511 
FXHW60 PHFO 131956
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
956 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-southwest through the week,
weakening the trade winds from Tuesday through early Thursday 
morning. Localized sea breezes will develop on Tuesday and 
Wednesday and could lead to afternoon showers over leeward and 
mountain areas. Instability associated with the upper low could 
lead to some heavy showers at times. Drier weather should return 
from Friday onward as the trade winds strengthen across the 
region.

&&

.DISCUSSION...
A ridge of high pressure far north of the state will maintain
moderate trade winds and fair weather across the state today.
Latest satellite imagery shows an upper level low located roughly
600 miles northeast of the state. This upper low is expected to 
gradually move west-southwest and will be our main weather 
influence during the next several days. 

At the surface we have a trough that is associated with the upper
level low and as this trough approaches tonight into Tuesday, 
showers will be on the increase. Showers will be primarily focused
over windward and mauka areas with leeward showers from time to 
time. Trade winds will also be on the decrease during this time 
mainly over Maui County and the Big Island. Localized sea breezes 
are expected over leeward areas of Maui and the Big Island by 
Tuesday afternoon and we could see scattered showers developing 
over the leeward slopes of these islands. For the western half of 
the state, moderate northeasterly trade winds will persist with 
scattered showers. 

On Wednesday and Wednesday night, the trough is expected to be 
passing just north of the state and will lead to lighter winds 
with localized sea breezes across the state. The low level winds
will also begin to veer toward the southeast and that will allow
for more moisture to move in across the state. Pockets of heavy
showers will be possible especially over the Big Island and the
leeward and mountain areas of the smaller islands Wednesday
afternoon through the evening. 

Thursday will be a transition day as the trough begins to move 
away from the state and high pressure builds north of the state. 
Showery conditions are still expected as the breezy trade winds 
fill back in starting from the eastern end of the state. By 
Friday, strong trade winds will return across the state with drier
conditions.


&&

.AVIATION...
Moderate to locally breezy trade winds will carry bands of clouds
and light showers towards the Hawaiian Islands. Mostly dry
conditions are expected during the day with increasing showers
overnight. Brief MVFR CIG or VIS is possible in passing showers,
otherwise VFR conditions will prevail statewide.

Meanwhile, leeward sea breezes will drive cumulus development 
along the south and west facing slopes of the Big Island this 
afternoon and early evening. The air mass remains moderately dry
and moderately stable, so little precipitation is expected in 
these areas.

No AIRMETs are currently in effect or anticipated at this time. 



&&

.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...Kino
AVIATION...Bedal
MARINE...Foster