Heavy Rain in Hawaii, April 20, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
827 PM HST Thu Apr 19 2018

.SYNOPSIS...
Wet weather and cloudy skies will remain in place through Friday,
particularly over windward areas. Some showers may be heavy at
times and isolated thunderstorms are possible. A typical trade
wind pattern will return Friday night, with showers favoring
windward and mauka areas. High pressure north of the state will
maintain moderate to breezy trade winds through the weekend,
before strengthening to locally windy levels next week.

&&

.DISCUSSION...
An upper level trough approaching the main Hawaiian Islands from
the northwest will destabilize our airmass, while an old frontal
boundary embedded within trade flow will supply low level
moisture from the east. This combination of features has prompted
continuation of a flash flood watch through Friday afternoon.
Mostly cloudy to overcast conditions will prevail across the
state, with an extensive shield of mid and high cloud currently
extending well southwest of the island chain. These layered clouds
will push east, raising chances of winter weather at the Big
Island summits. A winter storm warning remains in effect through
Friday afternoon.

Radar shows showers pushing into windward areas at this hour.
Models show a steady increase in showers this evening and
continuing into Friday morning. Showers will favor windward and
mountains areas through the night, but leeward locations will get
wet as well from time to time. The upper level trough will push
eastward across the islands tonight, introducing the possibility
of heavy rainfall and thunderstorms.

By Friday morning, the threat for heavy rain will focus across
the eastern half of the state with the upper trough over Maui
County and the Big Island. Kauai and Oahu may see some clearing
during the morning hours with more normal trade wind weather
returning. By Friday evening, enhanced showers will remain over
windward Maui and the Big Island, but drier air from the east and
increasing stability will gradually decrease shower intensity and
coverage.

A surface ridge to the north will maintain moderate to locally
breezy trade winds Friday night through the weekend. High pressure
will strengthen Monday bringing trades to locally windy levels
through Wednesday. Overall, a fairly typical trade wind pattern
is expected through the period, with bands of enhanced clouds and
showers drifting through the islands from time to time. Showers
will focus over windward and mauka areas with some showers riding
the strong trades to leeward locations.

&&

.AVIATION...
A ridge of high pressure north of the state will keep a moderate
trade wind flow in place through Friday. Abundant moisture over
the state along with an upper level trough moving overhead will
result in showery weather. The lowest clouds and most persistent
showers are expected across windward and mauka areas, where
widespread MVFR cigs/vsbys are expected. Conditions will begin to
improve from northwest to southeast down the island chain on
Friday.

AIRMET Sierra is in effect for mountain obscuration over north
thru east sections of all islands. These conditions are expected
to continue through the night.

AIRMET Tango remains in effect for low level turbulence below 070 over
and downwind of the terrain of all islands. These conditions will
likely continue through the night. AIRMET Tango is also in effect
for moderate upper level turbulence between FL200 and FL400 due
to a jet stream in the vicinity of the islands. This AIRMET will
likely remain in place through Friday.

AIRMET Zulu is in effect for moderate icing between 130 and FL250
from Molokai eastward to the Big Island. These conditions are
expected to continue through the night.

&&

.MARINE...
A surface high north of the area will move northeast through
Saturday. The high is producing locally strong trade winds over
Hawaiian waters, but a trough aloft is keeping winds weaker than
might be expected. The trough will bring a slight chance of
thunderstorms with locally heavy downpours through Friday.

A Small Craft Advisory (SCA) continues through Friday for strong
trade winds in the windier areas near Maui County and the Big
Island tonight. The SCA will likely be extended and expanded over
the weekend as trade winds strengthen further.

Surf is expected to remain below High Surf Advisory (HSA)
thresholds along all shores through this weekend. Rough surf will
persist along east facing shores through Sunday night due to the
trade winds, but heights will remain moderate through the weekend.
As trade winds strengthen early next week, surf may build to the
HSA threshold along east facing shores starting late Monday, and
persist into the middle of next week.

One moderate northwest swell is expected to peak on Friday, a
slightly larger swell will peak Sunday and another moderate swell
is possible next Thursday.

A series of small swells from the southern hemisphere will maintain
modest surf along south facing shores chain through this weekend.
Lows in the southern hemisphere may strengthen and shift east
enough to produce some bigger surf along south facing shores by
the end of next week.

See the latest Collaborative Surf Discussion for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Friday afternoon for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui-Kona-South Big Island-Big Island
North and East-Kohala-Big Island Interior.

Winter Storm Warning until 6 PM HST Friday for Big Island
Summits.

Small Craft Advisory until 6 PM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

DISCUSSION...Powell
AVIATION...Jelsema
MARINE...Donaldson