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Hawaiian Islands Synoptic Discussion and Guidance

972
FXHW60 PHFO 201927
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
927 AM HST Wed Nov 20 2019

.SYNOPSIS...
High pressure north of the islands will bringing increasing trade
winds to the region through the weekend. A wet trade wind pattern
is expected, with some rain reaching leeward areas at times with
the stronger winds.

&&

.DISCUSSION...
A high pressure system north of the islands is bringing trade
winds back to the region. The high is expected to move to the east
over the next 24 hours or so, and then stall far to the northeast
of the islands. Expect winds to be on the increase during that
time, and then stay elevated through the weekend.

The ECMWF and GFS both show an upper level trough at 250mb
dropping down over the islands over the next 24 hours or so, with
the GFS developing a low near Kauai at that level. Heading into
the weekend, the models differ on the placement and strength of
the trough/low, but both lift it out to the northeast by Sunday
afternoon. The models have some reflection of this upper trough at
500mb, with a 500mb low moving to the west during the weekend.
High pressure remains at the surface throughout, with strong trade
winds. Anticipating a fairly decent trade wind inversion to set
up. The upper level features will help to enhance the trade wind
showers, but at this time, not expecting any additional impacts.

Have lowered the upper level winds for Haleakala and the Big
Island Summits for tomorrow night through Saturday. While surface
winds are expected to be strong, the latest couple of runs of
the models show winds at those levels will not be as strong as
previously forecast. The current forecast reflects surface winds
reaching advisory levels in some locations by Friday morning, and
persisting into Sunday, which remains in line with the longer
range models. Will be fine tuning those areas as the event gets
closer, and factoring in the higher resolution models which don't
quite extend that far in time at the moment.

Longer range, the GFS and ECMWF both indicate a cold front with
associated upper level trough approaching the islands from the
northeast next week. The GFS is faster with the system, and has a
stronger system that gets closer to the islands than the ECMWF.
This set up could bring another round of thunderstorms to the
area. The current forecast does reflect an upward trend in
precipitation during this time, but will need to make further
refinements as the even draws closer.

&&

.AVIATION...
East northeast trades will strengthen today as surface high
pressure begins to build north of the state. This will allow for
most of the shower activity to occur along the windward coasts and
and slopes. Only isolated showers expected for the leeward sides.
Similar conditions will occur tonight with increased clouds and
shower along the windward coasts and slopes. MVFR conditions may
occur from time to time due to lower clouds causing mountain
obscuration and low vis in SHRA.

No AIRMETs are currently in effect.

&&

.MARINE...
The weakening low aloft has moved NE out of the area, acting to
stabilize the atmosphere some and end the thunderstorm chances
over the marine area. Trade winds will gradually strengthen today,
becoming breezy to strong tonight into the weekend, in response
to strong high pressure far north and northeast of the islands.
Also, combined seas are expected to exceed the 10-foot Small Craft
Advisory (SCA) threshold in most coastal zones tonight into Thursday
due to a building northwest swell and/or strong trade winds. Gale
conditions are possible in some channels toward the end of the
work week.

A new long-period northwest swell arriving this afternoon and
evening will produce High Surf Advisory (HSA) surf for exposed
shores of the smaller islands through Thursday night. If peak
swell heights are larger than WaveWatch guidance, as they have
been recently, surf could approach the 25-foot Warning threshold
for north facing shores. Another northwest swell arriving this
weekend may require brief HSA. Strong trade winds will likely
drive a steady increase in short-period wind waves beginning
tonight into the weekend, eventually leading to an HSA for east
facing shores. No other significant swells are expected. See the
Oahu Surf Discussion (SRDHFO) for details on swell sources.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 2 PM this afternoon to 6 AM HST Friday
for Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North
Shore-Oahu Koolau-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

Small Craft Advisory until 6 PM HST Friday for all Hawaiian
waters except Maalaea Bay-


&&

$$


DISCUSSION...M Ballard
AVIATION...Thomas
MARINE...TS

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