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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 031337

Area Forecast Discussion
National Weather Service Honolulu HI
337 AM HST Fri Apr 3 2020

An area of low pressure northwest of the state and a ridge of
high pressure to the east will keep a light wind regime in place
across the islands through early next week. Showers will favor
areas near the coast at night and the interior of the islands
during the afternoon and evening hours. Some deeper moisture will
get drawn northward Saturday night through early next week,
bringing the potential for some locally heavy downpours and a few
thunderstorms, mainly during the afternoon and evening hours. The
light wind regime featuring mainly afternoon and evening showers
looks to hold in place through the middle to latter part of next


Currently at the surface, a trough of low pressure is located
around 150 miles west-northwest of Kauai. Further displaced from
the islands, a 1010 mb low is located around 1050 miles northwest
of Kauai, with a 1029 mb high around 1600 miles northeast of
Honolulu. The resulting gradient is producing light to moderate
east to southeast winds from Oahu to the Big Island, strongest on
the eastern end of the island chain. Meanwhile, a light and
variable flow has set up around Kauai and some of the more
sheltered locations on Oahu and Maui County, where land breezes
are in place. Infrared satellite imagery shows partly cloudy skies
in most areas, with a bit more cloud cover over portions of Kauai.
Additionally, some scattered to broken jet stream cirrus is
streaming overhead. Radar imagery shows scattered showers moving
through Kauai in the southerly boundary layer flow, with mainly
rain free conditions elsewhere. Main short term concerns revolve
around rain chances the next several days.

The surface trough just northwest of the islands will gradually
dissipate today. Meanwhile the low far northwest of the state
will shift southward and closer to the islands, while the high
northeast of the area gradually weakens. This will result in the
low level boundary layer flow shifting more southeasterly across
the island chain, sheltering the smaller islands sufficiently to
allow for daytime sea breezes. Moderate southeasterly winds will
hold in place across the Big Island however. A few showers will
continue to move onshore across Kauai this morning, while the
other islands remain generally rain free. With daytime heating we
should see some showers develop over the interior of the all
islands this afternoon, with an isolated thunderstorm possible
over the Big Island slopes.

Tonight through Tuesday, model solutions are in generally good
agreement showing a relatively stationary low wobbling around well
north-northwest of the state, with a ridge of high pressure well
to the northeast. This is expected to keep a fairly persistent
south to southeast boundary layer flow in place across the island
chain, with daytime sea breezes and overnight land breezes
dominant across the smaller islands while light to moderate
southeasterly winds prevail around the Big Island. The pattern
will not change much tonight and Saturday, with lingering showers
ending in the evening in most areas. A few showers may continue
to roll onshore into Kauai and Oahu through the night and morning
hours Saturday, and a few showers may also affect windward
portions of east Maui and east and southeast facing sections of
the Big Island. Another round of daytime heating driven showers
can then be expected over the island interiors Saturday afternoon,
and a thunderstorm or two could even develop over the Big Island

A tap into some deeper moisture will begin to lift northward over
the islands beginning Saturday night with precipitable water
values climbing into the 1.5 to 1.7 inch range. This deeper
moisture will then linger over the islands in a broad convergent
south to southeasterly flow through Tuesday. The latest guidance
continues to support the potential for some locally heavy
downpours, particularly during the afternoon and early evening
hours each day, with showers most likely in areas near the coast
at night. A few thunderstorms can't be ruled out during this
period either, mainly over the island interiors during the
afternoon/early evening hours and over the coast waters at night.

Wednesday through next Thursday, the gradient appears to weaken
even further, with daytime sea breezes and overnight land breezes
dominant across the entire island chain. Model solutions differ on
how much lingering deep moisture will remain over the islands, but
it does appear that slightly more stable that over the weekend and
early next week. As a result, we expect showers to continue to be
driven by daytime heating over the island interiors during the
day, and showers hugging the coasts at night.


A low pressure system approaching the islands from the northwest
will keep light and variable winds in the forecast through the
weekend. Expect land and sea breezes over each island with wind
directions becoming more variable than normal as southerly flow
moves over the western islands.

Clouds and a few showers will build up over island interior
sections this afternoon. Isolated thunderstorms remain in the
afternoon forecast for the Big Island. More unsettled weather
will move up from the south over the next 24 hours, increasing
cloud and shower trends from Saturday to Sunday.

AIRMET Sierra in effect for Tempo Mountain Obscuration for Kauai.
This AIRMET will likely continue through the morning hours.


A front slowly approaching the region from the northwest will
cause the surface ridge far north of the area to weaken. This
will cause the winds to weaken and veer slightly more out of the
south over the western waters. However, the southeasterly winds
over some of the waters across the eastern end of the island chain
will strengthen slightly today. There is a chance the southeast
winds may be near the Small Craft Advisory criteria over the Big
Island Windward and Southeast Waters. From later this weekend into
early next week, a broad area of south to southeasterly
convergent flow will set up over the state. This will likely
bring unsettled weather with locally heavy rainfall and
thunderstorms possible from Saturday into early next week.

A new north-northwest swell spreading across the area Saturday
will produce moderate surf along most north and west shores over
the weekend, but surf heights are expected to remain below the
High Surf Advisory (HSA) criteria. As the weekend swell subsides,
a new west-northwest swell will spread across the area Sunday
night, and peak Monday. A reinforcing northwest swell arriving
late Tuesday night will likely cause another boost to surf
heights along most north and west facing shores Wednesday and

Southeast swells originating in the South Pacific are being
partially blocked by the islands of French Polynesia. However,
our current southeast swell appears to be passing through a
narrow swell window. This swell will continue to drop through
tonight. A small south-southeast swell is possible early next
week. Surf along east shores will slowly subside as the trade
winds weaken.






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