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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 232003

Area Forecast Discussion
National Weather Service Honolulu HI
1003 AM HST Mon Sep 23 2019

A band of deep tropical moisture will move across most islands
today, maintaining humid conditions and bringing a chance for
locally heavy showers and thunderstorms. The deep moisture will
move west of the state tonight as resurgent easterly trade winds
bring a brief decrease in humidity. The moisture will push back
over Kauai, and perhaps Oahu, on Wednesday and Thursday as winds
weaken and veer southeasterly again. An upper level disturbance
may put this moisture to work, leading to some downpours over the
western end of the chain. A return to trade winds is expected for
the weekend.


Humid and wet conditions persist for much of the state this
morning. A sharp mid to upper level trough west of Kauai is
enhancing a band of deep moisture that is currently focused from
Molokai to Kauai. Meanwhile, surface high pressure far northeast
of the state is maintaining moderate east to east-southeast winds
over the islands. Spotty heavy showers produced localized rainfall
in excess of an inch across some parts of Oahu during the past
couple of hours, with heavy rainfall moving onto Kauai as of this
writing. Across the rest of the island chain, the deeper moisture
is clearing but remains hung up along windward slopes, where
moderate shower activity persists. The tropical moisture has also
pushed dew points into the low to mid 70s, which translates into
very humid conditions.

For the rest of the day, expect the focus for spotty heavy
rainfall to be on Kauai and Oahu, which will remain under the band
of tropical moisture. Windward slopes from Molokai to the Big
Island should continue to see active showers, with leeward and
interior slopes generating some downpours in the afternoon,
especially on the Big Island. A thunderstorm or two could fire up
just about anywhere. In addition, all areas will feel the very
humid conditions as dew points remain elevated into the mid 70s
and temperatures remain above normal.

A more stable, easterly trade wind flow will build in from the
east tonight, though unsettled weather will likely linger on Kauai
through much of the night. A mid to upper level ridge will push
westward to at least Oahu, bringing in more stable conditions that
will decrease shower intensity and largely limit rainfall to
windward slopes by midnight. The exception will be Kauai, where
the deeper moisture will linger and maintain chances for heavy
showers through the night.

A brief period of trade wind weather and a bit of relief from the
humidity is due on Tuesday. The deep moisture will clear Kauai by
morning, leaving the entire state under easterly trade wind flow,
and a ridge aloft will provide stable conditions. Expect showers
to be focused over windward slopes, but trades are expected to
begin to weak late in the afternoon, leaving open a chance for
some leeward showers, especially on the Big Island. Dew points
will drop into the upper 60s to lower 70s, providing some relief
from the humidity.

Deeper moisture stalled just west of Kauai Tuesday night will
slosh back east over Kauai and possibly Oahu on Wednesday and
Wednesday night. Guidance suggests that a weak mid level shortwave
may activate this moisture band bringing a chance for heavier
downpours and even a couple of thundershowers once again. Highest
chance would be near and around Kauai, with a lesser risk around
Oahu. The band should then weaken and become less active during
the latter part of the week as the mid-level energy weakens.
Nevertheless, as the flow becomes more trade-like again next
weekend, moisture levels are expected to remain above normal so
somewhat wet trades are expected.


Increased moisture and instability moving up over the island
chain is resulting in MVFR conditions this morning for many
locations. An upper trough near the state will continue to spark
some heavier showers through the day as well as a slight chance of
afternoon thunderstorms. As this feature moves away from the
island chain, conditions will improve through the overnight hours.

Trade winds will be in the light to moderate range, with some
locations becoming breezy this afternoon. Trades may have a bit
of an east-southeasterly component at times.

AIRMET Sierra for mountain obscuration remains in effect.


A small craft advisory has been posted for winds through the
Pailolo and Alenuihaha channels as well as areas near South Point
on the Big Island. Current breezy trade winds will diminish
Tuesday and turn out of the southeast Wednesday as a weakening
front approaches from the northwest. Light south to southeast
winds will continue into Friday before a weak ridge builds to the
north and light trade winds are expected to return. An upper
level trough will maintain unsettled weather through the week
bringing the possibility of heavy showers.

Small south swells are expected through Tuesday with a bigger
south swell starting to show Wednesday and fill-in on Thursday.
Surf from this swell may reach advisory levels along south facing
shores Thursday into Friday. Surf along north facing shores is
expected to steadily trend down today. A few other small to
moderate northwest swells will be possible towards the end of the
week from a low developing northwest of the state over the next
few days.


Small Craft Advisory until 6 AM HST Tuesday for 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

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