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Hawaiian Islands Synoptic Discussion and Guidance

776
FXHW60 PHFO 241405
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
405 AM HST Fri Feb 24 2017

.SYNOPSIS...
Dry and breezy trade winds will slowly ease today and drop to
moderate levels by tonight. Clouds and limited showers will favor
windward and mountain areas through Saturday. Showers will
increase over the state from east to west starting Sunday with the
Big Island seeing the initial increasing rainfall over windward
and southeast areas. Enhanced moisture will push northwest over
the smaller islands as winds veer from the east southeast Monday
and Tuesday. Locally heavy showers may be possible with afternoon
showers. Scattered showers and light winds are expected for the
second half of next week.

&&

.DISCUSSION...
A high pressure ridge northwest of the state is maintaining
breezy trade winds over the islands, with only light passing
showers seen over Oahu and little to no rainfall elsewhere. The
overnight soundings from Hilo and Lihue both show a stable, dry
atmosphere over the state with a temperature inversion near 6
thousand feet and low precipitable water values under 1 inch.
Expect similar sensible weather conditions today with dry trade
winds prevailing.

Tonight, trades will diminish as low pressure to the north pushes
the ridge closer to the islands. This weakening pattern will
continue over the weekend. As the ridge approaches, trade winds
will decrease to light to moderate speeds while turning from an
east southeast direction. The ESE winds will maintain a pseudo
trade wind flow over the Big Island and windward areas of the
smaller islands with widespread leeward sea breezes expected
sunday afternoon. The low to the north, and the ESE flow will
bring in increasing moisture from the tropics, and this in turn
will bring increasing rainfall to the windward and southeast
sections of the Big Island. This moisture will slowly work its way
up the islands chain Sunday night and into next week bringing
increasing showers to the entire state.

Aloft, a quasi-stationary trough will stay in place in the
island's vicinity through the middle of next week. Models are
indicating embedded short-wave troughs sweeping over the islands
at times from Sunday into next week, bringing added instability to
the atmosphere. At this time, there is potential for heavier
afternoon convective showers starting Sunday for the Big Island
and then Monday due to the increasing low level moisture, leeward
sea breezes converging with windward trade winds, and the
atmospheric instability. One potential cap on the rainfall will
be how fast the enhanced moisture works its way over the smaller
islands.

This hybrid pattern of windward southeast trades combined with
leeward sea breezes looks to continue through the middle of next
week before light southerly winds take over for the second half of
the week. The upper level trough is expected to weaken for the
second half of the week and afternoon convective showers will
diminish as well. Both the GFS and ECMWF show a low forming in the
vicinity of the islands mid-week, but both models quickly speed
the low to the northeast away from the islands, leaving the state
under a light southerly flow with passing scattered showers.

&&

.AVIATION...
Mostly VFR conditions continue across the islands with brief
periods of light showers along north and east facing slopes.
Breezy trade wind speeds will slowly decrease today as a strong
East Pacific ridge will be weakened by two low pressure systems
north of the Hawaiian Islands. Expect lighter trade winds turning
into a sea breeze regime over the south through west facing
slopes of the Big Island this afternoon.

AIRMET Tango remains in effect for low level turbulence to the
South and West of the terrain. AIRMET will likely end later today
as northeasterly wind speeds decrease. No additional AIRMETs are
anticipated.


&&

.MARINE...
A Small Craft Advisory (SCA) remains posted for the typically
windier waters around the Big Island and Maui County, including
the water southeast of the Big Island, the Alenuihaha and Pailolo
Channels, and Maalea Bay. Winds will start to ease this afternoon
through these area as the high pressure north of the state
weakens and a low pressure systems develops far north-northwest
of the state. Winds are then expected to remain below the SCA
threshold (25 KT) into next week.

The current west-northwest swell will gradually decline through
the day, with current buoy reports reading close to 4 ft, 13-14
sec. A new west-northwest swell should start to fill in later
today. Spectral density at buoys 51001 and 51101 are hinting at
the long period energy starting to arrive northwest of the
islands, a little later than expected. Observations through the
morning will help gauge the incoming swell for the need and
possible timing of a High Surf Advisory tonight and Saturday for
exposed north and west facing shores. The swell will then
gradually decline through Monday.

A low north of the state will help provide a small north swell
today and Saturday before turning out of the northeast from Sunday
into early Tuesday. The northwest swell will help bump surf up
along the east facing shores with a northern exposure. The
combination of the incoming west-northwest swell and north swell
tonight may produce combined seas near the SCA threshold (10 ft)
for northern waters. This will be monitored. After Monday, north
and west facing shores should expect to see below normal surf.

A small long period south-southwest swell is forecast to fill in
today and tonight before gradually subsiding through the weekend,
which could provide a small bump to the relatively quiet surf
along the south shores.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Southeast Waters.


&&

$$

Morrison...Discussion
Bohlin...Aviation
Eaton...Marine

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