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Hawaiian Islands Synoptic Discussion and Guidance

456
FXHW60 PHFO 230645
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Tue Jan 22 2019

.SYNOPSIS...
Fair weather will prevail through Thursday as a ridge remains in the
island vicinity. Showers carried by the east to southeasterly wind
flow will reach the windward areas at times. Afternoon clouds and a
few showers will develop over the islands under daytime sea breezes,
with partial clearing at night under land breezes. A front will move
down the island chain Friday into Saturday, bringing a period of
rainfall and ushering in cool northeast winds for the weekend.

&&

.DISCUSSION...
A surface high pressure ridge has shifted north enough to allow
easterly trade winds to return to the Big Island, with weaker
southeast winds near Kauai. An upper level ridge overhead is
maintaining marginally stable atmospheric conditions, with a rather
dry airmass prevailing across the area. There are some clouds in the
island vicinity, though they are not bringing much if any
precipitation. Thus fair weather is expected through rest of the
evening into early Wednesday morning.

As similar weather conditions will persist through Thursday, mainly
fair weather will prevail across the area. A land and sea breeze
weather regime will continue to produce clouds and some showers each
afternoon, followed by mostly clear conditions overnight through
the early morning hours. Easterly trade winds will bring some
clouds and showers mainly to windward Big Island and east Maui.
Rainfall amounts will be limited.

Change in the weather is expected by Friday, as a front will move
down the island chain Friday into Saturday. Latest GFS and ECMWF
models are still in fairly good agreement, showing clouds and
showers along the front reaching Kauai Friday afternoon, and
advancing to the Big Island late Friday night or Saturday morning.
Expect rainfall to favor north facing terrain and windward slopes,
though rest of the areas may see some precipitation as well. Much of
the upper level support for this frontal system will stay to the
northeast of the state, lowering the probability of heavy rainfall
along the front.

Breezy northerly winds will likely develop behind the front,
ushering in a cool airmass with dew points in the mid to upper
50s. Showers will favor windward slopes, and even reaching the lee
portions of the smaller islands. This cool winter weather should
hold through Monday.

&&

.AVIATION...
VFR conditions will prevail as a surface ridge of high pressure
remains just north of the islands in a stable atmosphere. Light
winds will allow land and sea breezes to develop and modify wind
directions with some interior and mountain clouds over each island
during the afternoons.

No AIRMETs in effect and none expected.

&&

.MARINE...
A ridge will remain just north of Kauai through Thursday putting the
island chain in a light to moderate east southeast wind flow. A weak
cold front will move through the islands Friday and Saturday, with
moderate to strong northeast winds filling in behind on Saturday.

The northwest swell is struggling to reach 10 feet at buoys 51101
and 51001 as of 7 pm HST. It appears this swell is coming in
slightly smaller than both the ECMWF and WAVEWATCH III. Still, it
will be advisory level surf for the north and west facing shores of
Kauai by sunrise Wednesday. The swell will continue to spread down
the rest of the island chain the rest of Wednesday.

A High Surf Advisory (HSA) becomes effective at midnight tonight,
continuing to 6 pm HST Thursday for the affected shores of the
smaller islands. The leeward coast of the Big Island will feel this
above normal swell as well. So, a separate HSA goes into effect at 6
am HST Wednesday. A reinforcing west-northwest pulse of similar size
is due in Thursday night, and peak Friday, followed by a slow but
gradual decline through the weekend. The HSA will likely be extended
through at least Friday night.

No new northwest swell is expected over the weekend, but a moderate
west-northwest swell, from 300 to 310 degrees, reaches our north and
west facing shores Sunday night. It will max out at 7 feet and 15
seconds on Monday. This translate to surf heights near advisory
level of 15 feet along the north facing shores.

A Small Craft Advisory (SCA) for seas, goes into effect at midnight
tonight for near-shore waters exposed to both northwest swells. The
SCA, which is set to expire at 6 pm HST Thursday, could be extended
through at least Friday. We will see how big the next northwest is
before making this determination.

A slightly elevated east swell, generated by a pocket of moderate to
strong trade winds upwind of the islands, has arrived. The swell is
expected to peak Wednesday and Wednesday night, followed by a slow
decline through Friday. The surf may bump up to near advisory level
for east facing shores at its highest. Some south facing shores,
exposed to this swell, may see bigger surf as well. Otherwise, surf
along south facing shores will be small throughout the period.

Forecast models have scaled back the intensity of the gale low
north- northeast of the islands over the upcoming weekend. There is
a northeast pulse, but it is small at 4 to 5 feet and 10 seconds
Monday through Tuesday of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from midnight tonight to 6 PM HST Thursday for
north and west facing shores of Niihau, Kauai, Oahu, and Molokai and
for north facing shores of Maui.

High Surf Advisory from 6 AM Wednesday to 6 PM HST Thursday for west
facing shores of the Big Island.

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

&&

$$

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

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