Heavy Rain in Hawaii, December 1 to 2, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
529 PM HST Fri Dec 1 2017

.UPDATE...
Updated the forecast to increase PoPs and associated fields
for Kauai and surrounding waters for the first part of tonight.

&&

.SYNOPSIS...
Moderate to locally breezy trade winds will continue into Saturday,
due to high pressure north of the state. An upper level trough,
combined with a surface low to the northeast of the islands will
continue to provide the instability bringing enhanced showers on
the trade wind flow. Conditions are expected to gradually improve
over the weekend. The trade winds will ease on Monday as a cold
front approaches from the northwest. The cold front, with a band
of showers is expected to down the island chain Monday night
through Tuesday night. Look for cooler, drier northerly air behind
the front.

&&

.DISCUSSION...
The morning surface analysis showed a pair of high pressure
systems, along with a surface low. The highs were more than 1300
miles northeast, and about 1100 miles north of Oahu. The system to
the northeast is moving away to the northeast, while the system to
the north is moving east southeast. A low pressure system remains
between the two, about 1100 miles to the northeast of Oahu.

The high north of the islands is helping to strengthen trade
winds, particularly over the northern islands. An ASCAT pass this
morning clipped the waters near Kauai and showed some areas of 23
kts already moving in. The surface low is embedded within an upper
level surface trough, which is helping to provide the instability
near the islands for the enhanced shower activity.

The Big Island remained mostly shower free today, however clouds
and showers to the east and south will likely mean an increase in
showers overnight. Kauai and Oahu have been seeing most of the
shower activity today, and that has been focused over the windward
sides. The moisture over these islands is expected to spread to
Maui County tonight.

Satellite precipitable water (PW) shows the drier air to the
north and northeast which is expected to reach the islands
tomorrow, spreading from north to south. The upper level trough
mentioned above is expected to sink to the southeast, and bring
colder upper level temperatures which could help to maintain some
of the instability. The placement would suggest that showers could
linger near the Big Island late tomorrow.

As the high to the north edges closer, we'll see the trade winds
pick up tomorrow and Sunday. By Monday the winds will begin to
slacken some as a cold front approaches from the northwest. The
front will bring a band of showers down the island chain, and
timing remains with the band reaching Kauai Monday night, and to
the south of the Big Island by Wednesday morning. A high pressure
system behind the front will help to keep the front moving, and
will also bring northerly winds to the islands behind the front.

The models show colder dewpoints spreading down the island chain
behind the front. Tuesday shows dewpoints getting into the upper
50s, with northerly winds behind the front. The cooler, drier
airmass looks to stay in place for a few days, although winds will
turn more northeasterly by the end of the work week.

&&

.AVIATION...
AIRMET Sierra continues for tempo mountain obscurations along the
north through east sides of Kauai, Oahu, Molokai, Maui and the Big
Island as clouds and showers continue to blow in with the trade
winds.

AIRMET Zulu for moderate rime icing is posted for Kauai and Oahu
in layers 130-210. This is due to mid and high level clouds
ushered in by a jet along the eastern flank of an upper trough
west of the area.

Locally strong trade winds continue to produce turbulence to the
lee of the mountains on all islands. As such, AIRMET Tango
continues for mechanical turbulence south through west of
mountains statewide below 8000 feet.

These AIRMETs will likely continue at least through tonight, and
likely through the weekend for the turbulence threat.

&&

.MARINE...
Rough boating conditions will continue into the weekend due to a
combination of fresh to strong trades and a large north-northwest
swell. The Small Craft Advisory is in effect through Saturday for
all waters. Model guidance suggests the local winds will
temporarily relax and shift out of a more northerly direction
Sunday night through Monday as a front approaches and moves into
the area from the northwest. Strong northerly winds will fill in
across the waters in the wake of this boundary Monday night into
Tuesday, then relax through midweek as a ridge builds over the
islands.

The current north-northwest (330 deg) swell will hold through
tonight before slowly trending down over the weekend. The High
Surf Advisory for exposed north and west facing shores has been
extended through tonight, but is expected to drop below advisory
levels this weekend.

A new northwest swell (320 deg) is forecast to fill in Sunday and
Sunday night, then build into the upcoming week. This swell will
become reinforced from the north-northwest (340 deg) Tuesday
through midweek.

Surf along exposed north and west facing shores associated with this
active pattern could near advisory levels again by Monday night,
then potentially reach warning levels Tuesday night through
midweek as the reinforcing energy fills in.

Surf along east facing shores will remain rough into the weekend due
to the strong trades expected. The advisory for east facing shores
will likely be extended through Saturday in later packages.
Additionally, a reinforcing northeast (050 deg) swell, associated
with low pressure developing several hundred nautical miles
northeast of the state over the weekend, will likely keep the
surf elevated along east facing shores early next week as the
winds shift out of the north.

Surf along south facing shores will likely come up slightly over the
weekend due to a small, long-period swell sourced near New
Zealand.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory until 6 PM HST Saturday for all Hawaiian
waters-

&&

DISCUSSION...M Ballard
AVIATION...Foster
MARINE...TS