Area Forecast Discussion for Hawaii

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Issued: Mar 28, 2017 8:25 PM HST

Synopsis
A trade wind regime will persist through the upcoming weekend and into the early part of next week as a ridge of high pressure remains north of the islands. Showers embedded in the trade wind flow will favor the windward and mountain. The frequency of these showers will higher during the nights and mornings. The trades will turn windy over the weekend and lasting through early next week.

Discussion
The morning clouds and showers that affected some of the smaller islands this morning gave way to a brief period of clouds across the lee and interior sections early this afternoon. The trades picked up enough by mid afternoon to disperse these clouds towards evening. It was a rather wet morning in particularly the Hilo area of windward Big Island. Most of these showers have moved on. The lee side of the Big Island is mainly cloudy but dry at sunset. These clouds will gradually disperse during the evening hours.

During the overnight hours, the trade showers will be picking up especially after midnight as a batch of low level moisture approaches the Big Island from the east. Satellite imagery shows the leading edge of this area 160 miles east of Cape Kumukahi at sunset. Models have this shower area impacting windward Big Islands in the next couple of hours before moving to as far as Molokai before dispersing by mid morning Wednesday.

On Wednesday, we will see a repeat of today as a tinge of weakness in the trades will allow for a brief period of afternoon clouds and isolated showers to form over the lee and interior areas of the smaller islands. The models maintain this pattern through Thursday as the last of this area of moisture finally leaves the islands. This air mass will be followed by a pocket of slightly drier air mass Friday and Friday night.

On Friday night, the surface high feeding the islands with the gentle trades will be reinforced by another surface high. This will take place some 1200 miles NNE of the islands resulting in a strong 1034 mb high. This translate in a boost in the trades to moderate to strong speeds. At this time, there is a chance of a wind advisory for the typically windy areas of places like the Waimea Saddle on the Big island and or Lanai over the weekend. This boost in the trades will be accompanied by an uptick in the frequency of trade showers.

The weather pattern takes a change north of the islands Sunday into Monday where the high is shoved off far to the east. This will lead to a weaker trade wind flow by Tuesday. And with the weaker trades, there will be fewer trade showers.

Aviation
With a broad surface high far northeast of the state, trade winds will be light to moderate through Wednesday. Low clouds, with any accompanying showers moving along on the trades, will mainly affect windward and mauka sections on the individual isles, but conditions overall will remain VFR.

There are no AIRMETs currently in effect, and none are anticipated through the nighttime hours.

Marine
just over 1500 nm northwest of Kauai near the Date Line (around 40N) and a ridge axis extending westward from a 1030 mb high centered off the coast of California to just north of the islands. A very strong pressure gradient was depicted south of this low northwest of the state, that was supported by an ASCAT pass earlier today where a large area of strong west-northwest gales (focused within the 290 to 310 directional band relative to the islands) were shown out to around 500 nm south-southwest from the low (leading edge of the fetch was around 1300 nm northwest of the islands). According to the latest altimeter passes, seas associated with this system have dropped slightly from yesterday, but still upwards within the 28 to 33 ft range over the heart of the fetch region focused toward the islands. Despite some small differences between solutions, the latest model guidance is remaining in decent agreement with these latest satellite trends and shows this system continuing on an east-northeast track crossing the Date Line Tuesday night into Wednesday around 40N.

A large west-northwest swell (290-310 deg) associated with this system discussed above will result across the region, that should reach the islands Thursday night, peak through the day Friday and Friday night, then slowly ease over the weekend. Warning-level surf and advisory-level seas will be expected Thursday night into the weekend before trending down over exposed waters and beaches. For timing, the ECMWF-Wave, GFS-WAVEWATCH III and ensemble mean solutions all remain in close agreement and depict seas ramping up quickly around midnight Thursday night to Small Craft Advisory levels, peaking within the 10 to 14 ft range Friday, then slowly easing through the weekend. A few of the ensemble solutions are coming in slightly higher (by a couple of feet) at the northwest buoys and converge at a peak near 15 ft Thursday night. If the swell ends up coming in larger than predicted or later, warning- level surf could continue into Saturday before dropping to advisory levels along exposed north and west facing shores. Surf should dip below advisory levels Sunday.

Local winds and seas will likely remain below advisory levels through Friday as the ridge of high pressure north of the state slightly weakens in response to a cold front that is forecast to approach the region later in the week, then stall and weaken north and northwest of the area into the upcoming weekend. Advisory- level trade winds will return across portions of the waters over the weekend as high pressure builds north of the state behind this front.

Prior to the arrival of the large swell expected Thursday night through the weekend, surf along north and west facing shores will hold at levels below the advisory criteria Wednesday through Thursday due to a lingering northwest swell from earlier this week. Will continue to monitor buoy observations overnight for an expected small reinforcement out of the northwest (320 deg).

Surf along east facing shores will remain small and choppy through the week due to moderate to fresh onshore winds persisting. Surf will begin to build over the weekend as the winds strengthen locally and upstream of the islands and may near or reach advisory levels along east facing shores by the end of the weekend and into early next week.

A slight increase in surf along south facing shores will be possible Thursday through Saturday due to a long-period swell (190 deg) associated with recent activity across the southern Pacific. In addition to this southerly swell, wrap from the previously discussed large west-northwest swell will add to the mix by Friday at select spots along exposed southern facing beaches.

Watches/Warnings/Advisories
None.


Discussion...Lau

Aviation...Kinel

Marine...Gibbs