AFOS product AFDHFO
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Displaying AFOS PIL: AFDHFO
Product Timestamp: 2021-12-04 01:58 UTC

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FXHW60 PHFO 040158
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
358 PM HST Fri Dec 3 2021

.SYNOPSIS...

Periods of heavy rain potentially resulting in flooding will be
possible through Saturday. Widespread heavy rain capable of 
causing more significant flood impacts is then expected Sunday 
into the middle of next week.

&&

.DISCUSSION...

Today and tonight... 
A loosely organized area of moderate to heavy showers continues 
to march eastward within a convergent wind shift boundary over the
eastern part of the state. These showers prompted a flood 
advisory over Molokai this morning and over Maui this afternoon. A
similar magnitude of flood threat will exist over the Big Island 
this evening, particularly north and west facing slopes. Isolated 
thunderstorms will be possible over the slopes of Mount Haleakala 
and the Big Island this afternoon. Despite the areas of heavy rain
evident on radar imagery, the wettest spot so far today has been 
the north shore of Kauai within the onshore/northerly flow where 
the Kilohana rain gauge has 5.63" in the last 12 hours and and 
3.52" in the last 6 hours.

The cold front itself lags the wind shift by a few hours and now has
cleared Kauai as evidenced by the pronounced dry air mass that 
has moved over the island and the dewpoint which has fallen well 
into the 50s. The cold front will work through the islands
tonight with little fanfare other than dropping dewpoints into the
50s and low temperatures into the low to mid 60s for Kauai through
Maui. Likewise, high temperatures on Saturday will top out in the
mid to uper 70s for the same locations. The front is forecast to
be east of the Big Island by Saturday morning.

Saturday through Sunday morning...
 
Saturday will represent a transition day from the initial and
mostly benign cold frontal passage to the heart of the event. 
During the day Saturday, the closed upper low is forecast to be 
centered around 35N approximately due north of Kauai with the 
surface cold front positioned east of the Big Island. As the final
key piece of 180kt-or-so jet energy is injected into the upstream
side of the trough, it will quickly evolve southwestward. As the 
cold upper low nears the islands, southwesterly flow over the 
island chain will deepen from the top down, culminating in the 
development of a surface low west of Kauai by Saturday evening. As
this occurs, showers over the western end of the state will tend 
to deepen but should still remain rather benign. However, a marked
increase in shower coverage and intensity over central portions 
of the state will be possible late Saturday evening into late 
Saturday night. This will occur within a broad corridor of 
developing isentropic ascent, a consequence of deepening 
southwesterly flow, and increased orographic forcing focused over 
leeward zones. The lack of a mechanism to organize showers into
bands appears to be a limiting factor for significant flooding 
concerns for these locations during this time period.

Over the Big Island, uncertainty still exists with respect to the 
ultimate eastward position of the front, but at this time it 
expected to be east of the area Saturday into Sunday afternoon. 
However, with the upper low and associated 130+ upper jet parked 
over the western end of the state, right entrance dynamics will be 
increasing in a big way during this time. Thus, although the 
front should remain east of the Big Island at the surface, it will
slope backwards such that the 700mb front resides somewhere over 
Maui County. The global models indicate numerous waves of 
potentially organized showers moving from southwest to northeast 
over the Big Island and Maui Saturday through Sunday morning. 
Although the moisture associated with this activity will initially
be limited, the strengthening mid-level frontal circulation and 
increasing instability aloft will still be sufficient to cause 
periods of high rain rates. 

Sunday afternoon through Wednesday... 
The heart of the event will be driven by a deep, strong frontal 
circulation maintained by nearly stationary and intense right 
entrance dynamics associated with the upper jet near Kauai. As the
the plume of deep moisture east of the Big Island begins its 
westward slide across the state, it will come under the influence 
of increasing frontal forcing which will allow for greater 
organization and heavier rain rates over the Big Island. Then, as 
it progresses toward the central islands, including Oahu, the 
surface front will tend to reorganize further west directly 
beneath the strongest dynamics. The resultant front will be 
vertically steep and the surface front & deeper moisture will be 
much closer to the strongest forcing and cooler temperatures 
aloft. 

The peak of the event is therefore expected to initially feature 
widespread heavy rain with embedded isolated thunderstorms and 
intense rain rates over the Big Island beginning sometime Sunday 
afternoon and persisting through at least Sunday night and
potentially well into Monday. Several inches of rain can be 
expected and the attendant flash flooding potential will be 
maximized over the Big Island during this time period. 

The latest data suggests that as the moisture band works 
northwestward Sunday night through Monday, it will encounter an 
increasingly favorable environment in terms of instability aloft 
and deep layer forcing. As a result, the potential for banding and
the propensity for training cells containing intense rain rates 
may actually increase within the Oahu-Maui corridor. Significant 
flooding impacts will be possible over Oahu, Molokai and 
potentially Maui during this time. 

All headlines remain in effect. The Flash Flood Watch remains in
effect for the forecast reasoning outlined above and has been
extended through Tuesday. The Blizzard Warning likewise remains 
in effect as intense mid-level winds associated with the 
developing low will bring gusts in excess of 100 mph to the Big 
Island summits. Moisture will be sufficiently deep during this 
time to bring periods of snowfall amounting to a foot or greater. 
Snow levels will then rise to near or perhaps above summit level
as the band of heavy rainfall the island on Sunday.&&

.AVIATION...

A deep upper level trough moving in from the northwest will
destabilize the air mass over Hawaii today, creating conditions 
conducive for locally heavy rain and possible thunderstorms. Weak
low topped showers can be seen on satellite pushing up against 
the northern and western slopes of the smaller islands. 
Additionally, radar indicates stronger embedded showers, including
developing CBs, extending from Molokai to Maui and headed 
eastward. 

As of 3 pm, AIRMET Sierra for mountain obscuration was in effect 
for Oahu, Molokai, and Maui. Conditions are expected to improve by
evening over Oahu, but likely spread to the Big Island overnight.

AIRMET Tango remains in effect for low level mechanical 
turbulence over and downstream of the higher terrain of Kauai, 
Oahu, and the Big Island. Conditions are expected to persist 
overnight for the Big Island but end over Kauai and Oahu by late 
evening. 

Lastly, increasingly strong winds at higher elevations will 
likely necessitate an additional AIRMET for widespread strong 
surface winds across Big Island summits late tonight.

&&

.MARINE...

A vigorous cold front is currently located near Maui, with winds
becoming northerly and increasing in speed behind the front. The 
front is expected to stall near the Big Island tonight and Saturday
as a strong low aloft develops near Kauai. This low will spawn a 
surface kona low just west of Kauai Saturday night, which will 
linger near the islands for several days. A convergence band of 
showers east of the low will develop over the islands, with a 
strong potential for an extended period of unsettled weather. 
Although confidence is not all that high, generally expecting low 
pressure to linger west of Kauai through the middle of next week, 
keeping an unstable southerly flow in place over the area. 

Strong and gusty winds are expected to continue through this
evening, especially around Kauai and Oahu, as well as in and near
any heavy showers or thunderstorms. A Gale Warning remains in
effect for waters between Kauai and Niihau through 8 pm HST, with
Small Craft Advisory (SCA) winds and seas for most other marine 
zones. Although winds will ease Saturday, combined seas will be 
well above the 10-foot SCA threshold for most waters through the 
weekend. Winds will increase again Sunday, out of the south and 
southwest this time, as the surface low develops northwest of 
Kauai.

A large north-northwest swell will fill in this evening, then
shift more out of the north and continue to build through the 
weekend as the wave period increases. This fairly long-lived, 
large north swell will quickly build surf heights through tonight
and reach heights close to 30 feet Saturday along north facing 
shores, and holding through this weekend. Therefore, a High Surf 
Warning has been issued for north facing shores of Niihau, Kauai, 
Oahu, Molokai, and Maui, with a High Surf Advisory for west facing
shores of Niihau, Kauai, Oahu, and Molokai as well. As the swell 
becomes more northerly Saturday night and Sunday, high surf could 
affect north shores of the Big Island as well. High tide early 
Saturday and Sunday could lead to wave runup in some coastal areas,
as well as significant harbor surges in both Kahului and Hilo 
harbors. A Marine Weather Statement continues for those hazards.

Expect minimal surf along east facing shores through Saturday, 
with some increase later this weekend as the large north-northwest
swell turns more northerly, allowing some swell energy to wrap 
around the east sides of the islands. Surf along south facing 
shores will rise slightly over the weekend and early next week due
to locally- generated wind waves, as well as a small, long-period
south swell beginning Monday.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Monday afternoon for Niihau-Kauai Leeward-
Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central 
Oahu-Waianae Mountains-Lanai Mauka-Kahoolawe-Maui Windward West-
Maui Leeward West-Haleakala Summit-Kona-Kohala-Big Island 
Interior-Kauai North-Kauai East-Kauai South-East Honolulu-
Honolulu Metro-Ewa Plain-Koolau Windward-Koolau Leeward-Molokai-
Lanai Windward-Lanai Leeward-Lanai South-Maui Central Valley 
North-Maui Central Valley South-Windward Haleakala-Kipahulu-
South Maui/Upcountry-South Haleakala-Big Island South-Big Island 
Southeast-Big Island East-Big Island North. 

Wind Advisory until 10 PM HST this evening for Niihau-Kauai-

High Surf Warning until 6 AM HST Monday for north-facing coasts.

High Surf Advisory until 6 AM HST Monday for Kauai Leeward-
Waianae Coast-Molokai West.

High Wind Warning until 6 AM HST Monday for Big Island Summits.

Blizzard Warning until 6 AM HST Sunday for Big Island Summits.

Wind Advisory until 6 AM HST Monday for Haleakala Summit.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest 
Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters-
Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters.

Small Craft Advisory from 10 PM this evening to 6 PM HST Sunday 
for Kauai Leeward Waters.

Gale Warning until 10 PM HST this evening for Kauai Leeward 
Waters.


&&

$$


DISCUSSION...JVC
AVIATION...Bedal
MARINE...TS