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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 150621

Area Forecast Discussion
National Weather Service Honolulu HI
821 PM HST Sun Jul 14 2024

High pressure north of the Hawaiian Islands will hold through
early Monday, maintaining a dry trade wind pattern. Light windward
and mauka showers will primarily occur during overnight through
early morning hours each day. A mid to upper level disturbance
developing northeast of the state this week will weaken trades
from Tuesday onward.


Updated the forecast this evening to reflect drier conditions
through Monday night for the smaller islands. This is largely
based on the lack of rainfall over the last day or so. Many of the
models, both high resolution and global, show higher PoPs than
what we have been experiencing. The updated forecast leans heavily
on the HRRR model.

Otherwise, the forecast philosophy remains the same. A broad high
pressure system remain about 800 miles north of the state,
continuing to funnel moderate to locally breezy trades to the
region. An upper level ridge over the islands is helping to
maintain a dry and stable airmass over head, thus the update
mentioned above to dry out the forecast. Satellite continues to
show stable stratocumulus clouds riding in on the trades. Expect
this pattern to hold through Monday night.

A trough developing into a front to the far northeast of the
islands will help to disrupt the broad ridge to the north Monday
and Tuesday. Around the same time, an upper level trough
develops to the northeast, with a possible mid to upper level low
developing midweek. These features will help to weaken the trade
wind flow over the islands Tuesday onwards. The mid to upper level
low looks to remain well enough east of the area that the upper
level ridge remains anchored over the islands. This should
continue the stable and mostly dry airmass through the week.

Towards the end of the week, the upper level trough may approach
the islands, which could provide a little more instability leading
to a modest increase in showers, particularly over the eastern
end of the state which would be closest to the upper level trough.


Breezy to locally windy trades will continue through Monday
before easing slightly into mid week. Mainly light showers and
periodic low ceilings will favor windward and mauka locations with
a few showers making it to leeward locations. Some brief MVFR
conditions are possible in passing showers, otherwise VFR
conditions will prevail.

AIRMET Sierra may be needed with low ceilings and a slight increase
in showers expected tonight into the morning hours on Monday.

AIRMET Tango for moderate low-level turbulence lee of terrain is
in effect and will likely be needed through Monday due to the
strong trades under a strong inversion.


Surface high pressure north of the islands between 30N and 40N
will maintain fresh to locally strong trade winds the next couple
of days. A Small Craft Advisory (SCA) remains in effect for the
typically windier waters surrounding Big Island and Maui County
through Tuesday afternoon. Trades will drop off to more moderate
speeds, or fall below SCA criteria, Wednesday through Friday in
response to lower pressure northeast of the state.

Offshore NOAA buoy data from the past several days (i.e., the
PacIOOS Aunuu buoy off of American Samoa) has timed the island's
latest south swell moving around the chain through Monday...
peaking tonight through Monday morning. This swell will produce
frequent sets that exceed the 10 foot High Surf Advisory (HSA)
thresholds for southern shores. Nearshore southern buoys are
displaying a 4 foot, 16 to 18 second period rising swell with web
cams confirming HSA level surf this evening. The HSA currently
runs through Monday afternoon, but it may need to be extended
through Monday night if the swell holds larger than anticipated.
South shore surf will fall below the advisory levels by Tuesday,
then gradually decline through Friday.

Rough east-facing shore surf will remain slightly above July
averages into Monday. This short period wind wave energy will
slowly decline as mid week trades upstream of, and eventually
over, the islands weaken. East surf will fall below seasonal
averages Wednesday and will fade further into Friday. No
significant source for northerly swell is expected for the
remainder of the week.


High Surf Advisory until 6 PM HST Monday for Niihau-Kauai
Leeward-Waianae Coast-Kahoolawe-Maui Leeward West-Kona-Kauai
South-East Honolulu-Honolulu Metro-Ewa Plain-Molokai Leeward
South-Lanai Leeward-Lanai South-Maui Central Valley South-South
Maui/Upcountry-South Haleakala-Big Island South-Big Island

Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office