Kekaha-Waimea, HI Weather

Synopsis

Northeasterly trade winds will weaken through Monday. Brief passing showers will drift into the islands on the trade winds, favoring windward and mountain areas during the overnight to early morning hours. Deep tropical moisture will bring the potential for heavy rain and thunderstorms across the state starting early Tuesday and lasting through at least Thursday. The forecast for next weekend is a return of trades but continued above-normal rainfall.

Discussion

Trade winds were still present this afternoon, averaging 10 to 20 mph, with a few gusts around 25 mph at the usual breezy spots. Isolated showers were noted on radar, most numerous near Volcano on the Big Island, and over northeast Molokai. Skies were the usual mix of mostly sunny all the way to overcast, with the majority of the clouds windward and mauka.

There's a complicated weather pattern over the area, and it's just going to get more complicated over the next few days. This afternoon, there's a large closed low about 1000 miles to our northeast. At the surface, a closed low sits directly beneath it. A front extends to the east and then south of the low, and a trough extends southwest from the decaying end of the front toward Hawaii. A separate upper level trough to our northwest will move southeast, helping to push Tropical Storm Olana toward us. The good news is that Olana is forecast to weaken back into a depression by Tuesday, and the remnants are expected to remain west of the state. The trough extending from the low to our northeast will eventually extend all the way to the remnants of Olana, and then drift north over the state. The trough will bring a great deal of moisture with it. PW values are still forecast to be close to 2.5 inches, which is extremely high.

This trough will remain nearly stationary over the area from Tuesday through the end of the week. A series of small lows will form and slide east, with one or more potentially moving over the state. Models show different timings on these features, but the point is that each time one of these lows moves across the immediate area, the winds will shift direction and different parts of each island will briefly become "windward" and "leeward". As the wind blows toward the mountains, rain efficiency will increase, giving brief increases in rain rate and the potential for flooding. The exact locations and timings of these subtle features are not yet predictable with any confidence. Therefore, the QPF will undergo (perhaps significant) revision over the next couple of days. We have opted to go with another hydrologic statement today discussing the potential for heavy rain, as opposed to a flood watch. One other consideration in this decision is that the latest models are slower to bring heavy rain to the state. It now looks like it should arrive Tuesday as opposed to Monday night.

At this time, it appears the threat of heavy rain will extend from Tuesday through the end of the week, and possibly into the weekend. Obviously our confidence gets lower with time, so the current forecast does not explicitly depict high PoPs and heavy rain beyond Thursday. The best chance for heavy rain appears to be from Tuesday afternoon through Wednesday afternoon, but I wouldn't be surprised if that ended up being extended into Thursday or even Friday. Total rainfall amounts in our QPF grids from Tuesday through Thursday, using broad numbers, currently range from 2 to 8 inches, with the highest amounts in the higher elevations. These numbers will change over the next few days as higher resolution guidance becomes available (when the forecast is within 48 hours, generally).

There is also a chance of thunderstorms at times during this period, but we have adjusted the wording from "scattered" to "isolated", indicating a slight lessening of the threat based on the latest guidance. Upper level temperatures are cold enough for thunderstorms, especially considering the high moisture content of the atmosphere, but it's close. We feel that isolated should best convey what we're thinking at this time. As with any thunderstorm, brief gusty winds are possible, but overall this setup should not bring strong winds to large areas of the state.

Aviation

Moderate trade winds will prevail the rest of today with some isolated breezy conditions at PNHY and PHMK. Otherwise, windward and mountain areas on Big Island will see an increase in showers which may cause temporary MVFR ceilings and visibilities in any shower that passes through.

Moderate turbulence has been reported between 23,000 and 34,000 feet over the open water areas north and east of the islands. The moderate nature and forecast for continued weakening of the winds preclude any AIRMET or SIGMET headlines at this time.

Looking ahead, widespread MVFR/IFR conditions in heavy showers and low ceilings are possible beginning late Monday Night into Tuesday morning; see the main discussion for details on the upcoming heavy rain and flooding potential.

Marine

A surface low far northeast of the Hawaiian Islands will continue to dig south tonight and stall roughly 800 nm northeast of the state by Monday, resulting in relatively weak wind flow across the area this week. A low pressure trough will then approach the area from the southwest late Monday and bring heavy showers with embedded thunderstorms to the waters over the next several days. Winds may become locally breezy along the trough and around showers.

A short period north northwest swell will gradually lower over the next couple of days. An overlapping, moderate, mid-period northwest swell is expected to build through the day Monday, with surf reaching near advisory along north and west facing shores through Wednesday. A small north northeast swell should arrive during the second half of the week.

Overlapping south-southwest swells will keep surf along south-facing shores to around seasonal averages through Tuesday. A larger south swell may push surf up to near advisory levels Thursday and Friday.

Surf along most east-facing shores will be small through early next week, but areas exposed to wrapping northerly swells will experience periods of moderate surf through Monday. East-facing shores should also see a bump in surf if the north-northeast swell materializes during the latter half of this week.