Daily Weather

Nov 22, 2014 4:00 AM HST

Synopsis
Moderate trade winds will diminish today. Showers will be limited mainly 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.

Discussion
Observations around the main Hawaiian islands reported winds gradually becoming more southeasterly, but still remaining strong enough to require extending the small craft advisory in places around the Big Island. Aloft, the ridge languishing across the state from west to east thinned out between shallow troughing to south, and a deepening long-wave trough to the north. A jet stream associated with the trough to the south brought cirrus layers from the ITCZ as far north as Maui county, and cloud cover was increased accordingly. However, the mid-level ridge remained strong enough to limit the inversion to near its usual height.

Lower cloud cover consisted mainly of generally stable clouds. Various measures of precipitable water depicted a relatively dry air mass upwind of most of the state, excepting only south slopes of the Big Island. Rain gauges reported little accumulation through the night. On the other hand, radar detected enough shower activity over the water so that the current pop grids for later today nonetheless appeared plausible given the predicted breakdown of the ridge aloft.

Guidance still largely agreed that the deepening long-wave trough aloft to the north will start to displace the mid-level ridge toward the east, finally 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 and become increasingly southeasterly.

The front itself should reach Kauai by Sunday night, but solution consensus 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 along northeast slopes on Tuesday and Tuesday night. 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 local 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 again with time.

Marine
Although wind direction around Maui and Hawaii counties became more southeasterly, speeds did not diminish as quickly as expected earlier, except in the Pailolo channel and Maalaea Bay, where Maui blocked the wind flow. Thus, the small craft advisory was extended through the day today for the Alenuihaha channel and waters south through west of the Big Island. Wind speeds should start to diminish this evening, but will strengthen again behind the front early next week, likely reaching the threshold for an advisory again.

A series of northwest swells will arrive during the next several days. The largest surf is expected between Sunday and Tuesday, when heights may exceed advisory levels at times. Trade winds will continue to produce short-period choppy surf along east-facing shores.

Watches/Warnings/Advisories
Small craft advisory until 6 pm HST this evening for 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

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