Current Conditions
Temp-0.0 C
RH63 %
WindW 13 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 220633

Area Forecast Discussion
National Weather Service Honolulu HI
833 PM HST Wed Mar 21 2018

Cloudy conditions with breezy trades will continue through Friday.
Showers will focus over windward areas with highest chances across
the western portion of the state over the next couple of days. An
upper trough will approach from the west and bring another wet
period along with a few thunderstorms across the state late Friday
through Saturday night. Improving conditions will be possible Sunday
into early next week.


Cloudy and breezy conditions continue across the state this evening
due to strong high pressure to the north and an upper trough
positioned several hundred miles west of Kauai near Midway. Water
vapor imagery showed plenty of middle- to upper-level moisture being
drawn northward east of this upper trough around a mid-level ridge,
which supports the cloudy conditions in place. Although most of the
shower coverage expected will continue to focus over the typical
windward and mountain locations tonight, heavier showers over
Kauai and Niihau will remain possible as a weak pulse of energy
moves through aloft around the mid-level ridge.

Guidance remains in decent agreement through the short-term period
(tonight through Thursday night) and appears to be initializing well
with the current trends and patterns. Consensus supports breezy
trades holding as strong high pressure far north of the area shifts
southeastward across the northeast Pacific. The aforementioned upper
trough positioned to the west is forecast to begin shifting eastward
toward the region Thursday. This upper pattern will continue to
support cloudy skies. Highest rainfall chances will remain over the
western end of the state where the best source of deeper moisture
will reside. Elsewhere, showers will remain focused mostly over
windward and mountain areas with a few passing showers leeward

Conditions will begin to evolve Friday into the upcoming weekend as
a shortwave trough digs southeastward into the area bringing a
threat for heavy rainfall and even a few storms. Despite the model
spread between various solutions that has been shown in each cycle,
most continue to depict weak low pressure developing along an
approaching trough in the vicinity of the islands Saturday with an
abundance of deep moisture being drawn northward (2" precipitable
water values). A combination of falling upper heights, 0-6 km
bulk shear (40-50 kt) profiles, lift, and deep tropical moisture
will lead to a wet weekend with a few thunderstorms, especially
late Friday through Saturday. The one limiting factor appears to
be the instability (high shear/low instability event), which may
end up being out of phase with where and when the heaviest
activity ends up developing. The best chance for the heaviest
rainfall and storms will remain over the western end of the state
late Friday into Friday night, then shifting eastward to Maui
County and the Big Island Saturday afternoon through Saturday

Sunday through early next week, improving conditions are expected as
drier air fills in and the upper trough and surface low lift
northeastward and away from the area. Guidance depicts light west
winds, potentially giving way to overnight land breezes and
afternoon sea breezes by early next week. Forecast confidence,
however, remains low through this extended period provided the
spread between solutions shown.


Surface high to the north will gradually move SSE and slowly
weaken the E winds, but these are still strong enough to justify
AIRMET TANGO for low-level turbulence to the lee of the higher
terrain through the night. Clouds and a few light showers continue
to create MVFR conditions over N and E sections of all islands,
so AIRMET SIERRA is still in place for mountain obscuration. This
should continue overnight and into Thursday.


The large NE swell has peaked, but remains large enough to keep
the High Surf Warning going for both north and east facing shores.
The warning is likely to be lowered for the north facing shores
early Thursday morning, but continue for the east facing sector
due to the higher threshold of 15 feet or higher there. Surf is
expected to fall below the warning threshold of 15 feet late
Thursday afternoon. This surf event is of special concern to
east-facing shores which do not typically receive waves of this
size. Strong surges are also forecast for harbors exposed the

No other significant swells are expected through the rest of the
forecast period. The fading NE swell will be reinforced by a
small to medium pulse of 3 feet from the NNE between Saturday
night and Monday. This swell will be followed by a small to medium
NW swell between Sunday night and Tuesday. Surf will be well
below advisory level. South- facing shores can expect a slight
rise in the surf this weekend as a small southwesterly swell

An approaching front from the west will be in range Thursday to
where the trade wind starts diminishing. And with the dying NE
swell, expect a gradual to rapid reduction of the SCA beginning
Thursday night. But in the mean time, the SCA remains in effect
for all Hawaiian waters due to the breezy trades and or above
normal seas.

A thunderstorm or two will accompany the front as it makes it way
toward and down the island chain during the weekend.


High Surf Warning until 6 PM HST Thursday for north and east
facing shores of Niihau, Kauai, Oahu, Molokai, Maui and the Big

Small Craft Advisory until 6 PM HST Thursday for all Hawaiian




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