A trough moving over the island chain from the west will bring clouds and showers to the state through tomorrow night. An accompanying upper level disturbance provides the instability for the chance for thunderstorms and locally heavy rainfall. Conditions are expected to improve Thursday or Friday, with trade winds returning this weekend.
No major changes to the forecast with the afternoon package. The High Wind Warning for the Big Island summits continues, as winds have reached advisory levels late this afternoon and are expected to continue to increase tonight. There were some minor tweaks made to the PoPs and associated fields for the first part of tonight as the rain is not spreading across the islands as quickly as we anticipated this morning.
While clouds have been plentiful over the islands today, rainfall totals have been light. Radar this afternoon shows heavier rainfall moving over the Kauai Channel, with light rain moving over the leeward and south portions of Oahu, Molokai and Lanai. Expecting the moderate to heavier shower activity to spread across the islands in the next few hours.
Satellite derived precipitable water (PW) continues to show over 3 inches about 325 miles southwest of the islands. This is twice the normal PW for this time of year. The 2 inch satellite derived PW is approaching the islands of Maui County this afternoon. The afternoon sounding from Hilo have 2.30 inches of PW and Lihue had 1.76, both about the normal value around 1.5 inches.
The remnants of Olana are more than 1300 miles southwest of the islands, with a monsoon trough extending nearly 800 miles to the east-northeast of the remnants, and trough extending to the east and then northeast of that. The trough remains south of the islands, with some surface lows forming along the trough. The low of most concern was analyzed about 530 miles southwest of Oahu this afternoon. The GFS and ECMWF models forecast the low to be a little further north at this time, but both agree with a weak low moving towards the islands. Satellite shows a broad area of layered clouds with embedded thunderstorms near the low and in the area between the low and the main Hawaiian islands. This area corresponds with the elevated PW mentioned above.
A north-northeast to south-southwest oriented upper level trough is about 800 miles northwest of Kauai, and is expected to move to the east over the next 24 hours, and will help to destabilize the airmass over the islands as the jet moves over the islands.
The Flood Watch is in effect through tomorrow night. The highest rainfall totals are expected to be focused over Maui County and Oahu. However, all islands remain under the Flood Watch with the potential for thunderstorms and locally heavy rain. Slow moving cells may become anchored along the terrain of the islands, which could lead to flash flooding anywhere. Breezy east winds could form just north of the low, while gusty southerly winds form to the south. Strong thunderstorms with localized damaging gusty winds may develop along and just south of the surface trough.
The threat of heavy rainfall will diminish Thursday and Friday. The jet stream aloft will lift to the north as the upper level trough moves to the west and an upper level low develops along the trough. The weak surface trough, with 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.
VCSH/-SHRA increasing in coverage this afternoon with steady rain on track to arrive from the southwest at any time. The timing of the heaviest rain will be late this evening through much of the overnight hours for the western end of the island chain. MVFR/IFR cigs/vis are expected with the rainfall. TEMPO groups are included in most locales to account for a period of RA with IFR cigs/vis. There will likely be isolated instances of RA+. VCTS/TS not included due to low confidence at this time. By late tonight into early tomorrow morning, PHTO and PHKO will start to see rain moving in as the area of precip moves to the east. However there could be -SHRA anytime through the evening and night.
Winds will remain generally weak through the forecast period. However, thunderstorms could bring brief gusty and erratic winds.
AIRMET Sierra Mountain obscuration remains in effect for all areas of Oahu, Molokai, Maui, Lanai.
A vigorous surface trough and possible elongated area of low pressure will move over coastal waters surrounding Maui County and Oahu tonight and Wednesday as a broad surface low meanders roughly 800 nm northeast of the islands. Expect moderate easterly winds to develop north of the trough and southeast to south winds to form south of the trough. Close to the trough, these winds could become locally strong, especially if a surface low deepens near the islands, and a Small Craft Advisory (SCA) is in place for waters around Maui County and Oahu through Wednesday. In addition, strong thunderstorms along the trough could produce erratic wind gusts in excess of gale force. The trough and possible low will weaken near the Big Island Thursday, allowing ridging to the north to drive moderate to fresh easterly winds over most waters and southerly winds near Big Island. The trough will move south of Big Island Friday, and moderate to locally fresh trades will prevail Friday night and Saturday. Stronger northeast winds may develop on Sunday and require an SCA.
A moderate northwest swell will decline late tonight and Wednesday, allowing surf to fall below seasonal average along north- and exposed west-facing shores. This swell will fade by Thursday. A small, short-period north-northeast swell will persist through much of the week, and may even become moderate Thursday and Friday with the arrival of reinforcing energy from the northeast. A larger long-period northwest swell will arrive late Saturday and may push surf above the high surf advisory threshold Sunday and Monday.
A series of overlapping south-southwest swells will continue through the weekend. The current swell is producing south shore surf near seasonal average, and additional pulses on Wednesday and Friday could produce above average surf.
Surf along most east-facing shores will be small through the first half of the week, but areas exposed to wrapping north to north- northeast swells will experience periods of moderate surf. A moderate northeast swell is possible Thursday and Friday.
Flood Watch through late Wednesday night for all Hawaiian islands.
High Wind Warning until noon HST Wednesday for Big Island Summits.
Small Craft Advisory from midnight tonight to 6 PM HST Wednesday for Kaiwi Channel-Kauai Channel-Maui County Leeward Waters-Maui County Windward Waters-Oahu Leeward Waters-Oahu Windward Waters- Pailolo Channel.
