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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 210625
Area Forecast Discussion
National Weather Service Honolulu HI
825 PM HST Mon Sep 20 2021
A high pressure ridge north of the state will maintain moderate
to breezy trade winds across the region through Saturday. Wind
speeds will decrease on Sunday and Monday as the ridge north of
the region weakens. A few disturbances moving through the region
over the next several days will keep periods of passing clouds
and showers in the forecast, favoring windward and mountain areas,
and trending higher in the overnight to early morning hours.
The satellite picture this evening shows an upper level trough
just east of the Hawaiian Islands keeping periods of passing
showers in the forecast across the region. A weak upper level low
continues to drift in from the north passing through the western
islands over the next few days. A weak low level trough currently
southeast of the Big Island will also drift westward through the
area. Satellite imagery and model guidance continue to show
scattered to broken high level cirrus clouds drifting over the
islands through Tuesday. These high level ice crystal clouds will
enhance sunrise and sunset colors.
The high pressure ridge north of the islands will remain strong
enough to produce moderate to breezy trade winds each day through
Saturday. Slight variations in the strength of the trade winds
are expected from day to night, with decreasing wind speed trends
for most land areas in the overnight to early morning hours.
Decreasing trade winds are forecast across the region on Sunday
as the ridge north of the state weakens due to a cold front moving
into the Central Pacific basin.
A combination of upper level and low level disturbances across the
region will weaken trade wind temperature inversion heights and
produce periods of passing showers for all islands into the
weekend. Expect day to day changes in shower and cloud coverage
as these disturbances move through the state. A low level
disturbance southeast of the islands will drift westward and
brush through the Big Island from Monday afternoon into Tuesday.
Increasing moisture with this disturbance will enhance showers
along the east and southeast slopes Maui and the Big Island as it
passes. Higher moisture levels may add a few more showers to the
Kona slopes during the afternoon to early evening hours. A weak
upper low moving in from the north will approach Kauai and Oahu on
Tuesday and Wednesday. This system will move out fairly quickly
and continue to keep passing showers in the forecast. Another
unstable low level trough will move from east to west across the
island chain on Saturday and Sunday.
High pressure north of the state and a surface trough swinging by
to the south, will keep breezy trade winds in place during the
next 24 hours. Fairly typical trade wind weather is expected for
the smaller islands, with showers favoring windward and mauka
areas, particularly during the overnight and early morning hours.
Brief MVFR cigs/vsbys will be possible in passing showers across
these windward areas. Over the Big Island, more unsettled weather
is expected. An area of enhanced moisture on the northern fringe
of the surface trough will bring widespread MVFR cigs/vsbys to
windward sections later tonight through much of the day Tuesday.
Conditions should improve over windward Big Island Tuesday
AIRMET Sierra is in effect for mountain obscuration across
windward sections of the Big Island. This AIRMET will likely
remain in place tonight and possibly much of the day Tuesday.
AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain of all islands. This AIRMET will
likely remain in place through at least Wednesday.
The northern Pacific has begun to transition out its summer
doldrums featuring a series of gale- to storm-force lows driving
overlapping swells toward the islands this week. Surf will respond
along exposed north and west facing shores and trend up beginning
Tuesday. This first north-northwest pulse will peak Tuesday
night, hold Wednesday, then ease Thursday. A larger north-
northwest pulse should begin to fill in by Thursday night, peak
just under the advisory level Friday, then slowly fade into the
Surf along south facing shores will trend up by midweek as a new,
long-period south swell arrives and moves through. This event should
peak around Thursday before fading into the weekend.
Surf along east facing shores will remain small and choppy due to
the fresh/strong trades locally and just upstream of the state.
Fresh to strong trade winds will persist each day over the local
waters through the week. The Small Craft Advisory winds are
expected over the windier areas and channels from the Kaiwi
Channel (will be added to the advisory later tonight) to the Big
Island. Seas will near the advisory-level of 10 ft Friday due to a
combination of the long- period north- northwest swell and trade
Red Flag Warning until 6 PM HST Tuesday for Kauai Leeward-Oahu
South Shore-Waianae Coast-Oahu North Shore-Olomana-Central Oahu-
Waianae Mountains-Molokai Leeward-Maui Leeward West-Maui Central
Valley-Leeward Haleakala-Kona-South Big Island-Kohala.
Small Craft Advisory until 6 PM HST Wednesday for 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