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Issued: Feb 14, 2012 8:30 PM HST
Synopsis
A surface ridge near the islands will bring light winds through Wednesday. The light winds will allow clouds to form over the islands Wednesday afternoon, but the atmosphere will be stable, so rainfall will be light. Trade winds will return Wednesday night as the ridge moves north and strengthens. Windward showers will return with the trade winds. The showers may become more active Friday through the weekend as a low aloft approaches the Big Island from the southeast.
Discussion
A surface high is located far northeast of the area near 32°N 140°W. A ridge extends west southwest from the high to about 150 miles north of Kauai. Moderate trade winds are blowing near the Big Island with gentle east southeast flow over the smaller islands. A dry and stable atmosphere aloft is keeping showers suppressed.
Clouds remain over the islands this evening from earlier daytime heating. Radar also shows a couple of isolated showers present across Oahu and Molokai but should only produce very brief showers at best. Both the clouds and showers should dissipate in the next few hours as land breezes clear away much of the cloudiness over the islands. However, eastern sides of Maui and the Big Island could get clouds moving in from the east overnight given current satellite trends.
The ridge will remain near the islands through Wednesday evening. This means moderate trade winds will continue near the Big Island but southeast winds over the smaller islands will be partly blocked by the Big Island. This pattern will keep the prevailing flow light enough to allow land and sea breezes to dominate over the smaller islands Wednesday. The light prevailing flow will allow daytime sea breezes through Wednesday. The sea breezes will bring cloudy skies Wednesday afternoon but the atmosphere will be too stable for much rainfall to result.
Trade winds will spread back over the smaller islands starting Wednesday night as the surface ridge strengthens and moves north, so would expect more clouds across windward slopes tomorrow night versus tonight. The trade winds will become locally breezy on Thursday. Winds may reach advisory levels over Haleakala summit as the trade winds strengthen. Windward showers will return as the trades build back. Rainfall will be moderate on Thursday but showers will become more active starting Friday as a low aloft approaches from the southeast. This period of active trade wind showers will continue into the weekend. Windward slopes will be the focus, but elevated winds will carry showers to leeward sides of the smaller islands occasionally. Rainfall should be most active around the Big Island since it will be closest to the low. Moisture near the Big Island may get deep enough to bring precipitation to the Big Island summits and temperatures are likely to be cold enough for snow or ice.
The GFS and ECMWF models both show the surface ridge moving back down closer to the islands early next week. That will cause the winds to weaken and shift to southeast again. At the same time the low aloft should shift farther south so the atmosphere is likely to stabilize.
Marine
Surf has dropped below the advisory threshold. Another northwest swell arriving tomorrow will bring surf back up well above the advisory /hsa/ threshold along north and west facing shores. Monitoring buoys tonight and will likely issue an hsa tomorrow morning. Strengthening trade winds Thursday could produce big enough rough surf along east facing shores to require and advisory by the weekend.
As the northwest swell builds, a small craft advisory is likely to be required starting Wednesday night. The advisory is likely to be expanded Thursday as the trade winds strengthen.