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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 200125
Area Forecast Discussion
National Weather Service Honolulu HI
325 PM HST Sun Nov 19 2017
Cool, locally breezy northerly winds will diminish slightly
Monday and Tuesday. Low clouds streaming toward the islands will
focus along north facing slopes and coasts, while also dropping a
few showers. Trade winds will return on Wednesday and will become
breezy and gusty for the Thanksgiving holiday weekend, with
windward showers occasionally spreading leeward.
The islands remain under a cool northerly flow produced by a deep
low centered about 900 miles northeast of the state. Daytime highs
were around 80 today, which is a few degrees below normal for
this time of year, and dew points have dipped down into the upper
50s now that a somewhat dry air mass has moved in. However,
instability associated with the upper trough driving the low
maintained a weak band of showers that dropped less than one
quarter of an inch of rainfall across northwest and north facing
slopes of Oahu and Maui through the day. On the Big Island, spotty
showers, a few briefly heavy, flared along northeast and southeast
Another cool night is in store tonight, with shower chances
largely confined to north facing slopes. The cool northerly flow
will push overnight temps down to the low 60s, similar to last
night and about 5 to 10 degrees below normal. Aside from a few
lingering showers across the Big Island during the evening, shower
activity is expected to be modest and confined to northern slopes
of all islands.
The nearby upper level low continues to drive very strong winds
across the high summits of the Big Island. Westerly winds peaked
overnight and have been above 60 mph today. The High Wind Warning
runs through tonight, but northwesterly winds will be at border
line advisory levels for Monday.
The cool northerly flow will persist Monday and Tuesday. The
surface low currently 900 miles northeast of the state will be
replaced by another surface low dropping into the established
longwave trough over the eastern North Pacific. We will likely see
a couple of weak and shallow moisture bands dropping over the
state and delivering light rainfall to north facing slopes, while
leeward areas remain rather dry. Temps will remain a few degrees
below normal, meaning overnight lows in the mid 60s and daytime
highs around 80.
Winds will gradually shift toward a northeasterly trade wind
direction on Wednesday, then potentially become quite strong on
Thanksgiving as the low departs and strong high pressure builds
north of the area. This high may support strong trades into the
holiday weekend. Showers will be focused along windward slopes,
though some may periodically spread leeward.
Light to moderate northerly winds will remain over the island
chain over the next 24 hours, with bands of clouds and showers
moving through from time to time. Some brief MVFR cigs/vsbys will
be possible as the bands of clouds and showers move through, but
predominantly VFR conditions are expected at the terminals through
AIRMET Sierra is in effect for mountain obscuration over northeast
through southeast sections of the Big Island. These conditions
are expected to linger into the evening hours before improving.
AIRMET Tango is in effect for moderate low level turbulence
downwind of the higher terrain of the Big Island. These conditions
will likely remain in place through tonight.
A slow moving surface low, around 800 nm northeast of the
islands, produced a fetch of gale force winds directed at the
state over the past few days. This has translated to the north
(350-010) swell across the nearshore waters today. Thus far, the
swell has remained fairly aligned with model guidance, and looks
to have peaked at the outer NOAA buoys and should peak near shore
in the next few hours. Warning level surf along north facing
shores of Kauai and Niihau should hold through the night, before
transitioning to advisory level surf during the day Monday.
Advisory level surf along the north facing shores of Oahu,
Molokai, Maui, and the Big Island will continue through Monday
afternoon. The swell will then gradually subside through the
beginning part of the week. Additionally, combined seas through
the coastal waters will also exceed 10 feet, and a Small Craft
Advisory (SCA) is posted for all waters except the leeward and
southeast Big Island waters through Monday. Winds over the next
few days will remain below SCA levels.
The aforementioned low will begin to move northeast Monday as a
strong high pressure builds in from the northwest, and the
northerly flow will transition to northeasterly flow. Northeast
trade winds will ramp up late Wednesday into Thursday, increasing
to SCA levels across most waters, and potentially to gale force
through the Alenuihaha Channel. Winds should stay elevated into
the upcoming weekend.
A developing storm force low in the Gulf of Alaska will produce
another round of large surf. The new large north (350-010) swell
is expected to start filling in Wednesday and peak Thursday. The
swell is forecast to be slight larger in magnitude with a slightly
longer period, compared to the current swell, and will likely
bring warning level surf across most islands.
Small long period background south and southwest swells will
continue through the week, keeping small, above flat surf along
the south facing shores.
High Surf Advisory until 6 PM HST Monday for north facing shores
of Oahu, Molokai, Maui and the Big Island.
High Surf Warning until 6 PM HST Monday for north facing shores
of Niihau and Kauai.
High Wind Warning until 6 AM HST Monday for Big Island Summits.
Small Craft Advisory until 6 PM HST Monday for all waters except
Maalaea Bay and areas south and west of the Big Island.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office