Daily WeatherNov 21, 2014 4:00 AM HST
Moderate trade winds will diminish into the weekend. Most showers will be limited to windward slopes, especially over the Big Island. A front will bring modest rain and stronger, northerly winds to most of the state from Sunday night into Tuesday. The winds then will weaken and become northeasterly again as the cool and dry air mass moistens gradually into Friday.
Observations around the main Hawaiian islands still reported moderate to locally strong trade winds blowing, and a timely satellite scatterometer pass supported a small craft advisory around Maui and Hawaii counties. The fairly solid ridge prevailing aloft to the northwest through north expanded over the state overnight, lowering and strengthening the inversion. Together with land breezes, this development largely kept offshore the moderately active showers visible just offshore, especially to the southeast of the Big Island. Further north, cloud cover upwind shifted to a clearly more stable form, and satellite estimates of precipitable water showed solidly below-average precipitable water in the air mass about to move across the state.
Thus, pops have been reduced for all areas during the first period. Under these conditions, afternoon convection will be hard pressed to produce any thunder today, so no mention has been added. However, the mainly stable cloud cover should be at least as widespread as forecast earlier.
By Saturday, guidance still largely agreed that a deepening long-wave trough aloft will displace the mid-level ridge toward the northeast, supporting the approach at the surface of the front currently crossing 30°N near 170°W. As the surface ridge retreats to the southeast ahead of the front, the trade winds will diminish locally, becoming southeasterly in places by Sunday.
The front itself should reach Kauai by Sunday night, but recent solutions predicted the supporting mid-level trough to focus mainly to the east of the state. Thus, most development along the front should remain northeast of the islands, leaving only a fairly shallow boundary to move down the state from Sunday night into Tuesday. Some rain will accompany the front, mainly over the smaller islands. The Big Island more likely will experience a less organized increase in showers from the frontal remnants moving ashore on Tuesday and Wednesday. Moderate to strong northerly winds in the cooler and drier air mass behind the front may be more significant than the front itself.
From Wednesday into the latter half of the week, the subtropical ridge will reestablish itself north of the state gradually, returning the winds to a more familiar northeasterly direction, though wind speeds will drop off significantly until the ridge shifts far enough north. The cool and dry air mass will modify in place, allowing windward showers to increase with time again.
Wind speeds are expected to remain above the threshold for a small craft advisory /sca/ around Maui and Hawaii counties into tonight. After a brief lull.speeds likely will increase behind the front from Sunday into Monday, probably requiring an SCA again around much of the state.
Northwest swells will continue to produce modest surf through Saturday night. A slightly larger swell is expected to arrive by Sunday, possibly building surf to near the advisory level along north-facing and west-facing shores of the smaller islands through Monday. This swell also may contribute to the need for an SCA over the coastal waters.
Surf from trade-wind waves along east-facing shores should decline as the trade winds diminish.
Small craft advisory until midnight HST tonight for Maalaea Bay, Pailolo channel, Alenuihaha channel, Big Island leeward waters, Big Island southeast waters.
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