High Surf in Hawaii, March 8 to 10, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
947 AM HST Mon Mar 8 2021

.SYNOPSIS...
Widespread clouds and showers expected through the first half of
the week as an upper level disturbance interacts with moisture
moving in from the east and south. Breezy trade winds will give
way to lighter winds by midweek as a surface develops over the
islands. Winds will strengthen again with drier weather expected
this weekend.

&&

.DISCUSSION...
Water vapor imagery shows an upper low centered at 24N163W, or
about 300 miles northwest of Kauai. Infrared satellite shows midlevel
and high clouds persisting along the eastern flank of the low
covering the Big Island and Maui County this morning. 12Z soundings
show 500 mb temperatures ranging from -11 C at Hilo to -13 C at
Lihue. At the surface, a 1035 mb high is centered far north to the
northeast is driving fresh to strong trade winds statewide. The
trades are driving clouds and showers east to west. MIMIC Total
Precipitable Water satellite shows PW values around 1.4 to 1.5
inches moving over Big Island and Maui County and 1.3 inches over
Kauai and Oahu. The combination of the cold temperatures aloft and
incoming moisture will result in an extended period of heavy
showers and thunderstorms through mid week that could impact any
island at this point. Therefore, the Flash Flood Watch has been
expanded to include the entire state through Tuesday night. A
Winter Weather Advisory is also in effect above 11 kft on the Big
Island Summits through early Tuesday morning due to merging
layered clouds expected to fill today. Trade winds will focus more
of the heavy rainfall over windward slopes, but all portions of
the state could be impacted by the flash flooding. This overall
weather pattern will change very little through early Wednesday.

Differences between the GFS and ECMWF in the development of a
surface trough/low on Wednesday could vary the forecast
considerably from this point on. The GFS has a cutoff surface low
spinning out of the trough on Thursday and dragging most of the
moisture north of the state, whereas the ECMWF develops a weaker
trough just west of the state and keeps the upstream moisture
moving in. Depending on which scenario develops will result in
either the Flash Flood Watch ending or continuing into Wednesday
or Thursday. The GFS scenario will also result in winds weakening
and becoming light and variable Thursday and Friday, while the
ECMWF keeps winds at the moderate to fresh east southeast flow.

A strong high will set up far north of the state this weekend and
quickly force the remnant moisture to the southeast away from the
islands. Expect an increase in trade winds and a decrease in
showers this weekend.

&&

.AVIATION...
An area of enhanced moisture will continue to spread over the
islands from the southeast today. Increasing shower activity,
heavy at times, will lead to periodic MVFR ceilings and
visibility along windward sides of all islands and over elevated
terrain. Isolated IFR conditions may occur with any of the
heavier bands of showers, along with isolated thunderstorms.

Breezy easterly winds will generate tempo moderate turbulence
below 8000 feet over and immediately south through west of
mountains.

AIRMET SIERRA remains in effect above 2000 feet for north thru
east facing slopes of the Big Island, Maui, and Oahu due to
mountain obscuration from clouds and showers

AIRMET TANGO remains in effects below 8000 feet for areas
south thru west of mountains due to tempo moderate
turbulence.

&&

.MARINE...
A strong high pressure system far north of the Hawaiian islands
will continue to produce fresh to strong trade winds across the
region through Tuesday morning. An upper level disturbance will
produce a surface trough over the central islands, disrupting the
trade wind pattern from Tuesday through Thursday. Stronger trade
winds return from Friday into the weekend as the surface trough
weakens over the area.

Strong trade winds will continue to keep a Small Craft Advisory
in effect for all local waters through 6 AM HST Tuesday morning.
A combination of trade wind waves and a northerly swell will keep
elevated seas over exposed waters and channels. A High Surf
Advisory also remains in effect for exposed east facing shores
through 6 AM Tuesday morning. Both advisories will likely be
dropped on Tuesday as trade wind speeds begin to decrease.

Surf heights will remain elevated along east facing shores today
and tonight due to a combination of trade wind seas and a small
northerly swell wrapping into east facing shores. The trade wind
seas will remain elevated in some areas from Tuesday through
Thursday due to uncertainties in the strength of the surface
trough affecting the trade wind fetch area. The north swell will
continue to slowly decline over the next few days. Diminishing
northerly swells will cause surf heights along north and west
facing shores to decrease from Tuesday into Friday. A new
northeast swell and increasing trade winds will build surf heights
along north and east facing shores by this weekend. Small surf is
expected along most south facing shores through the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Tuesday for east facing shores
of Kauai, Oahu, Molokai, Maui and Big Island.

Flash Flood Watch through late Tuesday night for all Hawaiian
Islands.

Winter Weather Advisory until 6 AM HST Tuesday for Big Island
Summits.

Small Craft Advisory until 6 AM HST Tuesday for all Hawaiian
waters-

&&

DISCUSSION...Foster
AVIATION...Thomas
MARINE...Bohlin