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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 261407

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Fri May 26 2017

Winds will become light and variable in the next few days as a weak
high pressure ridge establishes just north of the state. The light
winds will allow more widespread afternoon sea breezes to develop,
with clouds and showers over the interior and lee areas, while land
breezes will develop at night, bringing some clearing. An upper
level feature passing over the islands could help enhance afternoon
showers over the weekend.


Winds have become rather light overnight across the islands as trade
winds continue to diminish. Not much changes in terms of cloud
coverage across the area since last night, as area of mid-level
clouds continues to affect Kauai, while some low clouds carried by
the weakening trade winds reached mainly the eastern islands at
times, affecting the windward areas. Latest satellite imagery
indicated there are still more low clouds northeast of Maui and the
Big Island, and these low clouds will bring some more showers to the
windward areas of those islands through the rest of the morning.
Elsewhere mainly fair weather will prevail. Morning soundings from
Lihue and Hilo are not much different from Thursday afternoons',
though the inversion looks to be slightly strong early this morning
on Hilo sounding. Meanwhile, Lihue sounding continues to reveal
multiple low to mid-level clouds, with moisture extending to over 14
thousand feet. Hilo sounding also showed a rather dry airmass in its
vicinity. And both soundings also indicated the airmass in the
island vicinity is only somewhat unstable.

As the high pressure far northwest of the islands continue to sag
south and weaken through the weekend, downward trend of the trade
winds will progress as pressure gradient slacks across the region.
With the synoptic winds easing, more widespread afternoon sea
breezes, and overnight land breezes will develop. Clouds will build
over the interior and leeward areas of the islands during the day,
with showers developing. Forecast models continue to indicate low
level moisture in the island vicinity to be moderate at most and
that may tamper the rainfall a little. But then, weakness in the mid
to upper levels over the area is bringing slight unstable conditions
to the islands. Furthermore, a deeper upper level trough is expected
to move in from the northwest Saturday and Sunday, bringing colder
upper level temperature to the area, resulting in more atmospheric
instability to the state. Latest model runs still indicated the
coldest upper level temperatures will stay north and northeast of
the islands. Therefore, showers will be enhanced in the island
vicinity, with locally heavy downpours possible, especially on
Saturday and Sunday. Dewpoint temperatures are also expected to
edge into the upper 60s late Sunday to close to 70 Monday and
Tuesday. Combined with the light winds, rather muggy conditions are
expected for the area early next week. The synoptic wind flow will
also turn more southeast, which will spread vog from the Big Island
to the smaller islands during the first part of the next week.

A slightly stronger high pressure may develop northeast of the state
during the middle of next week, though models have their differences
on how strong this high will become. Looks like trades may return
to the islands later part of next week, though how strong will
remain to be seen.


A high pressure center northwest of Hawaii will continue to produce
light northeasterly trade winds for the next 48 hours with a daytime
onshore sea breeze trend starting on Saturday afternoon over western
slopes of all islands. Mostly VFR conditions expected.

No AIRMETs currently in effect.


Surf along south facing shores will steadily rise through the day
today, peak tonight through Saturday, then gradually ease into next
week. The offshore NDBC buoys south of the islands continue to
reflect increasing long period energy within the 17 to 21 second
bands. Surf should reach advisory levels late by tonight, which will
hold through Saturday. A High Surf Advisory remains in effect for
exposed south facing shores through early Sunday. Please see the
Coastal Hazard Message (CFWHFO) on our website for more information
about the surf. Although the surf will likely dip below advisory
levels by the end of the weekend, this source will be long-lived and
continue into next week before completely fading. A small
long-period south (170-190 deg) is expected to arrive late next
week. Otherwise, no significant sources out of the south are
expected Wednesday through the following weekend.

Trade winds will continue to weaken over the upcoming holiday
weekend as a weakness within the ridge sets up in response to low
pressure that has developed well northeast of the state. Land/sea
breeze conditions are expected each day over the weekend and early
next week, which will allow the seas to trend down locally.

Surf along east facing shores will trend down through the weekend as
the trades relax. A small north-northeast swell, however, will
support small surf continuing at the exposed spots each day through

Models remain persistent and show a small to moderate northwest (330
deg) swell filling in Monday night through midweek due to a low
pressure system tracking east of the Date Line over the weekend and
deepening as it passes north of the state Sunday through Monday.
Otherwise, small surf is expected to persist each day, mainly at the
locations exposed to north-northeast swell.

In addition to the building surf along south facing shores, coastal
flooding impacts associated with record level King Tides will
continue each day along all shores through the Memorial Day weekend,
especially for Maui County and Oahu. Beaches and shorelines, boat
ramps and docks, and vulnerable low-lying roads and structures will
be impacted during these late afternoon and early evening high
tides. Please see the Special Weather Statement (SPSHFO) product on
our website for more information about the potential impacts from
the high tides and incoming south swell.


High Surf Advisory until 6 AM HST Sunday for Niihau-Kauai
Windward-Kauai Leeward-Oahu South Shore-Waianae Coast-Molokai
Leeward-Lanai Makai-Kahoolawe-Maui Leeward West-Windward
Haleakala-Leeward Haleakala-Kona-South Big Island-Big Island North
and East.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman