Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 180138

Area Forecast Discussion
National Weather Service Honolulu HI
338 PM HST Sat Mar 17 2018

Trade winds will spread westward across the island chain
beginning tonight. A moisture band will pass southward across the
islands tonight and Sunday, ushering in a slight increase in
showers and sky cover. An approaching front will stall west of
Kauai by midweek, causing local winds to veer a bit to
southeasterlies and re-introducing the possibility of wet weather
for the second half of the work week.


Light to moderate trade winds across far eastern waters will
spread westward overnight. Models show that weak low pressure
near 30N150W, northeast of the main Hawaiian Islands, will move to
the east tonight through early next week, allowing trade flow
induced by high pressure far to our north to spread westward
across the chain. The cloud cover and light showers which formed
this afternoon across mauka areas should decrease as the trades
spread westward this evening.

GFS depicts a narrow but distinct band of moisture moving
southward across the islands tonight and Sunday, likely an old
frontal remnant. In contrast, ECMWF shows a much less distinct
feature moving across the chain. Our forecast follows GFS but,
either way, the increase in trade flow mentioned above will be
most noticeable as this moisture band passes by. We also expect
increased associated shower activity overnight and Sunday.

A front expected to stall just west of Kauai will cause winds to
veer to southeasterlies near Kauai beginning Tuesday, while
moderate trades continue elsewhere. GFS and ECMWF are in rather
close agreement concerning the timing of this front`s movement,
with both stalling it 300 to 500 miles west of Kauai by midweek.
Moisture brought northward by the southeasterlies expected near
Kauai may increase rainfall across the western islands during the
second half of the coming work week. ECMWF wants to push the front
across Kauai this weekend while GFS keeps it stalled just west of
Kauai. While this difference would affect the local day 6 and 7
wind forecast, both scenarios paint a wet picture for next
Saturday and Sunday. We will stick with GFS for now, maintaining
trades or prefrontal southeasterlies through the forecast period
instead of long range postfrontal northerlies.


High pressure well to the north of the state will keep a light to
moderate trade wind flow in place through Sunday. A band of
clouds and showers will shift southward into the island chain
tonight and Sunday, bringing an increase in low clouds and
showers, particularly to windward areas.

No AIRMETs are currently in effect. AIRMET Sierra may be needed
for windward sections of Kauai later this evening.


No marine warnings or advisories are in effect for the next couple
of days. Light winds will give way to moderate trades tonight,
which will persist through Tuesday. Winds may approach Small Craft
Advisory (SCA) levels in the windy zones around Maui County and
the Big Island around Tuesday or Wednesday. Winds will begin to
shift out of the east southeast late Tuesday through the second
half of the upcoming week as a front approaches from the west and
stalls west of Kauai. The front may bring a period of wet weather
during the second half of the week and next weekend.

The current northwest will fade tonight with a more northerly
swell short period swell filling in on Sunday. Small long period
south swells should provide above null surf for the south facing
shores over the next few days. Surf heights will remain below
advisory levels on all shores over the next several days.

A broad low is forecast to develop and move into the northeast
Pacific late this weekend and direct a large swath of gale force
winds towards the islands. This should translate to a building
north-northeast swell late Tuesday, and could bring advisory
level surf to north and east facing shores through at least
Thursday, possibly longer for east facing shores.





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