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000 
FXHW60 PHFO 220625
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
825 PM HST Tue Nov 21 2017

.SYNOPSIS...
A poorly organized band of low clouds and showers will move down 
across the islands through Wednesday. Most of the rainfall will be
along north and northeast facing slopes and coastlines. A surface
high building far northwest of the state will move slowly southeast
into early next week, while an upper level trough over the region
gradually weakens. Gusty trade winds are expected starting 
Thanksgiving Day, with brief trade showers mainly along windward 
sections, occasionally spreading leeward on the smaller islands.

&&

.DISCUSSION...
A 1032 mb surface high centered near 39N 173W, or about 1540 miles
northwest of Honolulu, is moving slowly southeastward. The 
surface low, which has been responsible for bringing an extended 
period of cool, dry air across the Hawaiian Islands is located 
about 1740 miles northeast of Honolulu. This feature continues to 
move slowly northeastward away from the area, so it is gradually 
relinquishing its influence on the weather across the region. As 
a result, the pressure gradient is slowly rebounding across the 
islands, so light to moderate northeasterly trade winds are 
starting to dominate the low-level flow across the Aloha State 
tonight.

Aloft, middle- and upper-level troughs continue to reside above 
the islands according to loops of water vapor satellite imagery. 
The low-level inversion height was around 8 thousand feet at 
Lihue and Hilo according to the balloon soundings earlier this 
afternoon. The precipitable water (PW) value was near 0.9 inches 
at Lihue, while the PW was close to 1.1 inches at Hilo. Despite 
the troughs, the atmosphere remains dry and relatively stable 
early this evening.

The leading edge of a diffuse east-northeast to west-southwest
oriented band of broken low clouds and scattered showers appears 
to be across Oahu early this evening. This band, which is nearly 
80 miles wide, has been moving toward the south-southeast at 10 to
15 mph during the past few hours. Rainfall associated with this 
band is relatively light, but some sections of northeastern Kauai
had over one-half of an inch of rain over a 6 hour period earlier
this afternoon. This band, which will bring an increase in low 
clouds and showers especially across windward section, is 
expected to continue moving from Oahu this evening to reach the 
islands of Maui County later tonight. The band will remain across 
Maui County through early Wednesday morning, and will reach the 
Big Island late Wednesday. 

Over the next couple of days, as the pressure gradient continues 
to tighten across the area due to building surface high pressure 
far northwest of the region, the northeasterly trade winds will 
continue to strengthen. We expect the trades to become locally 
windy starting Thanksgiving Day, continuing through Friday. Wind 
speeds are forecast to weaken slightly starting Saturday. However,
we anticipate that trade winds will persist through early next 
week. The atmosphere will remain relatively dry and stable through
Saturday. So expect modest rainfall amounts mainly across 
windward and mauka sections during the start of the Thanksgiving 
Holiday period. However, the strong trades may carry brief showers
over to leeward sections of some of the smaller islands. Also, 
expect the typical cloud cover and a chance of showers over the 
upslope sections of the Big Island each afternoon once the trades 
return.

The forecast guidance continues to show an area of enhanced 
moisture may move up from the southeast toward the eastern end of 
the island chain early next week. If this occurs, we expect 
widespread cloud cover and showers could increase, especially over
the windward Big Island.

&&

.AVIATION...
Cool northerly winds will transition to strong northeasterly trade
winds on Wednesday morning. Expect continued bands of clouds and 
showers drifting across the islands tonight with scattered 
showers favoring mountain areas and the north through east slopes 
of all islands. VFR conditions prevail with only isolated MVFR in 
moderate showers. 

No AIRMETs are in effect. AIRMET Tango will likely be needed 
before 16Z Wednesday morning.

&&

.MARINE...
Combined sea heights continue hovering near 10 feet at the
windward nearshore PacIOOS buoys early this evening, as the north
swell is still running above Wavewatch guidance. Therefore the
Small Craft Advisory remains in effect through noon Wednesday for
all waters exposed to the north swell. This swell should finally
drop slightly below 10 feet late tonight or Wednesday morning, 
but a new larger north swell will be arriving into the area later 
Wednesday. This incoming swell combined with increasing trade 
winds will likely require the SCA to be extended/issued to cover 
all of the coastal waters from late Wednesday through at least 
Friday, with the SCA then continuing for some of the waters into 
the weekend. Wind speeds will probably reach near gale force in 
the windiest areas around Maui County and the Big Island from 
Thanksgiving Day through Friday.

The lingering north swell is likely maintaining surf near or just
below advisory level along north and some east-facing shores this
evening. This surf is expected to subside a bit more late tonight
into Wednesday morning, then rise rapidly Wednesday afternoon as 
a new large north-northeast swell arrives. This swell is forecast 
to produce warning-level surf on north and exposed east-facing 
shores, and a High Surf Warning has already been issued. The swell
and surf will peak Wednesday night into Thanksgiving Day, then 
slowly decline Thursday night and Friday. 

Yet another north-northeast swell is forecast to arrive late in
the weekend and continue into early next week. This swell appears
likely to produce advisory-level surf once again on north-facing 
and some east-facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Friday for north and east 
facing shores of Niihau, Kauai, Oahu, Molokai, Maui, and the Big 
Island.

Small Craft Advisory until noon 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-
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters.

&&

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