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Hawaiian Islands Synoptic Discussion and Guidance

375
FXHW60 PHFO 291942
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
942 AM HST Tue Sep 29 2020

.SYNOPSIS...
A weakened ridge just north of the islands will keep winds light
and variable into Saturday. Sea breezes will develop each
afternoon, increasing cloud cover across mauka areas and producing
light rainfall mainly in the late afternoon to early evening
hours. Overnight land breezes will clear out any cloud cover and
push lingering shower activity offshore. Increased moisture
moving into the area from the east may slightly increase shower
activity by Wednesday night.

&&

.DISCUSSION...
A stalled front north of Kauai has weakened the subtropical
ridge, keeping winds light and variable across local waters.
Overnight soundings show a rather dry and stable airmass, with 1.1
to 1.2 inches of PW and positive lifted index values. An area of
broken low clouds lies just northeast of Maui and the Big Island,
but satellite loop shows skies across the islands themselves are
mostly clear this morning. Increased moisture slowly drifting
into the islands from the east may reach us by mid week, possibly
increasing shower activity Wednesday night into Thursday morning.

Models show the current pattern will remain in place through the
rest of this week, with the subtropical ridge weakened and pushed
south near the islands by a stalled front farther to the north.
Winds will stay light, allowing local land and sea breezes to
develop. Expect increased cloud cover across mauka areas each
afternoon and evening, with possible light rainfall. Cloud cover
will decrease each night with the development of land breezes.

By Sunday, the front is forecast to weaken and lift out, allowing
the subtropical ridge to drift back northward. The pressure
gradient will then steepen enough across the state to drive
moderate to locally breezy trade winds briefly on Sunday. Light to
moderate trades are then expected from Monday onward.

&&

.AVIATION...
Land breezes will emerge again today with favored areas seeing VFR
inland ceilings and the potential for brief isolated showers. A
band of light to moderate showers may produce MVFR conditions
along portions of Windward Big Island and possibly Maui, but is
not forecast to affect any TAF forecast points at this time. Land
breezes will redevelop tonight.

No AIRMETs are in effect.

&&

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

Surf along south facing shores will gradually lower through the
rest of the week.

For the long range, a late season south swell may arrive locally
early next week (Oct. 5th) time frame from a storm-fore low
currently near Southern New Zealand. 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 today, then
trend up tonight through the weekend as back-to-back north-
northwest swells arrive. The first one is from a storm- force low
centered around 1500 nm north of Kauai this morning. This source
will arrive tonight and gradually become more northerly Wednesday
through 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. 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.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Powell
AVIATION...JVC
MARINE...Almanza/Kino

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



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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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