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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 181334
Area Forecast Discussion
National Weather Service Honolulu HI
334 AM HST Thu Oct 18 2018
Light to moderate trades will continue through the remainder of
the work week. The trades will then strengthen to moderate levels
over the weekend and possibly reach locally breezy levels next
week as high pressure builds north of the islands. Clouds and
showers associated with and old front will linger near the Big
Island and Maui through the weekend, keeping a showery trade wind
pattern in place. Meanwhile, drier conditions should prevail
across the western half of the state with mainly windward and
mauka showers through Saturday, with a more showery trade wind
pattern expected Sunday through early next week. A disturbance
aloft will bring unstable conditions and the potential for heavy
rain and thunderstorms to Maui and the Big Island Friday night
through Sunday as well, which could continue into early next week.
Currently at the surface, a weak ridge of high pressure is
centered around 500 miles north of Honolulu, and is driving light
to moderate trade winds across the island chain early this
morning. Meanwhile, a weakening cold front is located around 450
miles northwest of Kauai. Aloft, an upper level trough is evident
in water vapor imagery over Kauai, with a sharper and more potent
shortwave trough digging southward around 400 miles north-
northwest of Kauai. Infrared satellite imagery shows partly to
mostly cloudy skies across the state, with cloud cover most
prevalent over windward sections of Oahu eastward to the Big
Island. Radar imagery shows scattered showers moving into windward
areas, with shower coverage the highest across Maui County and
the Big Island. A few showers are also making the trip over the
terrain into leeward areas of the smaller islands, with a few
showers near the coast of leeward Big Island as well. Main short
term concern revolves around the potential for some locally heavy
rainfall and thunderstorms associated with a digging upper level
trough over the weekend.
The ridge of high pressure north of the state will continue to
erode over the next couple of days as a cold front passes by well
to the north of the island chain. Light to moderate trades are
expected to prevail today, then ease slightly tonight through
Friday night with local land and sea breezes possible in some of
the more sheltered areas. A ridge of high pressure will then
become re-established north of the state over the weekend,
bringing a return of moderate trade winds, with the trades
strengthening to moderate and locally breezy levels early next
As for sensible weather details, a plume of deep moisture
associated with the remnants of an old front, will linger over the
Big Island through the weekend and possibly into early next week.
In the near term through Friday, showery trade wind weather is
expected to persist over the Big Island, with drier trade wind
weather expected across the smaller islands. Clouds and showers
will favor windward and mauka areas, but we should see some
interior and leeward shower development as a result of the
weakening trades, particularly on Friday. Additionally, with the
airmass remaining unstable over the Big Island, will keep a
slight chance of thunderstorms in the forecast each afternoon.
The models are advertising quite a different scenario than they
were 24 hours ago during the weekend and into early next week.
Both the GFS and ECMWF now show a sharp upper trough digging
southward over the western islands Friday night, then closing off
over the central and eastern islands Saturday through Sunday. The
models to differing degrees, then show the weakness aloft
lingering over on in the vicinity of the island chain through the
first half of next week.
Deep moisture associated with the frontal remnants is expected to
linger over the eastern end of the state through the middle of
next week, so this should keep a showery trade wind pattern in
place here. Across the western islands the moisture won't be quite
as deep through Saturday night, so expect the shower coverage to
not be as great here at least initially. This will likely change
on Sunday as the deeper moisture begins to lift back across
western islands, making for a more showery trade wind pattern
which could continue through the middle of next week.
The upper trough/low overhead could also create a fairly unstable
airmass Friday night through Sunday, particularly across Maui
County and the Big Island. The main hazards would likely be heavy
rainfall and thunderstorms. We'll need to monitor model trends
closely over the next couple days however, as there is the
potential that we could see some severe weather particularly
Saturday afternoon and evening as the coldest air aloft rotates
overhead. At this time the main severe weather threats, if they
materialize, appear to be strong gusty winds and hail.
Light to occasionally moderate trade winds will continue across
the island chain through the day. Brief to TEMPO MVFR conditions
will mainly affect windward and mauka sections over all isles.
This will especially be the case over the Big Island, and, to a
lesser extent, Maui and Molokai. Low level remnant moisture from
an old front will affect those isles. Elsewhere VFR will hold
An AIRMET for mountain obscuration remains in effect for the
windward areas of the Big Island.
Trade winds are forecast to remain weak through Friday night.
The trades may gradually strengthen from Saturday into early next
week. The current forecast does not have Small Craft Advisory
(SCA) conditions over any of the coastal waters or channels
through Sunday night. SCA conditions may develop across the
typically windy waters adjacent to the islands of Maui County and
the Big Island early next week.
Unstable atmospheric conditions are forecast to develop over
parts of the state toward the end of the work week. This may
cause locally heavy downpours, and allow thunderstorms to form
across some areas from Friday night into this weekend. Mariners
should monitor future weather forecasts as this situation evolves.
The current north-northwest swell is gradually declining early
this morning. Therefore, the High Surf Advisory (HSA) has been
cancelled. The north-northwest swell will continue to slowly
subside through this weekend. The weak trades will cause surf to
slowly decrease along most east facing shores. However, some east
facing shores with a northerly exposure may continue to see small
breakers from the north-northwest swell energy during the next
couple of days.
A new long-period south swell arriving Friday, is expected to
peak this weekend. Based on the latest guidance, surf will likely
reach the HSA criteria along south facing shores starting Saturday.
There is also a possibility surf may approach the High Surf
Warning threshold along south facing shores this weekend, before
the swell begins to gradually decline early next week.
See the latest Collaborative Nearshore Swell and Wind Forecast for
Oahu (SRDHFO), which was updated earlier Wednesday afternoon, for
more details on swells and surf.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office