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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 221407
Area Forecast Discussion
National Weather Service Honolulu HI
407 AM HST Fri Jun 22 2018
A moderately strong high pressure system is slated to pass north
of the islands over the weekend, resulting in a boost in the trade
winds to breezy conditions. This ramping up of the trades will be
accompanied by an increase in trade showers, where the lee side
of the smaller islands will experience some passing showers.
Leeward Big Island will have some scattered showers each afternoon
and evening for the coming days.
The strengthening trades is being brought on by an approaching
surface high currently located 1515 miles northwest of Kauai. It
has a central pressure of 1028 MB, and is forecast to strengthen
to 1035 MB when it passes some 1150 miles north of the islands
Saturday and Saturday night. The trades will strengthen to
moderate to strong by this evening that will warrant an expansion
of the Small Craft Advisory, and possibly a limited Wind Advisory
to the notoriously windy areas such as the Kohala Mountains area
and Lanai. The trades are slated to tone down slightly, to moderate
to locally strong on Sunday night. Another round of breezy trades
may resumes on Tuesday night as this same high makes a turn to
the south and southwest, back toward the islands.
Satellite imagery shows plenty of open-cell clouds upwind of the
islands. They are moving west at 20 to 25 mph, with a cloud top of
between 8 and 10k feet. The upper low currently 730 miles east-
northeast of the Big island, continues to move west at 20 mph.
The ECMWF and GFS models are still in good agreement in the track
of this low, where they take it northward after reach 150w
longitude Saturday evening. The GFS solution has the marine layer
at 10k area wide tonight, and will lift higher to 14k feet over
the Big Island by this afternoon. It remains in the 10 to 14k
foot range through Saturday night when a mid level ridge comes in
and lowers the inversion to 8 to 10k feet. Although we may see a
slight drop off in the trade showers, orographic influence, can
still maintain a wet trade pattern.
Trade winds will be strengthening today as high pressure builds
far northeast of the state. VFR will prevail over most sections of
the individual isles, with brief MVFR conditions likely in
windward and mauka areas in passing showers, with the Big Island
seeing the most precipitation.
No AIRMETs are currently posted. However, one for low level
turbulence may be needed this morning as trades become stronger.
A 1029 high about 1300 nm northwest of the main Hawaiian Islands
is forecast to move east at 20 kt. The high will build to 1035 mb
as it reaches a point about 1000 nm north of the area Saturday
night. The pressure gradient near the main Hawaiian Islands is
already tight enough to produce locally strong easterly trade
winds. A Small Craft Advisory (SCA) is in effect through Saturday
for Maalaea Bay, the Kaiwi, Pailolo and Alenuihaha channels, the
windward Maui county waters and waters south of the Big Island.
Wind speeds may reach near-gale force in Maalaea Bay and the
Alenuihaha and Pailolo channels. As the high moves farther east
early next week, wind speeds will gradually subside.
Surf will gradually build along east facing shores as trade winds
strengthen, but is expected to remain below the advisory
threshold. Surf will remain small along other shores with no
significant swells through the weekend. A pair of swells from
southeast of New Zealand will reach the area next week. The first
swell is expected to peak Wednesday and the second on Thursday.
Surf along south facing shores may reach advisory levels.
Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay,
Pailolo and Alenuihaha channels, and waters south of the
Small Craft Advisory until 6 PM HST Saturday for Kaiwi Channel,
and windward Maui County waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office