437 
FXHW60 PHFO 110952
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1152 PM HST Sun Mar 10 2019

.UPDATE...
The Wind Advisory for Maui County has been extended through 6 AM
Monday.

&&

.SYNOPSIS...
A strong high will persist far northeast of the islands through 
Monday, then move away to the east. A weaker high far northwest 
of the area will move east through mid-week. A ridge trailing 
behind the high will shift southeast over the islands Saturday. A
shallow front will move southeast across the islands next Sunday 
through Monday.

&&

.PREVIOUS DISCUSSION...
A strong surface high persists far northeast of the main Hawaiian
Islands, while an upper trough extends from the Big Island to an
upper low far to our southwest. The pressure gradient between the
high and equatorial low pressure continues to drive locally 
strong easterly trade winds over the islands and across adjacent 
local waters this evening. Instability associated with the upper
trough is seen in afternoon soundings, with negative lifted index
values noted. The subsidence inversion has lifted noticeably from
24 hours ago, allowing shower-bearing clouds to build deeper as 
they move by.

Satellite loop shows a narrow band of broken low clouds, which
provided a bout of increased showers this afternoon, has passed 
west of the islands this evening. Radar shows isolated to
scattered showers pressing onto windward shores and slopes, with
only isolated showers noted across leeward areas. In general, skies
across the islands are clearer than they were 6 to 12 hours ago, 
except near Maui and the Big Island, where broken high clouds 
persist. The trade wind boost associated with the passage of the
showery cloud band prompted issuance of a Wind Advisory for Maui
County this afternoon. Winds remain particularly gusty across
Molokai and within the Maui saddle, so we have extended the Wind
Advisory through 6 AM HST Monday.

Breezy trade winds will continue through tomorrow morning. The 
trades will focus showers over mainly windward areas. Clouds and 
showers will decrease during the afternoon, but windward areas 
will remain showery. High clouds associated with the upper trough 
will persist over the islands through tomorrow morning. These 
clouds will not produce any precipitation, but will add to total 
cloud cover, mainly over Maui and the Big Island.

Winds will subside starting tomorrow afternoon as the high
northeast of the area moves away to the east. A weaker high far
northwest of the area will move east through the week, with a 
ridge trailing back to the west of the high. During the second 
half of the week, the ridge will shift south ahead of a front 
approaching from the north. As the ridge sags south, trade winds 
will weaken and windward showers will subside. Winds may become 
light enough for afternoon sea breezes and nighttime land breezes 
from Friday afternoon through Saturday night.

Models show a front moving over Kauai from the northwest Sunday 
morning. ECMWF shows a stronger front with breezy post-frontal
winds while GFS shows a weaker front with light winds behind it. 
We will depict post-frontal wind strength as an average of these
model solutions until confidence rises. 

&&

.AVIATION...
The rather strong surface high is still northeast of Hawaii and it
continues to produce northeasterly trade winds. AIRMET Tango is 
still in place for low-level turbulence below 8,000ft and will 
probably stay in effect through at least a good portion of Monday.

There are passing showers across the state, mainly affecting the
windward slopes but with the stiff winds, some of these showers
may spill over to leeward areas. Due to the isolated and passing
nature of these showers, AIRMET Sierra for mountain obscuration
may not be necessary through the night but there will be MVFR
conditions on the windward slopes from time to time with clouds
and showers. The rest of the state should remain VFR for the 
period.

An upper level trough moving over Hawaii will push to the east
through tonight. This should produce some upper-level turbulence
over the islands. The chance for this will reduce as we progress
through the night into Monday morning. AIRMET Tango for moderate
turbulence between FL250 and FL360 is currently in place.

&&

.MARINE...
Strong to near gale force trade winds are expected to continue 
into Monday then slowly decrease to moderate to fresh levels 
Tuesday. The moderate to fresh trades should remain through 
Thursday, then become light to moderate as a front far north of 
the state pushes the high pressure ridge closer to local waters. 
The strong trades will keep winds and seas above the Small Craft 
Advisory threshold through Monday night.

A large northwest swell that peaked Saturday night has been 
declining through the day based on surf observations and nearshore
buoy data. Additional swells from the northwest are expected to 
arrive during the week. The first swell should arrive late Monday
and peak Monday night into Tuesday, then lower into midweek. The 
second one is expected to begin filling in Thursday, peak by 
Thursday night, then lower into the weekend. Heights should remain
below advisory levels for both swells. A third northwest swell is
forecast to fill in Friday night and will be much smaller.

Surf along east facing shores will remain rough due to the strong
trades, likely holding around advisory levels through Monday 
night before lowering. This downward trend will continue through 
the week and into next weekend as the upstream fetch weakens and 
the local winds drop into the light to moderate range.

South shore surf will gradually lower Monday through Tuesday. 
Small background reinforcements expected through the week should 
be enough to support small surf continuing each day. 

For the long range, guidance has come into better agreement and
continues to advertise a gale- to storm-force low evolving over
the northwest Pacific Thursday through Friday, then quickly
tracking east-southeast across the central Pacific to an area
north of the islands over the weekend driving a cold front into
the islands Saturday night. A captured fetch within the 310-330 
directional bands relative to the islands is depicted through this
time with seas climbing into the 25 to 40 ft range over the
central Pacific. WAVEWATCH III, ECMWF-WAVE and ensemble guidance 
all reflect this and show seas building into the 18 to 22 ft 
range over the Hawaiian waters late Sunday through Monday 
accompanying the fresh to strong northwest-north winds behind the 
front. Although confidence in these details remains low this far 
out in time, small craft advisory conditions along with warning- 
level surf for north and west facing shores will result if this 
scenario materializes. Confidence will quickly increase later in 
the week once this system develops.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Tuesday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward 
Haleakala-South Big Island-Big Island North and East.

Wind Advisory until 6 AM HST Monday for Molokai-Lanai-Kahoolawe-
Maui Windward West-Maui Leeward West-Maui Central Valley-
Windward Haleakala-Leeward Haleakala.

Small Craft Advisory until 6 AM HST Tuesday for all Hawaiian 
waters-

&&

$$

DISCUSSION...Powell
AVIATION...Chevalier
MARINE...Gibbs