Joint Base Pearl Harbor Hickam, HI Weather

Synopsis

Chances for heavy showers will increase today as a trough or will pass over the island chain through Wednesday night. The trough will bring the threat of heavy flooding rainfall and thunderstorms before weakening weaken Thursday or Friday, though humid conditions with a chance of heavy showers may linger. Trade winds may briefly redevelop Saturday, followed by stronger northeast winds Sunday.

Discussion

Across the state this morning, clouds continue moving overhead from the southwest along with scattered showers. Winds on the Big Island summits have come down enough that the Wind Advisory was cancelled. However, these winds are expected to increase later today and will likely support another Wind Advisory.

Remnants of Olana will remain far to the west and not directly impact the area. A surface trough originating in the western Pacific extends through and several hundred miles east of the what remains of Olana. An upper level trough,well west of Kauai will advance eastward, tapping moisture aloft from thunderstorms along the surface trough and sending increased high clouds overhead.

Chances for heavy rainfall and thunderstorms will increase today. The upper level trough will drift closer to the state. Proximity to the jet aloft will erode this trough and pull the surface level trough toward the islands. This will spread the deep tropical airmass overhead with PWATs increasing to around 2 in by the afternoon. Chances for widespread rain should remain relatively low in the morning as the Flood Watch for all islands goes into effect, but as moisture increases later in the day, the flood threat will slowly increase.

The flood threat will be greatest this evening through Wednesday night. The jet stream aloft will provide its strongest ascent over the region, pulling the surface trough over the state and possibly generating an elongated surface low near or over the islands. A deep tropical airmass with PWATs in excess of 2.25 in will accompany this system and provide ample fuel for heavy rainfall. Guidance shows the surface trough stretching across the central portion of the island chain, favoring Oahu and Maui County for the greatest rainfall totals. However, all islands will have a heavy rain threat, and slow- moving cells anchoring on terrain could drop very heavy rainfall in a short period of time, leading to flash flooding anywhere. If a surface low does form along the trough, breezy east winds could form just north of the low, while gusty southerly winds for to the south. To further complicate the scenario, strong thunderstorms with localized damaging gusty winds may develop along and just south of the surface trough Wednesday.

The threat of heavy rainfall will diminish Thursday and Friday. The jet stream aloft will lift to the north, and weak upper level anticyclonic flow will develop, ceasing the large scale ascent over the islands. A weak surface trough and ample moisture will likely linger, providing some chance for spotty heavy showers that could require the Flood Watch to be extended.

Friday night into Saturday a brief period of trade winds may return as the lingering moisture associated with the trough is pushed south of the islands. The GFS and ECMWF are showing breezy northeast winds developing Sunday as a shallow front moves near or over the state.

Aviation

A band of deep tropical moisture will arrive from the southwest late this afternoon into this evening and will likely bring periods heavy rain to the islands with isolated thunderstorms. The heaviest rain bands are expected to arrive tonight into Wednesday. Expect periods of MVFR and IFR ceilings and visibility associated with this activity. Brief periods of LIFR will also be possible with the more robust storms.

Winds will remain generally weak through the forecast period. However, thunderstorms could bring brief gusty and erratic winds.

No AIRMETs are currently in effect.

Marine

A surface low around 800 nm northeast of the main Hawaiian Islands will remain nearly stationary as it weakens through Wednesday. A surface trough to the southwest of the islands moves into the offshore waters today. This feature will bring heavy showers with embedded thunderstorms over the next several days. While large scale wind flow will remain quite weak, northeasterly winds on the north side of the trough may reach fresh or locally strong levels, most likely in the central waters. Additionally, any thunderstorms that develop may produce localized erratic and gusty winds.

A moderate, long-period northwest swell will peak today, bringing surf to near advisory levels along north and exposed west-facing shores. This swell will begin to decline late today and fade by Thursday. A small, short-period north-northeast swell will maintain through much of the week, and may even become moderate for some time during the latter half of the week with the arrival of reinforcing energy from the northeast. A moderate, long-period northwest swell is forecast to arrive this weekend, with potential to reach advisory levels by Sunday.

Overlapping south-southwest swells will keep surf along south-facing shores near seasonal average today. A slightly larger south swell arriving tonight pushes surf above seasonal average for Wednesday. Additional south energy may prolong near to above average surf for south-facing shores on Thursday and Friday as well.

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