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Hawaiian Islands Synoptic Discussion and Guidance

948
FXHW60 PHFO 230703 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
903 PM HST Mon May 22 2017

.SYNOPSIS...
High pressure to the north of the islands will continue to
provide trade wind weather through the rest of the week. Moderate
to strong trades will weaken during the second half of the week,
becoming light and variable by the start of the holiday weekend as
a surface trough develops near the islands. An upper level
disturbance may bring some unsettled weather to the islands over
the Memorial Day weekend as well.

&&

.DISCUSSION...
The forecast models point to strengthening trades overnight
area wide but particularly Kauai and the surrounding waters.
This is in response to a broad area of high pressure moving into
a position 1230 miles N of Kauai. The surface high with a central
pressure of 1037 mb moves away but subsequent highs will maintain
this strength into Tuesday night.

A stalled front/shearline extends from 1000 to 1200 miles NE of
Oahu to 87 miles N of Kauai this evening. It is forecast to weaken
during the next 24 to 48 hours, becoming a trough or a weak
surface boundary. This boundary then drift down and stalls over
Kauai on Thursday, raising the chances for showers island wide.
Both the ECMWF and GFS suggest that this trough lingers around
Kauai through the Memorial Day. There is also the outside chance
that this trough will affect Oahu as well.

Trades will turn lighter during the second half of the week with
a hybrid convective pattern in place by Friday, particularly the
central islands. Further more, over the weekend, we may be
dealing with an upper level trough, as noted by both the GFS
and ECMWF. With this said, the air mass over the islands turns
unstable to where the the Memorial Day weekend could be showery
and an unsettled one.

In the mean time, scattered shower-bearing clouds will be riding
with the trades. It appears Kauai will be least affected by these
showers overnight, but the models are pointing to a showery rush
hour for Tuesday morning. The trade showers should be fewer but
not completely gone by noon for the remaining islands.

&&

.AVIATION...
High pressure well north of the state will keep a breezy trade
wind flow in place through Tuesday. Clouds and showers will favor
windward and mauka areas, but will spread leeward across the
smaller islands from time to time due to the strength of the
trades. VFR conditions are expected at most of the TAF sites, but
MVFR cigs/vsbys will impact windward areas, including PHLI and
PHTO from time to time.

AIRMET Sierra may be needed for mountain obscuration later tonight
across windward sections of some of the islands.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of all islands.

&&

.MARINE...
Trade winds continue to build across the state as high pressure
moves far north of the islands. The Small Craft Advisory remains
in effect for the typically windy zones around Maui County and the
Big Island, as well as additional areas further to the west,
through Wednesday. Trade winds are forecast to peak overnight and
Tuesday, then weaken beginning Wednesday.

Northwest swell should peak overnight at the nearshore buoys,
with resulting surf remaining below advisory levels on north and
west facing shores. This swell is forecast to gradually decline
while veering around to the NNW through Wednesday. A series of
small and short-period NNE swells are expected later this week
into the weekend.

The current small south swell will linger through Wednesday.
A larger south swell is expected to arrive Friday and continue
through most of the Memorial Day weekend. This swell may produce
advisory level surf of 8 feet or greater along south facing
shores. In addition, the combination of swell energy, high
astronomical tides associated with the new moon, and ongoing
higher than predicted water levels due to oceanographic factors,
will likely result in minor coastal flooding due to wave run-up
along south facing shores later this week and during the weekend,
especially near the high tide times.

East facing shores will see short-period swell and choppy surf
during the next couple of days due to the breezy trade winds.

For more detailed information on surf for Oahu, see the latest
Collaborative Nearshore Swell and Wind Forecast
(www.prh.noaa.gov/hnl/pages/SRF.php#Discussion).

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Wednesday for Kauai leeward
waters, Kauai Channel, Oahu Windward and leeward waters, Kaiwi
and Pailolo Channels, Maalaea Bay, Alenuihaha Channel, and waters
south of the Big Island.

&&

$$

DISCUSSION...Lau
AVIATION...Jelsema
MARINE...Jacobson

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

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