Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 170634

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Mon Jul 16 2018

High pressure north of the state will keep a moderate to breezy
trade wind flow in place through the upcoming weekend. Clouds and
showers will favor windward and mauka areas, with a few showers
spilling leeward from time to time.


Generally dry trade wind weather prevails on this Monday evening.
Satellite and radar shows areas of clouds and widely scattered
showers over the windward waters and moving into windward Oahu,
but rain gauges report only isolated very light rainfall totals on
land during the past six hours. Surface analysis depicts an east-
west oriented high pressure ridge located about 1100 miles north
of Kauai, with a weak and disorganized tropical disturbance
(Invest 91C) located about 800 miles east-southeast of the Big
Island. Moderate to fresh trade winds prevail over the state to
the south of the ridge. Aloft, a lengthy mid/upper level ridge
extends from the Northeast Pacific to the WSW over Hawaii, while a
large and persistent Tropical Upper Tropospheric Trough (TUTT) is
located to the west and northwest of the state. The ridging aloft
is providing stable conditions over Hawaii, with strong
inversions based between 6000 and 7000 feet as per the 00Z
soundings. PWs are near to slightly above normal (around 1.4 to
1.5 inches), and MIMIC-TPW satellite imagery shows similar PW
values to the east of the state, with a more moist airmass exiting
to the west of Kauai.

Little overall change is expected during the next several days,
with moderate to breezy trade wind weather continuing, and mainly
passing showers focused over windward areas. Model consensus is
in good agreement with the surface ridge holding far north of the
state during this time. Rather dry trades should continue
overnight through Tuesday evening, as mid-level ridging overhead
keeps the inversion strong and fairly low and PW values remain
near normal. Invest 91C is forecast to continue tracking westward,
passing about 350 miles south of the Big Island on Wednesday.
While the bulk of the deep tropical moisture is forecast to remain
south of the state, enough additional moisture may pass over the
islands to cause some increase in shower activity, mainly over
windward/mauka areas, from late Tuesday night through Wednesday
evening. A dry airmass spreading in from the east should result in
lower rain chances over the islands from Thursday through
Saturday. Another surge of tropical moisture associated with a
disturbance to our southeast and south may arrive over the state
late in the weekend, but there is still considerable uncertainty
that far out in time.


Moderate trade winds will continue over the next 24 hours with a
persistent high pressure ridge far to the north of the Hawaiian
Islands. Low clouds moving through the trade winds will bring
periods of MVFR clouds to the windward slopes of most islands
through the evening hours. Shower activity will remain light
through Tuesday in a drier weather pattern.

AIRMET Sierra issued this evening for Tempo Mountain Obscuration
with MVFR ceilings over north through east sections of Kauai,
Oahu, Molokai, Maui, and the Big Island. This AIRMET coverage will
likely be reduced later tonight.


Broad high pressure far north of the area will continue to produce
locally breezy east trade winds over the state for the next
several days. A Small Craft Advisory (SCA) is posted for the
typically windier waters around the Big Island and Maui County
through Tuesday afternoon, with an expansion of SCA conditions
likely thereafter. A low pressure system, which is expected to
pass south of the islands, Wednesday and Thursday, could also help
bump the trades up a notch, especially for the more southern

The locally breezy trade winds will continue to produce rough and
choppy surf along east facing shores, with surf heights nearing
the advisory level as winds increase by midweek. The
aforementioned low pressure system could also produce a short-
lived elevated east swell, mainly for the southeast and south
waters near the Big Island as it passes.

A series of reinforcing south and southwest swells will keep
small to moderate surf along the south and west facing shores
through most of the work week, with surf heights climbing a little
each day. Long term guidance shows a larger, longer period south
swell for the upcoming weekend, with surf heights nearing the
advisory level (8 ft) for south facing shores.


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



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