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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 222001

Area Forecast Discussion
National Weather Service Honolulu HI
1001 AM HST Fri Feb 22 2019

A surface trough near the Big Island will support increased
clouds and chances of showers over the eastern end of the island
chain through the weekend. Meanwhile, weak high pressure to the
northwest will support mostly dry weather over Kauai and Oahu. A
cooler northerly wind flow pattern is expected to bring drier air
to the state next week.


Overall quiet weather pattern for much of the state today.
However, the Big Island remains in the vicinity of a surface trough
located just to the east. Result of the upper level jet nearby is
strong westerly winds over the Big Island summits requiring a
wind advisory through the day. In addition to the winds, a thin
layer of clouds is intersecting with the summits at near freezing
temperatures, which may bring pockets of ice and fog, especially
this morning. The winds are expected to decrease below advisory
levels by tonight. Also, higher moisture levels across the rest of
the Big Island will bring an increasing chance for showers and a
slight chance of thundershowers through the rest of the day and
tonight to mainly the Hilo, Puna and Kau districts.

Otherwise, light winds today will allow afternoon sea breezes to
drive cloud formation over interior portions of Kauai, Oahu and
parts of Maui county, but much drier and more stable airmass will
likely yield little in the way of rainfall.

Model guidance shows little change in this pattern over the
weekend with an axis of moisture stuck near the Big Island, and
occasional, hard to accurately time, upper level shortwaves
bringing potential bursts of deep convection and locally heavy
rainfall Saturday and Sunday on parts of the Big Island and maybe
as far west as eastern Maui. The western portion of the state
remains under a much drier and more stable airmass during the
weekend for quieter weather.

Medium range ECMWF shows another trough developing far northeast
of the state by the middle of next week. This would put the state
in a cooler northerly flow, but without much moisture or
dynamical support for rainfall. If subsequent guidance shifts the
trough closer to the state than presently shown, that would
increase the threat for gusty north to northwest winds and rain
showers and summit snowfall around the middle to end of next week.
Will need to keep monitoring this period for shifts in guidance.


Patches of mid-level clouds continue to stream across the eastern
end of the island chain early this morning. The background low-
level flow will remain light today as a weak surface trough near
the eastern Big Island tracks slowly westward. An area of
enhanced moisture also remains in the vicinity of the Big Island.
This is expected to cause low clouds and showers to increase over
the windward Big Island this morning. AIRMET Sierra for mountain
obscuration due to these low clouds and showers is in effect for
the windward Big Island. By this afternoon, a slight chance of
thunderstorms is also included in the forecast for most of the
Big Island.

The low clouds and showers are expected to spread over portions
of windward Maui later today and tonight. As a result, AIRMET
Sierra may need to be extended to windward Maui. Elsewhere,
mainly VFR conditions are expected to prevail today.


Minimal marine hazards are expected through the weekend. A surface
trough near the Big Island and a bubble high north of the area
will keep light northeast winds over the smaller islands through
Sunday. The winds will be light enough for sea breezes to develop
during the day. Light to moderate southeast to south winds should
be expected through the windward and southeast Big Island waters.
Periods of heavier showers and a slight chance of thunderstorms
are also expected over these waters through tonight. The surface
trough will being to shift east over the weekend as high pressure
builds to the northwest and a front passes to the north.

A declining east swell will keep texture to the eastern waters
and produce surf just below advisory levels today. This swell will
fade through the weekend. Surf elsewhere will remain quiet until
the beginning of next week. A hurricane force low is expected to
develop off of Japan this weekend and send a large, long period
northwest swell towards the islands Tuesday through midweek. Surf
heights could near warning levels.

Additionally, a cold front will approach and move into the area
from the north Monday night through midweek, which will bring
fresh to strong north winds and rough seas (small craft advisory
conditions) across the waters Tuesday through midweek.


Wind Advisory until 6 PM HST this evening for Big Island Summits.




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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