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Temp0.6 C
RH20 %
WindESE 8 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 200648

Area Forecast Discussion
National Weather Service Honolulu HI
848 PM HST Sat Aug 19 2017

Light to moderate trade winds will prevail through early next
week as a weakened ridge remains to our north. Expect local sea
breezes along with increased afternoon clouds and showers across
leeward areas. Low clouds carried by the trades will continue to
affect windward and mauka areas. Tropical moisture from the east
will increase shower coverage and keep muggy conditions around
through Tuesday. Instability will introduce a slight chance of
thunderstorms Sunday through Tuesday. Drier and windier conditions
will return Wednesday as the ridge strengthens to our north.


The subtropical ridge far north of the main Hawaiian islands
continues to weaken as upper troughing between 140W and 165W takes
its toll. The ridge is strong enough to drive light to moderate
trade winds across local waters, but we expect trade flow will be
too weak to defeat development of local leeward sea breezes each
afternoon Sunday through Tuesday. Satellite loop shows patchy
broken low clouds embedded within trade flow moving across
windward and mauka areas of the islands, while radar shows
scattered showers across these same areas. Increased low cloud
cover is noted east of about 152W, along the leading edge of an
expected increase in tropical moisture. Afternoon soundings show
either no inversion or one much weaker than at this time
yesterday, another sign of increased troughing aloft. Hilo had
more moisture than Lihue, with 1.32 versus 0.98 inches of PW.

Models show the upper trough over and north of the islands will
dig to the southwest through early next week, keeping the ridge
weak and dragging a pool of cold air over the western half of the
chain. Combined with an expected increase in tropical moisture
from the east, this colder air aloft will introduce a slight
chance of thunderstorms to our forecast from Sunday afternoon
through the day Tuesday. This threat will start with the potential
for thunderstorms across leeward portions of the Big Island Sunday
afternoon, then morph into a broader threat including Kauai
and/or Oahu Sunday night through Monday night. As the pool of cold
air begins to pass west of the islands, the thunderstorm threat
will shrink once again to only leeward portions of the Big island
Tuesday afternoon before vanishing altogether.

Outside of the thunderstorm threat, the general weather pattern
Sunday through Tuesday will consist of windward clouds and
showers merging with afternoon and evening leeward clouds and
showers, thanks to the hybrid light trade/sea breeze setup. By
Wednesday, the upper trough, with its associated cold air pool,
will be far west of the main islands. This will allow the ridge to
our north to rebound and drive stronger trade winds across local
waters. In short, drier and windier conditions are expected
Wednesday through the rest of the forecast period.


VFR conditions will continue at all TAF sites with brief MVFR
conditions expected mainly for the windward sites this evening.
As an area of enhanced moisture from the east reaches the eastern
islands overnight, MVFR conditions will become more widespread
over windward areas. No AIRMETS are currently in effect but one
may be needed overnight for mountain obscuration over eastern
portions of the state.


Winds and seas are expected to remain below Small Craft Advisory
levels through early next week as a trough aloft helps weaken the
pressure gradient. The trough may make showers more active, and
could even produce some thunderstorms Sunday through Tuesday.
Wind speeds will increase again later next week.

Surf along south facing shores will continue to lower overnight
as a south-southeast swell that filled in and peaked late Friday
eases. Looking ahead, mainly small background long-period
reinforcing pulses out of the south-southwest (210-220 deg) and
south-southeast (130-160 deg) will keep the surf from going flat
each day along south facing shores. For more detailed surf
information, see the latest Collaborative Nearshore Swell and
Wind Forecast at

Water levels this afternoon ended up coming in and exceeding the
record water level measured at Honolulu back at the end of
April and May (the Honolulu station recorded a preliminary water
level of 3.284 ft relative to MLLW this afternoon). These extreme
water levels are in response to an oceanic eddy moving through
from east to west and the regular King Tide cycles. Water levels
will steadily trend down Sunday through the upcoming week. See
the Special Weather Statement for more details.






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