Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

717 pm hst Wed may 24 2017

Synopsis

Breezy trades will persist today, pushing clouds and showers across windward areas. Winds will gradually weaken through the second half of the week, becoming light and variable through the Memorial Day weekend. Daytime sea breezes and nighttime land breezes will take over this weekend and bring warm and humid weather with possible vog. A weak upper trough will pass over the state this weekend and may provide some enhancement to afternoon showers.

Discussion

Breezy trade winds have been pushing an old frontal boundary over windward Kauai through the day bringing showers and moderate rainfall. Showers will continue overnight on Kauai and increase over windward Oahu as those same showers affecting Kauai sag south. The boundary will continue bringing showers to the western end of the state into Thursday before finally dissipating tomorrow night.

A strong high pressure system centered far NW of the islands is providing the breezy trade winds over the state. Trades will weaken tonight and tomorrow as a surface low forms to the NE, diminishing the pressure gradient over the state. Winds will remain light and variable through the weekend as an approaching front from the NW turns the synoptic scale winds out of the SE. The light SE flow will allow for afternoon sea breezes and nighttime downslope winds to prevail over the islands from Saturday into early next week.

Windward and mountain areas will see the majority of showers through Friday, but sea breezes over the weekend will bring passing afternoon showers to interior and leeward areas. A weak upper level low will bring some instability to the atmosphere Saturday and Sunday. The instability will enhance rainfall, allowing for some isolated heavy showers to occur.

The light SE flow will bring volcanic haze, or vog, up over the smaller islands by Sunday and linger into the beginning of next week. The light winds will also bring hot and humid conditions for the Memorial Day weekend.

Aviation

Airmet Tango continues for mechanical turbulence to the lee of the mountains due to the breezy trade winds. Expect the turbulence to continue through tonight. Winds are expected to continue to decline over the next couple of days.

Clouds and showers continue to impact Kauai and Oahu this afternoon and have made little progress southward. Expect these clouds and showers to continue to bring periods of MVFR conditions. As such, AIRMET Sierra continues for mountain obscuration. Remaining areas should remain VFR through the first part of the night.

Marine

The latest surface analysis showed a large area of high pressure encompassing much of the northern Pacific. The high pressure is driving moderate to fresh trade winds over the state. A Small Craft Advisory (SCA) is in effect for locally strong trades in the typical windy zones around Maui County and the Big Island through tonight, and should begin to taper off Thursday.

Surf along north facing shores will steadily trend down tonight as the north, short period swell declines. For the long range, models are hinting at another moderate north northwest (340 deg) swell filling in early next week due to a low tracking east of the date line over the weekend and deepening as it passes well north of the state on Monday.

Surf along south facing shores is expected to maintain through Thursday night. The largest south swell this season is forecast to fill in Friday, peak Friday night through Saturday night, then slowly ease into early next week. A high surf advisory for south facing shores will be likely beginning Friday through Memorial Day weekend, before steadily easing into early next week.

In addition to the expected advisory-level surf through the weekend along south facing shores, coastal flooding is anticipated due to a combination of record level spring (king) tides (especially for Maui County and Oahu) and large surf. See the Special Weather Statement on our website for more details. Similar to the April 28th through May 1st weekend, when the highest tides on record were reported for Honolulu, impacts will include beach flooding and standing water on roadways and low- lying areas near the coast around the times of high tides each day through the weekend.

The following are the daily high tide times for Honolulu which will correspond to the maximum coastal impact:

5/25 Thursday 4:21 PM HST 2.4 feet

5/26 Friday 5:07 PM HST 2.5 feet (south shore surf expected)

5/27 Saturday 5:56 PM HST 2.4 feet (south shore surf expected)

5/28 Sunday 6:47 PM HST 2.3 feet

5/29 Monday 7:40 PM HST 2.1 feet

For more detailed information on surf for Oahu, see the latest Collaborative Nearshore Swell and Wind Forecast that was issued this afternoon (www.prh.noaa.gov/hnl/pages/SRF.php#Discussion).

Watches, Warnings, Advisories

Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.

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