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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 230649
Area Forecast Discussion
National Weather Service Honolulu HI
849 PM HST Tue May 22 2018
Refreshing, trade winds will continue into the upcoming weekend
as a high pressure system passes north of the area. The trades
will be particularly breezy through Thursday before weakening on
Friday and Saturday.
A new surface high pressure system with a central pressure of
1031 mb, is currently 980 miles NNW of Kauai. It will be moving
slowly E as it passes N of the islands between Wednesday and
Thursday. In doing so, rather breezy trades are in store for the
islands through Thursday night, and may be Friday.
The high will be some 1600 miles NE of the islands, far away
enough where the trades trend weaker to moderate to locally
strong, with the strong pertaining to Maui and the Big Island.
The trades may become light to moderate by Monday of next week.
A dissipating front is located 300 miles N of Kauai. It is not
expected to reach the islands. However, small, fragmented pieces
of shower-bearing clouds, embedded in the trade flow, will affect
the windward and mountain areas of about all the islands. Due to
the breezy trades, expect a few of these showers to venture
downwind into the lee areas of the smaller islands. Relatively
light rainfall amounts are expected with these showers.
The air mass is quite stable over the islands, with an inversion
between 8k feet for the smaller islands and to 10k over the Big
Island. This air mass is forecast to last through the Friday
before weakening over the weekend, where the inversion rises to
10k feet state wide. The GFS is ushering in an area of high
precipitable water (PW) on Monday with values around 1.65 inches.
Most of the PW is due to a dense layer of mid and high layered
clouds in association with an upper level trough just west of the
High pressure north of the state will maintain locally breezy
trade winds over the next few days. The afternoon soundings from
PHTO and PHLI show a strong low level inversion between 6-7 kft.
The combination of the breezy trade winds and stable atmosphere
will create conditions for more wide spread low level turbulence
and and AIRMET TANGO is posted over and immediately south through
west of the mountains.
Low level moisture embedded in the trades will focus clouds and
isolated to scattered showers along the windward and mountain
slopes. Some passing showers may introduce periods of MVFR
CIGS/VIS. However, cloud and shower coverage is not expected to be
widespread enough to warrant the need for mountain obscurations.
Otherwise, expect predominately VFR conditions.
Periodic explosive eruptions at Kilauea/Halemaumau crater
continue to produce periods of volcanic ash, with the plume of
emissions predominately rising to the inversion level and
extending to the southwest over Kau district on the Big Island.
MVFR VIS can be expected in this area. Volcanic ash SIGMET TANGO
series remains in place at this time.
High pressure passing by to the north of the state will keep a
breezy trade wind flow in place across the coastal waters through
Saturday. The trades are expected to trend down Sunday through
early next week as an approaching front shifts the high
northeastward and further away from the island chain. The Small
Craft Advisory has been expanded to include most marine zones
tonight and Wednesday, and the SCA may need to be expanded in area
and/or time with the Wednesday morning forecast package. SCA
conditions will likely continue into the weekend for the typically
windier waters around Maui and the Big Island.
No significant swells are expected, with surf remaining below
advisory levels into next week. A small, long-period south-
southwest swell will slowly build on Wednesday, and then continue
into the weekend. A small north-northwest swell arriving Wednesday
will diminish on Thursday, with another small northwest swell
possible over the weekend. Breezy trade winds will deliver choppy
short-period waves to east facing shores into the weekend.
Small Craft Advisory until 6 PM HST Friday for Maalaea Bay,
Pailolo and Alenuihaha channels, and waters south of the Big
Small Craft Advisory until 6 PM HST Wednesday for waters NW
of Kauai, Kauai Channel, Oahu Leeward and Kaiwi Channel.
Small Craft Advisory from 6 AM to 6 PM HST Wednesday for Oahu
Windward waters of Oahu and Maui County.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office