Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 290850 RRA

Area Forecast Discussion...DELAYED
National Weather Service Honolulu HI
1050 PM HST Fri Jul 28 2017

Moisture associated with the remnants of former Tropical Cyclone
Greg will continue to influence mainly the Big Island tonight and
Saturday, with drier conditions overspreading the smaller
islands. Showers will favor windward areas, but will spread
leeward from time to time. Drier weather will return to the entire
island chain Saturday night, with a typical trade wind weather
pattern then expected through much of next week. Trade winds will
continue through the next week as a ridge of high pressure remains
to the north of the islands.


Currently at the surface, the remnant trough of former Tropical
Cyclone Greg is located around 275 miles southeast of South
Point, while a 1026 mb high is centered around 1600 miles
northeast of Honolulu. The resulting gradient is driving moderate
trade winds across the island chain this evening. Infrared
satellite imagery and surface observations indicate mostly cloudy
conditions in windward areas of all islands as well as leeward
sections of the Big Island, with partly cloudy skies in leeward
sections of the smaller islands. Radar imagery shows scattered
showers moving into windward areas of the smaller islands, with
more widespread shower activity affecting windward sections of
the Big Island. Main short term concern revolves around rainfall

Tonight and Saturday,
The remnants of Greg will slide by to the south of the Big Island,
while high pressure remains anchored far northeast of the State.
This will keep a moderate trade wind flow in place across the
island chain. MIMIC total precipitable water imagery continues to
depict a very moist airmass in place over and upstream of the
islands, with PW values ranging from 1.7 to 2.1 inches. Despite
this moist airmass in place, there remains very little in the way
of showery low clouds upstream of the smaller islands. Upstream
of the Big Island is a different story, with quite a bit of
showery low cloud seen in satellite imagery. As a result of the
current trends, we have dried out the forecast considerably across
the smaller islands over the next 24 hours, with mainly scattered
windward showers and isolated leeward showers expected. Across
the Big Island, fairly wet conditions will likely continue in
windward areas tonight into Saturday morning, with conditions then
beginning to dry out Saturday afternoon. Moisture will likely get
trapped over leeward sections of the Big Island due to the
eddies around the northern and southern tips of the island, so
some localized downpours and even possibly a rumble of thunder
will be possible here Saturday afternoon.

Saturday night through next Friday,
High pressure north of the islands will keep moderate trade winds
blowing through the end of next week. Any lingering deep moisture
over the Big Island will shift southwestward Saturday evening,
with a drier and more stable airmass moving back in. Overall, we
should see a more typical trade wind shower pattern through the
period, with scattered windward showers and isolated leeward
showers, mainly during the overnight and early morning hours.


High pressure centered far north of the Hawaiian Islands will
keep moderate trade winds across the state. Moisture from ex-
Tropical Cyclone Greg will linger tonight, mainly across the Big
Island. As a result, extensive cloud cover and showers will
impact windward slopes here, with drier conditions across the
smaller islands. Drier air will move into the Big Island Saturday
afternoon or Saturday night, decreasing shower activity.

AIRMET Sierra is in effect for mountain obscurations continues
this evening for north and east slopes of Maui, and the entire
Big Island. No other AIRMETs are in effect or expected tonight.


Surface high pressure far northeast of the islands will maintain
moderate to locally strong trade winds across the islands into
early next week. Trade wind speeds are forecast to remain near
the Small Craft Advisory (SCA) criteria over Maalaea Bay and the
Pailolo and Alenuihaha Channels into early next week. The trade
winds will also eventually strengthen near the southern Big
Island, so the SCA will likely be added to the waters south of
the Big Island starting late Saturday.

The trade winds will keep moderate choppy surf along east facing
shores of most islands into early next week. In addition, wave
model guidance continues to indicate mid- to long-period east
swells from distant east Pacific Tropical Cyclones Irwin and
Hillary may reach the islands starting this weekend. These swells
will need to be monitored for timing details and resultant surf
heights in case surf might unexpectedly approach the High Surf
Advisory criteria.

A small mid- to long-period west swell, which was produced by a
distant typhoon in the west Pacific, is expected to spread across
the area from later tonight into this weekend. This swell may
produce a small bump in surf heights along some exposed west
facing shores. However, Niihau and Kauai will likely block much of
this swell energy from the west facing shores of the remaining

Elsewhere, a series of small mid- to long-period southeast and
southwest swells will maintain small background surf along most
south facing shores through the middle of next week.


Small Craft Advisory until 6 AM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.



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