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Hawaiian Islands Synoptic Discussion and Guidance

966
FXHW60 PHFO 190231
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
408 PM HST Mon Jan 18 2021

.SYNOPSIS...
The frontal moisture has stalled over Maui County and parts of the
Big Island this afternoon and is producing heavy rain across the
eastern half of the state. The heavy rain will continue overnight
and gradually shift westward towards Oahu and Kauai Tuesday and
Tuesday night. The strong north to northeasterly winds is
starting to fill in over Maui County this afternoon and is
expected to gradually decrease across the state by late tonight
into Tuesday. Moderate to breezy easterly trade winds are forecast
from Wednesday through the end of the week with periods of
showers favoring windward and mountain areas. Trades are expected
to strengthen to strong speeds over the weekend.

&&

.DISCUSSION...
A Flash Flood Warning remains in effect for the island of Molokai
and parts of Maui this afternoon. Rainfall intensities have picked
up since this morning and we are seeing widespread rain with rates
of 1 to 2 inches per hour with locally up to 4 inches per hour
across Maui County and parts of Big Island this afternoon. Over
the past 24 hours, we have seen over 12 inches of rain reported on
the Puu Alii gauge on Molokai and over 5 inches on Maui. These
amounts are expected to continue to increase tonight.

Further west on Oahu and Kauai, we have seen the strong north
winds fill in behind the front with several weather stations
reporting wind gusts of 50 mph.

For rest of this afternoon and tonight, we are expecting stronger
wind gusts to move into Maui County this afternoon and evening.
The Wind Advisory has been extended through tonight, but we should
see the winds gradually taper off during the second half of tonight.
Heavy rain will continue overnight across Maui County and the threat
of flash flooding will begin to expand further east into east Maui
and north Big Island this afternoon and evening. By late tonight,
the frontal moisture is expected to gradually shift westward and the
threat of heavy rain should begin to decrease over Big Island and
increase over Oahu into Tuesday morning and over Kauai by Tuesday
night. Although heavy showers will be possible Tuesday, we are
expecting these showers to be moving and not staying stationary like
today. Thus the threat for flash flooding will decrease, but still
remain possible on Tuesday.

By Wednesday, most of the frontal moisture should be west of the
state, and we should see our typical trade wind showers return
across the state. Some lingering heavy showers will be possible
over Kauai on Wednesday. Moderate to locally breezy trade wind
weather will return during the second half of the week with trades
potentially pick up to strong speeds over the weekend.

&&

.AVIATION...
A cold front is stalled between Maui and the Big Island.
Widespread MVFR cigs/vsbys can be expected along and near the
front, particularly in windward areas. Unsettled weather is
expected to continue tonight across portions of the state
especially Maui County.

AIRMET Sierra remains in effect for mountain obscuration for
windward sections of Kauai, Oahu and the Big Island, and for all
of Molokai, Lanai and Maui.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain from Kauai eastward to Maui. This AIRMET
will likely continue through the overnight hours.

AIRMET Zulu is in effect for moderate rime ice from Kauai eastward
to Maui. Conditions may improve over Kauai and Oahu later today,
but will likely hold over Maui County. This AIRMET may also need
to be expanded to include the Big Island later today.

&&

.MARINE...
The Sunday overnight passage of a cold front is ushering in
strong to near gale force north northeast winds in its wake.
Frequent wind gusts to gale have warranted a continued Gale
Warning (GLW) through the overnight Tuesday morning hours. The
GLW is in effect for the nearshore waters west of the Big Island
until 6 AM HST Tuesday. The front has become stationary near the
Alenuihaha Channel and will slowly retrograde west through
Wednesday. The lower level convergence associated with this front
weakening to a trough, along with forcing provided by its parent
western upper level trough, will provide the necessary ingredients
to maintain areawide showers, some becoming locally heavy, and
scattered to isolated thunderstorms into mid week. Lingering
moisture within this instability will likely keep maritime
precipitation chances high through Thursday.

The latest XL northwest swell is passing through and spreading down
across the Hawaiian Islands. This will be build surf back up to High
Surf Warning (HSW) criteria along smaller island north and west-
facing shorelines tonight through Tuesday morning. The HSW is in
effect through 6 AM HST Tuesday. Buoy 51101 (180 nm NW of Kauai) has
been consistently observing this latest northwest swell and has
increased swell heights above model guidance this afternoon (15-20
feet, 14-16 second energy). The PacIOOS Waimea buoy's swell
observations have jumped up over the past few hours with 20 second
energy focused at 310 degrees. Early Tuesday morning surf will peak
out in the dangerous 30 to 40 foot range along Kauai, Oahu, Molokai
and Maui's north-facing shores...20 to near 30 feet along the west-
facing shores of Kauai, Oahu and Molokai due to the more westerly
component to these swells wrapping around northwestern shores. This
swell and subsequent surf will be trending down through the day
Tuesday. Swell direction will be veering more northerly into Tuesday
and this will lessen Tuesday's surf heights along western-exposed
shores. Surf will lower to advisory levels for the north and west-
facing shores of the smaller islands by Tuesday afternoon. Swell
energy will also reach parts of the Big Island, but on a smaller
scale later tonight through Tuesday. A High Surf Advisory is in
effect through tonight for the exposed north and west-facing shores
of the Big Island. Strong and gusty northeasterlies behind a cold
front that is currently stalled out near the Alenuihaha Channel will
ultimately impact wave shape. This will result in poor surf quality
with this latest winter northwest swell event. Eastern shore surf
will stay up through mid week. This is primarily due to post-frontal
strong to near gale force northeast winds pushing more wind wave
swell onto eastern-exposed shorelines.

Strong to near gale force winds, along with this building northwest
swell, has produced very rough, high sea conditions. A GLW covers
all windward and leeward Hawaiian Island waters. The exception is
the ongoing Small Craft Advisory (SCA) that covers the greater than
10 foot significant seas across the east and southeastern windward
waters surrounding the Big Island. The SCA is currently in effect
through 6 AM HST Tuesday and may need to be extended into Tuesday
afternoon as northwestern swells subside through the morning hours.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Tuesday afternoon for Oahu-Molokai-
Lanai-Kahoolawe-Maui-Big Island.

Wind Advisory until 6 AM HST Tuesday for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-
Maui Central Valley-Windward Haleakala-Leeward Haleakala.

High Surf Warning until 6 AM HST Tuesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

High Surf Advisory until 6 AM HST Tuesday for Kona-Big Island
North and East-Kohala.

Winter Weather Advisory until 6 AM HST Tuesday for Big Island
Summits.

Gale Warning until 6 AM HST Tuesday for Kauai Northwest Waters-
Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters-Maui County Leeward Waters-Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters.

Small Craft Advisory until 6 AM HST Tuesday for Big Island
Windward Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Kino
AVIATION...Burke
MARINE...Blood

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

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