West Side

Today: Sunny and breezy. Isolated showers in the morning. Highs 80 to 86. Northeast winds up to 15 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 20 percent.

Tonight: Mostly clear. Breezy. Isolated showers in the evening. Lows 69 to 76. Northeast winds 10 to 20 mph decreasing to up to 15 mph after midnight. Chance of rain 20 percent.

Wednesday: Sunny and breezy. Highs 82 to 87. East winds up to 20 mph increasing to 10 to 20 mph in the afternoon.

South Side

Today: Sunny and breezy. Highs around 89. Northeast winds 10 to 20 mph.

Tonight: Mostly clear. Breezy. Lows 69 to 77. Northeast winds 10 to 20 mph.

Wednesday: Sunny and breezy. Highs around 90. Northeast winds 10 to 20 mph.

North Shore

Today: Partly sunny. Numerous showers in the morning, then scattered showers in the afternoon. Highs 79 to 85 near the shore to around 66 near 5000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Breezy. Scattered showers. Lows around 73 near the shore to around 52 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Wednesday: Mostly sunny. Breezy. Scattered showers in the morning, then isolated showers in the afternoon. Highs 80 to 86 near the shore to around 67 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Central Maui

Today: Sunny and breezy. Highs 82 to 89. Northeast winds 10 to 20 mph.

Tonight: Mostly clear. Breezy. Lows around 71. Northeast winds 10 to 20 mph.

Wednesday: Sunny and breezy. Highs 83 to 91. East winds 10 to 25 mph.

Upcountry

Today: Mostly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs around 65 at the visitor center to around 70 at the summit. East winds up to 10 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

Tonight: Partly cloudy with isolated showers. Lows 46 to 55. East winds up to 15 mph. Chance of rain 20 percent.

Wednesday: Mostly sunny. Scattered showers in the morning. Highs around 64 at the visitor center to around 71 at the summit. East winds 10 to 15 mph. Chance of rain 50 percent.

East Maui

Today: Partly sunny. Numerous showers in the morning, then scattered showers in the afternoon. Highs 79 to 85 near the shore to around 66 near 5000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Breezy. Scattered showers. Lows around 73 near the shore to around 52 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Wednesday: Mostly sunny. Breezy. Scattered showers in the morning, then isolated showers in the afternoon. Highs 80 to 86 near the shore to around 67 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Lanai City

Today: Sunny and breezy. Highs 74 to 82. Northeast winds 10 to 25 mph.

Tonight: Mostly clear. Breezy. Lows 64 to 69. Northeast winds 10 to 25 mph decreasing to 10 to 15 mph after midnight.

Wednesday: Sunny and breezy. Highs 75 to 83. Northeast winds 10 to 20 mph.

Kaunakakai

Today: Sunny and breezy. Highs 73 to 90. Northeast winds 10 to 25 mph.

Tonight: Mostly clear. Breezy. Isolated showers in the evening. Lows 60 to 75. Northeast winds 10 to 25 mph decreasing to 10 to 15 mph after midnight. Chance of rain 20 percent.

Wednesday: Sunny and breezy. Highs 74 to 91. East winds 10 to 20 mph.

Detailed Forecast

Synopsis

High pressure to the north will maintain trade winds through the week, with limited clouds and showers focused over the windward and mauka areas through Thursday. Increased shower activity is expected Friday and into the weekend.

Discussion

Have largely trended the forecast toward the National Blend of Models with the morning package. As a general rule, the GFS and ECWMF are in good agreement with the handling of the synoptic features over the next couple of days. There are some differences in the models as we head into Friday and the weekend, but both global models agree with an increase in instability coinciding with a substantial increase in precipitable water (PW).
PoPs were increased over windward portions of Maui this morning due to a showery cloud band making its way into the islands this morning. However, not expecting much in the way of rainfall amounts. Forecast PoPs have little change for tonight through Thursday.
High pressure will remain north of the islands through the upcoming weekend, maintaining a trade wind flow over the region. A cold front currently near 30N, about 500 miles north of Oahu, is associated with a low pressure far northeast of the islands. As the low lifts to the north, the front will move to the east, and leftover moisture associated with the front along 30N will become entrenched in the trade winds. This moisture is expected to ride in on the trades later this week. Both the GFS and ECMWF are in agreement with the main area of moisture reaching the islands Friday night. The GFS also brings in a smaller band of moisture Thursday night. An upper level low is expected to form Thursday along the upper level trough currently over the western islands.
Both the GFS and ECMWF show PW rising to near 2 inches with this band of moisture Friday night. At the same time, 500 mb temperatures drop to -9C near Kauai in the ECMWF, and -8 in the GFS. In either case, this is substantially colder than normal for September. This combination does raise the concern for thunderstorms and/or locally heavy rainfall. We have seen run to run consistency in the models, but there are still some differences. The ECWMF keeps the main focus over the central and western islands, while the GFS focuses more on just the western islands. Have held off putting thunderstorms into the forecast just yet, but with this trend, locally heavier showers and possible thunderstorms could be added to the forecast for Friday night into Saturday. Another round of moisture is possible to move through Sunday, with another upper level level low over the western end of the state.

Aviation

Moderate to locally breezy trade winds will continue through today. Expect some clouds and showers mainly over windward Maui and the Big Island this morning with a batch of moisture being carried by the trades. VFR conditions will generally prevail, with brief MVFR possible with passing showers.
No AIRMETS are in effect and none are anticipated today.

Marine

Weak high pressure north of the state will become reestablished by midweek. This will result in small fluctuations of trade wind speeds across the coastal waters for the next several days, but they will generally remain in the moderate to locally strong range. A Small Craft Advisory (SCA) that is in effect for the windier channels and waters around Maui and the Big Island has been through Wednesday afternoon.
North shore surf will hold at tiny levels for the next several days. A gale low that is currently in the Gulf of Alaska is expected to send a minor north swell towards Hawaii later this week, resulting in a small bump in surf heights for north facing shores beginning on Thursday. An additional small northwest reinforcement is expected this weekend.
The current small, medium period south swell will gradually decline through mid-week. A couple of very small reinforcements are expected throughout the next several days, though surf will remain small. A slightly larger long period swell, generated from gales and near-gales aimed at Hawaii this past weekend near New Zealand, is forecast to arrive in the islands by Friday night. This swell will boost south shore surf back up to near September averages this weekend.
East facing shores will experience typical trade wind generated energy throughout the forecast period.

Fire weather

Humidity gradually increases through the week as sustained winds briefly hover near to below the critical fire weather threshold each afternoon. Showers remain rather sparse in coverage until Friday.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov