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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 240649
Area Forecast Discussion
National Weather Service Honolulu HI
849 PM HST Thu Feb 23 2017
Dry and breezy trade winds will slowly ease through Friday and
drop to moderate levels by Friday night. Clouds and limited
showers will favor windward and mountain areas through Saturday.
Showers will increase over the state from east to west starting
Sunday with the Big Island see the initial increasing rainfall
over windward and southeast areas. Enhanced showers will push
northwest over the smaller islands as winds veer from the east
southeast Monday and Tuesday. Scattered showers and light winds
are expected later next week.
A high pressure ridge northwest of the state is maintaining
breezy trade winds over the islands, with little to no shower
activity seen on radar, or measured by our rain gage network. The
afternoon soundings from Hilo and Lihue both show a stable, dry
atmosphere over the state with a temperature inversion near 6
thousand feet and low precipitable water values under 1 inch.
Expect similar sensible weather conditions Friday with dry trade
Friday night, trades will diminish as low pressure to the north
pushes the ridge closer to the islands. This weakening pattern
will continue over the weekend. As the ridge approaches, trade
winds will decrease to light to moderate speeds while turning from
an east southeast direction. The ESE winds will maintain a pseudo
trade wind flow over the Big Island and windward areas of the
smaller islands with widespread leeward sea breezes expected
sunday afternoon. The low to the north, and the ESE flow will
bring in increasing moisture from the tropics, and this in turn
will bring increasing rainfall to the windward and southeast
sections of the Big Island. This moisture will slowly work its way
up the islands chain Sunday night and into next week bringing
increasing showers to the entire state.
Aloft, a quasi-stationary trough will stay in place in the
island's vicinity through the middle of next week. Models are
indicating embedded short-wave troughs sweeping over the islands
at times from Sunday into next week, bringing added instability to
the atmosphere. At this time, there is potential for heavier
afternoon convective showers starting Sunday with the increasing
low level moisture, leeward sea breezes converging with windward
trade winds, and the atmospheric instability. One potential cap on
the rainfall will be how fast the enhanced moisture works its way
over the smaller islands. Both the GFS and ECMWF are showing a
slow progression of the enhanced moisture reaching Oahu and Kauai
and this would hinder widespread convection Sunday and Monday
afternoons for the western end of the state.
This hybrid pattern of windward southeast trades combined with
leeward sea breezes looks to continue through the middle of next
week before light southerly winds take over for the second half of
the week. The upper level trough is expected to weaken for the
second half of the week and afternoon convective showers will
diminish as well. Both the GFS and ECMWF show a low forming in the
vicinity of the islands mid-week, but both models quickly speed
the low to the northeast away from the islands, leaving the state
under a light southerly flow with passing scattered showers.
Mostly VFR conditions continue across the islands with brief
periods of light showers along north and east facing slopes.
Breezy trade wind speeds will decrease on Friday as a strong East
Pacific ridge will be weakened by two low pressure systems north
of the Hawaiian Islands. Expect lighter trade winds with a sea
breeze regime for PHKO.
AIRMET Tango remains in effect for low level turbulence to the
South and West of the terrain. AIRMET will likely end later today.
No additional AIRMETs are anticipated.
A Small Craft Advisory (SCA) remains posted for the typically
windier waters around the Big Island and Maui County tonight for
northeast winds to 25 kt. High pressure north of the state will
continue to weaken as a low pressure system develops far north-
northwest of the state and drops south. Winds over all waters
should drop below the threshold by Friday afternoon. Light to
moderate trades are then expected to persist over the coastal
waters through the weekend.
The current west-northwest swell will continue to subside through
Friday as another, slightly larger, west-northwest swell fills in
and peaks Friday night into Saturday. This swell is expected to
bring low-end advisory level surf to the exposed north and west
facing shores during the peak.
A low north-northeast of the state will provide for a small
north swell Friday and Saturday. The swell will then shift out of
the northeast as the low moves east, with the northeast swell
filling in Sunday and peaking Sunday night and holding through
Monday night. A combination of the arriving west-northwest swell
and the north swells may produce combined seas near 10 ft, mainly
for the northern waters Friday and Saturday. This will need to be
monitored for the potential need of a SCA for seas.
The rough and choppy surf along east facing shores should trend
down as the trades weaken further. The small northeast swell early
next week should provide a bump to surf again along the exposed
east facing shores.
A small long period south-southwest swell is forecast for Friday
and Saturday, which could provide a small bump to the relatively
quiet surf along the south shores.
Small Craft Advisory until 6 AM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Southeast Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office