Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 290212

Area Forecast Discussion
National Weather Service Honolulu HI
412 PM HST SAT MAY 28 2016

A surface trough northwest of the state will keep winds light
through the weekend. A weak front will reach Kauai Sunday night
and Oahu on Memorial Day, then stall and dissipate over the
eastern islands by mid week. High pressure will build north of the
state behind the front, bringing a return to trade winds for much
of next week.


Water vapor imagery shows an upper level trough a couple hundred
miles northwest of the state, with a westerly jet on the south
side of the trough passing across the Big Island. At the surface,
a trough is located northwest of Kauai, extending to a front
farther to the north. 00z soundings show a stabilizing airmass
across the state, with an inversion around 7kft at Hilo and a
slightly stronger one near 14kft at Lihue. Precipitable waters
were 1.2 and 1.5 inches respectively. Early afternoon MIMIC total
precipitable imagery shows drier air over windward Big Island that
extends across windward waters as far northwest as Oahu, and a
band of higher moisture from Kauai northward, as well as across
the leeward coastal waters.

Showers flared up again across interior sections this afternoon.
The heaviest rainfall occured over Kauai/Oahu, but with some
showers across the remaining islands as well. These showers are
diurnally driven and will weaken with sunset, but we have seen
the past few days that they may linger longer that would otherwise
be expected, in part due to the surface trough and moisture axis
nearby. This may account for the showers that flared up overnight
last night, and we may see a similar situation for the next
couple nights. Therefore have kept a slight chance of showers for
many areas during the overnight hours as well.

Some drying is expected over the next day or two, with
precipitable waters decreasing a bit, but overall little change is
expected in the sensible weather pattern. The surface trough
northwest of Kauai will pass over the islands, keeping winds light
and leading to a continuation of the locally-driven sea breeze
pattern again on Sunday. Convergence and heating/instability
during the day will lead to afternoon/evening showers.

The 12z GFS and ECMWF are in better agreement with the weak front
expected to cross the area early next week, but these global
models are faster bringing the boundary southward than some of
the higher resolution guidance. Have shaded the timing of the main
cloud band back a bit, but we should still increased shower
coverage across Kauai/Oahu on Memorial Day, as well as an increase
in northeasterly trades behind it. One possible result of the
approaching front would be that subsidence ahead of it would
suppress afternoon convection across Maui County and the Big
Island on Memorial Day. This would be the case with a strong,
dynamically-driven front, of which this is certainly not the case,
so we have kept isolated/scattered showers in for areas ahead of
the front. From Monday night into Wednesday, the front is
expected to stall and dissipate over the eastern islands. With the
new high building north of the state, this will lead to more of a
wet trade wind pattern, with moisture from the old front enhancing
showers across windward sections.


Light winds on the large scale will will begin to favor a northerly
direction near Kauai and Oahu on Sunday as a weak surface trough
moves in from the N. However, winds are expected to remain
sufficiently light so as to be overridden by island-scale land and
sea breezes. Therefore, expect skies over the islands to gradually
clear after sunset this evening, with tempo MVFR and localized IFR
vis/cig in +SHRA, as well as localized mountain obscuration,
persisting until then. These showers bring the chance of brief
MVFR conditions at most of the TAF sites into this evening. VFR
conditions should prevail overnight, although the gradual approach
of the trough may prevent skies from completely clearing. Light
winds on Sunday will again allow sea breeze convergence to drive
cloud and shower formation over the islands from late morning into
the early evening hours, producing MVFR vis/cig in SHRA, and
possibly isolated IFR conditions over island interiors.


Seas will remain fairly quiet for the next several days. There
are no significant swells expected. The current small long period
southwest swell will continue to decline through the weekend.
Another small long period southwest swell is expected Tuesday
night into Wednesday. North shores will also see a small rise in
surf Sunday into Tuesday from a small northwest swell. All swells
will remain below advisory levels.

Winds will remain light into Sunday as a front passes north of
the islands. Moderate to fresh winds will return starting over
Kauai late Sunday and moving down the state into Monday. Current
forecasts keep the winds below Small Craft Advisory levels, but
some of the typically windier areas may near that threshold.





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