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Hawaiian Islands Synoptic Discussion and Guidance

190
FXHW60 PHFO 090646
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
846 PM HST Mon Mar 8 2021

.SYNOPSIS...
Widespread heavy showers, especially windward, are expected through
at least Tuesday night as an upper level disturbance enhances deep
moisture moving in from the east and south. Locally strong east to
southeast winds will continue Tuesday, then diminish Wednesday as a
trough develops near and north of the islands. The trough will
linger until the end of the week, keeping winds light and
maintaining the potential for showers. Strong high pressure
building north of the islands will bring breezy, dry and cool
northeast winds this weekend.

&&

.DISCUSSION...
Water vapor imagery shows an upper trough about 250 miles west of
Kauai. Infrared satellite shows midlevel and high clouds
persisting along the eastern flank of the low covering most of the
state. 00Z soundings show 500 mb temperatures ranging from -10 C at
Hilo to -14 C at Lihue. At the surface, a 1036 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 through Oahu to near 1.3 inches
over Kauai. 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 that includes 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, and may need to be extended and potentially
upgraded to a warning. Trade winds will focus more of the heavy
rainfall over windward slopes, but all portions of the state could
be impacted by 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, with dew points plummeting in the mid 50's, it
will feel quite chilly.

&&

.AVIATION...
An area of enhanced moisture will continue to spread over the
islands from the southeast through tonight and likely much of
Tuesday as well. Widespread shower activity, heavy at times, will
lead to MVFR ceilings and visibility, especially along windward
sides of all islands and over elevated terrain. Expect isolated
IFR conditions to 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 west of mountains. AIRMET
Tango remains in effect for this through at least tonight. Winds
are expected to trend down slightly through tomorrow night.

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

&&

.MARINE...
Strong high pressure centered far northeast of the state is
producing fresh to strong trade winds across and upstream of the
region. Strong trade winds will continue a Small Craft Advisory
(SCA) that is in effect for all local waters through 6 AM HST
Tuesday. The SCA may be allowed to expire Tuesday with weakening
trades and responding lowering seas. An upper level trough and
associated surface trough will not only maintain high chances for
numerous heavy showers and periodic thunderstorms but will also
disrupt the trade flow pattern. The location of this surface
trough will determine the magnitude and direction of this week's
coastal and offshore winds. If the trough does pass across the
chain, then expect more variable winds and an overall weakening to
mid to late week trades. As the surface trough weakens over the
area later this week, stronger trades are forecast to return by
the weekend.

A High Surf Advisory (HSA) is in effect for the east-facing shores
of Kauai, Oahu, Molokai, Maui and the Big Island to account for the
continued greater than 8 foot surf that will impact most eastern
exposures through Tuesday. East-facing surf heights will remain
elevated over the next several days due to a combination of trade
wind wave and eastern swell along with a small north swell wrap
around northeastern shorelines. Surf heights along north and east
facing shorelines will remain fairly uncertain this week due to a
surface trough of low pressure laying across the islands. The
location of this trough may disrupt the general large scale fresh to
strong trade flow generated by the large scale pressure gradient
from high pressure northeast of the chain. Diminishing northerly
swells will cause surf heights along north and west-facing shores to
fade through the week. Strong north winds setting up behind a passing
late week eastern Pacific cold front will follow a great circle
path directed back toward the state. This will generate a long
fetch region that may allow large, long period north northeast
swells to propagate into the islands this weekend. These swells
could produce XL surf along north and east-facing shores by the
end of the week. Very small, lower period south southwest swells
will continue to create small surf along south-facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late Tuesday night for all islands.

High Surf Advisory for East facing shores of Kauai Oahu Molokai Maui
and the Big Island.

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...Birchard/Foster
AVIATION...TS
MARINE...Blood

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office