Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 230632

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Sat Jul 22 2017

Wet trade wind weather conditions are expected into early Sunday
morning due to an upper level trough west of the islands. This
trough aloft is expected to gradually weaken Sunday, which will
cause a slight reduction in showers. The remnant surface trough,
which was formerly Fernanda, will move slowly westward across the
islands Sunday night and Monday. This trough will bring lighter
winds with very warm and muggy conditions. Locally heavy showers
are also possible in some locations, especially Monday afternoon.
Breezy trades and more settled weather will start to return by
Tuesday as surface high pressure far to the northeast of the area
regains control of our weather.


A 1028 mb surface high near 36N 141W has a surface ridge extending
west-southwest to a point about 825 miles north of Honolulu. The
remnant post-tropical surface low, which was formerly Tropical
Cyclone Fernanda, is evident about 380 miles east of Hilo. This
feature appears to be moving nearly due west at about 10 mph. The
relatively tight pressure gradient across the state ahead of the
surface low is maintaining moderate trade winds across the islands
early this evening. In addition to the trades, very humid
conditions remain in place over Hawaii.

Aloft, loops of water vapor imagery depict a weak mid- to upper
level trough digging southward just west of Kauai. The local
instability associated with this feature continues to produce an
area of enhanced showers and thunderstorms south through west of
Kauai and Niihau at dusk this evening. These thunderstorms have
recently had cloud tops over 40 thousand feet.

Elsewhere, satellite imagery indicates partly to mostly cloudy
skies across much of the state this evening. Radar reflectivity
data show there are some showers over windward sections of some
islands, but most of the showers are over the coastal waters south
and southwest of the smaller islands.

The forecast models suggest the weak trough aloft will continue
to dig west of the islands overnight. This is expected to maintain
enhanced showers and some thunderstorms over the offshore waters
west and southwest of the islands. This trough aloft will slowly
weaken west of the islands from Sunday into Monday, so its impact
on atmospheric stability over the islands is expected to gradually
diminish. With increasing tropical humidity move in from the east
and trade winds remaining across the area, mainly windward
showers will continue on Sunday. There will also be the
possibility of locally heavy downpours over the upslope sections
of the leeward Big Island and leeward Haleakala Sunday afternoon.

From Sunday night into Monday, the remnant surface trough and
enhanced tropical moisture associated with ex-Fernanda will be
tracking from east to west in the vicinity of the islands. As
this feature moves slowly westward, expect an interruption of the
trade winds. This will cause very warm and muggy conditions to
develop. Mid-level heights and temperatures do not appear to be
very favorable for widespread heavy rainfall. However, with the
very moist tropical air mass in place and daytime heating in a
conditionally unstable tropical environment, we have included the
possibility of locally heavy downpours for many areas Monday
afternoon. Some of these heavy showers may persist into Monday
evening, but they will likely diminish later Monday night as
night-time cooling takes place.

The forecast guidance continues to indicate the trades will
return soon after the departure of the surface trough associated
with ex-Fernanda, as the pressure gradient rebounds due to a
broad surface high far northeast of the islands. At the same time,
the air mass will begin to have decreasing humidity levels
spreading from east to west. In addition, mid-level ridging will
become established across the area next week. This will usher in a
more typical summertime trade wind weather pattern from the
middle of next week into next weekend.


Moderate trade winds will continue to drive showers into windward
and mauka areas overnight, with some showers also reaching leeward
areas. Temporary to widespread MVFR cigs, along with MVFR and
possibly IFR vsbys in SHRA will affect windward locations through
the night, with PHLI and PHTO the most likely to see sub-VFR
weather. Conditions should improve area wide on Sunday, with the
remnants of Fernanda possibly resulting in a return of reduced
cigs/vsbys moving into windward areas Sunday afternoon and Sunday

AIRMET Sierra remains in effect for mountain obscuration across
north thru east sections of all islands. These conditions will
likely hold in place through the night.


High pressure centered far NE of the islands will support trade
winds through the forecast period. However, the remnants of Post-
Tropical Cyclone Fernanda will pass between the islands and the
high Sunday night and Monday, which will likely cause trades
to ease. As the trough that marks the remnant of Fernanda moves W
of Kauai Monday night and Tuesday, a moderate to occasionally strong
trade flow will resume, and a Small Craft Advisory will likely be
necessary for the windier zones.

A High Surf Advisory remains in effect through 6 AM Monday for E
facing shores of most islands, due to the combined effects of a
fetch of strong trade winds upstream of the islands, and the
limited fetch directly associated with Fernanda. Surf
observations support the continuation of the advisory, with some
windward locations reporting waves as high as 9 feet.

The largest high tides of the month will occur each afternoon
during the next couple of days. Note that these high tides,
combined with the elevated surf along E facing shores, may result
in coastal flooding in some areas through Monday.

Over the Hawaiian Offshore Waters, there will be a slight chance
of thunderstorms through Monday as an upper trough and increased
moisture interact.


High Surf Advisory until 6 AM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.



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