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Hawaiian Islands Synoptic Discussion and Guidance

864
FXHW60 PHFO 260138
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 PM HST Mon Jan 25 2021

.SYNOPSIS...
Locally breezy east to southeast winds and wet conditions with
heavy showers and the potential for flash flooding will persist
through Tuesday. Showers will diminish from east to west on
Wednesday. Breezy trade winds and will focus showers over
windward areas from Thursday through the weekend.

&&

.DISCUSSION...
The deep moisture and heavy showers that have been focused along
windward and southeast slopes of the Big Island have spread to
other islands this afternoon. Strong high pressure centered about
1,300 miles northeast of the state and a deep trough several
hundred miles to the west of the islands are causing the breezy
winds to shift out of the east to southeast. The deep trough is
also producing a broad area of convergence, and the islands sit
along the northern fringe of this area of heavy showers and
thunderstorms. This feature has dropped 5 to 7 inches of rainfall
over the Kau, Puna, and Hilo Districts of the Big Island during
the past 12 hours, and heavy rainfall has developed over and
around Oahu and Kauai this afternoon. As a result, the Flash Flood
Watch has been expanded from the Big Island to cover the entire
state. While the primary threat will be over windward areas,
anywhere may experience flash flooding. Also, a Winter Storm
Warning is up for the high summits of the Big Island.

The weather pattern will change little tonight and Tuesday. The
low-level flow will remain out of the east to southeast, and the
islands will remains along the north edge of the deep moisture.
The GFS model is hinting at a developing area of divergence aloft
that could shift the highest chance for heavy showers around
Kauai and Oahu. Windward areas should experience more rainfall,
but the Flash Flood Watch is out for all areas of the state
through Tuesday afternoon.

The threat for heavy rainfall is expected to diminish from east to
west on Wednesday and Wednesday night. The GFS and ECMWF models
show the mid- to upper-level trough southwest of the state
filling, which should allow the area of moisture convergence to
slowly move off to the west. Meanwhile, the surface high to the
northeast will only weaken slightly, leading to breezy and gusty
trade winds. As a result, showers, some heavy, will be focused
over windward slopes, with some occasionally spreading to leeward
areas of the smaller islands.

Breezy trade wind weather will prevail Thursday through the
weekend. A mid-level ridge will lead to more stable conditions,
and showers will be focused over typical windward slopes.
Although timing varies, the GFS and ECMWF are showing a band of
enhanced trade wind showers moving through Friday or Friday
night.

&&

.AVIATION...
A temporary window of improving conditions is currently observed
over windward Big Island while widespread persistent MVFR is
noted for remaining windward areas. Likewise, moderate to locally
heavy rain has, for the most part, been focused over windward
areas. The exception has been Oahu where the Lanai and Molokai
plumes are both in full swing bringing steady moderate rainfall
and episodic IFR/localized LIFR within the heaviest showers. Radar
trends suggest this activity will persist over Oahu, including
PHNL, for at least a couple more hours. Further, this corridor of
activity is now advancing northwest of Oahu and is forecast to
reach the eastern and southern slopes of Kauai, including PHLI, by
around 04z/6pm HST. Additional moisture immediately upstream of
the Islands, and now apparent on regional radar imagery, will move
into the area tonight and will support a continued threat of
locally heavy rain and MVFR/localized IFR conditions, chiefly over
windward areas.

AIRMET Sierra remains in effect for windward mountain obscuration
for Big Island through Kauai. A forthcoming update will expand the
AIRMET over Oahu to include the entire island.

AIRMET Tango remains in effect for low-level mechanical turb and
strong surface winds over some adjacent coastal waters.

AIRMET Zulu remains in effect for moderate rime icing over Big
Island and adj waters to the east. These conditions will remain in
place through today and possibly through tonight.

&&

.MARINE...
Robust high pressure far north and northeast of the islands will
sustain breezy to strong east and southeast winds across local
waters through the week. Thus, a Small Craft Advisory remains in
effect for winds (at or above 25 knots) and/or seas (at or above
10 ft) across exposed waters.

Surf along east facing shores will remain rough due to the strong
onshore winds locally and upstream across the eastern Pacific,
driving large seas through the area. The advisory in place will
likely become extended into Wednesday. A slightly downward trend
will be possible through the second half of the week as the
upstream fetch diminishes across the eastern Pacific.

Surf along north and west facing shores will steadily lower
through tonight as a northwest swell eases. A large scale pattern
change across the Pacific over the past week featuring a blocking
pattern setting up east of the date line will translate to an end
of the active run of large swells that began in the second half
of December and continued through the first half of January. Surf
will respond and drop well below the January average Tuesday
through midweek.

For the long range, guidance shows a developing hurricane-force
low far northwest of Hawaii, tracking northward toward the
western Aleutians through Tuesday. Wave models reflect this and
show small, long-period energy arriving through the day Thursday
out of the west-northwest, then trending up out of the northwest
Friday into the weekend. Large errors typically associated with
these sources lead to low forecast confidence, however.

Surf along south facing shores will trend up through midweek, as
an out-of-season Tasman source arrives. Surf should drop and
return to normal levels through the second half of the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Tuesday afternoon for all Hawaii
islands.

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

Winter Storm Warning until 6 PM HST Tuesday for Big Island
Summits.

Small Craft Advisory until 6 PM HST Wednesday for all Hawaiian
waters.

&&

$$

DISCUSSION...Wroe
AVIATION...JVC
MARINE...Kinel

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