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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 172005
Area Forecast Discussion
National Weather Service Honolulu HI
1005 AM HST Wed Oct 17 2018
Moderate trades will slowly weaken through Friday, then strengthen
again on Saturday. Remnants of a front will linger near the Big
Island for the next few days. A trough aloft will bring somewhat
unstable conditions around the Big Island before moving away
Saturday night. Drier weather will persist for the western half
of the state this week.
A weak surface ridge is centered around 600 miles north of Kauai
this morning driving gentle to moderate trade winds across the
island chain. Visible satellite shows a remnant frontal band lined
upstream of the Big Island and Maui with trade winds carrying the
clouds and showers over windward sections of these islands. The
northern end of the shower band is also impacting windward Molokai
and Oahu. Rainfall amounts and intensity has been light.
Meanwhile, leeward sides of the islands are seeing mostly sunny
skies this morning.
Water vapor imagery shows a trough aloft over the western portion
of the state. 12Z soundings show weak and elevated midlevel
inversions at Hilo and Lihue. The upper trough will remain nearly
in place through Saturday and will act to create more enhanced
showers, particularly near the Big Island where moisture levels
will continue to be a bit higher from the remnant front. There is
also a slight chance of thunderstorms over the next several
afternoons on the Big Island. Elsewhere, expect similar weather
to continue, with windward and mauka clouds and a few passing
A front will pass by to the north of the island chain Thursday
through Friday, weakening the high pressure ridge, and easing the
trades slightly. Light trades should prevail across the entire
island chain by Friday, with some of the more sheltered areas
possibly seeing land and sea breezes develop. High pressure will
become better established to the northwest of the state over the
weekend, bringing a return of moderate trade winds to the island
chain that will last through early next week. The upper trough is
also forecast to move away to the east Saturday night.
Light to locally brisk trade wind flow continues. Bands of showers
embedded in the the flow will continue to impact mainly north and
east facing slopes and coasts while leeward coasts and adjacent
waters experience mostly clear skies.
A thick band of low clouds continues to shroud the Windward coasts
the Big Island and Maui. AIRMET Sierra remains in effect for north
through east facing slopes of both the Big Island and Maui.
Conditions are expected to gradually improve from the west, so
Maui will likely be removed from Sierra later today.
Mostly clear skies over Leeward Big Island this morning will give
way to sea-breeze driven cumulus build ups in the afternoon.
Shower activity should be limited to mountainous areas while the
coast remains excellent VFR.
The airmass over Maui and the Big Island remains moderately
unstable. Slight chance of thunderstorms again over interior Big
Island late this afternoon and early evening.
Over the remainder of the state, the air mass is becoming
increasingly stable. High based stratocumulus tops are not
expected to exceed about 8000 ft west of Molokai. Shower activity
will be light and limited.
A check of the nearshore buoys this morning show the current
north-northwest swell is still at its peak, but is starting to
show signs of declining. The High Surf Advisory (HSA) for north
and some west facing shores continues today, and will be
monitoring the swell decline today to see if surf will indeed
drop below advisory levels by sunset.
The swell continues to produce some seas in excess of 10 feet over
waters exposed to the northwest, but are expected to drop below 10
feet this afternoon. The trade winds are already on the decline,
with most areas below Small Craft Advisory (SCA) levels already.
The current SCA is in effect until 6pm, but may be able to cancel
that a little earlier if the winds and seas continue to subside.
Trade winds will continue to weaken through the end of the week.
After the current SCA event ends, no SCA is anticipated through
The next north-northwest swell is expected to arrive this weekend,
but remain below advisory levels. Surf along south facing shores
will continue to decline today, with a new south swell expected to
arrive Friday, peaking Saturday into Sunday at high end advisory
or low end warning levels.
See the latest collaborative surf discussion (SRDHFO) for more
High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala-Big Island North and East.
Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi
Channel-Maui County Windward Waters-Alenuihaha Channel-Big
Island Windward Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office