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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 220141
Area Forecast Discussion
National Weather Service Honolulu HI
341 PM HST Fri Sep 21 2018
A stable, moderate to breezy trade wind flow will continue to
build across the state tonight and will remain in place through
Saturday. Clouds and showers will favor windward slopes, along
with a few afternoon showers over leeward Big Island. Winds will
shift out of the southeast and south on Sunday and Monday,
bringing humid conditions over the state and potentially wet
weather across the western end of the island chain through the
middle of next week.
Trade winds are making a come back, but it will be brief. As a
weak surface trough northwest of Kauai lifted away, a surface
high far northeast of the state has exerting greater influence
locally. As a result, easterly trade winds have built in across
all islands, and ridging aloft edging in from the east has
brought increased stability. An inversion has developed between
3000 and 5000 ft according to the afternoon soundings and recent
aircraft data. This, combined with modest moisture in the trade
wind flow, has led to little shower activity today, even across
windward slopes. Some lingering moisture is triggering enhanced
showers south of Kauai, but this activity is expected to remain
south of the Garden Isle.
Stable trade wind weather will persist tonight and Saturday.
Expect typical late summer conditions for the equinox. Easterly
trades will hold at moderate to locally breezy strength. Ridging
aloft will produce stable conditions with an inversion around 4000
to 6000 ft, keeping modest rainfall confined largely to windward
slopes. With dew points dropping to the upper 60s and lower 70s,
a brief drop humidity will occur.
A humid southeast to south flow will develop on Sunday, with wet
conditions possible for the western end of the state into the
middle of next week. A broad slow-moving area of low pressure
will strengthen several hundred miles northwest of Kauai on
Saturday night and Sunday. Locally, winds will initially shift
out of the southeast, then veer out of the south for the western
half of the island chain by late Sunday. This will draw up a very
humid air mass from the deep tropics. Dew points will to rise
several degrees above normal into the mid 70s, creating very humid
conditions across all islands.
A convergence band with deep moisture will set up just west of
the state and may bring enhanced rainfall to the western end of
the island chain. There is still some disagreement between the GFS
and ECMWF on the position of this feature, with the GFS continuing
to bring the deep moisture as far east as Kauai by Sunday night.
As a result, there will be a chance of enhanced rainfall with
heavy showers over Kauai, and perhaps as far east as Oahu,
through the middle of next week. That said, uncertainty remains
elevated. Across the rest of the state, mid level ridging should
remain dominant, allowing modest showers along southeast facing
terrain and spotty afternoon showers elsewhere.
Uncertainty grows larger later in the week. The GFS allows a weak
trade wind flow to build back in by Thursday, while the ECMWF
drives a decaying front over the western end of the state.
Easterly trade wind flow will continue to build in from the east
into tonight. Expect isolated MVFR conditions mostly on the
windward slopes of most islands, otherwise VFR conditions will be
predominant over most other areas. Moisture associated with the
trough just to the west will mostly be kept at bay by the
strengthening easterly trades but a few showers may be triggered
on Kauai through the period.
Although the trade winds have strengthened through the day they
probably will not be strong enough to create low level turbulence
until tomorrow. Will monitor and if conditions warrant, AIRMET
Tango may be needed.
High pressure northeast of the state will keep a moderate to
locally strong trade wind flow in place through Saturday. As a
result, a Small Craft Advisory (SCA) remains in place for the
typically windy waters around Maui County and the Big Island. An
area of low pressure is expected to develop several hundred miles
northwest of Kauai over the weekend, allowing the winds to weaken
below SCA levels and shift around to the southeast Sunday through
early next week.
Surf will remain below advisory levels along all shores through the
middle of next week. The current south swell will slowly decline
through Saturday. Rough, short-period surf will remain around the
summer average along east facing shores through tonight, then
decline over the weekend. A north-northwest is expected late next
week, possibly producing advisory level surf along north facing
Small Craft Advisory until 6 PM HST Saturday 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