Flash Flood and Heavy Rain in Hawaii, December 30 to 31, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
1004 AM HST Thu Dec 30 2021

.SYNOPSIS...
Wet and rainy weather is expected to develop late today and
tonight as a surface trough is pulled north over the state. As the
trough moves through, our winds will switch from trades to
southeasterly. Rainfall will become more spotty and showery later
Friday behind the trough, but it will remain muggy. We will have
scattered showers and a couple of stray thunderstorms to ring in
the new year. A weak surface ridge will develop near the islands
next week, keeping winds light and variable and leading to
scattered showers, especially over island interiors each
afternoon.

&&

.DISCUSSION...

Blocked flow over the mid-latitude north Pacific continues
allowing the west Pacific jet to seed shortwave energy into the
low latitudes west of the islands. The latest iteration of this
pattern has resulted in the deep upper low currently positioned
southwest of Hawaii around 15N latitude. Deep layer southwest flow
ahead of this feature is the culprit for recent high overcast
skies. Some thinning of high clouds will be possible on Friday
afternoon, but for the most part expect no sun today or tomorrow.
For the remainder of the daylight hours today, expect a weakly
stable wet trade wind pattern characterized by winds veering from
the E this morning to SE by this evening. The Big Island and Maui
in particular will see steady trade wind showers today, some of
which may be locally heavy.

At present, regional satellite imagery reveals a healthy looking
extratropical cyclone with the first signs of a surface low emerging
on visible imagery and lifting toward 20N. This development
increases confidence in model depictions of overall system
evolution as this is very near the modeled placement by the
guidance yesterday. It also signals a transition toward
increasing dynamic forcing that will ultimately become more
focused over the western half of the state. Per GOES derived rain
rates, pockets of heavy showers are currently lifting toward the
Big Island and Maui. This is expected, but these will tend to
weaken today as greater forcing establishes further west in
closer proximity to the deepening surface low. The surface
low itself will be of little consequence other than to promote
slightly gustier southeast winds tonight, mainly over Kauai. By
this evening, the wing of sloped moist ascent supporting the
expansive area of clouds and showers to the south will begin
lifting through through the area. Rain may initially have more of
a stratiform character, but as instability increases, convective
showers (slight chance thunder) will become the dominant precip
mode. Still leaning against a Flash Flood Watch at this time as
the window for heavy rain in most locations should be no more than
about 6 hours centered in the 11pm-5am HST period. Locations that
are more exposed to southeast low-level flow such as the Big
Island and Kauai should see a longer duration of rainfall.
However, low confidence still precludes issuing a watch at this
time.

A secondary period of potentially showery weather will exist Friday
afternoon through Friday night, especially over the leeward zones.
This will occur as low-level flow becomes veers to southerly
coincident with the approach of the upper-level trough axis. Given
the abundance of moisture in place, it should be no problem for
this setup to at least result in showers focused over the leeward
terrain. However, there is ample opportunity for showers to
become more organized over the open water Friday evening,
particularly as they approach Leeward Kauai and Oahu - and mainly
in the 6pm-2am HST period. With that said, it should be noted
that the New Year's Eve portion of the forecast remains fluid.
Confidence in shower coverage remains low and precludes any
adjustment to the official forecast at this time.

Previous Discussion...Broad gentle southerly flow will continue into
the first week of 2022, with models suggesting weak surface ridging
building over or just S of the islands by Mon or Tue. This should
lead to an active convective pattern with daytime sea breezes giving
way to interior afternoon showers in many areas, followed by partial
clearing and fewer showers over land each night. Some of the models
hint that a cold front may try to push into the western main islands
on Wed, finally bringing drier and less humid air, but this is quite
far out in time and the details could change.

&&

.AVIATION...

A strong surface high located about 1400 miles NNE of Honolulu
will maintain breezy easterly surface winds today. As a result,
AIRMET Tango will likely remain in effect for tempo moderate
turbulence below 8,000 feet downwind of the mountains of all
islands through at least tonight.

Latest satellite trends and model data suggest that mid and upper
level cloud cover will continue to increase today across the
islands from the south while the breezy trade winds bring
enhanced low level cloud cover with occasional showers to windward
areas. Periods of MVFR ceilings and visibility are expected,
though isolated IFR is possible in the heaviest showers. As a
surface low west of the state gradually lifts northward, an
associated trough will pass over the islands late today through
tomorrow morning that will weaken winds as they turn more
southerly. Additionally, more widespread showers are expected
across the state and rain may be heavy at times, further reducing
ceilings and visibility. Isolated thunderstorms cannot be ruled
out with this activity.

AIRMET Sierra for tempo mountain obscuration above 2,500 feet
remains in effect for north through east sections of all islands due
to clouds and showers. AIRMET Sierra is likely to remain in
effect through tonight as low clouds and showers increase.

&&

.MARINE...

Deep high pressure remains far north-northeast of the state, and
a surface low is developing just outside of the offshore waters
about 375 nm southwest of Kauai. These features will maintain
fresh to strong easterly trade winds today, and a Small Craft
Advisory (SCA) remains in effect for all waters except Big Island
windward and Maui and Oahu leeward waters. The low will drift
northward and pass just west of the offshore waters tonight and
Friday, dragging a trough over the islands. This will cause winds
to veer out of the southeast late tonight and allow some areas to
fall out of the SCA, and as winds become more southerly on Friday,
the SCA will likely be dropped entirely. Moderate to locally
fresh southeast to south winds will ease to moderate strength or
less over the coastal waters on Saturday with a further decline
expected heading into next week.

As the low and trough move northward, they will bring a chance of
thunderstorms to the coastal waters tonight and Friday.
Thunderstorm chances will move north of the coastal waters on
Saturday.

A broad fetch of trade winds over and northeast of the islands is
producing elevated wind swell of 7 to 10 feet at 9 to 10 seconds.
These large seas are contributing to the need for the SCA over
mainly windward waters and are maintaining rough surf above the
seasonal average along east facing shores. As winds veer southerly
tonight and Friday, wind swell will only gradually decline due to
the persistence of trade winds east of the state. Expect east
shore surf to fall below seasonal average by Saturday and remain
low through much of next week.

Below seasonal average surf will persist for north facing shores
into early next week. A small northwest swell will receive a
boost to about 5 feet 15 seconds late Saturday, then decline
Sunday and Monday. A larger northwest swell is possible Tuesday
and Wednesday. No significant swells are due for south shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters.

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Big Island Southeast Waters.

&&

DISCUSSION...JVC
AVIATION...Vaughan
MARINE...Wroe