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RH9 %
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 290640

Area Forecast Discussion
National Weather Service Honolulu HI
840 PM HST Mon Sep 28 2020

A stalled cold front north of the islands will keep light and
variable winds in the forecast into Saturday. Afternoon sea
breezes will develop building clouds over island mountain and
interior sections with isolated to scattered showers mainly in the
afternoon to early evening hours. Off-shore land breezes will
develop after sunset, clearing out any cloud cover. A weak
disturbance moving into the area from the east may bring a slight
increase in shower activity by Wednesday night.


The satellite picture this evening shows a cold front stalling
roughly 400 miles north of Kauai. This front will remain well
north of the island chain, weakening the subtropical ridge, and
keeping light large scale winds in the forecast. A weak
disturbance, associated with the remnants of former TC Lowell, is
located roughly 450 miles east of Hilo. Higher moisture levels and
cloud bands associated with this disturbance will slowly drift
into the islands possibly producing a slight increase in shower
activity from Wednesday night to Thursday morning. Clouds over the
Hawaiian Coastal waters look fairly stable with less overnight
shower activity built into the forecast. Land breezes will also pick
up over all islands later tonight adding stability to the

The subtropical ridge that typically provides moderate trade winds
over the islands will remain in a weakened state through the week
due to the stalled cold front. In the absence of large scale
winds, local scale afternoon heating and overnight cooling of each
island will drive land and sea breeze circulations. Convergent sea
breezes will develop each day building clouds over island mountain
and interior sections. These convergent cloud bands will produce
isolated to scattered showers, especially in the late afternoon
to early evening hours as solar heating decreases, and the clouds
begin to collapse. Any rainfall activity will be brief and result
in light precipitation amounts. Divergent land breezes developing
after sunset will stabilize the weather pattern over each island
and clear out most of the cloud cover. Expect this land and sea
breeze weather pattern to continue into Saturday.

Warm temperatures and slightly higher humidity levels are forecast
during the daytime hours. Cooler temperature trends will develop
each night due to enhanced radiational cooling under mostly clear

The long range forecast shows little change as global models
remain in good agreement with the sea breeze pattern lasting into
Saturday morning. A band of tropical moisture may move in from
the southeast by the end of the week, increasing humidity levels,
and possibly producing higher showers along the southeast slopes
of the Big Island. Southeasterly flow aloft typically creates a
rain shadow affect for islands in the lee or west of the Big
Island. Lowering the potential for showers over the smaller
islands. There are early signs of a return to trade winds by
Saturday afternoon and breezy trade winds from Sunday into the
first half of next week. Stay tuned.


Weak easterly flow will lead to land breeze development across
the islands later tonight. Isolated showers will continue over the
smaller islands with scattered showers poised to affect windward
Big Island. Brief MVFR ceilings and visibility may occur with this
activity, but not expecting anything widespread.

No AIRMETs are currently in effect.


Light to moderate easterly trades will relax enough for a land
and sea breeze pattern to setup tonight that will continue
through the rest of the week. The background flow will shift out
of the north-northeast Tuesday through Thursday, then potentially
shift back out of the east Friday into the weekend. A return of
moderate to fresh easterly trade winds will be possible by then
end of the weekend.

Surf along south facing shores will remain up into Tuesday -
gradually lowering as the south swell that filled in over the
weekend eases. Buoy numbers were impressive today and remained up
slightly above predicted. The evening offshore buoy observations,
however, do reflect a downward trend, which will be observed
through the day Tuesday given the travel time. A mix of southeast
trade wind swell and background small, southerly energy should be
enough to keep the surf from going flat Wednesday into next

For the long range, the active trend continues down south around
New Zealand within Hawaii's swell window. The latest surface
analysis and satellite data reflect this and depict a storm-force
low near southern New Zealand with a broad area of gale- to storm-
force winds over our swell window from the Tasman Sea to south-
southeast of New Zealand. Altimeter passes showed seas up above
30 ft in this area earlier today, which is in line with drifting
SOFAR buoys in the general area. This will translate to a late
season south swell that should arrive locally early next week
(Oct. 5th time frame). More details will follow later in the week
once the swell moves through the nearshore PacIOOS spectral buoy
in American Samoa.

Surf along north facing shores will remain small through Tuesday,
then trend up once again Tuesday night through the weekend as
back-to-back north-northwest swells arrive. The first one is from
a compact storm-force low centered around 1200 nm north of the
islands this evening. This source will arrive Tuesday night and
gradually become more northerly by Thursday as it peaks near the
advisory level (15 ft faces). The second swell is forecast to
arrive Thursday night, which should keep the surf near the
advisory level through the day Friday. This swell combined with a
smaller, medium period northwest arriving Friday night into
Saturday will keep the surf up through the weekend.

Surf along east facing shores will pick up as a small, easterly
swell from former tropical cyclone Lowell in the eastern Pacific
builds across the islands from east to west. This swell is showing
at the Hilo PacIOOS buoy and should fill in across the western
end of the island chain Tuesday. A downward trend is expected
through the second half of the week due to the lack of trades
locally and upstream of the islands.

The large daily tide cycles will gradually trend down toward the
monthly minimum each day through the first week October.






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