FORECAST DISCUSSION

Synopsis

Trade winds will build slowly from east to west across the state through Wednesday as a ridge of high pressure pushes north away from the islands. The trades will then ease briefly Thursday and Friday as a front begins to approach from the northwest. Limited showers will favor the windward and mountain areas through the week. A change in weather is then expected over the weekend as a front pushes southward through the island chain, bringing an increase in showers and strong trade winds to the Aloha state.

Discussion

Currently at the surface, a 1023 mb high is centered around 1250 miles east-northeast of Hilo, with the ridge axis extending westward to a location around 160 miles north of Kauai. This is resulting in moderate trade winds over the Big Island and Maui County, with light trades over Oahu and Kauai. Infrared satellite imagery shows variably cloudy skies in place across the state, with radar imagery and rain gauge observations showing shower activity mainly limited to windward sections of the Big Island early this morning. Main short term concern for today revolves around summit winds on the Big Island. Today through Friday, The high pressure ridge axis will lift northward slightly through Wednesday, allowing the trade winds to gradually strengthen across the island chain. Light to moderate trades are expected across Kauai and Oahu today, while moderate trades continue across Maui County and the Big Island. The trades will peak at moderate levels on Kauai and Oahu tonight and Wednesday, with locally breezy trades expected across Maui County and the Big Island. A front will then approach from the northwest Thursday and Friday, with the trades easing back to light to moderate levels across the state. A dry and stable weather pattern is expected through the work week, with inversion heights remaining suppressed due to mid-level ridging just north of the state and precipitable water values remaining around or below 1 inch. The showers that do develop are expected to remain limited primarily to windward and mauka areas. Winds on the Haleakala summit have dropped below advisory levels early this morning, so the Wind Advisory has been cancelled here. The winds may increase to advisory levels once again late tonight and Wednesday. The Big Island summits have also seen a slight decrease in winds overnight, so the High Wind Warning has been replaced with a Wind Advisory which is in effect until 6 pm this evening. This advisory may need to be extended in time later today as winds will flirt with the advisory level threshold right through Thursday. Friday night through Monday, The weather pattern will change over the weekend as a front pushes southward through the island chain. There remains some timing differences between the GFS and ECMWF, so will utilize a blended forecast approach for forecast details. Overall, it appears that the front will be closing in on Kauai Friday night, then push southward through the island chain Saturday through Sunday. The GFS shows the front hanging up near the Big Island through early next week, while the ECMWF allows the front to clear the island chain. With the approach of the front and the break down in mid-level ridging Friday night, we should see an increase in shower activity mainly across windward and mauka areas. Showers coverage will increase in association with the front Saturday through Sunday, with showers likely reaching leeward areas of the smaller islands as well. Model solutions differ by early next week, so will keep more shower activity in place across windward sections of the Big Island, while showing a drying trend for the remainder of the island chain. In addition to the increase in showers associated with the front, the trade winds will become strong over the weekend and possibly lingering through early next week, with the trades potentially reaching wind advisory levels across much of the island chain.

Aviation

The low-level trade wind flow, which is slightly stronger near Maui County and the Big Island compared with Kauai and Oahu, will continue to transport low clouds and isolated light showers into the windward facing slopes and coasts of the islands today. As a result, brief periods of MVFR CIGS are possible in passing low clouds and -shra over some windward sections, especially windward Maui and the Big Island. Otherwise, VFR conditions are expected to prevail. There are currently no AIRMETS in effect, and none are anticipated this morning. As the trade winds continue to strengthen, there is a possibility that AIRMET TANGO may be required for TEMPO moderate low-level turbulence leeward of the mountains on some islands later today or tonight.

Marine

Doppler radar derived winds from the Kauai NEXRAD shows solid 15 kt trades blowing there this hour. So, the firmer trades have finally reached the western end of the island chain. However, the ASCAT pass overnight determined the winds in the Alenuihaha Channel and waters south of the Big Island to be blowing just below Small Craft Advisory (SCA) criteria. This condition will be short lived as we expect strengthening trades today. We expect the trades to reach SCA criteria of 25 kt this afternoon in the Alenuihaha Channel and waters south of the Big Island, and spread over to adjacent Maui County waters tonight and the windward waters of the Big Island. The strong trades will continue through Wednesday before easing off a bit Thursday and Friday. If not the wind that will warrant an SCA, the seas will, brought on by an elevated WNW swell slated to arrive very late tonight and spread down the island chain Wednesday. Therefore, the SCA will cover many of the marine zones. On Thursday, the trades will back off slightly while the WNW swell starts subsiding to where we may begin scaling down the SCA. As for the surf, as noted above, warning level surf is expected with the WNW swell for most N and W facing shores of the smaller islands starting as early as tonight and running through Wednesday night. The swell will peak late Wednesday, then drop to advisory level by Thursday night. Due to its west component, the Kona coast of the Big Island will likely have warning level surf Wednesday night into early Thursday. As noted above, the trades are slated to ease a bit Thursday through Friday as a front approaches from the northwest. The front will move through the islands Saturday and Saturday night, bringing with it blustery NE winds, and large, rough, short-period seas for at least the rest of the weekend. This will result in SCA conditions to near gale force winds over all Hawaiian waters, and a High Surf Advisory for E facing shores.

HFO Watches/Warnings/Advisories

Wind Advisory until 6 PM HST this evening for Big Island Summits. Small Craft Advisory until 6 PM HST Wednesday for Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters. Small Craft Advisory from 6 PM this evening to 6 PM HST Wednesday for Maui County Windward Waters, Maalaea Bay, Pailolo Channel, Big Island Windward Waters.

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DISCUSSION...Jelsema AVIATION...Houston MARINE...Lau

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