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Hawaiian Islands Synoptic Discussion and Guidance

433
FXHW60 PHFO 200651
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
851 PM HST Mon Aug 19 2019

.SYNOPSIS...
A strengthening ridge north of the islands will maintain a trade
wind regime through the upcoming weekend. Under this flow, showers
riding in with the trades will favor the windward and mountains
areas. The moderate trades tonight will be strengthening to
locally strong by Tuesday night and persist through the rest of
the week.


&&

.DISCUSSION...
The POPs were scaled back slightly for all islands tonight, and
for Kauai and Oahu Tuesday morning, to scattered based on the
latest infrared satellite imagery. On the satellite, it is
certainly not as clear and dry upwind of the islands this evening,
but it doesn't appear to be all that very showery, either. We do
expect the trade showers to pick up in frequency Tuesday, after
sunrise, for especially the Big Island and windward Maui, in
association with surface trough currently 260 miles east-southeast
of the Big Island. This feature is slated to pass south of the
islands Tuesday and Tuesday night. Some of its low level moisture
will likely graze the Big Island and windward Maui Tuesday and
Tuesday night. Also, the afternoon heating and light winds over on
the Kona side of the Big Island will lead to some scattered
afternoon and evening showers. No thunderstorms for the higher
elevation, though, as the mid level ridge will cap the moisture at
around 10k feet. The latest GFS precip field is painting a normal
trade wind shower pattern for the Big Island by Wednesday.

The trades are at moderate speeds this evening, and is expected to
strengthen to locally strong by Tuesday evening, due in part of
the trough passing south of the islands Tuesday and Tuesday night.
This boost could be brief, as a softening of the trades may occur
Thursday through Friday, due in part of a front cutting into our
northern offshore waters Thursday into Friday. The remnants of the
front gets pushed westward, away from the islands after Friday,
allowing for the the return of widespread moderate to breezy
conditions for the weekend.

An upper level trough is forecast to develop just west of Kauai
over the weekend. The trough will be close enough to the islands
allowing for the uptick in trade showers for the islands.

&&

.AVIATION...
A ridge of surface high to the north of the state will maintain
weak to moderate east-northeast trades through early Tuesday. A
surface trough passing south of the islands on Tuesday will lead
to a boost in the strength of trade winds due to tighter pressure
gradient across the areas.

Expect most shower activity to occur along the windward side of
the islands through early Tuesday morning, with brief MVFR
conditions at times, due to ceilings and lowered visibility.

No AIRMETs are currently in effect, though an area of low clouds
spreading over windward Big Island in the next few hours may
bring enough cloud coverage to warrant mountain obscuration for
that area. The anticipated strengthening in trade winds on Tuesday
may also result in AIRMET TANGO being posted due to possible
increasing low turbulence on the immediate lee areas of mountains.

&&

.MARINE...
A trough of low pressure sliding by to the south of the state and
a new high building to the northwest, will gradually strengthen
the trade winds over the next couple days. The trades are expected
to reach Small Craft Advisory (SCA) thresholds Tuesday night
across the typically windier waters around Maui and the Big
Island, with winds holding at these levels through Wednesday night
and possibly Thursday. A weak surface front approaching the area
from the north late this week is expected to cause a slight
disruption in the pressure gradient over the region, easing the
trade winds below SCA levels late Thursday through Friday. The
trades are expected to strengthen to moderate and breezy levels
once again over the weekend, with SCAs likely being needed for the
windier marine zones.

Surf along south facing shores will build tonight through Tuesday,
peaking late Tuesday and Tuesday night. Long-period forerunners
are beginning to register on the nearshore buoys this evening,
with a considerable amount of long-period energy seen at the
offshore NOAA buoy 51002 since mid-afternoon. This should
translate to advisory level surf later tonight along south facing
shores. A High Surf Advisory (HSA) remains in effect from midnight
tonight through 6 AM Thursday along south facing shores of all
islands. We will be monitoring observations closely in case a High
Surf Warning might be required during the peak of this event late
Tuesday into Tuesday night. Surf may not drop below the HSA
criteria along south facing shores until Thursday based on the
latest guidance.

Surf along north and west facing shores of the smaller islands
may increase slightly late Wednesday through Friday as small
northwest swells spread down the island chain. Small, choppy surf
may trend up slightly along east facing shores this weekend as
the trades strengthen.

&&


.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from midnight tonight to 6 AM HST Thursday
for Niihau-Kauai Windward-Kauai Leeward-Oahu South Shore-Waianae
Coast-Molokai Leeward-Lanai Makai-Kahoolawe-Maui Leeward West-
Maui Central Valley-Leeward Haleakala-Kona-South Big Island-Big
Island North and East.


&&

$$


H Lau/Hui/Jelsema

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