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Hawaiian Islands Synoptic Discussion and Guidance

649
FXHW60 PHFO 010200
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 PM HST Tue Mar 31 2020

.SYNOPSIS...
Trade winds will gradually weaken through Wednesday, then remain
light and variable into 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 lighter 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 will
shift east tonight and Wednesday.

&&

.DISCUSSION...
Although there is reasonable confidence as to how weather features
will evolve on the large-scale over the area the next couple of
days, there is some uncertainty as to the finer details of the
forecast, both in the short term and in the longer term.

Currently, light to moderate trade winds are being supported by a
surface high pressure cell centered far NE of the islands, and its
associated ridge N of the islands along 30N. Water vapor imagery
highlights a shortwave trough aloft near Kauai moving E at 20 to 25
mph. Thick mid- and upper-level clouds E of the trough are pushing
E, having cleared Kauai earlier this afternoon. With daytime heating
of the islands coinciding with the sooner-than-expected arrival of
the destabilizing trough, localized heavy showers and thunderstorms
have developed over the area from Oahu to the Big Island.

The forecast anticipates high clouds to shift E of the islands as
the shortwave trough aloft passes tonight. Trade winds will
gradually weaken through Wednesday in response to a surface trough
developing E of the islands. The overnight forecast is of low
confidence due to the passing shortwave, and our inability to
determine where it might spark a heavy shower or thunderstorm. Best
chances for rain will likely be shifting E with the shortwave
through the night, and will primarily be over windward areas as
light trade winds persist.

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. Although the shortwave trough will be well E of the islands
by tomorrow, a lingering trough aloft and associated cold
temperatures will likely keep the island atmosphere unstable.

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.

&&

.AVIATION...
A mid to upper level low passing over the state will enhance shower
activity over the islands for the next twenty-four hours. As of 3pm
HST, isolated thunderstorms could be detected via satellite along
the southern coasts and southern adj waters of Oahu, Molokai, and
Lanai. Lightning activity and locally heavy showers are expected to
spread across the remainder of the state early this evening.

High level turbulence and bands of cirrus clouds remain in the
forecast due to a passing jet streak. Conditions are expected to
gradually improve from the west as the jet shifts off to the
southwest. AIRMET TANGO for moderate turbulence aloft between FL250
and FL350 will likely be cancelled towards evening.

Light to locally moderate trade wind flow over the next few days
will allow sea breezes to predominate in the afternoons and early
evenings. Expect clouds and showers to be focused over island
interiors during the afternoon and early evening then along windward
slopes and coasts overnight.

&&

.MARINE...
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 develops just east of the Big Island and lingers.
This pattern will bring another round of heavy showers and a few
thunderstorms over portions of the area tonight, departing the area
to the east on Wednesday. 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 somewhat 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 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.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Birchard
AVIATION...Bedal
MARINE...TS

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office