Heavy Rain in Hawaii, March 21, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
401 PM HST Sat Mar 21 2020

.SYNOPSIS...
A few showers over interior sections of Kauai and Oahu will linger
into the evening and then dissipate. Moderate trade winds fill in
tonight, then become strong Monday through midweek. Clouds and
showers will focus over windward and mountain locations, especially
through the overnight and early morning periods. Dry and stable
conditions will keep rainfall accumulations light.

&&

.DISCUSSION...
The last bit of moisture along a convergent band associated with
Kona low that brought the active weather across Hawaii this past
week will bring a few showers across Kauai and Oahu through this
evening. Sea breezes this afternoon have focused these showers
across leeward and interior sections of both islands. Trade winds
are already filling in east to west across the island chain and
will act to push this moisture westward tonight. Look for a
decrease in showers overall, but a few showers could begin to
track in from the east through Sunday as the trade winds build
back in, particularly during the overnight and early morning
hours.

The trades winds are expected to become strong/gusty by Monday as
the driving high pressure system to the north strengthens. Drier
and more stable air is expected to fill in across the state
Monday through Tuesday with precipitable water values dipping
below an inch and dewpoints into the 60s. As a result, rainfall
accumulations should remain light through the upcoming week.

A Wind Advisory for the typically windier areas in Maui County
and the Big island will become a possibility Tuesday through
Wednesday or Thursday as the high reaches it strongest point.
Trades will trend down late in the week as the high weakens. An
upper trough dipping southward into the region may translate to
better rainfall chances by next weekend.

&&

.AVIATION...
Scattered showers continue on and around Kauai and Oahu this
afternoon, with interior clouds building up on the other islands
due to localized sea breezes. MVFR conditions are widespread
enough on Kauai and Oahu to warrant AIRMET Sierra for mountain
obscuration for a few hours this afternoon. However, trade winds
are returning from east to west across the island chain and have
made it as far as Oahu. Thus, the interior/lee clouds and showers
should begin to diminish late afternoon/early evening as the
trades increase. Once the trades are in place tonight, clouds and
showers will once again favor windward coasts and slopes of all
islands, with a few showers pushed leeward at times.

Trades are expected to fill into Kauai by tonight and increase
into the moderate to locally strong category overnight as high
pressure builds over the islands from the north. The locally
strong trades will require an AIRMET Tango for low-level
turbulence south through west of all islands by later tonight.

&&

.MARINE...
Strong high pressure building north of the state has led to the
return of moderate to fresh trades across most areas this
afternoon. Winds are expected to strengthen over the next several
days as the strong high builds southward toward the state causing
the pressure gradient to tighten over the Hawaiian waters. Trade
winds are expected to increase to locally strong speeds tonight
and becoming near Gale force by Monday. A Small Craft Advisory
(SCA) is in effect for tonight across the typically windy areas
around Maui County and the Big Island. This SCA will need to be
expanded across most Hawaiian waters by Monday due to the
strengthening trade winds. Some of the windier channels could see
Gale force winds during this time.

The current long-period west-northwest swell has been on a gradual
decline through the day today and will continue to decrease
through Monday. Minimal background swells are expected Tuesday
through the end of the week for north and west facing shores.

Surf along east facing shores will rise early next week due to a
combination of strong trade winds locally and a short-period northeast
swell. Surf will likely rise to advisory levels around Monday night
and remain elevated through most of the week.  Small surf will
continue along south facing shores through the weekend. An out of
season, long-period south-southwest swell will build Monday and
Monday night, then peak on Tuesday at 3 feet 16/17 seconds. This
may result in surf nearing HSA thresholds along south facing
shores Tuesday and Tuesday night, with the swell then gradually
lowering through Thursday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

DISCUSSION...Foster
AVIATION...TS
MARINE...Kino