925 
FXHW60 PHFO 131329
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
329 AM HST Mon Sep 13 2021

.SYNOPSIS...
A high pressure ridge north of the islands will keep moderate
trade winds in the forecast today. An upper level low northeast 
of the islands will drift slowly west through the week weakening 
the trade winds from Tuesday through early Thursday morning. 
Localized sea breezes will develop on Tuesday and Wednesday along 
the western slopes of each island. Instability associated with the
upper low will keep periods of showers in the weather forecast 
for all islands through Thursday. Drying trends return from Friday
onward as trade winds strengthen across the region.

&&

.DISCUSSION...
Looking at the satellite loops this morning we see bands of
cumulus clouds drifting into each island on the trade winds. An
unstable upper low, roughly 600 miles northeast of the state, is
drifting slowly westward. Numerous thunderstorms are developing 
within the cold core of the low as it moves closer to the state. 
Local radar shows scattered showers drifting through the area with
periods of numerous shower activity over the windward and mountain
slopes of each island.

A weak high pressure ridge north of the islands will continue to 
produce moderate trade winds today. This ridge will weaken a bit 
with light to moderate trade winds forecast from Tuesday into 
Thursday as the upper level low passes just north of the island 
chain. Lighter large scale winds will allow a sea breeze pattern 
to set up along the terrain sheltered western slopes of each 
island from Tuesday to Wednesday. Stronger trade winds will 
return after the low passes by the islands from Thursday afternoon 
through next weekend.

Expect continued periods of enhanced showers through Thursday as
a weakened ridge north of the state and the approaching upper 
level low will increase the trade wind inversion heights into the
7,000 to 10,000 foot range. The higher rainfall amount totals will
favor windward and mountain areas with some of the stronger 
showers pushing into typically drier leeward areas, especially in 
the overnight to early morning time periods. 

On Wednesday, forecast guidance continues to show the upper low 
passing just north of the state with increasing potential for 
heavy showers and isolated thunderstorms over the eastern islands 
from Wednesday into Thursday. Much of these weather impacts will
depend on the strength and track of the approaching upper low.

By Thursday, the low will pass to the west of the islands and we
will begin the transition to stronger trade winds. Drying trends
are forecast from Friday into the weekend as a high pressure 
system builds far to the north of the Hawaiian Islands. Shower
coverage was lowered a bit from Friday onward as the latest 
forecast guidance shows more stable conditions, with lower trade 
wind inversion heights, and stronger atmospheric subsidence.

&&

.AVIATION...
High pressure northeast of the the state will allow for moderate
trades to persist across the island chain this morning and into 
early afternoon. Scattered shower activity will be mainly  
confined to the windward coasts and slopes though a few showers 
may pass over on leeward areas. Expect brief periods of MVFR 
ceilings and visibility associated with this activity. 

There are currently no AIRMETs in effect. 

&&

.MARINE...
High pressure north of the state will keep moderate to fresh 
trade winds in place through Tuesday, with winds flirting with 
Small Craft Advisory (SCA) levels across the typically windy 
waters around Maui and the Big Island. The trades will briefly 
ease on Wednesday as a surface trough slides by to the north of 
the islands. As this trough exits to the northwest of the state 
Wednesday night and Thursday, the trades will gradually ramp back
up to locally strong levels, with winds reaching SCA thresholds 
over the typically windy Maui and Big Island waters by Thursday. 
The strong SCA level trades are expected to expand to most if not 
all waters Friday and continue into the weekend.

A series of long period south and southwest swells will keep surf
near the September average through Thursday, before dropping
to slightly below normal levels Friday through the weekend. Small
surf will prevail on north facing shores through most of the 
week, with a small bump possible this weekend. East shore surf 
will remain well below normal through Wednesday. As trade winds 
ramp up Thursday through the weekend, surf along east facing 
shores will rise back to seasonal levels and will be quite rough 
Friday through the weekend.

&&

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

&&

$$

DISCUSSION...Bohlin
AVIATION...JT
MARINE...Jelsema