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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 200632

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Sat Jan 19 2019

Winds will shift from trades to southeasterlies tonight ahead of
a weak cold front approaching the islands from the northwest. The
fronts forward motion stalls the clouds and showers near Kauai on
Monday with potential for some elevated showers over the island
from late Monday into Tuesday. A weakened ridge north of the
state will keep light winds will keep land and sea breezes in the
forecast through Thursday. Another weak front should move into
the region on Friday.


A 1022 MB surface high north of the islands will continue to slide
eastward as a cold frontal trough moves into the Central Pacific.
This front will weaken the ridge north of the islands overnight
turning the light to moderate trade winds towards a southeasterly
direction by Sunday morning. Satellite imagery this evening
already shows clouds moving in a more southeast to northwest
direction as the ridge to the north weakens. These lighter winds
will allow land and sea breezes to form over the islands in a
fairly dry weather pattern.

The cold front moves into the Hawaii region from late Monday into
Tuesday mainly enhancing showers of Kauai at this point. The
global weather models continue to struggle with the forward motion
on this weak frontal zone as it moves into a surface ridge. The
latest model solutions are trending towards a near miss for the
islands. We are currently showing scattered showers for most
populated areas in our precipitation forecast grids for Kauai and
Oahu, these grids will likely be adjusted slightly this evening
towards a later arrival time on the elevated showers over these
western islands. The cold front will then lift northward by
Tuesday afternoon in continued light southeasterly flow. Expect
light and variable winds across the state as land and sea breezes

Wednesday and Thursday, another cold front will approach the
islands from the northwest direction eventually merging with the
cold front. The Hawaiian Islands will remain just south of a
weakened high pressure ridge in light southeasterly flow. A drier
weather pattern will linger through Thursday with light winds
modified by land and sea breezes over each island. The Big Island
may see continued scattered showers along the eastern slopes.

Friday and Saturday, trade winds return on Friday as another high
pressure system migrates across the Central Pacific. A cold front
passing north of the islands may drift far enough south to enhance
some showers over the northwestern islands during this period. At
this point its a 50/50 coin flip that the front will reach the
islands of Kauai and Oahu as the low level winds steering the
front will flow more parallel to the frontal boundary. Less push
behind the front means the frontal movement towards the south will
stall as it approaches the state. This lack of steering flow
towards the islands creates a large degree of uncertainty in the
rainfall potential for this next frontal system. Recent extended
range model solutions are keeping the clouds and showers well
north of the island chain. Will likely trend towards a drier
solution at this time with scattered rainfall coverage favoring
windward and mountain areas in a trade wind flow pattern.


Trade winds will gradually weaken tonight and then veer to the
southeast as another front approaches the area from the northwest.
Cloudiness, and any accompanying showers (MVFR conditions), will
initially be concentrated over windward and mauka sections. As the
winds become lighter through Sunday, cloudiness and showers will
tend to form over interior, mauka, and leeward areas during the
afternoon hours. Predominantly, however, VFR will hold sway.

No AIRMETs are currently in effect.


Winds will gradually shift out of the southeast overnight through
Sunday as a cold front approaches the state from the west.
Terrain-induced wind accelerations that typically occur with
southeast flow will result in small craft advisory conditions for
the waters around Maui County (windward) and the Big Island
(Hamakua coast/South Pt waters) through Sunday before trending
down. Light to moderate southerly winds are expected over the
western waters surrounding kauai and Oahu, with land and sea
breeze conditions likely near the coasts. The front is expected
to stall and diminish near or just north of Kauai through the
first half of the week bringing scattered showers (mostly favoring
the Kauai waters). A similar pattern will hold through much of
the week as the ridge of high pressure remains in the vicinity of
the islands keeping the gradient weak.

Surf along north and west facing shores will steadily lower
through Sunday, then gradually rise Monday through Thursday as
moderate to large overlapping northwest swells move through the
Hawaiian waters. The latest analysis and satellite data showed a
broad and complex gale- to storm-force low positioned over the far
northwest Pacific with a favorable batch of winds focused at the
islands within the 300-320 deg directional band around 2500 nm
away. Altimetry data confirmed this with wave height estimates
reaching the 35 to 40 ft range. Guidance seems to be handling this
scenario reasonably well and depicts a long-lived event unfolding
locally with advisory-level surf beginning as early as Tuesday
night and continuing through the end of the week, potentially
reaching or exceeding warning levels around the peak Thursday for
exposed north and west facing shores.

Surf along east facing shores will remain small into the upcoming
week as the local winds trend down. A gradual rise associated with
a potential easterly groundswell will become a possibility
Wednesday into the second half of the week due to a an upstream
pocket of fresh to strong breezes within the trades.

Surf along south facing shores will remain near the seasonal
average through the week with mainly background short-period
southeast trade wind energy.


Small Craft Advisory until 6 PM HST Sunday for Maui County
Windward Waters-Alenuihaha Channel-Big Island Windward Waters-
Big Island Leeward Waters-Big Island Southeast Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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