Last Updated on Wednesday, 11 December 2013 07:27
Winds Statewide in a glance ...just go HERE(this link is also the 4th drop down under weather)
Issued: Dec 11, 2013 4:00 AM HST
Moderate trade winds will diminish gradually and become southeasterly into the weekend. Light windward showers will give way to afternoon cloud buildups and evening clearing. A cold low in the middle atmosphere, together with a front approaching from the northwest, will bring heavier rain, and a Risk of thunderstorms, from Sunday through the first half of next week.
Surface observations around the main Hawaiian islands still reported trade winds strong enough to require a small craft advisory for the typical areas around the Big Island. Slightly drier air moved over the state from the northeast, and subsidence from deep ridging overhead and to the northeast compressed the surface mixed layer, but light showers managed to squeeze out a few drops along windward slopes anyway. Continued drying and subsidence through the next 24 hours should limit further shower activity.
Otherwise, guidance predicted only gradual changes through about Friday. Consensus showed the mid-level ridge overhead weakening and moving away to the northeast, and a trough aloft deepening toward the state from the northwest. At the same time, the surface ridge will weaken and shift south over the islands, while a front approaches from the northwest.
This scenario has played out several times already this fall, and most practical effects this time likely will not differ much from the basic pattern. Prevailing surface winds will diminish and turn more southerly, ultimately drawing up increasing moisture from the deep tropics. Volcanic haze from Kilauea again will reach the smaller islands. With prevailing winds light, diurnal circulations will become increasingly important. All of these changes should be in place by around Saturday.
Consensus still depicted the cold pool accompanying the mid-level trough as unusually frigid. By Sunday, temperatures at 500 mb over Kauai could drop to -15 celsius, well below the typical threshold of -10 celsius often used as a benchmark value for thunderstorm development in Hawaii. The core of the low could bring temperatures approaching -20 celsius to the middle of the state by Tuesday night. With moisture streaming in from the deep tropics, and a front not too far to the northwest providing forcing, the potential for deep convection was substantial. Local algorithms based on model output suggested that thunderstorms could continue to develop into the middle of next week.
That said, the time in question was late enough in the forecast period to suggest a bit of caution. In particular, recent solutions diverged with respect to the development of this trough aloft. For example, the 06Z GFS run predicted a more easterly track, which would take the core of the cold pool, and thus the instability, east of the islands. For now, the official forecast of somewhat increased shower activity and a slight chance of thunderstorms around early next week represented a reasonable compromise.
In any case, the front, or its remnants, will move southeast across the state from Sunday into Monday, spreading rain to about Maui county. Much of this precipitation will take the form of heavy showers from deep convection, but stratus rain is likely too.
Trade-wind conditions will return gradually over the second half of next week as the low aloft moves off to the northeast. However, the accumulated moisture and volcanic haze will take longer to clear out.
Mixed wnw/nnw swell will produce 6-10 ft surf along north-facing shores through today. A gradual easing trend will follow through early Friday before another large NW swell builds rapidly, possibly reaching advisory levels around 15-18 ft by sunrise Saturday. Also a small out-of-season south swell will produce modest surf along south-facing beaches through Thursday.
Breezy trades will require a small craft advisory for the windier waters around Maui and the Big Island through this afternoon.