National Weather Service Text Product
AFOS product AFDHFO
Dates interpreted at 00:00 UTCDisplaying AFOS PIL: AFDHFO
Product Timestamp: 2021-12-04 01:58 UTC
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678 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