Current Conditions
Temp-1.3 C
RH23 %
WindS 6 mph
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 180708

Area Forecast Discussion
National Weather Service Honolulu HI
908 PM HST Sun Jan 17 2021

A strong cold front currently over Oahu will continue to move
southeastward down the island chain, stalling near the Big Island
on Monday. Strong sustained north to northeasterly winds to
around 30 mph with higher gusts will develop after the front
passes each island. Added instability from an upper level low
disturbance behind the front will add to the rainfall production.
High pressure building north of the island chain will bring trade
winds back into the state by Tuesday, pushing the diminishing
frontal band back towards the western islands with another round
of widespread rainfall for Molokai, Oahu and Kauai. Windward and
mountain areas will likely see the highest rainfall totals on
Monday and Tuesday due to strong northeasterly winds. Elevated
Flash Flooding threats are forecast with heavy rainfall developing
along the frontal zone from Oahu to Maui County, and into the Big
Island through Tuesday. Moderate to breezy easterly trade winds
return to the region during the second half of the week and into
the weekend with a bit of a wet flavor.


A strong cold front currently near Oahu this evening will produce
widespread rainfall across the state as it moves southeastward
towards Maui and the Big Island. Radar imagery this evening has
shown moderate to heavy rainfall developing along the frontal
cloud band with the highest reported storm total rainfall
estimates of up to 4 inches occurring this afternoon over the Na
Pali Coast on Kauai. Additionally, an upper level trough and
strong north to northeasterly winds moving in behind this front
will elevate flash flooding threats over most islands through
Tuesday. A Wind Advisory remains in effect from Kauai through
Maui lasting through Monday evening. Elevated threats for Flash
Flooding will keep a Flash Flood Watch in effect for Oahu and Maui
County tonight through Tuesday, expanding to Hawaii County by
Monday morning. Enough instability will keep isolated
thunderstorms in the forecast through Tuesday.

Short range models continue to show the forward movement on the
front stalling out near the Big Island on Monday. At the same
time in the upper levels, the strong upper trough forms into a
closed low over Kauai. Breezy north to northeasterly winds will
bring the highest rainfall totals over windward and mountain areas
during this time period. However, enough convective instability
and upper level forcing will continue to keep elevated threats for
flash flooding over all areas from Oahu to the Big Island. This
cold front becomes cut off from the upper level westerlies as the
high pressure ridge builds north of the island chain. Low level
steering flow along the weakening frontal boundary will become
more easterly on Tuesday, pushing the remnant frontal band
westward back over Molokai, Oahu and Kauai. Another round of
widespread rainfall will remain in the forecast on Tuesday and
Wednesday as this diminishing frontal cloud band brings another
round of showers to the western half of the state.

From Wednesday to Saturday, expect moderate to breezy easterly
trade winds with periods of showers favoring windward and mountain


A cold front moving through Oahu will shift slowly eastward
tonight, moving through Maui prior to daybreak, and stalling out
over or near the Big Island during the day on Monday. Conditions
will deteriorate over Maui County later tonight, while some
improvement is expected over Kauai and Oahu. Widespread MVFR
cigs/vsbys can be expected along and near the front as it moves
through. A few showers may move into PHKO later tonight, while a
land breeze should keep any showers offshore PHTO. Showers should
increase in coverage during the day Monday over the Big Island,
where a thunderstorm or two is not out of the question.

AIRMET Sierra is now in effect for mountain obscuration for all
islands. Some improvement is possible over Kauai and the Big
Island overnight, but conditions will likely remain in place over
Oahu and Maui County through the night.


The next northwest swell to affect the main Hawaiian Islands has
reached Buoy 51101, located some 180 nm NW of Kauai, and is pretty
much in line with guidance. We will be monitoring the swell's
progress in the coming hours, but we do expect this large swell to
spread down the island chain later tonight and Monday. The High
Surf Warning remains in place for the exposed N and W facing shores
of most islands, which is good through Monday night. This NW swell
is not as large as the previous one from Saturday, which reached
19 feet, but the expected maximum height of 15 feet is still very
large and poses an extreme danger to life and property.

The swell will be severely impacted by very strong and gusty N to
NE winds, resulting in very poor surf quality. Seas will not only
be elevated but very rough and choppy.

The swell energy will also reach parts of the Big Island, but on
a smaller scale. A High Surf Advisory is in effect through Monday
night for the exposed N and W facing shores of the island.

A strong cold front is ushering in these strong and gusty N and
NE winds, thus leading to a Gale Warning for the near- shore
waters west of the Big Island. This Gale Warning is good until 6
pm HST Monday.

A Small Craft Advisory covers the building seas from the next NW
swell, that is the near-shore waters of windward Big Island.
There is enough of a west component with this swell where seas
will touch the SCA criteria of 10 feet or more. So the SCA
will go into effect for the leeward waters of the Big Island at 6
am HST. This segment of the SCA is good to until 6 pm HST Monday,
but is expected to be extended, so is the one for the windward Big
Island waters.

As for the cold front, it is located over western Oahu as of 730
pm HST. The front is moving SE at 15 knots. There are some pre-
frontal showers well ahead of the front, around Maui County, and
the Kohala to the Waimea saddle to Kailua-Kona airport area of
leeward Big Island. The front is expected to stall around Maui
County later tonight. It will linger there through early Tuesday,
before heading west as a trough. The stalled front will get a
boost of energy from an upper level disturbance Monday through
Tuesday that will lead to some heavy showers and the potential for
thunderstorms, where a few can be severe. The surface trough is
slated to move west of Kauai by Wednesday afternoon, but lingering
moisture and instability aloft may delay any significant
improvement to the weather area-wide through Thursday.


Flash Flood Watch through Tuesday afternoon for Oahu-Molokai-

Wind Advisory until 6 PM HST Monday 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.

Flash Flood Watch from 6 AM HST Monday through Tuesday afternoon
for Big Island.

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

Small Craft Advisory from 6 AM to 6 PM HST Monday for Big Island
Leeward Waters.

Small Craft Advisory until 6 PM HST Monday for Big Island
Windward Waters.

Gale Warning until 6 PM HST Monday 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