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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 210700

Area Forecast Discussion
National Weather Service Honolulu HI
900 PM HST Wed Jun 20 2018

Trade winds will increase tonight as high pressure builds far
northwest of the state. Volcanic haze over the smaller islands
will be clearing out tonight. Trade winds will become breezy
Friday, then eases off slightly on Monday. The trade showers will
favor the windward and mountain areas, although the breezy trades
will help blowing some of these showers into the lee areas of the
smaller islands from time to time.


Easterly winds at the lower levels carried a sliver of the vog
from the Big island to the smaller islands today. Any leftover
of this vog this evening will not stay long as trades with a
slight north component chases it out of the smaller islands by
sunrise Thursday. There were winds today, mainly out of the east
at light to moderate speeds. Not much in the way of showers,
except for the leeward Big Island coast this afternoon.

This pattern will be changing gradually during the next 24 hours
as the trade winds pick up speed, and with this boost in the
trades, so will the frequency of the trade showers, which will be
increasing as well. An eastward moving high, will be strengthening
from a 1032 MB to 1035 MB as it passes north of the islands over
the weekend. In response, the trades will be building to locally
strong by Thursday evening, affecting mainly Maui County and the
Big Island, then becoming quite breezy area wide by Saturday.
So, we won't be ruling out a Wind Advisory at this time. But we
will see what the models do in the coming days, whether the models
backs off on strengthening the high or not as it passes north of
the islands. The trades eases back into the moderate to locally
strong range on Monday, and continues through Tuesday.

An upper level trough is expected to come close to the islands
from the east by Saturday evening before pulling north on Sunday.
This action will be enough to lift the marine layer, which is
capped at 5k feet this evening, to 10 to 12k feet by Friday
afternoon, making the air mass favorable for deeper trade showers
area wide. The air mass recovers some of the stableness on Sunday
with the inversion lowering to 8 to 10k feet across the smaller
islands, but remaining at 12k feet for the Big Island. This will
favor somewhat of a wet trade pattern of the Big Island and Maui.
Both the ECMWF and GFS are showing a broad precipitation field, a
sign of plenty trade showers, upwind of the islands next week.


Mainly dry weather with light background winds and local nighttime
land breezes will continue across the state through early Thursday
morning. Isolated MVFR ceilings are possible along some windward
facing slopes in brief showers. Areas of MVFR visibilities are
possible across parts of the Big Island due to volcanic haze /
emissions from Halemaumau/Kilauea crater and the East Rift Zone.
Otherwise, VFR conditions are expected to prevail. No AIRMETs are
currently in effect, and none are anticipated through early
Thursday morning.

The trade winds will strengthen on Thursday. This may result in a
slight increase in the coverage of low clouds and trade showers
over windward sections, especially late Thursday.


Fresh to strong trades are forecast to return starting late
tonight as high pressure strengthens far northeast of the area. A
Small Craft Advisory (SCA) will likely be needed beginning
Thursday afternoon for winds reaching 25 knots in the typically
windy areas around the Big Island and Maui. Breezy trades with
SCA conditions in those usual areas are then likely to continue
through the weekend and into next week.

Surf will remain small with no significant swells anticipated
into the upcoming weekend. Surf will gradually build along east
facing shores as trade winds return later tonight and increase
Friday through the upcoming weekend. Overlapping long-period south
swells are expected next week with the first swell expected to
arrive around Tuesday. The forecasted local surf heights will be
refined as the initial swell reaches the buoy near American Samoa
during the weekend.






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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