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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 231356
Area Forecast Discussion
National Weather Service Honolulu HI
356 AM HST Wed May 23 2018
Refreshing trade winds will continue into the upcoming Memorial
Day weekend as a high pressure system passes north of the area.
The trades will be particularly breezy through Thursday night
before weakening on Friday and Saturday.
A new surface high pressure system with a central pressure of
1031 mb is currently 915 miles N of Kauai. It will be moving
slowly E, passing N of the islands during the next 24 hours. In
doing so, rather breezy trades are in store for the islands
through Thursday night, and may be Friday. Trades are in the
forecast for Memorial Day, but is subject to change.
The high will be some 1600 miles NE of the islands by Saturday,
far away where the trades trend weaker to moderate to locally
strong, with the strong pertaining to Maui and the Big Island. The
trades may trend even lighter to light to moderate speeds by
Monday of next week. A dissipating front is located 300 miles N
of Kauai. It is not expected to reach the islands. However,
scattered, fragmented pieces of shower-bearing clouds, embedded
in the trade flow, will affect the windward and mountain areas of
about all the islands into the foreseeable future. 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 a pretty
strong inversion between 6500 feet over Lihue, and 8500 feet
over Hilo. 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
Kauai. There are some uncertainties and differences between the
ECMWF and GFS in the handling of the upper trough late in the
weekend and Monday. The GFS solution is faster than the EC with
the upper trough over Kauai Monday evening, while the EC has the
trough west of Kauai. The earlier 00z GFS run had low level ESE
winds over most of the area. The 12z run is quite different with
moderate to strong trades. The ECMWF shows an inverted trough over
Maui County. In due course, the model will iron out these
differences. At this point, as far as Memorial Day is concern, we
have sided with the GFS solution, that is for mainly moderate
trades. But, it should be cautioned that this is subject to change.
High pressure north of the state will maintain locally breezy
trade winds over the next few days. The early morning soundings
from PHTO and PHLI show a strong low level inversion between 6-8
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. 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 most of the coastal waters
through Thursday night before easing off slightly on Friday. The
trades are expected to trend down a little more over the weekend
as an approaching front shifts the high northeastward and further
away from the island chain. A Small Craft Advisory (SCA) is now in
effect for most marine zones through tonight, and all waters east
of the Kauai Channel with the exception of the windward Big
Island waters through Thursday night. SCA conditions will likely
continue into the weekend for the typically windy 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 today, then continue into the
weekend. A small north-northwest swell will peak today then slowly
diminish through Thursday. Another small northwest swell may give
surf a slight bump along north facing shores 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 AM HST Friday for Oahu windward and
leeward waters, Kaiwi Channel, and Maui County windward Waters and
Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest
waters and Kauai Channel.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office