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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 232000
Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Sat Jun 23 2018
High pressure north and northeast of the islands will support breezy
trade winds for the next week. The trades will deliver passing
clouds and showers that will primarily affect windward slopes and
coasts, with a few brief showers spreading to leeward areas of the
smaller islands. Leeward slopes of the Big Island will see afternoon
and evening clouds that will drop a few showers before generally
clearing at night.
A 1034 mb surface high far N of the islands will move steadily E
through Tuesday while maintaining a central pressure in the 1030's.
The high is then expected to wobble around to the distant NE of the
islands through the end of next week. The associated pressure
gradient will ensure seasonable breezy trade winds remain over the
islands for the foreseeable future, with some subtle fluctuations.
Water vapor imagery and model analyses indicate a mid- to upper-
level low centered about 500 miles NE of the Big Island. Upper air
soundings and latest visible satellite images indicate that this
feature is not significantly destabilizing the lower atmosphere, but
an increase in showery low clouds is noted just a little E of the
islands, and extending E of 150W. In the short term, there is a bit
of a break in the low cloud field immediately upstream of the
islands from Oahu to the Big Island, with showery low clouds in
greater concentration near and E of Kauai. This should keep Kauai
somewhat showery today while the other islands see fewer showers.
Model guidance appears to have initialized well, and depicts the mid-
level low gradually weakening as it drifts W over the next 24 to 48
hours, while the upper-level low moves N. This is expected to lead
to an increase in trade wind showers later tonight and on Sunday,
and potentially again around Monday night, as the mid-level low
weakens to a trough and moves over the islands. Most of the
associated moisture will focus over windward slopes and coasts, but
the locally strong trade winds will push a few showers to leeward
areas on the smaller islands. Leeward slopes of the Big Island will
see the typical afternoon and evening clouds and showers that tend
to gradually clear overnight.
While the trade winds will keep the Big Island's volcanic emissions
away from the smaller islands, they will continue to stream over
portions of the Puna and Kau districts. Additionally, eddies that
develop to the lee of the Big Island will occasionally lead to vog
moving ashore over the Kona and Kohala districts.
Breezy, occasionally strong, trade winds will persist through the
day as a ridge of high pressure remains north of the state. VFR will
predominate, with passing showers bringing brief MVFR conditions to
windward and mauka sections of the islands.
AIRMET Tango remains in effect for low level turbulence over and SW
of all islands.
Trades are forecast to steadily increase through the weekend and
hold in the fresh to strong category as high pressure builds north
of the state. A Small Craft Advisory remains in effect for most
waters east of Oahu through Sunday due to the breezy winds. Although
the trades may slightly back off briefly by Monday, locally strong
trades will remain likely over the typical channels around Maui
County and south of the Big Island.
Surf along east facing shores will become rough through the weekend
as the trades increase locally and upstream across the eastern
Pacific. After the trades briefly weaken around Monday, a surge
upstream and locally will likely translate to increasing surf that
may near the advisory level by midweek.
Surf along south facing shores is expected to trend back up next
week as overlapping long-period south swells fill in due to recent
activity across the southern Pacific. The first swell is currently
building at the American Samoa buoy within the 14-16 second bands
this morning, which should continue through the weekend as the
second source fills in. The latest WAVEWATCH III guidance depicts
this source peaking Monday at the Samoa buoy at 7 ft 15 seconds.
Locally, surf will begin to build along south facing shores from the
first source Tuesday, then become reinforced by Thursday as the
second/larger source fills in. Surf may reach the south shore
advisory level at the peak of this reinforcement by Thursday.
Looking ahead into the first week of July, the active trend could
persist along south facing shores. The latest ECMWF and GFS guidance
supports a gale-force low passing just south of the Tasman Sea and
New Zealand today through Monday with gales setting up within
Hawaii's swell window.
Small Craft Advisory until 6 PM HST Sunday for Oahu Leeward Waters-
Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters-
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward
Waters-Big Island Southeast Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office