Flash Flood and Heavy Rain in Hawaii, April 4 to 6, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
331 PM HST Sat Apr 4 2020

.SYNOPSIS...
A low to our northwest and a displaced ridge to our east will
keep winds light across the islands through next week. Showers
will favor mauka areas during the afternoon and evening hours.
Increased moisture will slide northward Sunday night through much
of next week, bringing the potential for locally heavy downpours
and isolated thunderstorms.

&&

.DISCUSSION...
The combination of a surface ridge far northeast of the islands
and a low far to the northwest continues to induce light south to
southeast wind flow across local waters this afternoon. Blocking
from the Big island keeps winds across the smaller islands light
and variable. Local sea breezes have developed this afternoon,
increasing clouds and showers across mauka areas. Satellite loop
shows patchy broken low clouds across the smaller islands, with
broken to overcast low clouds across most of the Big Island.
Convergence within southerly flow is fueling a band of showery low
clouds across the Kauai channel. Radar shows moderate showers
within this band.

The low to our northwest will intensify over the next couple of
days while the surface ridge slides southward and edges closer to
the state. The resulting continued light wind flow will allow
development of local land and sea breezes. Models show the low
will lift northeastward early next week, while the surface ridge
remains nearly stationary. This will keep the current wind pattern
in place through Monday night. A weak front is forecast to move
into the western islands on Tuesday, then drift back and forth
across the islands over the rest of the coming week, triggering
increased rainfall and keeping winds light. Expect scattered
showers to develop over island interiors again Sunday afternoon
and evening. There is a slight chance of thunderstorms on the Big
island tomorrow afternoon, with a slight chance of thunderstorms
across all islands Sunday night through Tuesday.

Most rainfall should ocur over the western end of the state
through Monday night, although showers are in the forecast
statewide during this time. Again, some showers could be locally
heavy and there is a slight change of thunderstorms statewide
Sunday night through Tuesday. The frontal boundary is forecast to
move into the western islands Tuesday, then stall somewhere
across the central islands Tuesday night and Wednesday. Models
show the boundary may remain nearly stationary or waver back and
forth across the islands through the end of the week. While
confidence is too low at this point to include it in the gridded
database, locally heavy rainfall and even some thunderstorms
are possible beyond Tuesday.

&&

.AVIATION...
Light winds and isolated to scattered inland showers during the
afternoons for this forecast period. Kauai will see off and on
showers with some embedded heavy showers. The rest of the islands
should stay relatively dry with just a few isolated showers.
Afternoon sea breezes will set up today and Sunday, especially for
the Big Island. Some of these showers could be heavy at times.
The Big Island may even see a couple of thunderstorms. Tonight,
the best chance for showers will be for Kauai and the Big Island
but on the rest of the islands, the chance for rain will diminish
as land breezes set up.

There are no AIRMETs for mountain obscuration and across most of
the state we should see relatively brief, passing mountain
obscuration. If the need for an AIRMET arises, will issue
accordingly. There is an AIRMET for upper level turbulence from
FL250 to FL400. No reports as of yet but given the decreased air
traffic, it will probably be a bit more difficult to get PIREPs.

AIRMET Tango for light to moderate turbulence between FL250 and
FL400, will continue until 0400z, in association with a jet
stream.

&&

.MARINE...
The current light to locally moderate wind regime is expected to
persist into next week. The overall background flow will generally
be from the southeast to south as a front slowly approaches the
area from the northwest. No Small Craft Advisory conditions are in
the forecast into the middle of next week.

A broad area of convergent southeast to south flow is expected to
set up over the state from late this weekend into early next
week. This will likely result in a period of unsettled weather
with locally heavy rainfall, and possibly thunderstorms.

The new north-northwest swell has been running from 1 to 2 feet
above guidance. This has already been reflected in the surf
forecast for Oahu.

This north-northwest swell will likely peak tonight, then will
gradually lower from Sunday through Monday. A new long period
west-northwest swell is forecast to arrive Sunday night, which
will maintain moderate surf along most north and west facing
shores into early next week. This will be followed by a
reinforcing moderate northwest swell arriving late Tuesday, which
will likely peak Wednesday, and then gradually lower Thursday.
Surf is expected to remain below the High Surf Advisory (HSA)
criteria along north and west facing shores from Sunday through
the middle of next week.

The current southeast swell will continue to gradually subside
this weekend. Additional swells arriving from the southern
hemisphere are expected to maintain small background surf along
south facing shores next week. Also, the lack of trade winds will
keep only small surf along east facing shores through the middle
of the new week.

&&

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

&&

DISCUSSION...Powell
AVIATION...Chevalier
MARINE...Kinel