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Hawaiian Islands Synoptic Discussion and Guidance

512
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.

&&

$$

Discussion...Houston
Aviation...Bohlin
Marine...Jacobson

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



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