National Weather Service Raw Text Product

064 
FXHW60 PHFO 180717
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
917 PM HST Tue Dec 17 2019

.SYNOPSIS...
High pressure northeast of the state will push eastward over the
next couple days, resulting in a gradual decrease in the trade
wind speeds across the state. A new high building north of the
islands will increase the trades once again Thursday night and
Friday, with very strong winds expected Friday night through
Saturday night. Showers will favor windward and mauka areas
through the period, with a few showers reaching leeward areas from
time to time. A developing area of low pressure west of the
islands will drag a cold front toward the state Sunday through
early next week, initially weakening the winds Sunday and Sunday
night, before strong Kona winds develop early next week. Rather
unsettled weather with locally heavy rainfall and thunderstorms
will be possible late Sunday through early next week. 

&&

.DISCUSSION...
Currently at the surface, a 1028 mb high centered around 1000
miles northeast of Honolulu, is driving breezy to locally windy
trades this evening. Meanwhile, a cold front is located around 
1000 miles northwest of Kauai, with a new building 1032 mb high 
around 2000 miles northwest of the islands. Infrared satellite 
imagery shows mostly cloudy conditions in most windward and mauka 
areas, with partly cloudy skies in leeward locales. Radar imagery 
shows scattered to numerous showers moving into windward areas, 
with a few showers reaching leeward areas as well due to the 
strength of the trades. Main short term concerns revolve around 
trade wind trends over the next few days, with longer term focus 
around the potential for a Kona weather event next week.

High pressure northeast of the state will shift eastward and 
gradually weaken over the next few days, as a cold front 
approaches from the northwest. This will lead to a downward trend 
in the trades through Thursday. We will keep the Wind Advisory up
through the overnight hours due to winds near advisory 
thresholds. A much stronger high will build in behind the cold 
front as it approaches the islands Thursday night and Friday, 
leading to strengthening trades once again. This high will pass 
north of the islands Friday night through Saturday night, with 
very strong trades expected across the island chain. Wind 
Advisories will likely be required for much of the state during 
this time. A developing area of low pressure west of the islands 
and the lifting northeast and away from the state, is expected to
shift the boundary layer winds around to the southeast Sunday and
Sunday night. As a result, we should see a decrease in wind 
speeds across most of the area due primarily to sheltering 
effects from the island terrain. The low to the west of the state 
will lift northeastward to the west of the islands early next 
week, dragging a surface cold front eastward toward the state. The
models have come into much better agreement on this scenario over
the past couple days, but there remains some uncertainty with 
respect to the exact details given this highly anomalous pattern. 
Overall, we should see winds swing around to the south Monday 
through Tuesday, and we could see a rather windy Kona wind event 
unfold across the islands. Stay tuned as details get refined over 
the next several days.

As for weather details, a fairly typical trade wind pattern should
hold in place through the weekend with showers favoring windward
and mauka areas, along with a few leeward spillovers from time to
time. Depending on the southward extent of the front approaching
the islands Friday through Sunday, we could see a rather wet trade
wind pattern set up. There remains some uncertainty in the timing
details with respect to a cold front moving eastward through the 
islands early next week, but overall, some unsettled weather is 
beginning to look more probable statewide. Locally heavy rainfall
and thunderstorms would be possible if this scenario materializes
as currently advertised by the models, and given the strength of 
the dynamics aloft, we could be looking at some rather strong
thunderstorms as well.

&&

.AVIATION...
A high pressure system northeast of the islands will move 
eastward overnight with trade wind speeds trending lower through 
Wednesday. Moderate trade winds are forecast from Wednesday 
through Thursday. A band of unstable clouds moving through the 
islands will produce MVFR cloud ceilings and showers in the 
overnight and early morning hours along the windward slopes of all
islands. VFR weather conditions are forecast across the state 
after 21Z.

AIRMET Sierra remains in effect for tempo mountain obscurations
above 020 along north and east facing slopes of Kauai, Oahu,
Molokai, Maui and the Big Island. AIRMET Tango remains in effect
for tempo moderate isolated severe turbulence below 090 south
through west of all island mountains.

&&

.MARINE...
High pressure northeast of the state will maintain strong to gale
force trade winds through tonight. A slight decrease in wind 
speeds is expected Wednesday into Thursday as a front passes by 
north of the state. Winds will strengthen once again Friday 
through Saturday as a strong high builds in to our north, with 
some areas reaching Gale force. Gales to near gales will be 
possible for all Hawaiian Waters Friday night into Saturday. Gale 
force trade winds are very unusual for this time of year, so 
mariners please take caution if you are going out to sea later 
this week. Seas will remain rather elevated through rest of the 
week due to the strong trade winds as well as northwest swells 
moving through the area. 

The current northwest swell is on its way down and the High Surf
Advisory has been cancelled for north and west facing shores.  
The next northwest swell is expected to fill in Wednesday night 
and peak near advisory levels for north and west facing shores. 
This swell should gradually dissipate Friday through the weekend. 

Afternoon ASCAT pass showed a wide fetch of strong to near gales 
over and upstream of the state. Buoys exposed to the wind swell
continue to run higher than guidance and a High Surf Advisory is 
in effect for east facing shores through tonight. Surf may 
slightly drop on Wednesday as the trades weaken a bit. But very 
strong trades are expected to return Friday and will likely bring 
advisory level surf Friday into Saturday along east facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Wednesday 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 Wednesday for Lanai-Kahoolawe-Maui 
Windward West-Maui Leeward West-Maui Central Valley-Windward 
Haleakala-Leeward Haleakala-South Big Island-Big Island North 
and East-Kohala-Big Island Interior-Big Island Summits.

Gale Warning until 6 AM HST Wednesday for Maalaea Bay-Pailolo 
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island 
Southeast Waters.

Small Craft Advisory until 6 PM HST Wednesday for Kauai 
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi 
Channel-Maui County Windward Waters-Maui County Leeward Waters-
Big Island Windward Waters.


&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Foster