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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 181957

Area Forecast Discussion
National Weather Service Honolulu HI
957 AM HST Mon Jan 18 2021

A strong cold front looks to be stalling out near north Big
Island this morning. Strong sustained north to northeasterly winds
to around 30 mph gust up to 50 mph is starting to develop across
the state and will continue to increase this afternoon. Added
instability from an approaching upper level disturbance will
enhance rainfall this afternoon through tonight, especially over
Maui County and parts of the Big Island. Confidence is increasing
for Flash Flooding to occur over select areas on Maui this
afternoon into the evening. On Tuesday, the frontal moisture is
expected to drift back toward the west and will likely bring
another round of widespread rainfall to the western half of the
state. 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 could strengthen over
the weekend.


A cloudy, windy and rainy day for most areas across the state
today. Forecast still remains on track for heavy rain to pick up
over Maui County and parts of the Big Island this afternoon and
persist through at least the first half of tonight. An upper
level trough approaching the state, combined with the stalled
frontal moisture over Maui and the Big Island will lead to the
development of heavy showers and isolated thunderstorms this
afternoon through tonight. The greatest threat for flash flooding
looks to be over windward and mountain areas, where showers could
anchor along the terrain. Select areas could see over 10 inches of
rainfall throughout this event. So far, we have seen over 6
inches of rain on the mountain of Molokai from the Puu Alii rain
gauge. Over 4 inches of rain has been reported by the West
Wailuaiki gauge on Maui during the past 24 hours. A Flash Flood
Watch remains in effect for Maui and the Big Island through

A Flash Flood Watch also remains in effect for Oahu, and though
the shower intensity will likely not be as strong as Maui County
and the Big Island, we could see showers anchor over the Koolau
mountain, which could potentially produce flash flooding.

A Wind Advisory also remains in effect for the smaller islands due
to the gusty northerly winds behind the front. Wind gusts of up to
50 mph is expected as the winds continue to strengthen during the
day today. The strongest winds will occur over ridges and through
valleys where winds can accelerate downslope.

For tonight, the threat of flash flooding will continue over the
Maui County and the Big Island through at least the first half of
tonight. As we get closer to Tuesday morning, we could see the
showers tapering off on the Big Island as the frontal moisture
begins to shift back toward the west. This will bring another
round of heavy showers to the western half of the state late
tonight through Tuesday night.

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.


A cold front will stall near the Big Island today. Widespread
MVFR cigs/vsbys can be expected along and near the front,
particularly in windward areas. Unsettled weather is expected to
continue tonight over Maui County and the Big Island, and may
spread back westward to windward sections of Oahu and Kauai.

AIRMET Sierra remains in effect for mountain obscuration for
windward sections of Kauai and Oahu, and for all of Molokai, Lanai
and Maui. AIRMET Sierra may also be needed later today across the
Big Island.

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 day, and may need to be expanded
to include the Big Island.

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.


The 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 has warranted a Gale Warning that is in effect for
the nearshore waters west of the Big Island until 6 PM HST. The
front is expected to become near stationary over the Alenuihaha
Channel today before slowly retrograding west Tuesday 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
isolated thunderstorms into mid week. Lingering moisture and this
instability will likely keep maritime precipitation chances high
through Thursday.

The latest XL northwest swell will be arriving this morning and
building surf back up to High Surf Warning (HSW) criteria today.
This swell will be spreading down and across the Hawaiian Islands
today and tonight, leading to another round of warning level surf
to exposed north and west-facing shorelines. 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 (16 feet,
14 second) since 4 AM HST. Although today's northwest swell is not
as large as this past Saturday's near 20 foot swell, it will
still create dangerous surf conditions. Surf will peak out in the
35 to 45 foot range along Kauai's north-facing shores, 30 to 40
feet along the north-facing shores of Oahu, Molokai and Maui this
afternoon. This swell and subsequent surf will be trending down
tonight through Tuesday. Surf is likely to lower to advisory
levels for the north and west-facing shores of the smaller islands
by late Tuesday morning and into the afternoon hours. Swell energy
will also reach parts of the Big Island, but on a smaller scale
latter today through Tuesday. A High Surf Advisory is in effect
through Monday night for the exposed north and west-facing shores
of the Big Island. Strong and gusty northeasterlies behind a cold
front that is currently stalling out in the Alenuihaha Channel
will ultimately impact wave shape and result in poorer surf

Strong to near gale force winds, along with this building northwest
swell, has produced very rough, high sea conditions. A Small
Craft Advisory (SCA) covers these greater than 10 foot significant seas
across most windward and leeward nearshore Hawaiian waters. The
SCA is currently in effect through 6 PM HST and will likely to be
extended through at least tonight.


Flash Flood Watch through Tuesday afternoon for Oahu-Molokai-
Lanai-Kahoolawe-Maui-Big Island.

Wind Advisory until 6 PM HST this evening 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 from noon today to 6 AM HST Tuesday for
Big Island Summits.

Small Craft Advisory until 6 PM HST this evening for Big Island
Windward Waters-Big Island Leeward Waters.

Gale Warning until 6 PM HST this evening 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.




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

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