Daily Weather

Dec 22, 2014 3:40 AM HST

Synopsis
A weakening cold front boundary will move across Oahu and stall near Maui today. Showers will affect most of the smaller islands with slight chances of thunderstorms through this afternoon. Best chances for rain remain over mountain and windward areas, with localized heavy downpours possible. Then the arrival of an upper level trough will combine with the formation of a sfc low near the Big Island to bring potential for periods of heavy rain and scattered thunderstorms. A flash flood watch will be in effect from tonight through Wednesday. For the second half of the week, the forecast will depend on the final outcome of the aforementioned low near the Big Island.

Discussion
There has been a significant change in forecast philosophy for the next 48 hours as the global models have shifted to a more aggressive weather scenario.

At 330 am HST, data from the doppler radars around Hawaii showed lines of showers with localized downpours moving across Kauai and the Kauai channel waters, where a weak cold front is now located. A band of clouds marking the leading edge of the front can be observed on latest satellite IR imagery. Showers are also developing along the windward and north coastline of Oahu. Although the overall movement of the cold front boundary is towards the SE, individual cloud elements inside the frontal cloud band are streaming from NE to SW across Kauai county, the Kauai channel and Oahu.

An upper level trough is also nearing the aloha state from the NW. Shower coverage will continue to steadily increase today as the frontal boundary pushes through the W half of the island chain. Model solutions bring the decaying boundary through Oahu, and stalling it near Maui around noon, producing widespread showers over Kauai, Oahu and Maui along with slight chances of thunderstorms. Rainfall accumulations should remain moderate today, although localized heavy downpours can not be ruled out.

For tonight and into Tue, there has been a dramatic shift in the global models. Both GFS and ECMWF have reached a remarkable agreement in a short period of time, with the latest solutions depicting the development of a sfc trough/low structure over the islands. This feature will be supported by the upper trough, which seems to push its axis right along the island chain in the most recent model runs. This new synoptic setup will tap into the abundant tropical moisture far S of the islands and bring a much more vertically saturated profile across the aloha state than previously anticipated. The current model depiction of the low resembles a Kona low forming just to the WNW of the Big Island. Needless to say, with this new development, the whole forecast has been modified to reflect a much more wetter and aggressive scenario for the next 48 hours as the moisture S of the islands is advected across the state by the low. Widespread showers, periods of heavy rain and scattered thunderstorms will bring potential for flash flooding, starting tonight. Highest chances of rain reside over mountain and windward areas. A flash flood watch will be in effect starting tonight and continuing through Wednesday afternoon for all the islands.

In addition, temps around 500 mb will drop to around -12 to -16 celsius and deeper moisture will move over the Big Island, with snow very likely over the Mauna Loa and Mauna Kea summits tonight and again on Tue night. Snow accumulations of around 2 inches are possible, even higher with thunderstorms. A winter storm watch has been has been issued for the Big Island summits starting this evening and continuing through Wed.

Winds will be mostly variable through Mon morning, then becoming stronger and veering to the N and NW today over the W edge of the island chain behind the fropa. However, it has become rather difficult determining the final outcome for the winds over the E half of the aloha state as the overall location, structure and intensity of the advertised low/trough complex on Tue and beyond remain highly uncertain at this time. For now, will keep a blend of model solutions with weaker SE winds around the Big Island and Maui frequently shifting direction through the next couple of days.

The uncertainty in the forecast models extends into the second half of the week, for which the previous forecast trend has been for the most part kept in place with NE winds and a drying trend gradually returning across the area. Surely the long range forecast will be adjusted, depending on the final outcome of the aforementioned low/trough complex. Stay tuned.

Aviation
Clouds and showers associated with a weakening and stalling cold front are affecting Kauai and to a lesser extent Oahu, where airmet sierra is posted for mountain obscuration. Airmet tango is posted for low level turb over Kauai, due to gusty N winds that will continue at least into tonight. Widespread MVFR vis/cig in SHRA can be expected in association with the frontal cloud band, with the possibility of IFR vis/cig in areas of heavier showers. Vfr conditions will prevail over Maui and the Big Island into tonight, with the frontal cloud band expected to be relatively slow in its southeastward progression across the island chain. Later today into Tuesday however, a developing low aloft will bring the statewide possibility of thunderstorms and their associated aviation hazards, including icing, turbulence, wind shear and IFR conditions.

Marine
A large northwest swell is keeping warning level surf over north and west facing shores of the smaller islands and west facing shores of the Big Island until 6 am HST Tuesday. This swell will also generate small craft advisory conditions over most coastal waters due to hazardous seas.

Strong north to northeast winds are expected to move over the area starting later today over the northern waters. But it is not clear if small craft advisory conditions will be experienced again beyond Tuesday morning. Therefore, the small craft advisory will be allowed to expire at 6 am HST Tuesday and reissued later on if conditions warrant it.

Watches/Warnings/Advisories
Flash flood watch from this evening through Wednesday afternoon for all Hawaii islands.

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, Kona, Kohala.

Winter storm watch from this evening through Wednesday afternoon for Big Island summits.

Small craft advisory until 6 am HST Tuesday for all Hawaiian waters except Maalaea Bay,.






Advisory and Warning Criteria
Location Advisory Warning
North-Facing Shores 15 Feet (8') 25 Feet
West-Facing Shores - Big Island 8 Feet (4'+) 12 Feet
West-Facing Shores - Remaining Islands 12 Feet (7') 20 Feet
South-Facing Shores 8 Feet (4'+) 15 Feet
East-Facing Shores 8 Feet (4'+) 15 Feet

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