textproduct: Honolulu

SYNOPSIS

A cold front moving into the Hawaii region from the northwest today will cause southerly winds to strengthen over all islands through the day. A convergent band of clouds associated with the remnants of the last cold front will spread bands of prefrontal showers across the islands riding in along the southerly winds ahead of the main cold front. The front will move swiftly from west to east down the island chain with a brief period of wetting rainfall starting tonight near Kauai and then spreading eastward to the Big Island by Thursday afternoon. Easterly trade winds bring a return to drier, cooler and more stable weather conditions from Friday through Sunday. Another cold front will likely move down the island chain, producing another round of wet rainy weather, from next week Wednesday through Thursday.

DISCUSSION

Satellite imagery this morning shows bands of high level cirrus clouds moving over the islands ahead of a cold front approaching the Hawaiian Islands from the northwest. All islands will likely see a decent amount of rainfall with this next frontal passage. A few thunderstorms are possible mainly along the frontal band, however the heaviest rainfall for each island will be brief due to the rapidly moving frontal cloud band, limiting any flooding concerns. The front will move swiftly from west to east down the island chain, reaching Kauai by early evening, then through Oahu just after midnight HST, and then on to Maui around sunrise, and finally through the Big Island by Thursday afternoon.

Southerly winds ahead of this approaching cold front will strengthen over the region today creating some wind gust impacts. Southerly winds with this system will produce stronger wind gusts of 40 to 50 mph range over mountain ridgelines and along north and east sections of most islands today. A Wind Advisory was issued to cover these impacts as these unusual southerly wind directions tend to produce larger local scale impacts for these areas. Strong winds just below advisory levels are also forecast for Haleakala National Park on Maui with southerly winds forecast to strengthen just below our summit wind advisory thresholds. The highest summits of Mauna Kea and Mauna Loa on the Big Island will see the strongest winds with this event, and Wind Advisories were issued this morning for both summits. Brief periods of icing conditions are also possible on these higher summits by Thursday.

High pressure building in behind the front will herald a return to drier and more stable northerly trade winds on Friday, becoming more easterly trade winds from Saturday into Sunday. This drier air moving in after frontal passage will produce much cooler temperatures for all islands, especially during the overnight hours when the drier atmosphere can more efficiently radiate heat into outer space. These lower humidity levels and cooler overnight to early morning temperatures will linger into the weekend.

Another round of wet weather remains in the long range forecast for the middle of next week. The latest extended model forecast guidance continues to show good agreement on another period of cloudy skies and wet weather as yet another cold front moves eastward down the island chain from next week Wednesday through next Friday. Stay tuned as the island by island weather impacts from this next frontal band will likely evolve over time.

AVIATION

Gentle south to southeast winds across the islands will strengthen through the morning and veer as strong southwesterly winds are expected ahead of a cold front that is currently to the northwest. The front will arrive across the western end of the state tonight and then progress down the island chain. Showers are expected along the frontal boundary, with prevailing MVFR conditions as the front moves through. A few isolated cant be ruled out either. Behind the front, breezy northwest winds will fill in, along with drier conditions.

AIRMET TANGO is in effect over the entire state for tempo mod turb FL260/360. No other AIRMETS are currently in effect. However, AIRMET Sierra for mountain obscuration will likely be needed this evening and tonight due to increasing clouds and showers, starting on Kauai and moving east down the island chain.

MARINE

A strong cold front will sweep from west to east across the coastal waters from late today into Thursday, then stall near the Big Island by Thursday afternoon. Southerly to southwesterly winds ahead of the front will become gusty today, and the Small Craft Advisory (SCA) that was previously in effect has been extended in time and expanded in area to include most coastal water zones due to the gusty winds today. Winds will shift to become northerly behind the front as it moves down the island chain. Beginning tonight, an extra large northwest swell will elevate seas above SCA thresholds once again just as wind speeds decrease. The SCA remains in effect through Thursday for now, but will likely need to be extended in time as seas remain elevated through at least Friday night. A passing high pressure system far north of the islands will bring back easterly trade winds across Hawaiian waters from Friday through Sunday. Winds will weaken and veer from a more southeasterly direction over the northwestern waters by Monday as another cold front approaches the islands from the northwest.

The northwest (310-330 degree) swell that peaked yesterday will continue to gradually fade today, though surf heights are expected to remain above advisory levels along exposed north and west facing shores. The next extra large, long period northwest (310-330 degree) swell will build into the region late tonight into Thursday, likely producing another round of warning level surf along exposed north and west facing shores lasting through Friday. Surf along north and west facing shores will then linger near advisory levels Saturday before another reinforcing northwest swell briefly boosts surf heights back to near warning levels on Sunday and Sunday night. The current High Surf Advisory remains in effect today, but this will need to be upgraded to a High Surf Warning by tonight to account for the incoming swell.

Surf along east facing shores will remain small through most of this week due to shifting wind directions from the passing fronts. However, some east facing shores sensitive to northerly swells may experience a slight uptick in surf heights this weekend as the fading northwesterly swell becomes more northerly. South shores will see a slight bump from wind wave energy today due to the strengthening south to southwesterly winds. Otherwise, no noteworthy swells are expected for the next few days.

HFO WATCHES/WARNINGS/ADVISORIES

High Surf Advisory until 6 PM HST this evening north and west facing shores of Niihau, Kauai, Oahu, Molokai, and north facing shores of Maui.

Wind Advisory until 6 PM HST this evening for most north and east sections of Kauai, Oahu, Molokai and Maui.

Wind Advisory until 6 PM HST Thursday for Big Island Summits of Mauna Kea and Mauna Loa.

Small Craft Advisory until 6 PM HST Thursday for most Hawaiian Coastal Waters and Channels.