Synopsis20 Oct 2018 06:51:00 GMT: A weak trade wind flow will gradually strengthen through Saturday night. A disturbance at the upper levels of the atmosphere will lead to some unsettled weather with thunderstorms across the island chain, but especially for the Big Island over the weekend. Locally strong trades return by Monday and persist through at least the middle of next week. Increase rainfall, however, could return to parts of the state at mid week.
Discussion20 Oct 2018 06:51:00 GMT: An unstable trade wind flow will likely persist through most, if not all of the weekend. The current rather weak trades will be gradually strengthening through Saturday night into the moderate range, and to locally strong by Monday daybreak. The strengthening trades is in response to a front dissipating 450 miles north of Kauai. The trades are shallow now, only within the lower 6 to 8k feet.
The problem or cause of the unsettled weather is a deep upper level short wave trough slated to move from west to east across the main Hawaiian Islands on Saturday. The showers and isolated thunderstorms came into full bloom this afternoon on Maui, Molokai, Oahu, and even Lanai. The showers have ended on Molokai and Lanai, but they are still active, coming and going, across the windward areas of Oahu and Maui. With the trades bringing in additional moisture, these showers may last for several more, which will heighten the potential for flash flooding. To the contrary, the Big Island had some weather there today, although not to the extreme of the past few days. There is one cell that we are watching closely, near Waipio Valley. Otherwise, it is quiet across the rest of the Big Island this hour. Ditto for Kauai which has been rather quiet all day. There were a few interior showers this afternoon, but they have since dissipated.
All said but Saturday will likely be another active day as the short wave moves across the island chain. The axis of this trough is currently over Kauai, and should be over Maui County Saturday afternoon, then over the Big Island Saturday night. The deep moisture axis is still over the Big Island, and this will be acted upon Saturday. The ECMWF has joined up with the GFS in pushing the shortwave trough east of the Big Island Sunday evening. This means improving weather for the start of the new work week with the return of regular trade wind weather. This benign trade wind weather will be threatened during the mid to second half of the week as the GFS solution some tropical moisture heading toward the Big Island.
Aviation20 Oct 2018 06:51:00 GMT: The light trade wind regime will persist through at least Saturday. The result of this weak flow will be the presence of nighttime land breezes and daytime sea breezes, especially over leeward sections of the isles. Also, overhead of the island chain is a destabilizing upper trough that will move slowly from west to east. This feature will enhance the chances of showers and thunderstorms statewide over the weekend, most particularly the Big Island. With the precipitation, areas of MVFR weather will become more widespread in lower ceilings and/or visibilities.
No AIRMETs are currently in effect. Mountain obscurations are more likely Saturday into Sunday, especially for the Big Island.
Marine20 Oct 2018 06:51:00 GMT: Light to moderate trade winds are expected tonight, with a gradual increase in northeasterly trades expected on Saturday as high pressure builds northwest of the islands. Winds should remain below Small Craft Advisory (SCA) levels through Sunday, with moderate to locally fresh trade winds potentially requiring a SCA for the windier zones around Maui and the Big Island Sunday night through much of next week.
A persistent upper-level trough over the area will keep conditions unstable through the weekend, bringing the potential for heavy showers and thunderstorms, particularly over the waters surrounding Maui County and the Big Island.
A significant, long period south swell will build tonight and hold through early next week. A High Surf Advisory remains in effect for all south facing shores. Surf heights are expected to build to advisory levels during the overnight hours. Peak surf heights with this long-lived swell could approach the south shore High Surf Warning threshold (15 feet) over the weekend, and buoy observations will be monitored to fine tune the forecast. Near shore buoys exposed to the south swell show the longer period wave energy increasing.
Elsewhere, surf heights will remain below advisory levels for at least the next several days. A moderate short period north northwest swell is expected to hold through Saturday, then gradually drop through Sunday. See the Oahu Surf Discussion (SRDHFO) for additional details.
HFO Watches/Warnings/Advisories20 Oct 2018 06:51:00 GMT: High Surf Advisory from 6 PM this evening through Monday afternoon south facing shores of all islands.
Flash Flood Watch through Saturday afternoon for Big Island.
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