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Hawaiian Islands Synoptic Discussion and Guidance

301
FXHW60 PHFO 180637
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
837 PM HST Tue Jan 17 2017

.SYNOPSIS...
High pressure northeast of the state will keep moderate to
locally breezy trade winds in place through Wednesday night. The
trades will ease a bit on Thursday, then become light and
variable Thursday night and Friday as a front approaches from the
northwest. A dry and stable airmass will keep showers limited to
mainly windward and mauka areas through the work week. Big
changes in the weather are in store over the weekend, with a
front pushing southward down the island chain Friday night
through Saturday night, then stalling out near the Big Island
through early next week. The front will bring increasing showers
to windward and mauka areas with some of the showers reaching
leeward areas as well. Additionally, very strong trade winds are
expected to overspread the state this weekend, before easing a
bit early next week.

&&

.DISCUSSION...
Currently at the surface, a ridge of high pressure is oriented
east-west to the north of the island chain, with the ridge axis
around 180 miles to the north of Kauai. This is resulting in
moderate to locally breezy trade winds across the state this
evening. Infrared satellite imagery shows stable stratocumulus
trapped below a sharp low level inversion as evident in the 00z
soundings from PHTO and PHLI, with cloud cover more prevalent in
windward locales. Meanwhile, radar imagery and rain gauge
observations show light showers primarily limited to windward and
mauka areas. Main short term concern revolves around the incoming
west-northwest swell.

Tonight through Friday,
The high pressure ridge axis northeast of the state will keep a
moderate to locally breezy trade wind flow in place through
Wednesday night. The trades will ease on Thursday as an
approaching front well northwest of the islands begins to slacken
the gradient. As the front draws closer Thursday night and
Friday, the winds will become light and variable, with local land
and sea breezes expected. A dry and stable airmass is expected to
remain in place through the period as mid-level ridging north of
the island keeps inversion heights suppressed, and precipitable
water values remain near or below 1 inch. As a result, showers
will primarily be limited to windward and mauka areas with
rainfall amounts remaining light.

Winds on the Big Island summits are running below advisory levels
this evening, but an increase in 600 mb winds is expected to
push winds back up into the advisory range later tonight through
Wednesday. As a result, the Wind Advisory remain in effect through
6 pm Wednesday. Winds at the summits will continue to flirt with
advisory levels into Thursday, so the advisory may eventually need
to be extended in time.

Friday night through next Tuesday,
Big changes are in store for the weekend as a cold front moves
down the island chain, bringing an increase in showers and strong
trade winds to the Aloha state. In fact, the trades will likely be
strong enough to warrant a Wind Advisory over portions and
possibly the entire island chain Saturday through Sunday. The
front is expected to first reach Kauai late Friday night, then
continue to push southward down the island chain through Saturday
night. Showers will be most prevalent across windward and mauka
areas, but with the increasing trades and rising inversion
heights, some of the showers will reach leeward areas of the
smaller islands as well. There remains some uncertainty regarding
the southward push of the front, with indications in the latest
model solutions that it may hang up near or over the Big Island
Sunday through early next week. As a result, will show a drying
trend across the smaller islands Sunday into early next week,
while keeping windward sections of the Big Island and Maui a bit
more unsettled.

&&

.AVIATION...
Locally breezy trade winds will continue to transport low
clouds and isolated light showers over windward facing slopes
and coasts of the islands from tonight through Wednesday.
Brief periods of MVFR CIGS are possible due to these passing
low clouds and showers. Otherwise, VFR conditions are expected
to prevail.

AIRMET Tango remains in effect for TEMPO moderate low-level
turbulence leeward of the mountains on all islands due to the
breezy low-level trade wind flow. This AIRMET will likely
continue into Wednesday until the trades weaken on Thursday.

In addition, AIRMET Sierra remains in effect for the windward
Big Island from Upolu Point to Cape Kumukahi to Apua Point, as
well as along the windward sections of Molokai, Lanai, and Maui.

&&

.MARINE...
Easterly trade winds will build tonight as the high pressure
ridge axis lifts slightly farther to the north of the island
chain. An ASCAT pass this morning showed no Small Craft Advisory
(SCA) strength (25 kt) winds over local waters, but due to the
expected increase in the trade winds, an SCA remains posted for
waters around the Big Island and Maui County through Wednesday
night. A large west-northwest swell will build down the island
chain late tonight and Wednesday, producing widespread seas above
the 10 foot SCA threshold. As a result, the SCA covers all waters
Wednesday and Wednesday night.

A High Surf Warning remains in effect for exposed north and west
facing shores from Kauai to Maui, while the west shore of Oahu
remains under a High Surf Advisory. Uncertainty in peak surf
heights is slightly higher than normal, since guidance from the
GFS-based Wavewatch model is running several feet higher than the
European model. Large surf will reach west facing shores of the
Big Island Wednesday night, and we will get a better sense of the
heights once the swell reaches NOAA buoy 51001 later tonight.

Trade winds will gradually diminish on Thursday and Friday as the
area of high pressure to the north is eroded by an approaching
front. The front is expected to produce strong northeast winds and
large, short-period seas during the weekend, leading to widespread
SCA conditions and local gale warnings. The large, short-period
north-northeast swell will likely produce advisory-level surf along
north and east facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning from noon Wednesday to 6 AM HST Thursday for
Niihau-Kauai Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Surf Advisory from noon Wednesday to 6 AM HST Thursday for
Waianae Coast.

Wind Advisory until 6 PM HST Wednesday for Big Island Summits.

Small Craft Advisory until 6 AM HST Thursday for Maui County
Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory from 6 AM Wednesday to 6 AM HST Thursday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-
Kaiwi Channel-Maui County Leeward Waters.

&&

$$

DISCUSSION/MARINE...Jelsema
AVIATION...Houston

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



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