Drought in Hawaii, April 1 to 21, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
934 PM HST Tue Mar 31 2020

.SYNOPSIS...
The light trade winds will be becoming southeast by Thursday,
then light south and southeast on Friday, continuing into the
weekend as a front approaches the islands from the west. Expect
the weather to become more unstable over the weekend as the front
gets support from an upper level low forecast to settle in
northwest of the islands.

&&

.DISCUSSION...

Changes have been made to the evening forecast package, as a
eastward advancing short wave moves through the main Hawaiian
Islands. This shortwave is currently over Molokai, and is expected
to clear the Big Island mid Wednesday morning, taking most of the
dense mid and high clouds away with it.

Meanwhile, the trade winds will easing, becoming light over the
western islands overnight, while trades will be a tad stronger
over on the Big Island. This diminishing trend opens the door for
a sea breeze/ land breeze regime to settle in. Although the
shortwave would be east of the Big Island Wednesday morning, there
will still be enough instability, where combined with daytime
heating, light winds, and ample moisture, result in some scattered
afternoon showers and a slight chance of a thunderstorm,
especially over the interior and lee areas of the smaller islands.
Both the ECMWF AND GFS are in fair agreement with this scenario,
although they are both drier. We will see how things pan out
Wednesday morning, but there is a hint, where there could be less
showers and thunderstorm Wednesday afternoon.

The trades will then be turning southeast Thursday, in response
to a advancing front from the northwest and an upper level low
setting up shop 880 miles northwest of Kauai Friday evening. This
southeast flow continues into the weekend, and becomes wetter
each day as instability increases. The upper low may be pulling
some tropical moisture toward the islands late in the weekend.

The models disagree at the moment as to when the front arrives.
The latest ECMWF is faster with it, having it over Kauai Monday
morning. The GFS is slower by about 24 hours. But before the
front's arrival, there could be plenty of showers around. Stay
tune.

In the mean time, skies have cleared Kauai, while Oahu and Maui
counties, continue to thin out. There does not appear to be much
trade wind showers. But if there is any, the developing land
breeze will minimize them moving ashore. Overnight lows will
likely be a couple of degrees cooler as the land radiates out due
to the lack of cloud cover, such as Kauai.

&&

AVIATION...
An upper level trough passing over the eastern half of the state
will give Maui a chance for a few showers, but the main chance for
rain tonight will be for the Big Island. For the Big island, any
threat of thunderstorm ends by midnight, but the scattered showers
for the windward areas remains. Leeward Big Island will be dry.

AIRMET Sierra is in effect for mountain obscuration on the
windward side of the Big Island but ceilings have lifted quite a
bit and should be able to cancel that AIRMET soon. Will keep
AIRMET Zulu in place for light IC icing until the thick cloud
layer dissipates later tonight.

Light trade winds are expected through the rest of the forecast
period. The light trades will promote daytime sea breezes that
might trigger a few inland showers.

&&

MARINE...
A surface high far northeast of the area is maintaining easterly
trade winds across Hawaiian waters, but a trough over the eastern
offshore waters will keep the winds gentle to locally fresh though
tomorrow. A trough aloft moving east across the area is making the
atmosphere unstable, so there will be a chance of thunderstorms
over the eastern offshore waters and over the coastal waters east
of Molokai through tonight. As the trough moves away to the east
tomorrow, the chance of thunderstorms will subside.

A surface low south of the Aleutians is forecast to move south
for the next couple of days. As the low approaches, winds will
veer out of the southeast starting Thursday. Winds will weaken
over the western part of the coastal and offshore waters but winds
may strengthen over eastern waters, especially where the
southeast flow accelerates around the Big Island and Maui. A front
or convergence band associated with the low will likely move over
the northwest offshore waters Saturday and could produce
strengthening south winds over the western waters and bring a
chance of more thunderstorms.

The current small northwest swell will slowly subside through
Friday night. A new northwest swell building Saturday will produce
moderate surf along north and west facing shores over the
weekend, but surf is expected to remain well below the advisory
threshold. Swells from the southern hemisphere will maintain small
surf along south facing shores through tomorrow. Swells from the
distant southeast Pacific will be partly blocked by the islands of
French Polynesia but will produce small surf through the rest of
the week. Surf along east facing shores will remain up through
midweek due to an upstream fetch, despite the trades relaxing
locally. Surf will subside by the end of the week as the upstream
fetch weakens.

&&

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

&&

H Lau/Chevalier/Donaldson