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Hawaiian Islands Synoptic Discussion and Guidance

259
FXHW60 PHFO 291332
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
332 AM HST Wed Jan 29 2020

.SYNOPSIS...
Stable and rather dry conditions will persist through Thursday,
with light and variable winds expected across the western half of
the state. East to southeast winds will push showers across
exposed portions of 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...
There is little change in forecast philosophy from last evening.
The proximity of the subtropical ridge, just north of Kauai,
caused winds to veered to southeasterlies yesterday, allowing sea
breezes to prevail across the smaller islands. An upper ridge
keeps our airmass stable and dry, with PW values in a the 0.85 to
0.95 inch range. Patchy broken low clouds cling to windward and
southeast facing slopes of the islands, while radar shows isolated
to scattered showers across windward waters mainly from Molokai
eastward.

Stable and rather dry conditions will dominate through Thursday.
An approaching front will push the subtropical ridge over the
islands later today, 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...
Weather pattern will continue to be dry with light winds through
today into tonight. We may see a few isolated showers on windward
sides of the islands, the best chance the Big Island and maybe
Maui. 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...
A east to west oriented surface ridge just north of Kauai will be
dissipating through tonight. This comes about as an advancing
cold front is forecast to to enter the far northwest offshore
waters by mid Thursday morning. The front stalls near Kauai Friday
and dissipates there on Saturday, all the while a brisk northeast
wind flow spreads over the surrounding waters of Kauai County. As
the front dissipates, moderate trades return to other parts of
the island chain. An upper level disturbance over the islands
Friday may bring an uptick in showers.

The new northwest swell will be spreading down the island chain
today and tonight. This swell will dip slightly for Thursday but
still hang on at advisory levels for the north and west facing
shores. The swell arrived at the Hanalei buoy just before midnight
HST. The swell may have peaked at buoy 51001, getting to as high
as 11 and a half feet with a 15 second period. A new reinforcing
pulse is slated to reach the buoy Thursday night, meaning more
elevated surf for the north and west facing shores on Friday.
This northwest swell has a west component that will boost the surf
to just below advisory level for the leeward coast of the Big
Island later today, and lasting into Thursday.

The High Surf Advisory is in effect to until 6 am HST Thursday
for the north and west facing shores of most islands, but will
likely be extended into the weekend as a series of back-to- back
elevated northwest and north swell arrives, respectively. This
north swell will be subsiding gradually through Monday. The lack
of persistent trades means small surf for Oahu. The return of
moderate trades over the weekend, though, will bring a small boost
to the east facing shores. A background south swell will result
in small surf along south facing shores through the forecast
period.

Lastly, a Small Craft Advisory (SCA) for seas 10 feet or higher,
is in effect for the windward waters, stretching from the Big
Island to Kauai. The leeward waters of Kauai is also affected. The
SCA goes through 6 am HST Thursday as well, and is likely to be
extended. An ASCAT pass in the evening revealed small craft
advisory level winds of 25 knots over the waters south of the Big
Island, and models points to a continuation into this morning.
Therefore, an SCA has been issued for the waters south of the Big
Island to until 6 pm HST this afternoon.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 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 until 6 PM HST this evening for Big Island
Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Alenuihaha Channel-Big Island Windward Waters.

&&

$$

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

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



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