Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 062009
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1009 AM HST Thu May 6 2021

.SYNOPSIS...

Overcast skies and relatively cool temperatures will persist
through Friday. Tonight through late Friday night, an upper-level
low approaching from the southwest will bring the potential for
thunderstorms and locally heavy rainfall capable of producing
flash flooding. There is also potential for accumulating snow over
the Big Island summits.

&&

.DISCUSSION...

Mid and high clouds associated with an upper low positioned about
450 miles to the southwest of Honolulu will maintain overcast
conditions and relatively cool temperatures through Friday. Trade
wind showers remain shallow and mostly light this morning owing to
the solid 700-500mb dry layer noted on this morning`s Lihue and Hilo
soundings. This layer of mid-level stability will gradually erode
this afternoon through tonight giving rise to increasingly deep
convection, mainly tonight once deep layer moisture increases and
stronger dynamic forcing impinges on the islands. Trades will also
undergo modest strengthening and veer to the southeast during this
time. For this afternoon, this will favor a modified sea breeze
scenario whereby the potential for showers will be increased over
leeward and sheltered areas. However, limited instability owing to
opaque cloud cover will mute the intensity of any inland convection
today.

Increased moisture embedded in ESE trades is forecast to reach the
islands after midnight tonight. The superposition of wet trades and
increasing dynamics associated with the approaching upper low will
focus the greatest heavy rainfall and subsequent flash flooding
threat over the windward areas of all the islands. However, lighter
blocked southeasterly flow and favorable background forcing will
still support some degree of a heavy rain threat over leeward areas
as well. The Flash Flood Watch remains in effect this evening
through late Friday tonight.

The Winter Storm Watch for the Big Island summits likewise remains
in effect. Given the lack of deep moisture within the colder thermal
profile above summit level over Big Island, accumulating snow will
be dependent on deeper trade wind showers occurring within a much
warmer and generally less favorable section of the profile. The
afternoon package will evaluate prospects for upgrading the WSW to
an advisory or warning, though at this time the former appears most
likely.

Conditions should improve and trend back toward a typical trade wind
pattern statewide over the weekend through early next week as drier
air moves in and the upper disturbance lifts northward away from the
state.

&&

.AVIATION...
A low pressure system southwest of Kauai will continue to move
slowly westward towards the state today. Broken/overcast mid and
upper level clouds will continue to stream over all islands during
the forecast period. Moderate trade wind flow in the lower levels
will concentrate isolated to scattered showers along windward
facing locations through this afternoon and into the evening.

By tonight, expect more widespread showers and isolated
thunderstorms to affect mainly windward facing areas, trough wet
conditions will be possible on leeward sides too. Periods of MVFR
ceilings and visibility are likely with this activity. Brief IRF
conditions will be possible too, associated with any heavier
showers or storms.

AIRMET TANGO remains in effect below 9000 feet south through west
of mountains on all islands due to tempo moderate turbulence.

AIRMET ZULU for icing may need to be issued later today as mid
and upper clouds thicken and increase in coverage.


&&

.MARINE...
With strong high pressure far northeast of the islands, and
surface and upper troughing to the west and southwest of the
state, winds will be east to east-southeast in the moderate to
locally breezy range over the coastal waters. Winds will tend to
veer more southeastly over the western portion of the island chain
with the current features in place. Also, the troughs will
destabilize the atmosphere and may induce periods of heavy rain
and thunderstorms over the next couple of days.

No significant swells are forecast into next week. A small boost
in surf along north and west facing shores is expected from
Saturday night through Monday as a northwest swell traverses
local waters. East facing shores will receive a bump beginning
about Sunday as local and upstream trades increase. And surf along
south facing shores will be up moderately beginning Saturday as a
small southern hemisphere swell is expected to move through.



&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 6 PM HST this evening through late Friday
night for all Hawaii islands-

Winter Storm Watch from 6 PM HST this evening through late
Friday night for Big Island Summits.


&&

$$


DISCUSSION...JVC
AVIATION...Thomas
MARINE...Kinel


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