FORECAST DISCUSSION

Synopsis

An unsettled weather pattern will prevail through the weekend and early next week, as a front stalls over the islands and a potent low aloft significantly destabilizes the atmosphere. While cool north winds will limit shower coverage and intensity over Kauai county, and the front will likely focus the heaviest rainfall over Maui county, thunderstorms will be possible statewide through the weekend. Unsettled weather is expected to continue early next week, with a trend toward a more typical trade wind weather pattern expected by mid-week.

Discussion

The current weather pattern over the area is rather dynamic, and is expected to remain that way over the weekend into early next week. While forecast models continue to indicate a period of unsettled weather in our near-term future, with a special emphasis on Maui county, there remains some uncertainty as to the specifics. A Flash Flood Watch (FFA) has been issued for all islands except Kauai county, and a Winter Storm Watch has been issued for the summits and upper slopes of the Big Island peaks. The forecast reasoning follows. Latest water vapor imagery shows a sharp N-S oriented trough aloft with an axis roughly along 160W that extends from just W of Kauai to about 45N. A surface trough has developed over the islands in the last 24 hours, and this N-S oriented boundary is currently analyzed to be over Molokai, Lanai and Kahoolawe and adjacent waters. North winds to the W of the trough axis have been fairly strong at Buoys 51001/51101, with sustained winds between 20 and 25 kts shunting a cooler and drier low-level air mass toward Kauai and Oahu. To the E of the trough axis, light to moderate E to SE winds prevail over most of Maui county and the Big Island, delivering a moister air mass. Forecast models continue to depict a closed cutoff-low aloft hatching out of the sharp trough near 30N161W tonight, then moving straight S toward and over the islands through the weekend before stalling SW of the islands early next week. Models differ dramatically as to the evolution of the low after that point, and the focus for now will be on the impacts of the low through the weekend due to the increased uncertainty thereafter. In addition to the low bringing unusually cold mid-level temperatures - leading to a significant destabilization of the island atmosphere - the surface trough is expected to sharpen over the area as a diffuse and poorly-defined cold front near Kauai moves SE over the islands and stalls. Low-level moisture convergence will be greatest over Maui county, but the Big Island, and to a lesser degree, Oahu, will be close enough to warrant being included in the Flash Flood Watch. The greatest potential for heavy rain appears to be from Saturday night into Sunday, but the latest ECWMF indicates that weather could become active as early as Saturday. With a cool and dry air mass streaming over Kauai (at least initially), the potential for flooding rain is diminished, and Kauai county has been excluded due to this forecast philosophy. If the low-level boundary pushes further E than expected, then Oahu could be sufficiently dry to limit the flooding rain potential as well. Uncertainty remains high as to the details in the sensible weather, including surface winds, as a slight shift in the position of the features will lead to dramatic differences in the weather that occurs. With a pool of cold air associated with the low overlying warmer than normal water temperatures, lapse rates will become quite steep, and there is a high potential for thunderstorm development. Although their coverage is expected to be limited in the cool dry air over the W end of the island chain, this is where the coldest air aloft is expected, and any showers that develop may quickly develop into thunderstorms. The forecast will be changed by this afternoon to increase the weekend thunderstorm coverage. The general forecast philosophy for Sunday into next week is for the low aloft and the surface trough to gradually weaken. However, the potential for unsettled weather will remain high with enhanced low- level moisture in place over most of the chain, and the low continuing to provide instability. A more settled trade wind weather regime is in the long range guidance for the middle to latter part of next week.

Aviation

A weak front along with a trough aloft will cause occasional MVFR conditions today especially over interior and mountain areas. Locally heavy showers can be expected along with a slight chance of thunderstorms. AIRMET Sierra is currently in effect for portions of Kauai, Oahu and The Big Island. AIRMET Tango is currently in effect for moderate turbulence between flight level 270 and flight level 360 over and around Kauai and Oahu as several aircraft have reported moderate turbulence.

Marine

A complex weather pattern is expected for the next several days, introducing a slight chance for thunderstorms across most of the coastal waters today through Monday. A front and upper level trough just west of Kauai will continue to move southeast before stalling over the central islands Saturday. Small Craft Advisory winds and seas will fill in over the waters surrounding Kauai today and tonight as the front passes. A small break in thunderstorm chances should occur behind the front, before spreading across all waters again late Saturday as the upper low and developing surface low move over the island chain. The forecast confidence remains low on the exact placement of thunderstorm activity, but mariners should stay tuned to the updated forecast today and this weekend for updates. Surf along north facing shores bump up some tonight into Saturday from a short-period, choppy swell associated with the stronger winds behind the front. Surf will stay below advisory conditions. A series of small, longer-period northwest swells are expected for next week. A series of long-period south swells will impact the south facing shores into next week. The first of these swells will bring advisory level surf (8 ft) to exposed south facing shores today into early Saturday morning. A High Surf Advisory has been posted for these areas. Please refer to the Coastal Hazard Message, CFWHFO, for more information. A reinforcing south swell will fill in Monday and Tuesday, but keep surf borderline for advisory conditions.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 AM HST Saturday for South facing shores of Niihau, Kauai, Oahu, Molokai, Lanai, Maui, and the Big island. Flash Flood Watch from Saturday morning through Sunday afternoon for Oahu, Molokai, Lanai, Kahoolawe, Maui, Big Island. Small Craft Advisory until 6 AM HST Saturday for Niihau, Olomana. Winter Storm Watch from Saturday morning through Sunday afternoon for Big Island Summits. Small Craft Advisory until 6 AM HST Saturday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters.

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Discussion...Birchard Aviation...Burke Marine...TS

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