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Hawaiian Islands Synoptic Discussion and Guidance

962
FXHW60 PHFO 212011
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1011 AM HST Fri Feb 21 2020

.SYNOPSIS...
High pressure far northeast of the state will track further away
from the islands, resulting in weakening trade winds and less
passing showers this weekend. A front will approach from the
northwest and stall before reaching Kauai on Monday. Trades will
gradually strengthen during the first half of next week.

&&

.DISCUSSION...
A 1030 mb surface high is centered about 1200 miles northeast of
Hawaii slowly tracking eastward. This high brought strong winds
over the past few days, and winds are expected to weaken through
Saturday as the high drifts further away from the islands. Visible
satellite and radar show scattered clouds and a few showers moving
in with the trade wind flow anchoring more of the cloud cover and
showers across windward slopes. Downstream wave clouds are
observed on satellite across leeward Oahu and Kauai due to the
breezy trade winds. The subtropical jet setting up southeast of
the area is producing some cirrus clouds over the Big Island this
morning. 12Z soundings shows below average precipitable water
(PW) values around 1 inch with strong inversions in place.

For the rest of today and tonight, expect mostly dry weather with
breezy trade winds trending weaker. A few passing light showers
may impact windward areas at times. Winds will be noticeably
weaker by Saturday and veering out of the east southeast as a cold
front approaches from the northwest. Winds will become light and
variable by Saturday night continuing into Sunday, allowing sea
breezes and interior clouds to set up Sunday afternoon.

The front will weaken and stall just northwest of Kauai on Monday
with little direct impact to the state. High pressure behind the
front will usher in trades that will overrun the front and will
reach the islands Monday into Monday night. Trades will strengthen
during the first half of next week and will bring additional
clouds and showers to windward locations, with a few passing
showers reaching leeward sides of the smaller islands.

&&

.AVIATION...
AIRMET Tango remains in effect for tempo moderate low-level
turbulence over and downwind of the terrain due to breezy trades.
The surface high northeast of the state causing these winds will
continue to move east over the next day or so, as a front approaches
the islands from the northwest. As this happens, winds will switch
to a more southeasterly direction and weaken beginning Saturday
evening. In the mean time, stratocumulus with scattered showers
will push into windward areas bringing occasional MVFR ceilings,
while leeward areas see partly cloudy skies and only isolated
showers.


&&

.MARINE...
The 8 AM surface analysis shows a 1032 surface high about 920 nm
northeast of the Hawaii Coastal Waters near 35N 148W. A ridge
trails southwest from the high to 550 nm north of the area. The
pressure gradient south of the ridge is producing fresh to near
gale force easterly trade winds over the coastal waters. A Small
Craft Advisory (SCA) remains in effect through today for the
coastal waters except for the Kauai waters.

The high will move southeast at 10 knots over the next couple of
days. Winds over Hawaiian waters will subside as the high moves
farther away. Meanwhile a front will approach from the northwest
with a trough forming ahead of the front over the northeast
coastal waters. The front will enter the northwest offshore waters
Sunday night, then dissipated. A new high moving east behind the
front will bring a return of trade winds across Hawaiian waters by
Monday night.

A High Surf Advisory remains in effect through this afternoon for
east facing shores of the islands due to strong trade winds
maintaining elevated surf. This surf will subside over the
weekend as the trades weaken.

The current small northwest swell will gradually lower through
today. A slightly larger, northwest swell arriving late today
will likely peak on Saturday, then gradually lower Sunday through
Monday. Surf from these swells will remain well below the advisory
threshold. A large northwest swell is expected to arrive Monday
night. Surf from this swell will peak well above the advisory
level on Tuesday and the surf could reach warning levels.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for east facing
shores of Kauai, Oahu, Molokai, Maui and Big Island.

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

Small Craft Advisory until 6 PM HST this evening for Oahu
Windward Waters-Oahu Leeward Waters-Kaiwi Channel.


&&

$$


DISCUSSION...Foster
AVIATION...TS
MARINE...Donaldson

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



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