Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 090202

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

Widespread clouds and showers expected through at least Tuesday
night, potentially longer, 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
trough develops near the islands. Winds will strengthen again
with drier weather expected this weekend.


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 this afternoon. 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 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.


An area of enhanced moisture will continue to spread over the
islands from the southeast this evening trough tonight.
Widespread shower activity, heavy at times, will lead to periodic
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 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.

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


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.
The SCA will likely be cancelled on Tuesday as trade winds
decrease and easterly seas lower. The High Surf Advisory (HSA)
was cancelled this afternoon due to surf below advisory thresholds.

Surf heights will remain elevated along east facing shores this
week due to a combination of trade wind seas and a small
northerly swell wrapping into east facing shores. Surf heights
along east facing shores will remain elevated in some areas even
as trade wind speeds drop 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 north to northeast swell will
build larger surf heights along north and east facing shores by
this weekend. Small surf is expected along most south facing
shores through the week.


Flash Flood Watch through late Tuesday night for all Hawaii

Winter Weather Advisory until 6 AM HST Tuesday for Big Island

Small Craft Advisory until 6 AM HST Tuesday for all Hawaiian



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