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RH7 %
WindNE 3 mph
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Hawaiian Islands Synoptic Discussion and Guidance

768
FXHW60 PHFO 230650
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
850 PM HST Thu Jun 22 2017

.SYNOPSIS...
Moderate trade winds will trend lighter over the weekend, but will
continue to deliver passing clouds and showers to windward areas,
mainly during nights and mornings. The lighter winds over the
weekend will likely allow afternoon sea breezes to drive the
formation of clouds and a few showers over leeward areas of all
islands, and a weak disturbance aloft may allow a few briefly heavy
showers to develop, especially on Sunday. Breezy trade winds are
expected to develop early next week.

&&

.DISCUSSION...
High pressure cells remain centered far NNW and NNE of the islands,
with a low and associated stalled front separating the two. The low-
level gradient S of the highs is supporting trade wind flow over the
islands, with speeds ranging from light in sheltered leeward
locations to breezy in the more exposed areas. Soundings indicate
that the island atmosphere remains capped at a height near 8 kft,
which is sufficiently low to prevent significant rainfall, but also
sufficiently high to allow efficient warm rain processes to produce
light to moderate showers. Although recent rain gage reports
indicate little in the way of shower coverage and rainfall amounts,
the typical diurnal trend toward increased shower coverage is
expected overnight, with showers favoring windward slopes and
coasts. On water vapor imagery, a weakening low aloft about 1150
miles ENE of the islands near 27N140W is noted to be moving N while
a weak trough aloft is approaching from the distant NW.

Little significant change to the overall weather pattern is expected
through Friday, with moderate trade winds delivering passing clouds
fueling a few showers, mainly over windward areas. A trend toward
weaker trade winds is anticipated Saturday into Monday as a weakness
in the low-level gradient, and potentially a weak low-level trough,
moves over the islands from the E. At the same time, a subtle
increase in low-level moisture is expected to arrive on the trade
flow.

Although the low aloft to the NE is expected to dissipate soon, it
leaves behind a weakness as the approaching trough aloft to the NW
helps to cool the mid-levels over the islands. The combination of
slowly and steadily cooling mid-level temperatures and the arrival
of increased moisture is expected to maintain passing showers over
windward areas. In addition, the light winds will likely allow
afternoon sea breezes to drive the development of clouds and showers
over leeward and interior areas, with the weakness aloft potentially
allowing a few showers to become briefly heavy. Latest guidance
indicates that greatest instability, although still modest, will be
over Kauai and Oahu, and centered on Sunday. Light winds and slower
shower movement means there will be a chance for higher localized
rain totals where any showers do develop. The combination of light
winds, the extreme sun angle, and dew points near 70F will likely
make for sticky conditions over the weekend.

Early next week, the high to the NNW will move E, and models agree
that breezy ENE trades will prevail, delivering a few showers to
windward areas. A ridge aloft is expected to build over the area
from the NW as the trough aloft dissipates and moves SW.

&&

.AVIATION...
Light to locally moderate trade winds will persist through the
period. Ragged clouds and scattered showers will continue to favor
windward and mountain areas. Brief MVFR conditions possible in
passing showers, especially at night. Otherwise VFR conditions will
prevail all areas. No AIRMETs in effect or anticipated at this time.

&&

.MARINE...
A ridge of high pressure north of the islands will keep moderate to
occasionally breezy trade winds in place through the weekend, but
winds and seas are expected to remain below Small Craft Advisory
(SCA) levels. The trades are expected to strengthen early next week
as high pressure builds northeast of the islands, and SCAs for winds
may be needed for the typically windy waters at that time.

A new reinforcing SSW swell will arrive tonight, peaking near or
just below advisory levels Friday. Surf along south facing shores
will remain above average Saturday and Sunday, and into early next
week, but remain below advisory levels.

In addition to the new southerly swell, tides have been running
about a half a foot above predicted levels. These elevated tide
levels will increase the risk for minor coastal flooding when
combined with the above average south swell. For the next few
afternoons, the predicted high tides are around 2.5 feet MLLW, which
does not include the extra 1/2 foot mentioned above. See the latest
Special Weather Statement (SPSHFO) for additional details on the
elevated tide levels and potential impacts.

Small trade wind and SE swells will continue into the weekend. In
the longer range, models indicate a northeast swell developing for
the beginning of next week. A High Surf Advisory may eventually be
needed for east facing shores if the swell comes in bigger than
anticipated.

For more details on the surf, please refer to the Oahu Collaborative
Surf Forecast (SRDHFO) prepared by Pat Caldwell under WMO header
FXHW52 PHFO.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

Discussion...Birchard
Aviation...Bedal
Marine...Kinel

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