Heavy Rain in Hawaii, May 7 to 8, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
826 PM HST Wed May 6 2020

.SYNOPSIS...
Trade winds will finally deliver a few showers, mainly windward and
mauka, after an extended mostly dry period. Little overall change to
the weather pattern is expected through the weekend, with moderate
to locally breezy trade winds occasionally delivering windward
clouds and showers.

&&

.DISCUSSION...
After an extended period of minimal rainfall both windward and
leeward, the trade wind flow will be a little more showery (at
times) over the next couple of days. Most of the low clouds and
showers will focus over windward areas, but the moderate to locally
breezy trade winds will likely blow a few showers to leeward areas.

The trade winds will increase out of the NNE over Kauai and Oahu in
the short term, while remaining a little lighter and out of the ENE
over Maui County and the Big Island. This weak low-level convergence
is partially responsible for the increase in showers, as is low-
level moisture supplied by a long-dissipated frontal boundary now
embedded in the trade flow, and a passing short wave trough aloft
that is well-defined in latest water vapor imagery. Some "enhanced"
trade wind showers are seen approaching Kauai (where radar data is
unavailable to ongoing maintenance), with cloud tops approaching 20
thousand feet likely leading to a few briefly heavy downpours.

The trough aloft will depart rather quickly to the NE Thursday, with
a subsidence inversion keeping the island atmosphere rather stable
through the weekend. Meanwhile, high pressure to the NW of the
islands will move slowly E, with moderate to locally breezy NE to E
trade winds prevailing through the weekend. Low pressure passing far
N of the area will act to embed areas of increased low-level
moisture in the trades that will occasionally fuel periods of
showers over windward areas. Long range guidance currently features
weaker winds toward the middle of next week.

&&

.AVIATION...
The front that was near Kauai early this morning has dissipated.
In its wake, it is has ushered in a moderate northeast breeze to the
western half of the island chain. Supplied by a broad surface high
located far northwest of area, the northeast breeze, although
slightly lighter, will spread to the Big Island overnight and
Thursday.

Satellite imagery shows scattered to locally broken low clouds
across the smaller islands with embedded showers affecting the
islands from Molokai westward. Maui is currently in the clear, but
will fill in later. Broken to overcast low clouds covered all sides
of the Big Island slopes, coast, and adjacent waters. Clouds over on
the lee side of the Big Island will gradually clear out as the
evening progresses, while the windward side can expected little
change. Showers have been detected with all these clouds, area wide.
Tops of the showers are around 8 to 10k feet, but higher over the
adjacent waters, such as the ones off the Kona coast, which is up to
15k feet. Otherwise, the trade showers will be favoring the windward
and mountain areas of all islands, bring brief periods of MVFR
ceilings and vis through the rest of night. No AIRMET Sierra for
mountain obscuration is needed at this point but will be monitored
closely. The winds are weak enough not to warrant AIRMET Tango.

&&

.MARINE...
Moderate to fresh north-northeast winds have filled in across the
western end of the state today and should be in place across the
eastern end Thursday through Friday as high pressure builds north-
northwest of the area. Winds will shift back out of the east over
the upcoming weekend as high pressure shifts eastward over an area
north of the islands. Trade winds will fluctuate in intensity
through this time from place to place - with some areas potentially
reaching the fresh to strong category briefly (mainly in the windier
channel waters). For next week, guidance shows a weakness in the
ridge developing as a front passes far to the north. If this
evolves, light winds will become a possibility by the midweek time
frame.

Surf along north facing shores will gradually trend down overnight
through Thursday as the small, medium-period north-northwest swell
fades. An upward trend is expected Friday from a gale that passed
far north of the state over the past couple of days. This source
should hold through Saturday, then fade through the day Sunday. Low
pressure currently developing off the coast of Japan is forecast to
intensify as it tracks north-northeastward toward the western
Aleutians Thursday through Friday night. Favorable fetch aimed at
the islands within the 300-320 deg directional band that is forecast
will be the source for a small, long-period west- northwest swell
arriving by Tuesday of next week if it materializes.

Surf along east facing shores will remain small into the weekend due
to the lack of trades upstream of the state across the eastern
Pacific.

Surf along south facing shores will trend up Friday through the
weekend as a small south and southwest swell fill in from recent
southern hemisphere activity. Latest data and analysis show a decent
sized fetch setting up over Hawaii's swell window southeast of New
Zealand tonight. Guidance reflects this and depicts seas climbing to
around 30 ft through the day Thursday. A long-period south-southwest
swell arriving locally by Thursday/Friday of next week will be
possible.

&&

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

&&

DISCUSSION...Birchard
AVIATION...Lau
MARINE...Gibbs