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fxhw60 phfo 210625 
afdhfo

Area forecast discussion
National Weather Service Honolulu HI
825 PM HST Fri Jul 20 2018

Synopsis...
surface high pressure far north-northeast of the islands will
maintain a relatively dry trade wind weather pattern across the
Aloha state through Saturday night. Low clouds and light showers
transported by the trades will continue across windward and Mauka
areas. An area of enhanced moisture embedded within the trades is
forecast to spread westward across the islands from late Sunday
into early next week. This will likely cause an increase in the
coverage of low clouds and showers over windward sections early
next week.

&&

Discussion...
a 1034 mb surface high centered near 46n 140w has a surface ridge
extending southwest through a point about 1140 miles north of
Hilo. There is also a weak surface front approaching the western
end of the ridge due north of the islands. In addition, a weak
surface low appears to be near 13n 163w, or about 650 miles
south-southwest of Lihue. These surface features north and south
of the islands are maintaining a relatively tight pressure
gradient across the region. As a result, the trade wind flow is
moderate to locally breezy across the islands early this evening.

Aloft, an upper-level low is centered near 28n 165w, or about 550
northwest Lihue, while an upper-level ridge is east of The Big
Island. The flow aloft between these features is transporting
patches of cirrus clouds across the state from the south and
southwest. The atmospheric conditions across the state are also
relatively stable this evening, since the low-level trade wind
inversion is around 6 thousand feet. Precipitable water (pw)
values range from 1.0 to 1.2 inches, so the atmosphere is
relatively dry. Radar reflectivity data show isolated showers
moving across the coastal waters into the windward sides of the
islands, especially from Oahu to The Big Island. Rainfall totals
are rather meager, so these showers are mostly light.

The low south-southwest of the state is forecast to weaken as it
moves slowly westward through Saturday. In addition, the surface
front far north of the islands will continue to push slowly
eastward and erode the western end of the surface ridge. This
will cause a slight weakening of the pressure gradient across the
region, which will result in a modest decrease in trade wind
speeds this weekend. The forecast models also show the upper-
level ridge east of The Big Island will propagate slowly westward
over the islands this weekend. This will produce stable
atmospheric conditions across the area. With the low precipitable water values
already in place, this will mean only brief trade shower coverage
over some windward sections through early Sunday morning.

Changes are expected in the dry trade wind weather pattern
starting later this weekend. Looking upstream of the islands, the
leading edge of an area of enhanced moisture is evident in
satellite imagery about 850 miles east of Hilo. The forecast
models are in general agreement that the low-level trade wind
flow will eventually transport this area of moisture over the
eastern end of the island chain by late Sunday. This area will
continue spreading westward across the remainder of the state
from Sunday night into early next week. The forecast indicates
that trade shower coverage will likely increase over windward and
Mauka areas due to this enhanced moisture. Some brief showers may
also occur over leeward sections of the smaller islands, as well
as over the Kona slopes of The Big Island each afternoon. Since
the upper-level ridge will be in the vicinity of the islands, we
do not currently anticipate heavy rainfall early next week.

A trade wind weather pattern is expected to remain in place
through the remainder of next week. The trade wind speeds will
generally be moderate to locally breezy with low clouds and brief
showers favoring windward and Mauka areas.

&&

Aviation...
persistent east winds will focus clouds and showers over mainly
east areas of the islands. Cloud and shower coverage will be
greatest overnight and during the early morning. VFR conditions
will prevail. Isolated MVFR ceilings and visibility are expected,
but we do not anticipate that an airmet will be required.

&&

Marine...
moderate to fresh trade winds will hold through the weekend.
A disturbance is expected to pass south of the state early next
week, which will tighten the pressure gradient over the state, and
result in an increase of winds speeds and seas.

A new south swell is expected to build through the day Saturday
and peak on Sunday. This swell may cause surf to approach the high
surf advisory (8 ft) threshold along south facing shores Saturday
into Monday. The south swell is expected gradually drop Tuesday
and Wednesday. No other significant swells are expected for next
week.

See the latest Oahu surf discussion (srdhfo) for additional details
on surf and swell.

&&

Hfo watches/warnings/advisories...
none.

&&

$$

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