Current Conditions | ||
Temp | ![]() | 0.1 C |
RH | ![]() | 22 % |
Wind | ![]() | NE 17 mph |
Road | ![]() | Open (4x4) |
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Hawaiian Islands Synoptic Discussion and Guidance 037 FXHW60 PHFO 160656 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 856 PM HST Fri Jan 15 2021 .SYNOPSIS... Although mostly dry conditions will continue into the weekend, a few showers near and along a diminishing front will remain possible around Kauai through the day Saturday. Rainfall chances will quickly rise through the day Sunday as the winds shift out of the south to southwest ahead of a stronger cold front approaching. This front will quickly advance down the island chain Sunday night, then stall around the Big Island Monday through Tuesday. Heavy showers and a few thunderstorms will be possible along ahead of this front. Cool and breezy northerly winds are expected following the frontal passage for the western end of the state early next week. Trade winds may return late next week. && .DISCUSSION... Short-term guidance has initialized well with the current pattern and depicts mostly dry conditions with light to moderate trade winds continuing across much of the area through Saturday. The exception will be over and around Kauai where showery conditions will remain possible due to a diminishing frontal boundary that has stalled nearby. A transition period is expected Saturday night through Sunday as a cold front associated with a gale currently north of Midway approaches. Low-level winds will quickly shift out of the south and southwest ahead of this approaching boundary - potentially increasing to moderate levels over and around Kauai/Oahu later Sunday. Increasing instability and deep layer forcing for ascent out ahead of this front and its attendant shortwave trough aloft will be enough to trigger spotty pre-frontal showers Sunday afternoon over Kauai and Oahu. The cold front is forecast to reach Kauai late Sunday, then move down the island chain Sunday night through Monday. The front will be accompanied by a period of showers. Model reflectivities are in decent agreement and depict the leading edge of the main line of showers along the front reaching Kauai late Sunday afternoon, then to Oahu and Maui County Sunday night. Lowering upper heights combined with 40-50 kt 0-6 km shear ahead of this upper trough will support a few thunderstorms Sunday night into Monday. The one limiting factor with regard to the strength of these storms could be the phasing between the upper trough and deep moisture axis along the front (appears to be a lag in the latest model cycle). If these were to align, a few strong storms can't be ruled out - especially over the coastal waters Sunday night. Breezy northerly winds and cool conditions will follow the frontal passage across the western end of the state early next week. Guidance reflects this and depicts dewpoints dipping into the low 50s for Kauai and Oahu. There remains subtle differences in the global models on how far east the front will progress before stalling. The GFS moves the front to just east of the Big Island on Tuesday while the ECMWF has the front stalling over the Big Island. Either way, the Big Island and possibly east Maui will remain rather wet on into Wednesday, which could result in localized flooding concerns. Winter weather conditions can't be ruled out over the Big Island summits Monday night into midweek due to a combination of the deep moisture axis associated with the front hanging up or stalling and cold temperatures aloft arriving with the shortwave trough. As we head on into Thursday, high pressure will build in northeast of the area resulting in strengthening trade winds. Moisture associated with the frontal boundary is then forecast to move back westward likely providing for a wet trade wind pattern to set up across the state. With the differences in the long range models continuing, changes to the forecast our possible next week. && .AVIATION... A weakening front located northwest of Kauai will continue to approach the islands this evening. The close proximity of this feature is keeping a light wind regime over the western end of the state, with weak low-level trade flow in the vicinity of the eastern islands. Low clouds and showers ahead of the front, which is expected to stall later tonight, will primarily affect Kauai through early Saturday morning. This will likely result in periods of MVFR ceilings / visibilities on Kauai. Elsewhere, expect low clouds and isolated showers along some of the windward coasts and slopes of the islands east of Kauai. This may result in brief periods of MVFR conditions along some windward sections on these islands. Otherwise, expect primarily VFR conditions to prevail through Saturday morning elsewhere on the islands from Oahu to the Big Island. On Saturday, the front is expected to dissipate northwest of Kauai. The weak trade wind flow will continue to transport some low clouds and a few showers into windward sides of the islands. But VFR conditions will again likely prevail over most areas Saturday afternoon. No AIRMETs are currently in effect. We will monitor conditions on Kauai in case AIRMET Sierra is required for mountain obscuration later tonight or on Saturday. && .MARINE... In summary, an active winter pattern this holiday weekend with giant surf on Saturday, and a potent cold front arriving late on Sunday. The largest NW swell of the winter season so far will rapidly build overnight into Saturday, with peak surf heights of 40 to 50 feet (with potentially higher sets) expected along exposed shores, where a High Surf Warning is in effect. Long-period forerunners (around 26 seconds) are arriving at NDBC buoys 51001/51101 to the NW of Kauai, indicative of the powerful swell's arrival, although the swell (as is typical of late) is running a little behind WaveWatch III guidance. After peaking Saturday and Saturday night, the swell will gradually diminish Sunday, but a new NW swell arriving into Monday will likely push surf to warning levels once again. In the meantime, the giant NW swell has also prompted the issuance of a Small Craft Advisory for all waters except Maalaea Bay as seas build rapidly, and a Marine Weather Statement, which is highlighting the potential for surges and significant wave action in and near exposed harbors and their entrances. Due to the period and size of the NW swell, some (much reduced) swell will wrap into select S and E facing shores. A surface ridge is near and E of Kauai, while a weak and dissipating front is nearly stationary just NW of Kauai, leading to light winds over Kauai waters, and light to moderate trades elsewhere. The forecast anticipates the ridge moving N overnight as the front dissipates, leading to a temporary increase in E to SE winds on Saturday, potentially becoming locally strong. The ridge will move S over the islands again on Saturday night as what is expected to be a vigorous cold front approaches. The ridge will move SE of the Big Island Sunday and Sunday night, leading to a period of light to moderate SW winds, until the front's passage ushers in near gale force N winds Sunday night. The front will bring the potential for locally heavy showers and thunderstorms, with the front expected to stall over the islands early next week. A strong trough aloft is expected Monday and Tuesday, maintaining the potential for thunderstorms along the stalled frontal boundary. && .HFO WATCHES/WARNINGS/ADVISORIES... High Surf Warning until 6 AM HST Sunday for Niihau-Kauai Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward Haleakala. High Surf Advisory until 6 AM HST Saturday for Big Island North and East. Small Craft Advisory from 6 AM Saturday to 6 AM HST Sunday for Maui County Leeward Waters-Pailolo Channel-Big Island Leeward Waters-Big Island Southeast Waters. Small Craft Advisory until 6 AM HST Sunday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel- Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Alenuihaha Channel-Big Island Windward Waters. && $$ DISCUSSION...Gibbs AVIATION...Houston MARINE...Birchard Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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