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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 292003
Area Forecast Discussion
National Weather Service Honolulu HI
1003 AM HST SUN MAY 29 2016
A weak front will reach Kauai late this afternoon, and move into
Oahu tonight and Memorial Day. Moisture associated with the
gradually dissipating front will remain stalled over the island
chain for much of the upcoming week. High pressure building
north of the state the next couple of days will bring gradually
strengthening trade winds, which will focus most of the clouds
and showers over windward slopes and coasts. After prevailing
through the week, trade winds may diminish again next weekend.
Light winds today will allow afternoon clouds and showers to develop
over the islands as sea breezes converge in response to daytime
heating. Water vapor imagery highlights a weak trough aloft over the
island chain, while MIMIC TPW imagery and overnight soundings
indicate modest drying of the island air mass over the past 12 to 24
hours. Despite this trend, we are still expecting increasing clouds
through early afternoon, with showers developing over the slopes and
island interiors, with a chance of a few locally heavy downpours.
The 1000-700 mb steering flow is light and out of the north, at
least on Kauai and Oahu, so showers may tend to drift toward the
south sides of these islands this afternoon and evening.
The light winds are the result of a surface ridge extending toward
the islands from a high to the distant NE, with the terminus of the
ridge analyzed to be just N of Maui county this morning. The ridge
has been pushed S of it's climatological position by a low passing
far N of the area, and an associated front that is approaching Kauai
from the N this morning. The front is currently marked by a NE-SW
oriented band of broken to overcast low clouds that is about 75
miles wide at our longitude, with the leading edge located about 50
miles N of Kauai. The band is moving S at about 10 mph.
The fairly weak boundary is forecast to move S through Monday as a
high currently NW of the area moves E. Therefore, expect increasing
clouds and showers over Kauai overnight to spread to Oahu by Monday,
before spreading to Molokai and Maui Monday night and Tuesday. Trade
winds will return and gradually strengthen, keeping most of the
showers focused over windward areas. As the modestly strong high
slowly moves into a position N of the islands through mid-week,
moisture associated with the gradually dissipating front will become
E-W oriented near the islands. The trade winds will ensure this
moisture primarily dampens windward slopes and coasts, with a few
showers spreading leeward. Although latest guidance differs
slightly, the general expectation is that the moisture axis will
move very slowly southward down the island chain from Tuesday into
A mid- and upper-level trough currently over the islands is
providing modest instability, and will gradually weaken the next
couple of days, but will be slow to clear the area. The presence of
this feature may lead to some soaking showers as the moisture axis
moves through the islands, but widespread heavy rainfall is not
anticipated. A low aloft may develop nearby Thursday/Friday, and
enhance the moisture that is expected to be near the Big Island at
that time. This could potentially fuel heavy rainfall, especially
over the Kona slopes. Another front may approach from the NW late in
the week to kill the trade winds for next weekend, which could bring
a weather pattern similar to what we've experienced the last couple
of days, with land and sea breezes and locally heavy afternoon
A weak front just north of Kauai this morning is moving slowly east.
The front will keep high pressure at bay, far off to the northeast.
The lack of trade winds means light winds will continue across the
state with daytime sea breezes and nighttime land breezes forming.
Afternoon cloudiness due to daytime heating may lead to scattered
MVFR conditions due to lowered ceilings and visibilities from clouds
and showers with some areas seeing isolated IFR conditions. Clearing
to take place at night with the land breezes kicking in. But the
windward sections of Kauai and possibly Oahu may see some clouds and
moisture from the weak front skirting by tonight.
There is no AIRMET in effect at this time, but an AIRMET for mtn
obsc may be needed again this afternoon, depending on how extensive
the afternoon clouds become due to convection over the interior
sections of the islands.
The small southwest swell will continue to subside through today.
A small northwest swell will arrive throughout the day and peak
Monday before easing into midweek. Buoy 51101 is showing hints of
the northwest swell since this morning. Surf along north facing
shores should gradually increase into Monday, but remain well below
advisory levels. Another bump in south shore surf is expected by
midweek as a small southwest swell is forecast to peak Wednesday.
Winds will remain light into the afternoon as a weakening front
moves into the region. Moderate to locally fresh trade winds will
start to return this evening, first near Kauai then follow down the
rest of the island chain into Monday and Tuesday...as high pressure
fills in behind the front. Current forecasts keep wind speeds below
the threshold for small craft advisory, however some of the
typically windier areas may approach these levels.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office