A trough moving over the island chain from the west will bring increased clouds and showers 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.
The forecast has been updated this morning to account for the various models bringing the rain into the islands a little sooner. Although some of the models have the rain clearing out faster tomorrow afternoon, haven't been convinced that will be the case, so have left the rain in for tomorrow. Additionally, winds over the Big Island summits are expected to ramp up tonight to High Wind Warning levels. Have issued the warning to start this evening and carry through midday tomorrow.
Clouds are spreading across the islands this morning from the southwest. Satellite shows deeper layered clouds are about 40 miles to the southwest of the smaller islands, and are moving towards the islands. As indicated above, this area of moisture is moving a little faster than previous model runs indicated, so the forecast update was made to account for this.
Satellite derived precipitable water (PW) shows over 3 inches about 325 miles southwest of the islands. This is twice the normal PW for this time of year.
The remnants of Olana remain about 1200 miles southwest of the islands, with a monsoon trough extending nearly 700 miles to the east 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 650 miles south-southwest of Oahu this morning. The GFS forecast the low to be a little further north at this time, and the ECMWF doesn't have it forming a little later today. Both of these global models bring the low towards the islands today. Satellite shows a broad area of layered clouds with embedded thunderstorms near the low and to the area to the north where the global models form the low. This area corresponds with the elevated PW mentioned above.
A north-northeast to south-southwest oriented upper level trough is about 825 miles west 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, and continues through tomorrow night. As mentioned above, shower activity will increase today, with the rain moving in a bit faster than we thought 24 hours ago. By tonight, expecting widespread rain across the islands, with the highest rainfall totals 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.
Rain is on track to arrive from the southwest sometime after 00z. 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. 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.
Winds will remain generally weak through the forecast period. However, thunderstorms could bring brief gusty and erratic winds.
No AIRMETs are currently in effect, but AIRMET Sierra for mountain obscuration/IFR conditions may be needed later today.
A surface low centered about 800 nm northeast of the islands will remain nearly stationary as it weakens through Wednesday. A strengthening surface trough to the southwest of the islands will move across the offshore waters today. This feature will bring heavy showers with embedded thunderstorms later today through at least Wednesday. 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 around Oahu and Maui County where a Small Craft Advisory has been issued. Additionally, any thunderstorms that develop may produce localized erratic and gusty winds.
A moderate, long-period northwest swell will peak today, producing surf around seasonal average 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.
Flood Watch through late Wednesday night for all Hawaiian Islands.
High Wind Warning from 6 PM this evening to 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.
