FORECAST DISCUSSION

Synopsis

A weak upper level disturbance passing over the islands this weekend will weaken the trades and increase showers for some areas. The upper level disturbance will weaken and move away early next week as a a ridge of high pressure approaches the islands. This will return more settled weather, and it will turn locally windy starting about Tuesday.

Discussion

Gentle to locally breezy trades are blowing across the islands, thanks mainly to a high pressure area about 1500 mi NNW of the islands. The afternoon soundings show a capping inversion around 8-10kft, a little more elevated than previously. An area of scattered to broken showery low clouds has been affecting the islands today, but a lot of these have faded during the daytime. Would expect to see showers fill in again tonight somewhat in this environment. Some relatively minor changes will occur this weekend. A weak 700 mb trough about 300 mi ESE of the Big Island will be drifting W toward the islands at the same time a weak trough in the mid-to- upper levels of the atmosphere will be digging toward the islands from the NW. Neither of these two features is especially impressive, but will be enough to weaken the trade wind inversion further and could induce a weak surface trough over the islands this weekend that will push just W of the islands by Mon. There is some model disagreement as to how strong the surface trough might be, will affect how much the low level flow weakens and veers to E or ESE and thus where showers will be most likely. Hybrid patterns with weak trades can make for challenging forecasts when you dig into the details. A middle of the road approach suggests that there will be some leeward and interior afternoon and evening showers due to sea breeze convergence, along with night and morning showers favoring windward sections and possibly south shores as well. With MLCAPE forecast to increase to 500-1000 J/kg by Sunday over the northern main islands, any of these could be briefly heavy, but moisture will be somewhat limited and excessive rainfall appears unlikely. By Tue, the 700 mb trough will be well NW of the islands and lose it's hold on our weather. At the same time, a surface ridge will be approaching us from the N and NW, and models agree that a drier and more stable airmass will overspread the state. This should lead to more settled trade wind weather for the middle to latter part of the week, with the tightening pressure gradient leading to breezy to locally windy conditions.

Marine

A ridge of high pressure north of the islands will keep moderate to occasionally breezy trade winds in place through the weekend, but winds and seas are expected to remain below Small Craft Advisory (SCA) levels. The trades are expected to strengthen again early next week as high pressure builds northeast of the islands, and an SCA for winds will likely be needed for the typically windy waters at that time. A reinforcing SSW swell will peak this afternoon and evening. Surf along south facing shores will remain above average into early next week, but remain below advisory levels. In addition to the new southerly swell, tides have been running about a half a foot above predicted levels. These elevated tide levels will increase the risk for minor coastal flooding when combined with the above average south swell. For the next few afternoons, the predicted high tides are around 2.5 feet above MLLW (not including the extra 1/2 foot mentioned above). See the latest Special Weather Statement (SPSHFO) for additional details on the elevated tide levels and potential impacts. Small trade wind and SE swells will continue into the weekend. In the longer range, models still indicate a northeast swell developing for the beginning of next week, but have scaled back on the peak swell height by a foot or two. A High Surf Advisory is looking less likely for this event, but may still be needed for east facing shores if the swell comes in bigger than anticipated. For more details on the surf, please refer to the Oahu Collaborative Surf Forecast (SRDHFO).

Aviation

Moderate trade winds will continue tonight with a slight weakening of the winds tomorrow with localized sea breezes will become more probable. Clouds and showers will tend to favor the windward and mountain areas. Windward areas can expect passing showers to carry periods of MVFR CIGS/VIS tonight.

HFO Watches/Warnings/Advisories

None.

Footer

R Ballard/Stall/Eaton

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