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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 180625

Area Forecast Discussion
National Weather Service Honolulu HI
825 PM HST Sun Nov 17 2019

An unsettled weather pattern will persist across the island chain
through Tuesday, with locally heavy rainfall and thunderstorms
possible statewide. The airmass will stabilize a bit Tuesday night
and Wednesday, but a showery wet trade wind pattern is expected
to hold into next weekend. Breezy to windy conditions could
develop late next week into next weekend.


Currently at the surface, a 1026 mb high is centered around 1000
miles north of Kauai, with a weak north-south oriented trough of
low pressure over the eastern end of the state. Moderate to breezy
trade winds have developed across the island chain this evening.
Infrared satellite imagery shows partly to mostly cloudy skies
across the state, with cloud cover most prevalent over the eastern
and western islands. Radar imagery shows scattered showers over
windward sections of Kauai and the Big Island, with isolated
showers or rain free conditions elsewhere. Water Vapor imagery
shows a closed upper level low spinning around 175 miles north of
Oahu, and it is this feature that will keep unsettled conditions
in place through the first half of the work week. Main short term
concern revolves around the potential for heavy rainfall and
thunderstorms over the next couple days.

High pressure will build eastward well to the north of the
islands tonight, keeping moderate to locally breezy conditions
in place across the state. A slight easing of the trades is
expected Monday, with a further easing into the light to moderate
range expected Monday night through Tuesday night as a new front
approaches from the northwest. Models show the trades should then
strengthen again Wednesday and Wednesday night, with breezy to
windy conditions expected Thursday into next weekend.

The moist unstable airmass will affect us through at least
Tuesday as the upper level low moves slowly southwestward and over
the western islands. Unsettled conditions, including the
possibility for heavy rainfall and thunderstorms, will continue
through at least Tuesday and a Flash Flood Watch remains in effect
for the entire state through Monday afternoon. This Flash Flood
Watch may need to be extended into Tuesday for all or portions of
the island chain. Models indicate that the upper low will open
into a trough Tuesday night, then linger over the state through
the remainder of the week. As a result, we should see conditions
stabilize a touch beginning Tuesday night, but a showery wet trade
wind pattern will likely continue into next weekend.


The potential for heavy showers and a few thunderstorms will
remain in the forecast through Monday due to an upper low
drifting southward into the area. Although clouds and showers
will be most active over north and northeast facing slopes with
the moderate to breezy trades in place, some will manage to spill
over into leeward areas periodically. Trades will gradually trend
down late Monday into Monday night. MVFR conditions are possible
in and around showers. AIRMET Sierra for mountain obscuration is
now in effect over the Big Island and Maui.


The current northwest swell will continue to subside and the High
Surf Advisory (HSA) for most north facing shores of the smaller
islands should be cancelled later tonight. A small north-northwest
swell is expected to fill in late Monday and peak Tuesday well
below HSA levels. A larger northwest swell will begin to build on
Wednesday and will likely reach the HSA criteria along most north
and west facing shores of the smaller islands from late Wednesday
through Thursday.

The remnants of a dissipating front continue to linger over the
western part of the state, while a surface trough near the Big
Island is moving slowly west. The threat of thunderstorms will
continue to exist over the next day or two across the coastal and
offshore waters due to an upper level low developing over the

The current Small Craft Advisory (SCA) for the typical windy zones
around Maui County and the Big Island should be cancelled sometime
on Monday as winds drop below SCA speeds. Winds will ramp back up
to SCA levels towards the middle of the week as high pressure
strengthens north of the area. Winds may reach gale force by the
end of the week across the typical windy zones of Maui County and
the Big Island, with SCA level winds expected elsewhere.

Surf will remain small along east facing shores through the middle
of the week. The surf will increase along east facing shores
during the second half of the week as the trades become more
robust. Expect choppy surf to increase along east facing shores
with surf heights possibly nearing advisory levels this coming
weekend. Minimal background swells are expected along south facing
shores through most of the week with a few tiny southwest swells
possible near the middle of the week.


Flash Flood Watch through Monday afternoon for all Hawaii

High Surf Advisory until 6 AM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Molokai-Maui
Windward West-Maui Central Valley-Windward Haleakala.

Small Craft Advisory until 6 PM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.




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