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Temp7.9 C
RH46 %
WindNNE 8 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 151953

Area Forecast Discussion
National Weather Service Honolulu HI
953 AM HST Sun Jul 15 2018

High pressure far north of the area will continue to produce trade
winds across the area through the week and on into next weekend. A
slightly unstable airmass in the island vicinity today may allow
for a few heavy showers over portions of the Big Island and Maui
today. A more stable airmass is forecast to be over the area for
the remainder of the week. Clouds and passing showers will favor
windward and mauka areas with just a few showers expected over some
leeward locations.


High pressure is currently centered far north of the state
placing the islands in a moderate trade wind environment. Water
vapor imagery shows an upper low centered several hundred miles to
the north northwest of Kauai. This has caused a slight
destabilization of the airmass across the area. A slight chance
for thunderstorms is in the forecast today for portions of the Big
Island as well as over the leeward slopes of Haleakala. Otherwise
mainly passing showers over windward and mauka areas can be
expected today.

The upper low is forecast to pull further away tonight and Monday
with an upper ridge building in across the area. This will cause
to airmass to become more stable with a downward trend in shower

With high pressure remaining north of the area, moderate trade
winds will continue to blow through the remainder of the week. An
area of low pressure is seen on satellite far to the southeast of
the state. This area of low pressure is currently being monitored
for possible development during the next few days. Latest model
guidance currently shows a weak low or trough passing by south of
the state during the Wednesday through Thursday time frame. Most
of the moisture associated with the system is forecast to remain
south of the state but some increase in showers is possible over
the Big Island as it passes by since they will be closer to the

Otherwise just passing showers, mainly over windward and mauka
areas, can be expected through the remainder of the week and on
into next weekend.


Moderate trade winds will continue through tonight. In addition,
relatively unstable atmospheric conditions persist across the
state. This instability, combined with moist conditions, resulted
in the development of at least one thunderstorm over the Puna
District on the Big Island Saturday afternoon. There continues to
be a chance of thunderstorms in this same area through tonight, as
well as over the remainder of the Big Island and portions of the
south central valley on Maui this afternoon.

Otherwise, low clouds and showers transported by the low-level
trade wind flow will remain focused over windward and mountain
areas of the state today. The unstable conditions may result in
locally heavy downpours over some windward sections, so periods
of MVFR conditions may occur due to lower ceilings and reduced
visibilities along windward facing slopes. A few brief showers
may be carried over to leeward sections of some of the smaller
islands, but VFR conditions are expected to prevail there.

AIRMET Sierra remains in effect this morning for mountain
obscuration on Kauai, Oahu, Maui and the windward Big Island.
Expect these conditions to continue, possibly into this
afternoon, for most of these same windward areas.


A ridge of high pressure remains strongly entrenched far north of
the islands, resulting in fresh to strong trade winds over the
coastal waters that will last through next week. A Small Craft
Advisory (SCA) is in place over the typically windy waters around
Maui County and waters south of the Big Island through Monday
afternoon. Additional extensions of the SCA are likely to follow.
An area of low pressure associated with a tropical disturbance
will be pass south of the islands about the middle of the week,
which may bump up the trades another notch or two as well
expanding the coverage of the SCA.

With the surf, the choppy trade wind swell will produce surf
mainly in the moderate range along the east facing shores through
most of next week. The strengthening trades during mid week will
bump up the surf to near the advisory level for the east facing
shores. An incoming south southwest swell is expected to build
today producing small to moderate surf along the south and west
facing shores. This swell will slowly fade through Monday night. A
moderate south swell is due in Wednesday night with a similar
sized reinforcement late Friday.

An upper level trough northwest of Kauai that will lead to the
potential for a thunderstorm over the offshore/coastal waters
north of Kauai and Oahu today and Monday. The potential tropical
disturbance encroaching on the southeast offshore waters will
bring on a slight chance of a thunderstorm around the middle of
the week for that portion of the offshore waters.


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