Winter Storm Watch 4, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Sun Nov 26 2017

.SYNOPSIS...
Windy and wet trade winds will strengthen today as robust high
pressure to the north drifts closer to the state. Trades will
decrease slightly Monday and Tuesday but will remain breezy and
gusty through the week. Moisture within trade flow will deepen
tonight and Monday, especially over the southern end of the island
chain where periods of heavy rainfall will be possible during the
week. Winter weather is possible at the Big Island summits Monday
and Tuesday.

&&

.DISCUSSION...
A 1032 mb high far north of the main Hawaiian islands continues
to drive blustery trade winds across the area this morning. Radar
shows numerous showers across windward and mauka sections of the
islands, with the strong trades blowing a few of these showers
across leeward areas as well. Overnight soundings show inversions
beginning to weaken and rise as upper troughing across the state
decreases stability. PW has increased with the arrival of
additional low level moisture within showery cloud bands, reaching
1.27 inches at Lihue and 1.56 inches at Hilo.

Trade winds will increase today as the surface high moves to
within 800 miles north of the state, causing the local pressure
gradient to tighten. The Wind Advisory covering the lower
elevations of all islands will remain in place through the day
today. The current wet trade pattern will continue as well as
satellite loop shows bands of ragged low clouds lining up at
irregular intervals upwind from the islands.

Trade winds will decrease slightly on Monday, while rainfall
intensity will likely increase across the eastern end of the
island chain through Tuesday. In addition to the approaching
cloud bands noted above, an area of deep moisture along an old
frontal boundary several hundred miles east and southeast of the
Big Island will begin to drift toward the islands late today and
Monday. The Big Island will likely see the bulk of this deep
moisture, while the rest of the state will be on the northern
fringe. The upper level trough is expected to retreat westward
during this time, but it will likely remain close enough to
produce enough instability to generate spotty heavy showers in the
trade wind flow, especially over windward portions of the Big
Island. Due to higher QPF values noted in the latest global ECMWF
model, we now have enough confidence to issue a Flash Flood Watch
for Maui and the Big Island from 6 am Monday through Tuesday
afternoon. We will also issue a Winter Storm Watch for the Big
Island summits for the same time frame.

The current pattern will persist through at least Wednesday and
Thursday. The deepest moisture is expected to remain stalled over
the eastern half of the island chain, though windward portions of
all islands will continue to be rather wet. The upper level trough
over the islands will weaken. As a result, chances for heavy
showers will be lower after midweek, while chances for snow on
the high summits of the Big Island will continue.

Global ECMWF no longer wants to develop a low southwest of the
islands Thursday and Friday, instead coming more in line with the
GFS depiction of stronger local trades. Oddly, GFS now wants to
develop a low southeast of the islands next Saturday, then move it
sharply northeastward away from the islands. This would also
result in stronger trades for the islands. Either way, we are
sticking with a forecast calling for continued trade flow through
the forecast period, with speeds staying in the breezy to windy
range.

&&

.AVIATION...
As strong high pressure persists well north of the area, trade
winds will remain strong across the island chain. They will be
carrying low clouds with embedded showers, primarily affecting
windward and mauka sections with MVFR conditions, though leeward
areas will see lower ceilings and showers from time to time as
well.

An AIRMET has been issued for mountain obscuration over much of
the state because of the lower cloudiness and showers. Another
AIRMET has been posted to cover low level turbulence and strong
surface winds. These AIRMETs will be reissued later this morning.

&&

.MARINE...
Strong and gusty trade winds, combined with an incoming north
swell, will maintain Small Craft Advisory (SCA) conditions across
the Hawaiian coastal waters. Gale force winds are expected today
in Maalaea Bay, as well as the Pailolo and Alenuihaha channels. A
Gale Warning remains in effect for these areas through this
afternoon, but SCA level winds and/or seas will persist beyond
that time through Monday. Expect trade winds to subside somewhat
Monday night and Tuesday as high pressure to our north weakens.

The strong trade winds will cause surf along east facing shores
to remain at or above advisory levels today through Monday, so the
High Surf Advisory continues for the east facing shores of Kauai,
Oahu, Molokai, Maui and the Big Island.

The incoming north swell is expected to arrive later today, peak
late this afternoon into Monday morning, then slowly subside
Monday afternoon through Tuesday. Resulting surf on north facing
shores is currently forecast to peak just below advisory levels
this afternoon through Monday morning. The day shift will need to
monitor the incoming swell as it arrives at the buoys and may need
to issue a high surf advisory if it is any larger than forecast.

Another longer period north northwest swell is expected to arrive
Tuesday, peaking Tuesday night. A similar or slightly larger
northwest swell is expected to arrive Thursday. These swells may
also produce surf near advisory levels on north facing shores.

A small but long-period south swell will cause a slight bump in
surf on the south facing shores today into Monday. This swell
will fade out by midweek.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-
Maui Central Valley-Windward Haleakala-Leeward Haleakala-Kona-
South Big Island-Big Island North and East-Kohala-Big Island
Interior.

High Surf Advisory until 6 PM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-Big Island North and East.

Flash Flood Watch from Monday morning through Tuesday afternoon
for Maui Windward West-Maui Leeward West-Maui Central Valley-
Windward Haleakala-Leeward Haleakala-Kona-South Big Island-Big
Island North and East-Kohala-Big Island Interior.

Winter Storm Watch from Monday morning through Tuesday afternoon
for Big Island Summits.

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

Gale Warning until 6 PM HST this evening for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel.

&&

DISCUSSION...Powell
AVIATION...Kinel
MARINE...Jacobson