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Hawaiian Islands Synoptic Discussion and Guidance

235
FXHW60 PHFO 221957
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
957 AM HST Wed Jan 22 2020

.SYNOPSIS...
Variable winds and a stable land and sea breeze pattern will
prevail over most of the state through Friday afternoon. Expect
clouds and isolated light showers each afternoon followed clearing
overnight and during the early morning hours. A weak front will
stall near Kauai late Friday, bringing an increase in clouds and
showers for the northern end of the island chain into Saturday.
Moderate trade winds will return for the weekend, then decrease
early next week.

&&

.DISCUSSION...
Stable and mainly dry conditions prevail this morning. The
subtropical ridge has been pushed southward to a position roughly
100 miles north of Kauai, putting the state under an east-
southeast background flow. Stable clouds moving onto windward and
southeast Big Island and southeast Maui are producing scant
rainfall. The remainder of the state is blocked from the
background flow, leaving light land breezes and mostly clear
skies while a small batch of showers clips Niihau. A ridge aloft
is maintaining very stable conditions with a strong inversion
between 4,500 to 6,500 ft.

Afternoon sea breezes will develop over the next couple of hours
and dominate through most of the afternoon, resulting in interior
clouds and only isolated light showers. Rainfall will be minimal.

Little change is expected over the next 48 hours. The subtropical
ridge will be pushed over the islands by and approaching front. A
stable land and sea pattern will hold over most areas with
minimal shower activity favoring afternoon hours. An area of
moisture will move up over the Big Island from the southeast
Thursday, providing a slight boost in shower activity over mainly
the Kau, Puna, and Hilo Districts. We will also watch the small
pocket of moisture currently clipping Niihau, as it may provide a
small increase in showers over interior Kauai tomorrow afternoon.

The weakening front will drift southward and stall near Kauai
late Friday and Saturday. Kauai will likely experience an
increase in clouds and showers, while surface high pressure
building to the northwest drives increasing trade winds state-
wide. A persistent mid level ridge will maintain stable
conditions, with modest showers confined to windward slopes.

Trades will decrease and likely shift out of the east-southeast
early next week.

&&

.AVIATION...
Another round of light daytime sea breezes and overnight land
breezes is in store for the Aloha State today. Currently, the
majority of the clouds are situated over portions of Maui and the
Big Island. Additional cloud build ups are anticipated across
mainly interior sections of the island chain by this afternoon,
but VFR conditions will prevail. These clouds will clear land
areas during the overnight hours as land breezes take over.

No AIRMETS are in effect and none are anticipated.

&&

.MARINE...
The main marine hazard the next couple of days will be related to
a pair of large NW swells (and associated seas/surf), with the
first swell in the process of arriving. This swell has been
steadily building since last night at buoy 51001, which recently
measured seas of over 10 ft at 19 seconds, while the PacIOOS
Waimea buoy recently registered forerunners at nearly 5 ft 20
seconds. Although surf along most N and W facing shores started
off on the smaller side this morning with long lulls between
sets, surf will steadily increase through the day, and peak at
dangerously high levels tonight into Thursday before gradually
lowering later Thursday into Friday. A High Surf Warning and a
Small Craft Advisory remain posted for exposed shorelines/marine
zones. See the Coastal Hazard (CFWHFO) and Marine Weather Messages
(MWWHFO) for additional details, as well as the Oahu Surf
Discussion (SRDHFO). The second large NW swell is expected to
arrive Friday night and peak at warning level heights Saturday,
then gradually diminish Sunday and Monday.

A surface ridge will be near or over the islands into Friday,
keeping winds generally light and variable, but favoring a
southerly direction near Kauai, and an southeasterly direction
near the Big Island. A weak front will move into the area from the
N Friday into Saturday, pushed S by a high building NW and N of
the islands. This high will support light to moderate trade winds
over the weekend as the front stalls and dissipates near the
northern end of the island chain.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Thursday for north and west
facing shores of Niihau, Kauai, Oahu, and Molokai, for north
facing shores of Maui, and for west facing shores of Big Island.

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

&&

$$

DISCUSSION...Wroe
AVIATION...Dye
MARINE...Birchard/Wroe

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



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