Thursday, September 18, 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 17, 2014 3:54 AM HST

Synopsis Weak surface high pressure north of the islands will maintain gentle trade winds across the state today, with localized afternoon sea breezes developing. The ridge will break down from Thursday into the weekend, allowing for widespread sea breezes along with afternoon clouds and showers. Dry and stable air aloft will keep most rainfall amounts light. A tropical wave passing south of the area may bring an increase in rainfall, especially across the Big Island, with gentle trade winds resuming across the state by the later half of the weekend and into early next week.

Discussion Shower activity was most concentrated across Oahu during the night, with most windward and a number of interior sites receiving measurable rainfall. The other islands have remained almost entirely dry. Currently, showers are most prevalent over the leeward coastal waters well west of Oahu, and the waters southeast of the Big Island. Surface analysis depicts low pressure centered far northeast of the state, with a dissipating front/trough trailing SW to east and south of the Big Island. A 1022 mb high is centered about 1000 miles northwest of Kauai and is building slowly eastward, providing gentle trade wind flow across the state. Aloft, a weak upper level low is centered near Kauai, with a large mid/upper level ridge sprawling northwest of our region from west of the dateline to near 160°W. Mimic-tpw satellite imagery shows a stripe of higher moisture along the dissipating surface trough extending across the southern tip of the Big Island and south of the smaller islands, with another area of higher moisture approaching the state from the north. The 12Z soundings depict a rather typical profile, with inversions based around 7000 feet, and near normal pwats between 1.3 and 1.5 inches.

Surface high pressure will build slowly eastward to the north of the state today and tonight, maintaining a gentle background trade wind flow. Trades will remain light enough to allow localized sea breezes to develop once again this afternoon. Expect a hybrid pattern similar to that observed on Tuesday, with showers and clouds focusing on windward slopes during the early morning and nighttime hours, and across some of the leeward and interior areas during the afternoon. The area of increased moisture moving SW across the western and central parts of the state today and tonight will probably allow for a bit more leeward shower activity this afternoon, and windward shower activity tonight, than has been observed during the past 24 hours. A drier airmass appears likely to persist across the Big Island, and would expect showers to be less prevalent there.

From Thursday into the weekend, another cold front is forecast to drop southward across the north central Pacific. This will 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 a period of more widespread daytime sea breezes and nighttime land breezes. This pattern will bring mostly clear nights and mornings with spotty showers over interior areas during the afternoons and evenings. The light winds will allow volcanic haze to build up around the Big Island, but it does not appear as if there will be enough southerly component in the flow to carry the haze over the smaller islands.

Forecast uncertainty increases during the weekend and into next week, when the models bring a tropical wave and associated surge of deep moisture northwestward toward the state. The 00Z guidance has generally trended a bit south with the wave, with the deeper moisture affecting mainly just the Big Island or possibly remaining south of the state altogether Sunday into Monday. It also appears that the increased pressure gradient between lower pressure to the south, and a narrow high pressure ridge between the islands and a stalled frontal boundary to our north should induce some increase in easterly flow over the state during this time. Have nudged pops down a bit for the smaller islands, keeping the highest pops over the Big Island.

Uncertainty increases further for next Tuesday and beyond, when the GFS drops another cold front southward into the central Pacific, weakening and veering our local wind flow toward the southeast. The ECMWF has a completely different solution for this time period with a building high pressure ridge in the central Pacific, causing increasing trades over the state. Will have to await future model runs which will hopefully resolve these differences.

Marine Overnight observations from the Barbers Point buoy show the south swell continuing to slowly decline, with peak energy values gradually shifting into the shorter wave period bands. This swell will slowly fade away through Friday. Surf along south facing shores will remain somewhat elevated through today as another small south swell arrives, but is expected to remain below advisory levels. Otherwise, a small northwest swell arrived late yesterday, as indicated by Waimea bay buoy data. Another small northwest swell will arrive later today, but surf along north facing shores will remain well below advisory levels.

An 0806Z ascat pass found gentle to locally moderate trade winds across the Hawaiian coastal waters, with a small area of winds greater than 15 knots in the Alenuihaha channel. These will probably be the strongest winds seen in our waters through the next several days, with wind speeds expected to begin decreasing later today. Winds and seas are expected to remain below small craft advisory levels through at least early next week.

Watches/Warnings/Advisories None.

 

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