Flash Flood Warning 5, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
841 PM HST Sat Jan 28 2023

.SYNOPSIS...
A surface trough over Maui County this evening will continue to
slowly drift westward through Monday combining forces with a
deepening upper level trough that will deepen and enhance shower
activity across the state through Monday. Periods of heavy showers
and thunderstorms currently developing along the trough over Maui
County will expand slowly westward into Oahu and Kauai by Sunday
afternoon. Breezy northeast trade winds over the western islands
will tend to enhance rainfall amounts over north and east slopes
of Kauai and Oahu. The Big Island will also see increasing shower
activity as deeper tropical moisture moves into the region. Wet
weather conditions will begin to improve from Monday night through
Tuesday as the upper low and surface trough drift slowly westward
away from the Hawaiian Islands.

&&

.DISCUSSION...
Satellite imagery this evening shows a large band of enhanced
showers and thunderstorms lingering over Maui County, extending
northeastward to waters north of the Big Island and southwestward
over waters south of Oahu and Kauai. Shower activity is starting
to spread westward into Oahu, however local radar imagery shows
the heavier showers remain near a surface trough currently over
Molokai and Maui. These shower trends, heavy at times, are
expected to increase over Oahu by Sunday morning and into Kauai
County by late Sunday afternoon as an upper trough deepens into an
upper level low just west of the state.

A combination of a slow westward moving surface trough, a
deepening upper level low and breezy northeast trade winds over
the western half of the state will shift the focus of heavy
rainfall westward on Sunday to the islands of Niihau, Kauai and Oahu.
However, additional tropical moisture moving up from the south,
as the upper low deepens to the west, will likely keep the
islands in Maui and Hawaii Counties under a continued threat for
periods of heavy rain and flooding lingering into Monday. A Flood
Watch (FFA) remains in effect for all islands except Kauai County.
This FFA will likely be expanded in coverage to include Kauai
County and extended in time through Monday across Maui and Hawaii
Counties.

Deep moisture over the Big Island has already produced snow and
freezing rain to the summits of Mauna Kea and Mauna Loa this
afternoon. A Winter Storm Warning remains in effect for both Big
Island Summits through Sunday. Additional wintery weather and
snow accumulations are expected to continue above the 12,000 foot
elevation level into Monday.

Medium range forecast guidance continues to improve on Tuesday and
Wednesday as the surface trough and upper low will continue to
weaken and drift westward away from the state. These pattern
changes will produce a significant improvement in decreasing
rainfall trends for all islands on Tuesday. Wind speeds will also
weaken as a cold front passing north of the state weakens the
ridge. A high pressure system north of the islands will build in
from the west on Thursday and Friday producing moderate to breezy
northeast trade winds, increasing windward shower trends, and
cooling temperatures statewide.

&&

.AVIATION...
A surface trough over the central islands will act as a focal
point for convective activity through Sunday. The air mass along
and to the east of the trough is well saturated and moderately
unstable. The air mass to the west of the trough is markedly drier
and more stable.

This evening, radar imagery shows that the heaviest precipitation
is occurring across Molokai and Maui. The islands of Maui County
will continue to receive the bulk of the wet weather overnight,
though showers across windward Oahu and northern sections of the
Big Island remain possible as well. Prevailing MVFR conditions and
light to moderate showers can be expected to prevail at most
locations, though isolated IFR conditions within heavier showers
will be possible through the night and into the day on Sunday.
With ample instability over the islands, isolated thunderstorms
are possible through Sunday night from Oahu through Big Island,
and could expand to include Kauai by Sunday afternoon as well.

This evening, SIGMET X-Ray for embedded thunderstorms includes the
central islands and extends across adjacent waters to the
northeast and southwest. Numerous PIREPs of moderate mid-level and
upper level turbulence have been received within this area
throughout the afternoon and early evening. As a reminder, a
convective SIGMET implies that lightning, severe turbulence,
icing, and low-level wind shear are all possible within the warned
area. AIRMET Tango also remains in effect from Oahu through the
Big Island for moderate mid and upper level turbulence to account
for areas not included within the SIGMET. As surface winds
increase through Sunday from Kauai to Oahu, an AIRMET Tango may
be needed for moderate low-level turbulence as well.

AIRMET Sierra for mountain obscuration remains in effect for Maui,
Molokai, and Oahu. Conditions will likely persist through the next
24 hours and beyond. AIRMET Sierra for mountain obscuration on the
Big Island remains in effect this evening, though this should
diminish over the next couple of hours. Finally, tempo light
icing will remain possible over much of the state with thick cloud
cover in place.

&&

.MARINE...
A trough over the eastern end of the state will continue to bring
rain along with the chance for thunderstorms over the coastal and
offshore waters through at least Monday. Thunderstorm potential
remains over the offshore waters through Wednesday. The trough is
expected to slowly drift to the west over the next couple of days.

A surface high pressure to the northwest of the state will drift
to the south through Sunday. The tight pressure gradient between
the high and the trough is resulting in an uptick in winds over
the western waters, with Small Craft Advisory (SCA) level winds
over the coastal waters near Kauai and Oahu. Winds will be a touch
higher over the adjacent offshore waters. Coastal water winds
should begin to decrease on Monday as the trough moves over the
western end islands.

A new moderate, long-period northwest swell (310 degrees) reached
buoy 51001 around 6pm, and is expected to reach Kauai by 2am.
Buoy 51001 reported 3 feet 19 seconds, which is in line with the
GFS wave data, and a couple feet below the NWPS guidance. The
forecast has been trended down towards the GFS solution. As this
swell moves into the coastal waters, exposed seas are expected to
rise above the SCA 10 foot criteria. These 10 foot seas are
expected to remain over the coastal waters into Tuesday.

In addition, moderate overlapping north swells (340 to 040
degrees) will move through the coastal waters through the first
half of the week. As the new northwest swell arrives, this will
help to boost surf to low-end advisory levels along most north
and west facing shores later Sunday into Monday, before subsiding
Tuesday through the rest of the week. Will continue to monitor
the swell's arrival at the buoys to the northwest tonight to
determine if a High Surf Advisory will be warranted or not.

Surf along east facing shores will become rough over the next
couple of day with the local wind swell mixes with some wrap from
the north swells. Meanwhile, surf along south facing shores will
remain small through Sunday, then pick up a bit Monday into
Tuesday as a small, long-period south swell (190 degrees) fills
in.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Sunday afternoon for Oahu Molokai Lanai Maui
Kahoolawe and the Big Island.

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

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel.

&&

DISCUSSION...Bohlin
AVIATION...Vaughan
MARINE...M Ballard