Daily Weather

Jan 31, 2015 8:30 PM HST

Synopsis
Trade winds are gradually returning to the aloha state this weekend as a surface ridge builds north of the area. Low clouds and showers will favor windward and mauka sections, but isolated showers may develop over leeward sections each afternoon. Starting late Monday, a front approaching the islands from the northwest will cause gusty southwesterly winds and wet weather to spread across the island chain. After the front passes, the circulation around a broad surface low north of the islands will maintain an extended period of gusty westerly winds and wet weather across most of the state. These unsettled weather conditions may continue through the end of next week.

Discussion
The tail end of a weakening north northeast to south southwest oriented frontal system is evident in the vicinity of the Big Island according to satellite imagery. In addition to the former front, a weak surface ridge is just north of the islands. The close proximity of these synoptic features is keeping winds across the islands light and variable early this morning. In the upper levels above the state, water vapor imagery and the early morning balloon soundings from Hilo and Lihue indicate the upper level flow is nearly zonal. However, a mid-level trough located west of Kauai is propagating slowly toward the islands. This feature is producing some dynamic lift across the region, so the coverage of low clouds and showers has increased during the past few hours across many parts of the island chain compared with earlier tonight. The atmosphere is rather dry near Kauai according to the early morning sounding, with higher moisture values over the eastern islands. As a result, there are low clouds and isolated showers mainly over the eastern half of the state, especially in the vicinity of the windward and southeastern Big Island. Radar reflectivity returns appear to indicate the showers are relatively light.

The forecast models indicate the surface front will continue to dissipate northeast of the Big Island. They also show the surface ridge will continue to build north of the area this weekend. As this occurs, the pressure gradient will increase across the islands, which will allow weak trades to develop this weekend. Relatively dry air will also be over the islands, and the trades will carry some low clouds and light showers mainly into windward and mauka sections of the islands. Some showers may also develop over leeward sections during the afternoon hours.

The forecast guidance shows the brief period of trades will continue into Monday. By late Monday, a front will approach Kauai from the northwest. This front will not only bring wet weather to the islands, but it will increase southwest winds across the state. These strong southwesterly winds ahead of the front may produce locally gusty downslope winds over northeast and east facing slopes of some of the smaller islands through Tuesday.

The GFS and ECMWF models differ on the exact placement of the parent surface low associated with this front. However, both agree that westerly surface winds will dominate our weather after the front passes through late Tuesday. Both models also indicate cloudy and wet weather will continue, especially over the smaller islands. The models appear to bring 1 or 2 additional fronts down across the islands from Wednesday night through early Friday. The GFS model shows an extended period of unusually strong westerly winds across the state as the broad surface low spins north of the area. The ECMWF model shows somewhat weaker westerly winds, so we will need to monitor the future changes in the model output before we can be certain that they have converged on a realistic solution.

With some uncertainty remaining in the model solutions, the current forecast beyond Tuesday is a blend of the models. However, during time periods with significant differences in the guidance, the forecast has been trended towards the GFS solution. Further refinements will be needed as additional model runs become available.

Finally, assuming there is an extended period of westerly winds, this wind direction is very unusual in the islands for more than a day or two. Note that winds from the west may produce local accelerations and strong gusts due to downsloping on the downwind sides of higher terrain, such as the east and northeast facing slopes of the Waianae and Koolau mountain ranges on Oahu. Should these unusually gusty conditions occur over a period of days, there may be potential for some structures and vegetation to be adversely impacted. Localized power outages are also a possibility, especially if the strong gusty winds are accompanied by heavy rain that adds additional stress to weak tree limbs. Anybody living in these potentially vulnerable locations should monitor future forecast updates for the potential of damaging local wind effects developing next week.

Aviation
While VFR conditions are expected at most of the terminals, intermittent MVFR conditions are occurring across northeast-facing slopes of the eastern islands this morning. Cloud cover is sufficient to warrant issuance of airmet sierra for mountain obscuration across all Big Island slopes above 3000 feet and below 7000 feet. This cloud cover is expected to last through the morning.

Marine
The current northwest swell continues to produce elevated surf along most north and some west facing shores of the smaller Hawaiian islands early this morning. The most recent observations from national data buoy center buoy number 51101 located northwest of Kauai indicates the swell height is 11 to 12 feet with a period of around 14 seconds. Closer to the islands, the Waimea buoy readings were 8 to 9 feet with a period of around 13 seconds. West facing shores of Oahu and Molokai have been added to the high surf advisory. The high surf advisory may need to be extended beyond 6 am HST Sunday morning if buoy readings show the swell remains elevated later tonight. This north northwest swell should start to gradually subside starting Sunday afternoon.

In addition to high surf across parts of the state, the elevated swell has increased seas across portions of the Hawaiian coastal waters to the small craft advisory /sca/ threshold through tonight. Therefore, a SCA remains in effect due to elevated seas for the waters surrounding Kauai, the Oahu windward waters, the Kaiwi Channel and the Maui county windward waters.

Winds are currently forecast to remain below the SCA criteria this weekend. However, as southwest winds ahead of the front begin to increase, winds are forecast to reach the SCA criteria across parts of the western coastal waters starting late Monday. Strong southwest to west winds may continue across most of the coastal waters from Tuesday through the end of the new work week. In addition to the winds, seas will also become elevated again next week, especially starting late Wednesday.




Small craft advisory through 600 pm Thursday 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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