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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 030711

Area Forecast Discussion
National Weather Service Honolulu HI
911 PM HST Thu Apr 2 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 200 miles northwest of Kauai. Further displaced from the
islands, a 1011 mb low is located around 1100 miles northwest of
Kauai, with a 1030 mb high around 1700 miles northeast of
Honolulu. The resulting gradient is producing light to moderate
trade winds from Oahu eastward to the Big Island, strongest on the
eastern end of the island chain. Meanwhile, a light and variable
flow has set up around Kauai, where sea breezes are in the process
of transitioning to land breezes. Infrared satellite imagery shows
some lingering cloud buildups from earlier in the day, along with
some jet stream cirrus moving through overhead. Radar imagery
shows some decaying shores associated with daytime heating from
Oahu to the Big Island, while scattered showers are moving through
Kauai in the southerly boundary layer flow. Main short term
concerns revolve around rain chances the next several days.

The surface trough just northwest of the islands will gradually
weaken tonight and then dissipate on Friday. Meanwhile the low far
northwest of the state will shift southward and closer to the
islands, while the high northeast of the area will weaken. 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 overnight land breezes and
daytime sea breezes. Moderate southeasterly winds will hold in
place across the Big Island however. Lingering shower activity
from Oahu to the Big Island should largely end over the next
couple hours, with relatively dry conditions continuing through
the morning hours Friday. A bit stronger southerly boundary layer
flow near Kauai however, should keep a few showers moving onshore
tonight through Friday morning. With daytime heating Friday
afternoon, we should see some showers develop over the interior of
the all islands.

Friday night 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 Friday night 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 slopes.

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. Clouds and a few
showers will build up over island interior sections by Friday
afternoon. Isolated thunderstorms remain in the Friday afternoon
forecast for the Big Island.

More unsettled weather will move up from the south over the next
48 hours, increasing cloud and shower trends by late Saturday
through the weekend.

No AIRMETS are in effect.


A surface high far northeast of the area and a surface trough
northwest of Kauai will continue to produce light east to
southeast winds across the area through early Friday morning. A
front slowly approaching the state on Friday will cause the wind
direction to 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 may strengthen slightly
on Friday. There is a chance winds may be near the Small Craft
Advisory criteria over the Big Island Windward and Southeast
Waters on Friday.

From later this weekend into early next week, a broad south to
southeasterly convergent flow will set up over the area, and this
will likely result in heavy rainfall and the possibility of a few

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 west-northwest swell will slowly build, peaking around Monday
night, followed by a northwest swell on Wednesday. Surf from
these swells will also likely remain below the HSA criteria.

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
Friday. 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