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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 011958
Area Forecast Discussion
National Weather Service Honolulu HI
958 AM HST Thu Oct 1 2020
Winds will remain light through Saturday as a weak trough of low
pressures passes from east to west across the area. Scattered
showers and isolated thunderstorms are expected over the Big
Island with a few interior and mauka showers expected elsewhere. A
more typical trade wind weather pattern will return on Sunday and
linger for a few days. Clouds and passing showers will tend to
favor windward and mauka areas. Winds will become light once again
around Wednesday as a front approaches from the northwest.
A north thru south oriented low level trough along with its high
precipitable water values is seen moving into the east side of
the Big Island. Radar imagery is showing scattered showers and
thunderstorms falling over portions of the Hilo and Puna
districts. This feature along with daytime heating should allow
for more widespread coverage of showers and thunderstorms this
afternoon and evening. The remainder of the state will likely see
some afternoon clouds build up with some showers possible mainly
over interior locations.
The low level trough is forecast to move slowly westward over the
next couple of days. Winds will be light through Saturday with
daytime seabreezes and nighttime land breezes dominating. This
should allow for modest clearing overnight with clouds and showers
expected to develop during the day Friday. Abundant moisture
along with daytime heating should allow for a slight chance of
thunderstorms once again mainly over the leeward Big Island
slopes on Friday. The trough and its associated moisture field
will be west of the area by Saturday with a drier weather pattern
High pressure will be building in back to our north on Sunday.
This will allow for light to moderate trade winds to make a return
but some sheltered locations will continue to experience daytime
sea breezes and nighttime land breezes. Passing light showers are
expected to favor windward and mauka areas.
By Wednesday, a frontal system will be approaching the area from
the northwest. This will once again cause our winds to drop off.
The airmass is expected to remain dry so just a few showers are
expected mainly over interior areas during the afternoon hours.
A weak surface trough currently sits in the vicinity of Maui. It
is noted as a patch of low clouds lingering around Maui, with a
patch of low clouds lingering there. Another, stronger surface
trough is closing in on the Big Island from the east and southeast.
An area of convection is currently over the Puna and Hilo districts,
with some heavy showers and a few thunderstorms.
These two troughs are forecast to merge this afternoon, spreading
the showers westward in the process. A slight chance of thunderstorms
remains for the Big Island today, including the adjacent waters.
There is a good possibility of AIRMET Sierra for the east facing
slopes of the Big Island later this morning and this afternoon. In
the afternoon, scattered showers and a thunderstorm or two, as
well as showers expanding westward to Maui could require expansion
of AIRMET Sierra to those areas as well. Expect scattered showers
to spread to Oahu later this evening.
Otherwise, generally light winds will allow for development of
afternoon sea breezes (interior clouds) and nighttime land breezes
(clearing inland), especially over the western end of the state.
The passage of a northern Pacific cold front today will maintain
currently light to gentle easterlies through Friday. Late week
high pressure building in northeast of the islands will generate a
tight enough pressure gradient back across the state to produce
moderate trade flow. Greater areal coverage of moderate trades
will persist through early next week with locally fresh trades
possible through those traditionally windier channels and bays
surrounding Maui County and waters south of the Big Island this
The eastern approach and western passage of a surface trough
through tomorrow will keep rain chances and thunderstorm
probabilities higher over over those waters to the south and
southwest of the smaller islands and waters surrounding the Big
Island Friday through Saturday.
The current north to northwest swell will slightly drop off
today with a larger north to northwest swell arriving tomorrow.
This next swell is expected to push surf to (or near) advisory
levels tomorrow along the northern shores of the smaller islands.
Swell direction looks to be just north enough for island
shadowing to keep most west-facing shores below advisory levels.
The exception would be Kauai where west facing shores will be more
exposed to this north swell. Surf will gradually lower Friday
night through early next week.
Surf along south-facing shores will trend lower through the rest
of the week. A late season south swell is expected early next
week that will likely boost surf above seasonal averages Monday
through the middle of next week.
Surf along east-facing shores will be primarily driven from wrap
around associated with a series of north to northwest swells
moving through the islands this weekend. East shore surf may
substantially increase Sunday night through early next week as a
long period east swell associated with eastern Pacific tropical
cyclone Maria moves through. This could result in surf reaching
advisory levels along most east-facing shores Monday through the
middle of next week.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office