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Forecast Discussion for Honolulu, HI

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FXHW60 PHFO 312015

Area Forecast Discussion
National Weather Service Honolulu HI
1015 AM HST Tue Mar 31 2020

Trade winds will gradually weaken through Wednesday, then remain
light and variable through the weekend. While trade winds continue,
they will deliver a few showers to windward areas, some of which
could be heavy as a disturbance aloft passes tonight and Wednesday.
With the light winds, clouds and showers will tend to favor interior
and upslope areas during the afternoon hours, with mostly clear and
dry nights and mornings. High clouds over the islands today will
shift east tonight and Wednesday.


Currently, moderate trade winds are being supported by a surface
high pressure cell centered far NE of the islands, and its
associated ridge along 30N. Water vapor imagery highlights a
shortwave trough aloft about 200 miles W of Kauai near 24N 163W
moving E at 20 to 25 mph. The trough is delivering thick mid- and
upper-level clouds to island skies this morning, while also inciting
thunderstorm development near its axis. A forecast update was sent
earlier this morning to increase sky cover in the short term.

The forecast anticipates high clouds to linger into this evening
before shifting E of the islands as the shortwave trough aloft
passes. Trade winds will gradually weaken overnight and Wednesday in
response to a surface trough developing E of the islands. Although
the trade wind flow is somewhat dry at the moment, some increase in
windward showers (and a slight chance of thunderstorms area-wide) is
expected as the passage of the shortwave brings a period of
instability later today into Wednesday.

Although some briefly heavy downpours are possible, the meteorology
doesn`t appear to support a Flash Flood Watch (FFA) as the heavy
rain threat looks to be short-lived. On the flip side, the
hydrologic conditions probably do support a FFA for Kauai, and to a
lesser extent Oahu, where grounds have dried little since recent
soaking rains. This means that streams and rivers will react more
quickly due to increased runoff should heavy showers occur. Will
refrain from FFA issuance at this time.

With the light winds Wednesday, afternoon sea breezes are expected
to bring clouds and showers to interior and upslope areas, with the
potential for some heavy showers that should be relatively short-
lived. High clouds are expected to be too thick, and trade winds
too strong, for leeward sea breezes to drive much convection today.

From Wednesday into Thursday, the weak trough E of the islands will
either move or re-form, but end up W of Kauai. This will result in a
light SE wind flow that will likely allow island-scale land and sea
breezes to dominate winds over the islands, and also strongly
modulate weather over land. From Thursday into Friday, the surface
trough will move N of the area and dissipate as a cut-off deep-layer
low pressure system develops well NW of the islands. The surface
ridge will move over the islands, leading to light winds that will
favor a S direction near Kauai, and E/SE near the Big Island. The
slow-moving low to the NW complicates the forecast thereafter, with
forecast guidance offering varying solutions for the upcoming
weekend, but a land-sea breeze weather pattern looks to be the most
likely outcome through Saturday, while a convergence band associated
with the low may affect NW portions of the island chain from Sunday
into Tuesday.


An upper low drifting in from the west will keep unsettled weather
in the forecast for the next 24 hours with periods of showers and
brief MVFR conditions. Lighter winds over the next few days will
result in land and sea breezes developing along more sheltered west
slopes of each island.

An upper level jet stream is keeping cirrus clouds and upper level
turbulence in the forecast. AIRMET TANGO remains in effect for
moderate turbulence aloft between FL250 and FL350. This AIRMET is
expected to remain necessary through the afternoon hours and
possibly beyond.


Moderate to fresh easterly trades will trend down through midweek,
becoming light and variable through the second half of the week as a
broad trough and an upper disturbance move over the islands. This
pattern will bring another round of heavy showers and a few
thunderstorms over portions of the area tonight and Wednesday. The
trough will lift northward and away from the area as a cold front
approaches and stalls west of the state this weekend with background
southeasterly winds Thursday through the weekend.

Surf along north and west facing shores will trend up through
Wednesday as a small, medium-period northwest swell builds down the
island chain. Although confidence is low, a moderate, medium- period
north-northwest swell is possible Friday night through Saturday,
declining through Monday.

Surf along south facing should get a bump by Wednesday as a small,
long-period south-southeast swell fills in. Surf along east facing
shores will remain up through midweek due to an upstream fetch,
despite the trades relaxing locally. Heights will trend down by the
end of the week as the upstream fetch eases.






Previous Forecast Discussions may be found at
NWS Honolulu, HI (HFO) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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