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Hawaiian Islands Synoptic Discussion and Guidance

766
FXHW60 PHFO 241942
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
942 AM HST Mon Jul 24 2017

.SYNOPSIS...
The remnants of Fernanda are moving across the region from east to
west, bringing showers to the the area. This will continue to
disrupt the trade wind flow statewide, and bring muggy conditions
to the region. There remains the possibility for heavy showers or
thunderstorms today. More settled conditions should start to
return tonight as drier air and more stable air return. Breezy
trades, more sunshine, and less humid conditions are expected for
the rest of the week as high pressure well to the north of the
state regains control of our weather.

&&

.DISCUSSION...
Have done a few updates so far this morning to expand the area of
thunderstorms, based on satellite and radar, and to increase PoPs
mainly from Molokai westward. Have also issued a Flash Flood Watch
for Kauai.

The remnants of Fernanda continue to bring showers to the region,
with the focus to the northeast of the islands. As Fernanda has
opened into a surface trough currently near Oahu, we have seen
heavy showers and thunderstorms form to the southwest of the
islands as well. Over the last hour or so, we have seen a bit of a
downward trend, however plenty of heavier showers and thunderstorms
remain, and the bulk of moisture is east of Kauai. Kauai has
already received some rain overnight, so have issued a Flash Flood
Watch for Kauai County with the expectation of more rain as the
day wears on.

As the trough continues on its westward track today, winds will
become light and variable over Kauai and Oahu, with southerlies
becoming southeasterly elsewhere. This could lead to some
thunderstorm development over interior areas this afternoon. The
mid-level support is fairly weak and the surface trough is moving
right along, so if heavy showers or thunderstorms do occur, they
are not expected to last too long in any one location.

After today, the trough should exit the area, and the weather
should quiet down fairly quickly. The low level flow will return
to regularly scheduled summertime breezy trades by Tuesday,
moisture will decrease, and the trough aloft will weaken further.
Fewer showers are expected overall beyond today, but some of the
models are showing an modest increase in low level trade wind
moisture Thursday night through Friday night. This is indicated in
the grids by a little increase in PoPs for this time period. A
sprawling, persistent area of high pressure to the north of the
islands will keep breezy trades blowing locally into the weekend.

&&

.AVIATION...
A north to south surface trough, the remnant of Fernanda, is near
Oahu moving west at 10 kt. The trough and a low aloft are combining
to produce thunderstorms near the main Hawaiian Islands. The
SIGMET OSCAR series is in effect for these thunderstorms.

The trough is producing widespread MVFR conditions and localized
IFR conditions from Lanai to Kauai. AIRMET SIERRA for IFR and MTN
OBSC is in effect for these conditions.

There will be gradual clearing from east to west as the trough
continues moving west.

&&

.MARINE...
High pressure centered far northeast of the islands will support
trade winds through the forecast period. A surface trough near
Oahu this morning will cause the trade winds to temporarily
weaken across the western portion of the state today. Stronger
trade winds will begin filling in across the eastern end of the
state as early as this afternoon. The trade winds will strengthen
to Small Craft Advisory levels by Tuesday morning for the
typically windy zones around Maui County and the Big Island.

Moisture associated with the incoming surface trough will interact
with a broad upper air feature near the islands today. This
interaction could result in isolated thunderstorms over coastal
and offshore waters through tonight (and possibly through Tuesday
for the western offshore waters). A Marine Weather Statement is
posted on our website with more information (see MWSHFO product).

One final large high tide of the month will occur tonight. This
tide may result in coastal flooding in some areas. A Special
Weather Statement (see SPSHFO product on our website) is posted to
highlight the potential hazards.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 6 PM HST this evening for Niihau-Kauai.

&&

$$


DISCUSSION...M Ballard
AVIATION...Donaldson
MARINE...Foster

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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