Heavy Rain in Hawaii, March 16 to 20, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
406 AM HST Tue Mar 16 2021

.SYNOPSIS..
The trade winds are back, spreading from Kauai to as far as Maui
County overnight. The trade winds will continue to gradually
strengthen through Thursday, becoming rather windy on Friday.
Windy trade winds will persist through Sunday, followed by a rapid
weakening Sunday night through Monday as a front approaches the
islands from the west. A couple of upper level disturbances
passing through will briefly enhance the trade showers through
Thursday. A drier trade wind regime may arrive over the weekend.

&&

.DISCUSSION...
The demise of the surface trough near Oahu/Molokai has allow the
trade winds, which has been hanging around Kauai for the past few
days, to spread east, reaching Maui a short time ago. Breezy
northeast trades will spread to Oahu later today, Maui County
Wednesday, and finally eastern Big Island on Thursday. Another
boost in the trade winds will come on Friday as a strengthening
surface high settles in about a thousand miles north and northeast
of the islands. The high is expected to reach 1037 mb on
Saturday. So, for the islands, it means very wind conditions that
will likely warrant a Wind Advisory for the typically windy areas
of Maui County and northern Big Island. Other islands/areas could
be included. We will evaluate how things are progressing in the
next 48 hours. The strong trade winds is expected to collapse
Sunday night into Monday as a front approaches the islands from
the west.

A series of upper level disturbances will be passing through
the islands through Thursday. The first of two impulses will be
arriving later this morning, causing an uptick of trade wind showers.
The impulse may also lead to a thunderstorm over the Big Island
summits this afternoon. The second impulse will be moving through
the islands Thursday and Thursday night, lifting the inversion
from 10k feet to 15k feet, or an enhancement to the trade wind
showers. It won't be until Saturday afternoon when the inversion
lowers back down 10k feet across the entire island chain. Hopefully
this is a sign of a drier trade wind pattern.

Weather radar shows a new influx of showers riding in with the
trade winds from Maui westward. There will be some wet roads on
Kauai and Oahu this morning. There will be a slight drop in trade
showers this afternoon, but it will still be wet.

&&

.AVIATION...
As a surface trough near the islands has been dissipating, surface
high pressure far to the north-northwest has been taking hold
across the area. Trade winds have been persisting over Kauai and
adjacent waters, and are now beginning to become reestablished
from Oahu to Maui this early morning. And showers have already
begun riding on the returning trades and will be mainly focused
over windward and mauka sections today. An upper trough heading
this way from the northwest will enhance the showers to some
extent, along with their accompanying MVFR restrictions. The
instability from the upper feature may also trigger isolated
thunderstorms over portions of the Big Island by the afternoon
hours.

An AIRMET for mountain obscuration remains posted for Kauai, and
another AIRMET has been issued over Kauai for low level turbulence
below 8000 feet due to the strengthening trades.

&&

.MARINE...
A strong high pressure system northwest of the state will drive
fresh to strong trade winds over the western half and moderate
trades over the east today. The breezy trades will fill in for all
coastal waters tonight and Wednesday. A moderate to large north
swell will continue today, then decline tonight. A High Surf
Advisory is posted for east facing shores of Kauai, Oahu, Molokai,
Maui and Big Island due to exposure to the north swell today and
building trade winds through Wednesday. This advisory will likely
be extended to the end of the week as winds strengthen and spread
to all islands. A small to moderate northwest swell will build
later today and linger until the end of the week.

A Small Craft Advisory is is also in effect for the waters
surrounding Kauai, Oahu and Maui County due to a combination of
winds and seas over the western portion of the state. A couple of
the Maui zones drop out tonight as the north swell declines, but
will come back Wednesday or Wednesday night and the wind
strengthens. Trade winds are expected to build to near gale
strength Friday or Saturday, which may prompt the issuance of a
Gale Warning for accelerated areas.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Tuesday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-Big Island North and East.

Small Craft Advisory until 6 AM HST Tuesday for Alenuihaha
Channel-Big Island Windward Waters-Big Island Southeast Waters.

Small Craft Advisory until 6 PM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Pailolo
Channel.

&&

H Lau/Kinel/Foster