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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 160621

Area Forecast Discussion
National Weather Service Honolulu HI
821 PM HST Tue Jun 15 2021

Moderate trade winds will prevail through Friday, but will ease a
little over the weekend, and may become even lighter next week.
The trade winds will deliver an area of showery low clouds to
Kauai and Oahu tonight and Wednesday, primarily dampening
windward areas, and bringing a slight chance of thunderstorms to
Kauai. Otherwise, limited moisture will result in just a few
showers over windward areas. Periods of high clouds, associated
with a disturbance aloft, can be expected the next couple of days.


Currently at the surface, a 1029 mb high is centered around 1400
miles northeast of Honolulu, and is driving moderate trade winds
across the island chain this evening. Infrared satellite imagery
shows a mix of high and low clouds resulting in mostly cloudy
conditions across the state. Meanwhile aloft, water vapor imagery
shows a broad mid/upper level low several hundred miles northwest
of Kauai. Radar imagery shows scattered showers moving into
windward areas of Kauai and Oahu, as well as over leeward Big
Island. A few decaying showers are reaching leeward areas of Kauai
and Oahu, with very little rainfall noted elsewhere across the
state. Main short term concerns revolve around thunder chances the
next couple days.

High pressure northeast of the state will lift slowly northward
through Friday, keeping moderate trade winds in place across the
island chain. A couple fronts approaching and stalling northwest
of the islands Saturday through next Tuesday, will suppress the
trades down into the light to locally moderate range. The lighter
winds will allowing for some localized land and sea breezes to
develop during this time as well.

As for the remaining weather details, the remnants of an old front
will pass through Kauai and Oahu tonight and Wednesday, bringing
an increase in shower activity primarily to windward and mauka
areas. We will need to keep a slight chance of thunderstorms in
the forecast for Kauai late tonight through Wednesday evening due
to a disturbance aloft moving overhead while the old frontal
remnants move through. Meanwhile across Maui County and the Big
Island, a fairly dry trade wind pattern will prevail, although
showers may increase a bit over windward Big Island during the day
on Wednesday.

A more typical trade wind pattern will resume Wednesday night and
then continue through early next week, with clouds and showers
favoring windward and mauka areas. Showers will be most likely at
night and in the early morning, with a few showers spilling
leeward at times as well. A few showers may also develop each
afternoon over leeward areas in the lightened trade wind regime
Saturday through next Tuesday. Some lingering instability may
allow for a thunderstorm or two to pop on the Big Island Thursday
afternoon as well.


Moderate easterly trades will persist tonight and concentrate
most shower activity along the windward coasts and slopes. In
addition, a moisture boundary moving south towards the islands
will bring an increase in shower activity for Oahu and Kauai.
Relatively unstable air aloft will lead to a slight chance of
thunderstorms over Kauai and surrounding waters on Wednesday.

No AIRMETs are currently in effect.


High pressure north of the islands will maintain moderate to
locally strong trade winds through the week. Showers will
increase over the northern waters tonight into Wednesday and may
produce a few isolated thunderstorms around the waters of Kauai.
Small Craft Advisory is in effect through Thursday afternoon for
the windier waters near Maui and the Big Island. Trade winds are
expected to decrease over the weekend as a front develops well
northwest of the state.

Surf along south facing shores will remain small through Wednesday,
then slowly rise and approach High Surf Advisory levels over the
weekend. The current small short-period north swell will decline
Wednesday. Surf along north facing shores will be minimal through
the rest of the week, but a small northwest swell is possible
early next week. Trade winds upstream of the islands will produce
small and choppy surf along east facing shores.


Small Craft Advisory until 6 PM HST Thursday 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