Drought in Hawaii, January 15 to 31, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
804 PM HST Mon Jan 14 2019

.SYNOPSIS...
A stable and mostly dry weather pattern will remain in place with
light and variable winds through Tuesday. On Wednesday an
approaching cold front from the northwest will cause winds to
shift towards a more southerly direction with scattered showers.
On Thursday and Friday a cold front sweeps through the state from
west to east, spreading clouds and showers over each island.
Expect a return to trade winds on Saturday with typical windward
and mauka showers. Another cold front approaches the islands for
the first half of next week with another round of wet weather.

&&

.DISCUSSION...
A weak subtropical ridge axis remains just north of Hawaii
producing a light east to southeasterly wind flow over the state
leading to a light and variable wind regime. Strong subsidence
from the upper level ridge over the region will keep the trade
wind inversion heights low, capping cloud heights around 5000 to
6000 feet through Wednesday morning. These shallow cloud heights
will limit rainfall possibilities over the state with only
isolated shower coverage in the forecast. Diurnal surface heating
and cooling over each island will cause the light east to
southeast flow over the region to become modified by local scale land
and sea breezes through the day and overnight hours.

On Wednesday a low pressure system moving through the Central
Pacific will drag a cold front along for the ride, approaching
Hawaii from the northwest. This frontal trough will break down the
ridge over the islands and cause winds to shift towards a more
southerly direction. An upper level trough associated with this
system will reduce large scale downward subsidence and lift the
trade wind inversion heights to around 8000 feet deepening the
cloud depths. Expect scattered showers to develop by Wednesday
afternoon mainly over the western half of the state.

On Thursday and Friday the cold front will sweep from west to
east through the island chain. Breezy southwesterly wind speeds
will increase ahead of the cold front with breezy northerly winds
developing behind the frontal boundary as it passes through the
islands. Instability and lift along the frontal boundary will
create a brief period of isolated thunderstorm activity as the
front passes. Expect a roughly 8 to 12 hour period of showers for
each island starting with Kauai by Thursday around noon. The
frontal cloud band and showers will then reach Oahu by late
Thursday afternoon into the early evening, arriving over Maui
County by Thursday night into early Friday morning, and finally
pushing through the Big Island by Friday morning around sunrise.
This frontal system will bring widespread rain to each island with
most areas likely to observe at least a tenth or even a quarter
of an inch of rainfall with this event.

By Saturday, a migratory high pressure system passing through the
Central Pacific will bring one day of more typical moderate to
breezy trade winds with scattered showers mainly over windward and
mountain areas. On Sunday, large scale winds will veer towards a
more southeasterly direction with mostly dry conditions as a
second cold front approaches the islands from the northwest.

Wet weather remains in the forecast for the first half of next
week as long range forecast guidance continues to show another
cold front moving into the western Hawaiian Islands. Extended
range forecast models continue to struggle with the arrival timing
and impacts with this next frontal system. Expect more details on
this second system later this week as the forecast time period
grows shorter and weather guidance improves.

&&

.AVIATION...
Ridging over the area will keep the atmosphere dry and stable.
VFR conditions will prevail. Rather light background winds will
allow land breezes to develop over the islands overnight, then
becoming sea breezes by mid-day Tuesday.

No AIRMETs are in effect and none is expected through early
Tuesday.

&&

.MARINE...
A ridge of high pressure just north of the islands will keep
light to locally moderate trades in place through the Tuesday
night. An approaching cold front will shift winds around to the
south on Wednesday, with moderate to strong south to southwest
winds expected for Thursday ahead of the front. The cold front is
expected to sweep eastward through the islands late Thursday
through Friday, with moderate to strong northerly winds following
the frontal passage. Moderate to strong trade winds are expected
to fill in late Friday through Saturday as a quick moving high
builds eastward to the north of the state. A new approaching cold
front may then shift winds back around to the south and southeast
late in the weekend through early next week.

A large northwest swell will build tonight and is expected to peak
during the afternoon and evening hours on Tuesday, then slowly
begin to ease late Tuesday night. A High Surf Warning (HSW)
remains in effect for exposed north and west facing shores of the
smaller islands through Tuesday night, and a High Surf Advisory
(HSA) remains in effect for the west facing shores of the Big
Island through Tuesday night as well. This swell will continue to
gradually lower Wednesday through Thursday. A large, short-
period, northwest shifting to north swell will build Thursday
night and Friday, then slowly fade over the weekend.

The Small Craft Advisory (SCA) for elevated seas is expected to
continue through Tuesday night for portions of the marine area
exposed to the series of northwest swells moving through. This SCA
may need to be continued through Wednesday. Additionally, SCA
conditions develop ahead of and behind the cold front across
portions of the marine area Thursday through Friday for winds. A
period of strong trades may also produce SCA level winds across
the typically windy zones around Maui and the Big Island Friday
night through Saturday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Surf Advisory until 6 AM HST Wednesday for Kona-Kohala.

Small Craft Advisory until 6 AM HST Wednesday for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi
Channel-Maui County Windward Waters-Alenuihaha Channel-Big
Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

DISCUSSION...Bohlin
AVIATION...Hui
MARINE...Jelsema