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000
FXHW60 PHFO 060634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Wed May 5 2021

.SYNOPSIS...
Cloudy skies and increasing moisture associated with an upper
disturbance and surface trough setting up in the area will lead to
increasing rainfall chances statewide Thursday through early
Saturday. Heavy showers and a few storms will become a possibility
through this time, which increases the threat for localized flash
flooding. In addition to the potential for heavy rainfall, a
wintry mix appears possible at the summits of the Big Island
Thursday through Friday night. Conditions should gradually begin
to improve and trend back toward a typical trade wind pattern over
the weekend through early next week.

&&

.DISCUSSION...
Guidance remains persistent and continues to advertise a wet
pattern unfolding over the state Thursday through early Saturday
as an upper disturbance and surface trough influence the local
weather. Some showers may become heavy and lead to localized
flooding concerns where they become focused for any duration. In
addition to the rainfall, a few thunderstorms will be possible as
instability associated with upper low increases and the trade wind
inversion erodes. As the surface trough sets up near the western
end of the state, low-level flow will veer out of the east-
southeast direction and slightly weaken - likely giving way to a
land and sea breeze regime over sheltered leeward and interior
locations beginning Thursday afternoon.

Despite the high sun angle over the islands in May, the one
limiting factor to the flooding rain threat could be the canopy of
mid-to-high clouds draped over the area limiting daytime heating
and boundary layer instability over those shelter leeward areas
through the afternoon hours. High-resolution rainfall guidance
supports this and shows the bulk of rainfall accumulations setting
up over windward and mountain locations with peak 48-hr totals
reaching the 3-5 inch range.

For the summits of the Big Island, deep tropical moisture lifting
northward into the area east of the upper low combined with
freezing temperatures will likely translate to a wintry mix late
Thursday through Friday night and Winter Storm Watch may be issued
later tonight. Conditions should gradually improve at the summits
over the weekend.

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

&&

.AVIATION...
Expect VFR conditions to prevail through the evening and on
through the overnight hours. Clouds and showers embedded in low
level trade wind flow will interact with island terrain causing
brief periods of local MVFR conditions over windward slopes.
Conditions are not expected to be prolonged or widespread.

AIRMET Tango is currently posted for low level mechanical turbulence
over and downwind of terrain on all islands. Winds are expected to
diminish later tonight and Thursday.

&&

.MARINE...
Strong high pressure located approximately 1,100 miles north of
the islands and its resultant pressure gradient directed back
toward the state is producing fresh to strong easterly trade
winds this evening. Locally strong trades are occurring over the
bays and through the channels surrounding Maui County as well as
those waters south of the Big Island. This has warranted a Small
Craft Advisory that is in effect through early Thursday morning.
An upper level low moving into the area from the southwest, along
with its associated broad western water surface trough, will
weaken and veer this evening`s locally strong east trades more
southeasterly on Thursday. Little movement of west surface
troughing and the further strengthening of far northeast high
pressure will create fresh east trade flow over more eastern
island windward waters while maintaining gentle to moderate
southeasterlies over western island waters. The upper low may
trigger periods of heavy rainfall and thunderstorms as it is
forecast to slowly meander northeast toward the islands late
Thursday into Friday.

Surf along north and west facing shores will remain small into the
weekend. Broad northwest Pacific low pressure located near the
western Aleutians is forecast to track east toward the Date Line
through Thursday. Model guidance shows an area of strong to gale
force winds along its southern periphery that will be focused
along a northwest (310-320 degree) great circle path directed at
the islands. This small, short to medium period northwest swell is
scheduled to arrive and boost north and west facing shoreline
surf Saturday night into Sunday, hold into Monday and then drop
off by Tuesday. Surf along east facing shores should slightly
trend up through the rest of the week and weekend as fresh trades
expand well upstream of the islands. South facing shore surf will
remain small through Friday but will pick up a bit over the weekend
with the arrival of a long period south swell. Expect this trend
to hold next week with another small, long period south swell
possibly arriving Tuesday and holding through the middle of next
week.

&&

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

Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Gibbs
AVIATION...Powell
MARINE...Blood