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Hawaiian Islands Synoptic Discussion and Guidance

578
FXHW60 PHFO 161349
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
349 AM HST Wed Jan 16 2019

.SYNOPSIS...
A ridge over the islands will weaken and southerly winds will
develop as a cold front approaches the islands from the
northwest. The front will reach Kauai on Thursday morning, move
across Oahu late Thursday afternoon and over Maui County Thursday
night. The remnants of the front will reach the Big Island on
Friday. A brief period of trade winds is forecast for Friday
afternoon through Saturday as a migratory high passes eastward
through the weekend. Another cold front will move into the Hawaii
region for the first half of next week bringing some additional
showers to the western islands.

&&

.DISCUSSION...
The subtropical ridge axis just to the north of the Hawaiian
Islands will weaken today and winds will shift towards a
southerly direction as a cold front approaches the islands from
the northwest. An upper level ridge will produce strong stable
subsidence (downward vertical motions) over the state today. This
subsidence will continue to maintain a strong trade wind
inversion at roughly 6000 feet, elevation effectively capping
vertical cloud development. These low cloud heights under this
strong thermal inversion will keep shower activity at minimum
levels with only isolated showers in the forecast.

On Thursday and Friday, the cold front will sweep through the
Hawaiian Islands from west to east producing wet weather for all
islands as it passes through the region. The front will move into
Kauai in the late morning hours on Thursday spreading clouds,
showers and thunderstorms across the island. This front will
continue on its eastward journey reaching Oahu by late Thursday
afternoon and evening, then move into Maui County from Thursday
night into Friday morning, and finally arriving over the Big
Island by Friday afternoon. Expect an 8 to 12 hour period of
scattered to numerous rain showers and thunderstorms along the
frontal cloud band. An upper level trough will provide extra
dynamic lift producing isolated to scattered thunderstorm coverage
for most islands with this system. Rainfall will become locally
heavy at times, however due to strong winds aloft increasing the
storm motion these convective showers will move quickly as they
pass through the state. Upper level temperatures will drop snow
levels below the summits of Maunakea and Maunaloa. Any
precipitation developing over these two peaks will produce light
snowfall amounts mainly above 12,000 foot elevation. A brief
period of trades winds will return to the islands starting Friday
afternoon with elevated windward and mountain showers.

Over the weekend our weather pattern changes again from trade
winds on Saturday to a drier southeasterly wind pattern on Sunday
as a second cold front approaches the islands from the northwest.
For Saturday, a migratory high pressure system passing through
the Central Pacific will bring one more day of typical moderate to
breezy trade winds with scattered showers mainly over windward
and mountain areas. On Sunday, large scale winds will veer towards
a more southeasterly direction with wind directions modified by
land and sea breezes.

Scattered showers remain in the forecast for the first half of
next week as long range forecast guidance continues to show
another cold front moving into Kauai. Extended range forecast
models continue to struggle with the arrival timing and impacts
with this next frontal system as the fronts forward motion tends
to stall out. The elevated moisture and showers with this next
system will mostly increase showers over Kauai and Oahu.

&&

.AVIATION...
Ridging over the area will keep the atmosphere dry and stable with
VFR conditions prevailing today. Land breezes in the morning will
turn to sea breezes by mid-day under a light background wind flow.
Winds will turn more southerly and increase late today as a front
approaches the islands from the northwest.

No AIRMETs are in effect, and not expected today.

&&

.MARINE...
An east to west oriented ridge of high pressure just north of the
islands will give way to a front approaching the islands from the
west in the next 24 hours. Our light winds will turn southerly
today as the front approaches the islands. The southerly winds
will also strengthen to Small Craft Advisory (SCA) for the
windward waters of Kauai tonight and Thursday, ahead of the
arriving front. With SCA winds for the northern waters of Kauai,
gale force winds, ahead of the front, is likely to blow through
the offshore waters northwest and north of Kauai late tonight
through Thursday. The front is expected to move into the western
offshore waters tonight, and down the main Hawaiian Islands
between Thursday afternoon and Friday, with the front gradually
weakening and the winds becoming lighter during its eastward
transit. A slight chance of a thunderstorm is possible for the
northern and western waters, including the near- shore waters west
of Oahu ahead of the front. A fast moving high, following the
front, will usher in a short duration of SCA northeast winds
between Friday night and Saturday night. Another approaching front
will turn the winds around again over the weekend. This front
will enter the northern offshore waters early Monday, stalls, and
falls apart, leaving an area of light variable winds and
troughiness over the western- most offshore waters.

The current northwest swell has peaked with a lowering trend now
in motion. Thus, the warning level surf will be downgraded to a
advisory. The advisory for leeward Big Island will stays in place
through today. Expect the surf to be further lower to just below
advisory criteria by sundown this evening. If so, we can cancel
the advisory for all affected shores late this afternoon.

A new bump from the northwest is slated to reach our north and
west shores Thursday night where it will peak at . A gradual
decline follows through Saturday. This swell may boost the surf to
at or near advisory levels for the north and west shores during
this stretch. WAVEWATCH III is indicating a swell height of 9 feet
and 12 second period. The source of this swell is from a gale low
now passing just north of Midway Atoll. This gale low is the
parent of the front forecast to reach the main islands on
Thursday.

Some of the marine zones under the SCA due to elevated seas, have
been cancelled. These zones are the leeward waters and waters
south of the Big Island. Additional marine zones will be dropped
as the northwest swell continues to fill. However, a SCA will
likely be hoisted for the windward waters of Kauai later today for
strengthening southerly winds as the front approaches the islands
from the west.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-Kona-Kohala.

Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Channel-Oahu
Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Alenuihaha Channel-Big Island Windward Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Hui
MARINE...Lau

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