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Area forecast discussion
National Weather Service Honolulu HI
936 am HST sun Jul 23 2017

Synopsis...
an upper level trough to the west of the islands, and the remnants
of Fernanda to the east of the islands, will bring some showers to
the islands, some of which may be heavy this afternoon and
tomorrow afternoon. The remnant surface trough, that was once
Fernanda, will move slowly westward across the islands through
Monday night. Breezy trades, less humid conditions and a more
settled weather pattern will return by Tuesday as a surface high
far northeast of the area regains control of our weather.

&&

Discussion...
satellite, radar, and lightning data show thunderstorms over the
Kauai leeward waters, and over the offshore waters west of Kauai,
along with some pockets west of The Big Island in the offshore
waters. An upper level trough axis to the west of the Kauai
coastal waters is helping to fuel these thunderstorms. Satellite
data shows the low cloud circulation that was once Fernanda, about
230 miles west of Hilo this morning. More than 200 miles north of
that swirl, isolated thunderstorms and towering cumulus are
noted.

Closer to the islands, most areas are starting the day with plenty
of sunshine. The exceptions are Kauai, the west Maui mountains,
and the Kohala Mountains on The Big Island. Have made some tweaks
to the pops/sky/wx/qpf grids to better reflect this current
situation, and make some tweaks for this afternoon. With the
mostly clear skies over the west side of Haleakala and The Big
Island this morning, combined with the somewhat unstable air mass
overhead, have continued to include the mention of heavy showers
for those areas this afternoon.

The remnants of Fernanda are expected to further weaken into a
surface trough over the next 24 hours. This system is helping to
disrupt the trade wind flow, and as it moves to the west, will
continue to do so through most of Monday night. Beyond Monday
night, we should see trades returning from south to north across
the region due to a broad high pressure system far to the
northeast of the islands. There is good agreement with the GFS
and European model (ecmwf) with regards to trade winds building back in across the
islands for the rest of the week, with passing trade wind showers
impacting mostly windward locations from Tuesday onwards.

&&

Aviation...
VFR conditions prevail over the main Hawaiian islands. Significant meteorological information
Zulu remains in effect for an area of thunderstorms nearly
stationary over water just west of the area. An area of showery
clouds associated with the remnant of Hurricane Fernanda is
approaching from the east.

The thunderstorms west of the area are expected to weaken and move
away to the west this morning. The clouds and showers associated
with the remnant of Fernanda will spread over the area starting
this evening, and thunderstorms may develop. Widespread MVFR
ceilings and visibilities and isolate IFR conditions are expected
overnight, especially over windward areas. Airmet Sierra for
mountain obscuration will likely be needed overnight with possible
sigmets required for thunderstorms.

&&

Marine...
high pressure centered far northeast of the islands will support
trade winds through the forecast period. Post-tropical cyclone
Fernanda will weaken to a trough and pass east to west over the
island chain tonight and Monday, which will cause the trade winds
to temporarily weaken. Stronger trade winds will begin filling in
across the eastern end of the state as early as midday Monday.
The trade winds will strengthen to Small Craft Advisory levels by
Tuesday for the typical windy zones around Maui County and The Big
Island.

A high surf advisory remains in effect through 6 am Monday for
east facing shores of most islands, due to the easterly Fernanda
fetch pointed towards US. Buoy reports show borderline-advisory
conditions this morning and will assess through the day if the
high surf advisory should continue into tonight.

The largest high tides of the month will occur this afternoon and
tomorrow early evening. Note that these high tides, combined with
the elevated surf along east facing shores, may result in coastal
flooding in some areas through Monday. A Special Weather Statement
is posted to highlight The Hazards.

Moisture over the state is expected to increase as Post-tropical
cyclone Fernanda arrives. This moisture will interact with a broad
upper trough over the islands and result in possible thunderstorms
over coastal and offshore waters through Monday night (possibly
Tuesday for the western offshore area).

&&

Hfo watches/warnings/advisories...
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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