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Hawaiian Islands Synoptic Discussion and Guidance

927
FXHW60 PHFO 302001
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1001 AM HST Mon May 30 2016

.SYNOPSIS...
High pressure building north of the islands will bring gradually
strengthening trade winds today and tomorrow. The high will push
moisture associated with a weak front over the islands for the next
couple of days, fueling windward clouds and showers. Moderate trade
winds will continue until Friday, with light and variable winds
expected again next weekend.

&&

.DISCUSSION...
Clouds and showers associated with a weak frontal boundary have
moved onto Oahu over the past couple of hours, primarily affecting N
and E facing coasts and slopes. In the gradually increasing, post-
frontal NE flow on Kauai, scattered to locally broken low clouds,
and a few showers, are over windward areas and adjacent waters. In
Maui county and on the Big Island, a surface ridge just to the E is
maintaining light winds, allowing overnight land breezes to
completely clear skies. The expectation for today is for NE trade
winds to focus clouds and showers along windward slopes of Kauai,
Oahu and Molokai, while afternoon sea breezes lead to increasing low
clouds, and a few showers, over the slopes of Maui and the Big
Island. Trade winds may still be sufficiently light to allow
afternoon leeward sea breezes on Kauai and Oahu to fuel some clouds
and showers. The island atmosphere is not strongly capped, nor is it
extremely unstable, as an E-W oriented trough aloft remains over the
island chain. With moisture extending as high as 10-12 kft, there
could be a few brief heavy showers along the front, or with the
afternoon convection.

The forecast calls for high pressure now centered about 900 miles NW
of the islands to move slowly E through Tuesday, and then gradually
weaken from Wednesday through Friday N of the islands along 30N. The
high will continue to help push the frontal boundary S and SE down
over the island chain, with recent GFS guidance trending toward the
faster ECMWF. The 12Z runs of both models are now indicating the
bulk of the moisture will move S of the Big Island by Thursday
night, although the ECMWf remains a little faster. Will hold off
from making significant changes to the current forecast package, but
if current guidance trends continue, the timing of the feature in
the official forecast will need to be accelerated.

Although the low cloud field associated with the front currently
appears somewhat meager in latest satellite images, guidance
indicates that another surge in low-level moisture/winds N of the
boundary later tonight into Wednesday will help to energize the
boundary and lead to greater shower coverage. A mid-level shortwave
is also forecast to pass over the area at this time, and guidance is
indicating that this feature will also aid in re-energizing clouds
and showers along the boundary. So, while earlier forecasts
indicated that the front would be gradually weakening as it moved
through the island chain, it now appears that the opposite may occur
over the next 12-60 hours. Moisture is still expected to remain
confined to the lowest 12 kft, but if it were to train over one area
for a prolonged period of time, some localized flooding issues could
develop.

As the surface high to the N weakens and dissipates Friday into next
weekend, winds over the islands will once again become light and
variable. Although it's too early to have confidence in the details
of the forecast, current forecast grids indicate a land/sea breeze
weather regime, with clear skies nights and mornings giving way to
afternoon and evening clouds and showers. Gradually building mid-
level heights may bring increasing stability by late in the weekend.

&&

.AVIATION...
VFR will prevail over most areas through Memorial Day. However, a
weak cold front passing over Oahu today may generate ISOL MVFR
conditions in showers as it continues its slow march east
southeastward. Clouds and showers will favor the northern and
windward sections of Kauai and Oahu both today and tonight.
Meanwhile over Maui County and the Big Island, convective buildups
are expected to develop by early afternoon over island interiors in
response to the approaching front. ISOL MVFR will mainly affect
interior and mauka locales on those islands. Partial clearing is
expected along leeward coasts after midnight.

No AIRMETs are currently in effect or anticipated at this time.

&&

.MARINE...
A weak front over Oahu will continue to move down the island chain
over the next few days. Moderate to fresh trades are expected to
fill in over the state and the coastal waters as the front
progresses. Moderate northeast winds are already being reported at
Barking Sands, PHBK, and Lihue, PHLI. The current forecast does not
have the winds reaching the small craft advisory threshold. However,
some of the more typically windy areas will be monitored, as they
may see winds nearing this level.

The current southwest swell continues to hold, with the Lanai buoy,
51203, reporting just above 2 feet with a 13-14 second period. This
swell should gradually decline into Tuesday before another small
southwest swell bumps surf up late Tuesday into Wednesday evening.

A small northwest swell is slowly arriving and a bit undersized.
The Waimea Buoy, 51201, reads 2.3 feet 13 seconds, we are expecting
only a slight bump up by this afternoon. Swell at at Buoy 51101 has
been reading close to 4.3 feet 12 seconds for most of the night,
with a peak early this morning. Morning reports show the period
dropping to near 11 seconds. This small swell will then decline into
Tuesday, with no other swells on the horizon.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

Discussion...Birchard
Aviation...Bedal
Marine...Eaton

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



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