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Hawaiian Islands Synoptic Discussion and Guidance

149
FXHW60 PHFO 221416 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
416 AM HST Wed Nov 22 2017

.SYNOPSIS...
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 forecast
starting later today, and continuing through the Thanksgiving
Holiday Weekend. Expect brief showers mainly along windward
sections, occasionally spreading leeward on the smaller islands.
An area of enhanced moisture moving up from the southeast starting
Sunday may bring an increase in low clouds and rainfall,
especially over the windward Big Island.

&&

.DISCUSSION...
A 1031 mb surface high centered near 39N 173W, or about 1500
miles northwest of Honolulu, is moving slowly eastward. The
surface low, which has been responsible for bringing an extended
period of cool, dry air across the Hawaiian Islands is located
more than 1900 miles northeast of Honolulu. This feature
continues to move slowly northeastward away from the area, so it
is finally relinquishing its influence on the weather across the
region. As a result, the pressure gradient is rebounding across
the islands, so moderate northeasterly trade winds are starting
to dominate the low-level flow across the Aloha State early this
morning.

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 7 thousand feet at
Lihue and Hilo according to the balloon soundings this morning.
The precipitable water (PW) value was near 0.95 inches at Lihue,
while the PW was about 1 inch at Hilo. Despite the troughs, the
atmosphere remains somewhat dry and relatively stable early this
morning.

Satellite imagery and radar reflectivity data continue to depict
diffuse east-northeast to west-southwest oriented bands of
scattered to broken low clouds with embedded scattered light
showers moving down across the smaller island early this morning.
Rainfall associated with these bands has been relatively light
over Oahu, but some sections of northeastern Kauai had more than
three-quarters of an inch of rainfall over a 12 hour period since
Tuesday afternoon. These low clouds and showers will continue to
affect windward sections of the most of the smaller islands early
this morning, before spreading down toward the Big Island from
later this morning through this evening.

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. Breezy conditions will continue
from Friday into this weekend. The atmosphere will remain
relatively dry and stable through Saturday. So expect modest
rainfall totals mainly over 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 and evening.

The forecast guidance continues to show the trades will persist
into early next week. Also, an area of enhanced moisture will
likely move up from the southeast toward the eastern end of the
island chain from Sunday into early next week. This suggests that
widespread cloud cover and locally heavy showers could develop
over the windward Big Island starting Sunday or Monday. This
enhanced moisture may also eventually spread up to some of the
smaller islands, such as Maui. We will monitor the latest trends
in the forecast guidance to determine if additional areas may be
subject to heavy rainfall early next week.

&&

.AVIATION...
A high pressure ridge building in from the north will bring
moderate northeasterly trade winds this morning, becoming strong
trade winds later this evening. Expect continued bands of clouds
and showers drifting across the islands today.Scattered showers
will favor 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 for moderate low level
mechanical turbulence will likely be needed by 16Z.

&&

.MARINE...
Combined sea heights are slowly easing just below 10 feet at the
windward nearshore PacIOOS buoys early this morning. However,
a new large north-northeast swell will be spreading into the area
later today. This incoming swell combined with increasing trade
winds will bring Small Craft Advisory (SCA) conditions to almost
all of the coastal waters from this afternoon through Friday, and
we have expanded/extended the previous SCA accordingly. SCA
conditions will likely continue for many areas through the
weekend. Wind speeds may approach gale force in the windiest areas
around Maui County and the Big Island from Thanksgiving Day
through Friday, but there is not sufficient confidence in this
possibility to issue a gale watch at this time.

The lingering north swell is likely maintaining surf near or just
below advisory level along north and some east-facing shores
early this morning. Surf along these shores is expected to rise
rapidly this afternoon into the evening as the 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 remains in effect. The swell and surf will peak
Wednesday night into Thanksgiving Day, then slowly decline
Thursday night and Friday.

A Marine Weather Statement has also been issued to cover possible
harbor surges for the north-facing harbors due to the large
incoming swell.

Yet another north-northeast swell is forecast to arrive late in
the weekend and continue into early next week. The heart of this
swell is forecast to pass east of the state, but it still appears
likely to produce advisory-level surf 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 6 PM HST Friday 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.

Small Craft Advisory from noon today to 6 PM HST Friday for
Maalaea Bay-Big Island Southeast Waters.

&&

$$

Houston

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