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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 250130

Area Forecast Discussion
National Weather Service Honolulu HI
330 PM HST Thu Sep 24 2020

After a breezy couple of days, trade winds will diminish Sunday,
then remain light for most of next week. An area of enhanced
moisture passing through the islands will depart this afternoon.
Expect a few brief windward showers over the next couple of days,
with light winds keeping mostly dry weather in place Sunday into
next week.


There is no change in forecast philosophy from this morning. A
surface ridge far north of the main Hawaiian Islands will maintain
locally breezy trade winds into the weekend along with a few
brief showers. Low pressure passing north of the ridge will push
it closer to the islands beginning Sunday, causing winds across
local waters to decrease. Winds will then remain light for a good
portion of next week. Local land and sea breezes may form,
especially over leeward areas.

Satellite loop shows significantly clearer conditions across the
islands versus 24 hours ago, while radar shows isolated to
scattered light showers across windward and some mauka areas.
Expect a rather dry trade wind weather pattern statewide through
the forecast period as a stabilizing upper ridge caps deep
convection and allows just a few windward showers with limited
total rainfall.

Light winds beginning Sunday and Monday will persist for most of
next week as remnants of TS Lowell pass northeast of the islands
and as troughing far north of the islands keeps the ridge weak.
Decreasing flow Sunday will give way to light and variable winds
by Monday, with land and sea breezes expected, especially across
leeward areas. This will result in a daily weather pattern
characterized by clear nights and mornings, with afternoon
interior clouds possibly dropping a few showers. While this looks
to be mostly a dry weather pattern, models hint at an area of
moisture developing west of Lowell that could be near the islands
Sunday and Monday. This could delay the drying trend for a few
days if this materializes.


Drier conditions will overtake the area for the next several
days, though occasional light showers will continue to move
through windward areas in the trade flow through tonight. A few
light showers could be pushed leeward at times as well. Trades
will continue locally breezy through tonight as the inversion
quickly lowers to about 5000 ft, then slowly decrease into the
weekend as the pressure gradient relaxes over the state.

AIRMET Tango is in effect for tempo low-level turbulence over and
south through west of the mountains of all islands due to the
strong trades and will likely continue at least through tonight.


A zonal orientated surface high whose axis is centered near 35 N
latitude is forecast to move eastward towards the U.S. West Coast
over the next few days. This will be in response to a southeastern-
advancing Northern Pacific cold front. The associated western upper
ridge will slow the southern movement of the front. The boundary
will reach our northern-most offshore waters Tuesday providing
brief westerly winds over those waters Wednesday. Tropical Storm
Lowell's remnant low will be nearing the far eastern offshore
waters early next week. This weak circulation will not be of any
significance as it interacts with the aforementioned cold front
and dissipates.

The most recent ASCAT pass showed 25 knots winds present in the
Alenuihaha Channel with around 20 knot sustained winds over the
channels and bays surrounding Maui County and the waters south of
the Big Island. These winds are forecast to slightly strengthen
overnight so the Small Craft Advisory (SCA) remains in effect
until 6 AM HST Friday for those zones including the Alenuihaha
Channel. With the ridge of high pressure traveling south and
weakening the pressure gradient over the state, early week trades
will be on the downward trend to just a gentle easterly breeze by

Deep ocean buoys 51001 and 51004, northwest of Kauai and southeast
of the Big Island respectively, are both recording near 6 foot
swell at 12-15 second periods this afternoon. While much of this
swell is not being realized along high north faces, side lobes of
this energy clipping the chain have produced 4 to near 7 foot size
surf along the North Shore of Oahu today. These swells will produce
similar surf along north-facing shores through tomorrow (Friday)
before gradually lowering this weekend.

The current offshore 6 foot/10 second period south-southwest
swells will be reinforced by a series of slightly longer period
swells generated from large gales south of New Zealand a week ago.
Spectral density plots of ample near 16 second energy across
American Samoa these past two to three days will be realized along
south-facing shores from Friday through Sunday. 3 to 6 foot wave
heights may will approach surf advisory levels during this Friday
through Sunday period.

Due to the a weakening trade regime this weekend, surf along
east-facing shores will be lowering to small. We may receive a
small bump in easterly swell resulting in a small lift of surf
early next week from the energy produced from weakening Tropical
Storm Lowell.


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




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