the Big Island's leeward and 
southeastern waters. The SCA is in effect through 6 AM HST Monday.
A slight strengthening and subtle southern movement of the northern
Pacific high will maintain or slightly strengthen trades to more 
areawide moderate to locally strong magnitudes across the bays and
inter-island channels. Thus, the ongoing SCA will likely need to 
be expanded westward to encompass more Hawaiian coastal water zones
to account for stronger trades and the resultant rough seas. Trades
will begin to exhibit minor weakening with agitated seas lowering
to under SCA heights from Thursday into next weekend.

This morning's ASCAT satellite pass confirmed that the deepening 
surface low 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 will generate a fetch region that will promote higher swell. 
This moderate, relatively longer period northeast swell will be 
directed towards the state and will reach the islands on Monday, 
peaking surf along north and northeastern shorelines Tuesday with 
a slow fall through late week. The combination of this swell and 
the boost in trade winds will result in elevated seas that will 
exceed the 10 foot SCA criteria into mid week. East facing shores 
will experience greater than 8 foot breakers that may prompt an 
early week High Surf Advisory (HSA) for many eastern exposures. 
Due to the northerly component of this swell, surf along north 
facing shores will also pick up to more moderate 7 to 11 foot 
heights. North surf is expected to remain under the 15 foot HSA 
criteria. A small, long period south swell will maintain near 
seasonable surf along the southern shores.

&&

.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...Wroe
AVIATION...Burke
MARINE...Blood