Area Forecast Discussion
Issued by NWS Honolulu, HI
FXHW60 PHFO 201956
Area Forecast Discussion
National Weather Service Honolulu HI
956 AM HST Fri Jan 20 2017
Light winds this morning will become northeast and increase across
the state this afternoon and evening as a front approaches the
state. This front will move through the smaller islands late
tonight and Saturday, then stall near the Big Island from
Saturday night into early next week. Expect increasing showers
across the state with the front. Behind the front, very strong
trade winds are expected for the state this weekend, with winds
easing early next week.
Dry and stable conditions with rather light winds will prevail
today. A surface ridge parked a couple hundred miles north of the
islands will erode as a front approaches from the northwest. The
result will be weakening trade winds, with a possible shift out of
the north near Kauai late this afternoon. Ridging aloft is
producing very stable and rather dry conditions. Overnight
soundings and recent aircraft showed a strong inversion between
4000 to 5000 ft and precipitable water values well below an inch.
A field of broken to overcast stratocumulus clouds moving in on
the trade wind flow remains banked up along the islands this
morning and is squeezing out a few hundredths of an inch of light
rainfall along windward slopes. Expect these clouds to largely
burn off during the day, and although some interior clouds will
reform as afternoon sea breezes develop, little to no rain will
Northeast winds will steadily increase overnight ahead of the
approaching front. Breezy northeast winds will develop over Kauai,
and possibly Oahu, as the front reaches the Garden Isle in the
predawn hours. The somewhat shallow front will be accompanied by
rainfall over Kauai`s north and east facing slopes, and although
the ridge aloft will be steadily eroded by the broad upper level
trough driving the front, only a modest increase in windward
rainfall is expected from Oahu to the Big Island tonight.
The front is forecast to advance southeastward across the smaller
islands on Saturday, nearing the Big Island by late Saturday. The
main impact will be the windy northeast flow developing late
tonight and Saturday, as strong high pressure moves north of the
state. These winds will be strongest at higher terrain, through
mountain gaps, and downslope of mountains. Winds are expected to
be strong enough to reach advisory levels across many areas, and
we cannot entirely rule out high wind warning level winds in the
windiest locations. A Special Weather Statement has been issued to
highlight the expected windy conditions this weekend. An increase
in shower activity, especially for windward areas but with some
spillover leeward is expected along and behind the front.
The front is forecast to stall near the Big Island Sunday, with
windy conditions continuing statewide.
Heading into the early part of next week, longer range models
show winds veering to the east and southeast and easing as the
old front dissipates and another front moves quickly eastward
across the north Pacific. A somewhat wetter pattern than we have
observed recently is likely to continue through the early week
period as models keep some added moisture lingering around the
islands, with a weak trough aloft providing a weakened inversion
as well. The next front seems likely to pass mainly north of the
state around Tuesday, then a developing deep mid level trough to
our east pushes a drier airmass southward across the islands
Wednesday and Thursday. Moderate to fresh and rather dry trade
winds should prevail during that time.
The high pressure ridge north of the Hawaiian Islands will
continue to weaken today as a weak cold front approaches Hawaii
from the northwest. Expect a weak trade wind regime through the
afternoon. High pressure moves in behind the front increasing our
northeasterly trade winds later tonight. Mostly VFR conditions
are forecast with only isolated MVFR mainly over windward
No Airmet is expected today. However increasing winds behind the
cold front will require an Airmet for turbulence this weekend.
As the current large west-northwest swell subsides, a reinforcing
northwest swell is expected to arrive today with advisory level surf
continuing along north and west facing shores, including the west-
facing shores of the Big Island. Another large northwest swell is
expected Monday night and Tuesday. Current guidance indicates that
surf from this swell will reach High Surf Warning levels.
A front is forecast to move southeastward over the area today
through Sunday. The front is moving through the northwest offshore
waters today, reaching the coastal waters late tonight, and moving
southeast of the Big Island on Sunday. The global models show
northeast winds increasing tonight ahead of the front, with strong
to gale force winds by tomorrow. A Small Craft Advisory (SCA)
remains posted for waters exposed to large northwest swells.
Strengthening winds associated with the front will produce gales in
the windy areas around the Big Island and winds above the 25 kt SCA
threshold over the rest of the coastal waters.
The strong northeast winds will produce large choppy surf along
northeast facing shores, with surf possibly reaching the advisory
threshold along east facing shores.
High Surf Advisory until 6 AM HST Saturday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Gale Watch from Saturday afternoon through Sunday afternoon for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.
Small Craft Advisory until noon HST Saturday for Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Oahu Windward
Waters-Kaiwi Channel-Maui County Windward Waters-Big Island
Small Craft Advisory from 6 PM this evening to 6 PM HST Sunday
for Kauai Channel-Oahu Leeward Waters-Maui County Leeward Waters.
Small Craft Advisory from 6 PM this evening to noon HST Saturday
for Maalaea Bay.