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Hawaiian Islands Synoptic Discussion and Guidance

286
FXHW60 PHFO 210632
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Wed Mar 20 2019

.SYNOPSIS...
Stable and dry conditions will persist across the islands tonight,
due to high pressure to the northeast. Trade winds will increase
Thursday and Friday as a cold front approaches from the northwest.
This front will be close enough to usher in increased clouds and
showers across the area Saturday and Sunday.

&&

.DISCUSSION...
A dry and stable airmass remains over the area, with afternoon
soundings showing less than an inch of PW, positive lifted index
values and a strong inversion base between 6000 and 8000 feet. The
subtropical ridge lies just north of the islands this evening and
the slack pressure gradient keeps winds quite light. The pressure
gradient should strengthen over the next day or so as a cold front
approaches from the northwest, which will increase our trade
winds. The front will continue to move towards the islands
through the end of the week, and then dissipate and/or move to the
east of the islands.

Models show the front will stall just north of the islands, but
will be close enough to bring increased moisture, along with
stronger winds, a fair distance ahead of it. Moisture will
continue to increase as the front comes closest to the islands
Saturday and Sunday. With winds expected to remain out of a trade
wind direction, rainfall will focus over windward and mauka areas.
However, leeward areas will get wet as well from time to time.

Models show high pressure will approach from the northwest on
Monday, then move north of the islands Monday night and Tuesday.
This will maintain trade wind flow and, with leftover frontal
moisture across the region, we expect a wet trade wind pattern to
last from Tuesday through the rest of the forecast period.

&&

.AVIATION...
Trade winds will be returning overnight, and so will be
the trade wind showers, especially toward daybreak Thursday.
Currently, afternoon clouds are clearing out mainly the lee areas
of all islands. Clouds over the windward and mountain sections
appear to be clearing out as well, but they will likely stay in
place and be the source of trade showers later tonight. Cloud tops
are around 7k feet, with higher tops at 10k feet in showers. No
AIRMETs at the present time, but with the strengthening trades to
moderate to locally strong through 21/20z, we probably need AIRMET
Tango for low level turb after 16z.

&&

.MARINE...
Trades will steadily fill in across the state overnight into
Thursday as a ridge of high pressure noses eastward in the wake of
a front passing to the north. Trades will reach the moderate to
fresh category early Thursday, then the fresh to strong category
Thursday night through the weekend as high pressure sets up to
the north. Strongest winds are expected over the typical windier
channel waters and areas around Maui County and the Big Island. A
combination of overlapping north to northwest swells and wind
waves rising from the strengthening trades will lead to Small
Craft Advisory (SCA) conditions over exposed waters late Thursday
through early next week. Trades should begin to weaken Tuesday
through the midweek time frame as the ridge shifts south toward
the area and another front passes to the north.

Surf along north and west facing shores is expected to hold around
advisory levels through early Saturday due to the active pattern
over the northern Pacific this week, then steadily ease over the
late Saturday through Monday.

The new long-period northwest swell that filled in through the
day has peaked at the Hanalei and Waimea nearshore PacIOOS buoys
and near the peak at Pauwela this evening. Observations at the
offshore buoys northwest of the state have been steadily trending
down through the afternoon hours, which should be reflected at the
local buoys later tonight into Thursday. This downward trend will
be short-lived due to a subtle reinforcement expected Thursday,
which should be enough to keep the surf up around advisory levels
into Thursday night along north and west facing shores.

As this source eases Friday, a new long-period north-northwest
swell will fill in, which will support advisory-level surf
continuing along north facing shores late Friday into Saturday
before dropping below by Saturday afternoon. Heights along west
facing shores will likely drop below advisory levels by Friday
night due to the more northerly angle of the reinforcement.
Although the surf will remain below advisory levels late Saturday
through Sunday, a smaller reinforcement out of the northwest will
continue to deliver solid surf for north and west facing shores
Sunday for late March.

For next week, a small moderate period north swell expected
Monday night will keep the surf up for locations that favor this
direction through the day Tuesday. A moderate long-period
northwest swell will become a possibility Wednesday through
Thursday, that could generate near advisory level surf by
Thursday.

Surf along east facing shores will remain small through Thursday,
then rise Friday through the weekend as fresh to strong trades
return. A downward trend will be possible by the midweek time
frame of next week as the trades weaken.

Surf along south facing shores will remain small each day with
mainly background long period southerly pulses and short-period
southeast energy expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Friday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

&&

$$

DISCUSSION...Powell
AVIATION...Lau
MARINE...Gibbs

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