Wednesday, September 17, 2014 596-SURF , 596-WAVE , 922-BONG , 638-RUSH , 572-SURF(MAUI) , 241-SURF (KAUAI) , 324-RUSH (BIG ISLAND)
   
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Winds Statewide in a glance ...go HERE (this link is also the 4th drop down under weather)

Sep 16, 2014 3:38 AM HST

Watches/Warnings/Advisories
High surf advisory until 6 pm HST this evening for south facing shores of all islands.

Synopsis
Muggy conditions will continue through the rest of this week. Gentle trade winds will prevail today through Wednesday, but will still allow for localized afternoon sea breezes and interior clouds and showers. The trades are forecast to break down again Thursday into the weekend, leading to more widespread sea breezes along with afternoon clouds and showers.

Discussion
Scattered showers have developed across the windward coastal waters and around the Big Island during the last several hours, with shower motions and 88D VWP data indicating that gentle trade winds are becoming established once again across the state. Most land areas have remained dry since sunset, with rain gauge data showing just isolated very light amounts. Surface analysis depicts low pressure centered far northeast of the state, with a trailing cold front/trough extending SW to the southern end of the Big Island. Mimic-tpw imagery also clearly shows a stripe of higher precipitable water values along the front/trough, although the mimic estimated values are running a bit high compared to the measured 12Z Hilo pwat of 1.57 inches. A drier airmass continues to spread into the state from the north behind the trough, as shown by the 12Z Lihue sounding with a measured pwat of only 1.15 inches, which is down from 1.77 inches 24 hours ago and below the 25th percentile for this time of year. Another area of somewhat higher moisture is moving southward toward the state, with the leading edge of this moisture about 200 miles north of Kauai as of 11Z. A 1024 mb surface high centered far northwest of the state is building eastward behind the front/trough, allowing the gentle trades to resume over the islands. Aloft, a weak upper level trough lies across the state, with a large deep layered anticyclone centered well to our northwest, just to the east of the dateline.

Surface high pressure will build slowly eastward to the north of the state today through Wednesday, maintaining a gentle but definite background trade wind flow. Trades should remain light enough to allow localized sea breezes to develop each afternoon. Therefore we would expect a hybrid pattern, with showers and clouds focusing on windward slopes during the nights and early mornings, and across some of the leeward and interior areas during the afternoons. The area of higher moisture initially to our north is forecast to brush across at least the western half of the state late tonight into Wednesday. This may help to enhance shower activity a bit in that area. A rather dry airmass is forecast to spread across the Big Island later today through Wednesday, so showers should be less prevalent there with rainfall amounts remaining light. With ocean waters to the north of the state significantly warmer than normal, rather muggy conditions are likely to continue despite the prevailing trade wind flow.

From Thursday into the weekend, another cold front is forecast to drop southward across the north central Pacific. This will likely push the subtropical ridge south and east, disrupting the pressure gradient across the state. Local trade wind flow should weaken further or disappear entirely during this time, allowing for another period of more widespread daytime sea breezes and nighttime land breezes. With more of a pure convective pattern, we should see more prevalent interior clouds and showers during each afternoon, and more clearing across the islands each night. Model consensus eventually stalls the frontal boundary a couple hundred miles north of the state by the end of the weekend and into early next week, with gentle trade winds possibly resuming over the islands by Sunday or Monday.

Marine
A high surf advisory remains in effect for south facing shores of all islands through 6 pm today. Surf reports reached high-end advisory levels at many beaches on Monday. Wave heights at the Barbers Point buoy continue to run slightly above wavewatch guidance currently, but peak swell periods are gradually decreasing. Would expect that the resulting surf will remain above advisory criteria at least through today. Will let the day shift evaluate later trends and the surf observations to see if the advisory may need to be extended beyond today. Otherwise, a small northwest swell appears to be near its peak now as indicated by Waimea bay buoy data and wavewatch guidance. This swell will diminish later today, but another couple of small north northwest swells are expected to arrive today and late tonight into Wednesday. Resulting surf should remain well below advisory levels on north and west facing shores.

With either light/gentle trades or a land/sea breeze pattern forecast to prevail through the remainder of this week and into the weekend, winds and seas are expected to remain below small craft advisory levels.

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