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Hawaiian Islands Synoptic Discussion and Guidance

073
FXHW60 PHFO 290637
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
837 PM HST Tue Jan 28 2020

.SYNOPSIS...
Stable and rather dry conditions will persist through Thursday,
featuring light and variable winds across the western half of the
state and east to southeast winds with light windward showers on
the Big Island. A front will stall near Kauai on Friday, bringing
wet and unsettled conditions across the western end of the state
into Saturday as trade winds rebuild. Trades will veer
southeasterly again early next week.

&&

.DISCUSSION...
With the subtropical ridge just north of Kauai, easterly trade
winds veered to southeasterlies this afternoon, allowing sea
breezes to prevail across the smaller islands. An upper ridge
keeps our airmass stable and dry, with PW values in a the 0.6 to
0.9 inch range. Except for lingering low clouds across Oahu, skies
across the islands are mostly clear this evening. Radar shows
little or no shower activity within viewing range.

Stable and rather dry conditions will dominate through Thursday.
An approaching front will push the subtropical ridge over the
islands by late Wednesday, further weakening and veering the
background flow and allowing land and sea breezes to dominate. The
upper ridge will remain in place and maintain stable conditions.
As a result, expect light windward showers on the Big Island and
east Maui. The rest of the state will be under a stable land and
sea breeze pattern, featuring afternoon clouds and a few light
showers followed by clearing at night.

Wet and unsettled weather will likely develop across the western
end of the state late Thursday night and Friday and may last
through Saturday. A front will approach from the northwest and
stall near Kauai, while surface high pressure passing north of the
front ushers in building trade winds. An upper trough will pass
over the islands on Friday and could create enough instability to
trigger heavy showers. Expect wetter conditions to favor windward
areas of Kauai and Oahu with moderate, mainly windward showers
elsewhere. Heading into next week, expect trades to veer
southeasterly again with drier conditions.

&&

.AVIATION...
Quiet weather pattern will continue through this forecast period
with rather dry air and light winds. We may see a few isolated
showers on windward sides of the islands, the best chance on Maui
and the Big Island. Winds will continue to be light enough to have
afternoon sea breezes and nighttime land breezes.

There are no AIRMETs in effect or expected through this forecast
period.

&&

.MARINE...
An east to west oriented surface ridge lies some 80 nm north of
Kauai this evening. It is forecast to drift a little further south
coming close to Kauai by this time tomorrow, where it will then
gradually dissipate on Thursday. Through Wednesday, a light
southeast wind flow is expected across the smaller islands and a
moderate easterly wind flow for the Big Island through Wednesday.
Light southeast winds should be in place across all islands.
It should be noted the southeast winds will be more moderate in
strength over the open waters north of the islands tonight through
Wednesday night.

This change is being caused by an advancing cold front, forecast
to to enter the far northwest waters by mid Thursday morning. The
front weakens and stalls near Kauai Thursday night, while a brisk
northeast wind flow gradually spreads over the surrounding waters
of Kauai County. The front may edge on to Oahu on Saturday before
stalling and turning into a trough with northeast winds to the
west of the trough axis and a moist southeast wind flow to the
east. To complicate things, an upper level disturbance over the
islands may bring on some unsettled weather to the main Hawaiian
Islands and the surrounding waters over the weekend. There are
many uncertainties to the forecast at this time. But they will be
ironed out in the next couple of days.

A northwest swell is currently on its way to the islands. Swell
data from Buoy 51101, located some 180 nm northwest of Kauai,
showed the swell reached 10 feet an hour ago, matching Wave Watch
III prediction. A High Surf Advisory (HSA) is in place for the
north and west facing shores for most of the islands, which is
good through at least 6 am HST Thursday. This swell is a prolong
one, meaning the HSA is likely to be extended into Friday. A Small
Craft Advisory has also been issued for waters exposed to the
swell for seas 10 feet or higher.

A more northerly swell, from 340 degrees, is slated to affect the
islands north facing shores over the weekend that will extend the
existing High Surf Advisory for the north facing shores, while
lowering the advisory for the west facing shores. This north swell
will be diminishing slowly through the early part of next week. A
small to moderate pulse is expected to reach our north and west
facing shores Tuesday night.

The lack of persistent trades means rather small surf to continue
along the east facing shores through Thursday. The arrival of the
front, ushering in some brisk northeast winds to Kauai, will bump
up the surf along the island's east facing shores on Thursday. An
increase in surf is possible for the other islands over the
weekend should the easterly winds build back in.

Surf along south facing shores will remain small through the week.
However, background south swells should be enough to keep things
from going flat.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from midnight tonight to 6 AM HST Thursday
for Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North
Shore-Oahu Koolau-Olomana-Molokai-Maui Windward West-Maui
Central Valley-Windward Haleakala.

Small Craft Advisory from midnight tonight to 6 AM HST Thursday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Oahu Windward Waters-Maui County Windward Waters-Big
Island Windward Waters.

&&

$$

DISCUSSION...Powell
AVIATION...Chevalier
MARINE...Lau

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