Daily Weather
Jan 29, 2015 3:30 AM HSTSynopsis
Clouds and a few showers associated with a weak front will move over Kauai and Oahu today, and gradually spread to Maui county tonight and Friday. This moisture will linger through the weekend, and a brief return of light trade winds will focus clouds and showers over windward slopes and coasts. Trade winds will diminish early next week, and prevailing westerly winds will usher in slightly stronger fronts around Tuesday and Thursday.
Discussion
In summary, winds over the area will remain on the light side into early next week, but the large-scale wind direction will be somewhat dynamic, changing slightly from day to day. While heavy rain is not expected through the weekend as the island atmosphere will remain strongly capped at 6 to 8 thousand feet, a couple of weak frontal cloud bands moving over the islands will bring areas of clouds and showers into Friday. This relatively shallow moisture will linger over the area into next week, as the low level flow will remain light. The potential for heavier showers increases next week as westerly winds carry what are expected to be somewhat stronger fronts over the islands.
Currently, what had been a pair of slow-moving ne-sw oriented cloud bands just NW of Kauai Wednesday evening is now a solid mass of broken to overcast stable and showery low clouds that extend for several hundred miles NW of Kauai. Gravity waves propagating through this cloud field have made the identification of a surface frontal boundary near impossible. The islands lie on the southern periphery of a large gyre of low pressure that fills the N Pacific between Hawaii and Alaska. The presence of this gyre has led to a southward suppression of a surface ridge that extends over the area from the NE, with the ridge now analyzed near the Big Island. This has resulted in weak S to SW flow over islands NW of the Big Island, with clouds and showers moving over Kauai and Oahu in weakly convergent SW flow.
With the surface ridge near the Big Island, clear skies and dry weather over Maui county and the Big Island this morning will give way to a few clouds and isolated showers over the slopes this afternoon. For Kauai and Oahu, weak low level convergence and generally westerly flow will lead to clouds and a few showers, with radar currently detecting small showers moving toward and over Kauai and Oahu from the SW.
As low pressure systems rotate around in the gyre to the N, weak fronts extending southward will move over the islands from the NW. The first weak and difficult to analyze frontal cloud band is currently leading to increased clouds and a few showers near Kauai and Oahu, and this moisture is expected to spread to Maui county through tonight as another weak frontal impulse passes just N of Kauai. With the low level flow favoring a westerly direction, showers will favor the normally drier leeward sides of the islands. Also vog is expected to muddle skies from Oahu to the Big Island today before diminishing on Friday, when winds shift to the NW and N.
Moisture associated with the stalling weak front is expected to linger over the central part of the chain on Friday, as weak convergence SE of the front brings an increase in clouds and a few showers to the Big Island. The shift to northerly winds will be due to a weak ridge passing N of the area, with this ridge supporting light trade winds Saturday through Monday. With trade winds blowing, the lingering moisture will fuel clouds and a few showers over windward areas. This return to a weak trade flow will develop as the large gyre shifts northward.
As a new large scale gyre develops NW of the islands early next week, winds will weaken and shift to the s/se again as a surface ridge moves over the islands. On tuesday/wednesday, the ridge will slide far S of the islands, leading to prevailing westerly winds for most of the chain. These west winds will usher a couple of fronts over the islands, as the parent lows pass N of the islands. One front is slated to arrive around Tuesday, with another by Thursday, with these boundaries expected to contain a little greater moisture depth and dynamic support. Winds over the Big Island summits are also expected to increase around this time. Worth noting that global models still differ as to the timing and strength of these fronts, thus the longer range forecast will potentially be subject to large changes. The current long range forecast represents a blend of recent model guidance.
Aviation
A weak front NW of Kauai will push toward the western islands today. Ahead of this front, a convergence band is bringing showers and cloudy conditions to Oahu and parts of Kauai and Molokai. Radar shows some quick northeasterly moving showers, with cloud tops between 8 and 13 kft over western Oahu and the Kauai channel. The showers are embedded with periods of MVFR cigs and vis. There is potential for mountain obscuration this morning, mainly along the southwestern slopes of the Waianae range in Oahu, if showers and lower clouds continue to stream in. The approaching front is fairly shallow and weak, and is expected to bring mainly VFR with isol MVFR cigs and vis with minimal shower activity.
The other islands remain fairly quiet this morning and expect VFR conditions to prevail.
Marine
A moderate NW swell will gradually diminish through tonight, based on guidance and observations at buoy 51101 NW of Kauai. Recently observed swell heights of 5 to 6 feet at the Waimea buoy support the cancellation of the current high surf advisory for exposed N and W shores. However, reinforcing swells from the NW and NNW will arrive Friday and Saturday respectively, leading to elevated surf along N facing shores, and less so along W facing shores, especially as the swell turns toward the N on Saturday. Both swells will produce surf near advisory levels, with surf remaining elevated through Sunday before gradually diminishing Monday and Tuesday. The Oahu surf discussion /srdhfo/ was updated Wednesday and contains detailed insight on the source of the swells.
While winds are expected to remain below small craft advisory /sca/ criteria through at least the weekend, combined seas may top out near 10 feet on Saturday in marine zones exposed to the NNW swell.
Watches/Warnings/Advisories
None.
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
