Area Forecast Discussion for Hawaii

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Issued: Oct 30, 2014 3:55 AM HST

Synopsis
A front will bring an increase in clouds and showers as it moves slowly southward across the western and central parts of the state today and tonight. Locally heavy downpours will be possible during this time, and there will also be a slight chance of thunderstorms over Kauai this afternoon. Trade winds will increase today and tonight as strong high pressure passes far north of the state. A more typical trade wind weather pattern should return during the weekend as the front dissipates and high pressure moves further northeast and away from the state.

Discussion
Shower activity remained focused across windward sections from Kauai through Maui during the overnight hours, with showers decreasing somewhat recently just ahead of an approaching front. Most of the showers have been light to moderate, which combined with quick movement has kept rainfall amounts from becoming too heavy in any given location. A lone cumulonimbus has developed during the past couple of hours about 80 miles north of Oahu, with satellite/radar estimated tops up around 35-40k feet. Surface analysis depicts a weak front about 70 miles north of Kauai, with strong high pressure centered about 1150 miles to the NNW. This high is forcing the front to move slowly southward toward the state. Aloft, a longwave trough is located between 140°W and 150°W, .with water vapor imagery indicating a shortwave dropping southward along 160°W along the west side of the larger trough. Mimic-tpw imagery indicates a stripe of enhanced moisture along the front, with another area of higher moisture shifting south and west of the state. The 12Z soundings reflected fairly typical conditions present ahead of the front, with pwats near normal at just over 1.3 inches and somewhat elevated inversions based between 8000 and 10000 feet.

Today and tonight, the front will move slowly southward across the smaller islands, as the strong surface high moves eastward well to our north. Although low level convergence will be lacking along this feature with increasing trade wind flow already in place, enhanced moisture along with increasing instability due to a developing mid/upper level trough overhead should allow for some of the showers to become locally heavy. The best chances for heavy downpours would appear to be across Kauai and Oahu this afternoon and tonight, possibly spreading into Maui county as well tonight. Locally derived moisture-stability indices indicate the potential for isolated thunderstorms as well today, mainly across northern and western sections. The recent CB development north of the coastal waters adds confidence to this idea, and have included thunder in the forecast for Kauai this afternoon. Thunder chances appear to decrease tonight, but cannot be totally ruled out for this evening.

The front will then gradually dissipate across the state Friday through Saturday, with the mid/upper trough lingering near or just to the southwest of the area. Fresh trade winds will continue to drive showers into windward areas, carrying over into some leeward sections. A few heavier downpours will remain possible as well due to lingering instability aloft, but rapid shower motions should keep most rainfall amounts from becoming excessive.

Trades should decrease somewhat from Sunday into early next week, as the surface high shifts further to the east and another cold front moves slowly southeast toward the state. Would generally expect a typical trade wind weather pattern to prevail, although some of the models continue to entrain deeper tropical moisture northwestward across the state ahead of the next front early next week, which could result in higher rain chances. This front may eventually reach the western/central parts of the state by next Wednesday or Thursday, but it is too soon to have much confidence in this.

Aviation
Expect VFR to rule today, except for Kauai where clouds and showers from a dissipating front moves in early this afternoon that may warrant airmet sierra for widespread mtn obscuration. We expect the trades to strengthen area wide as front breaks up. However, lo lvl turbc is a question mark since the inversion is expect to be weak. So we will hold back on issuing airmet tango for lo lvl turbc and continue to monitor for signs of it.

The front is about 70 nm north of Kauai moving south slowly. After reaching Kauai early this afternoon, the front will continue to fall apart upon reaching the central islands during the first half of tonight. Airmet sierra is likely to expanded to include these islands.

Currently, conds are relatively dry statewide. The patchy low clouds and embedded showers harassing Maui county unexpectedly dissipated. Scattered showers are moving through Oahu now but this area continues to shrink and dissipate. Low clouds with embedded showers have increased across Kauai but is not widespread to warrant airmet sierra. The showers contains brief MVFR ceilings and vis, and they will be favoring the windward and mtn areas. Windward Big Island still has some shallow clouds along its shoreline and adj waters between Upolu Point and Hilo, while the afternoon clouds of yesterday, have departed ht Kona side. We expect the clouds to return to the Kona side this afternoon.

The leading edge to a band of dense high clouds is just touching southernmost Big Island. It is threatening to spread north as the day progresses.

Marine
The expected north swell has continued to arrive across the Hawaiian coastal waters overnight, as depicted by data from the cdip mokapu point buoy. This swell is expected to peak later today, with another reinforcing north swell arriving tonight into Friday. These swells, in combination with strong trade winds expected to build today and tonight, will bring hazardous surf conditions to east facing shores of the state through early Saturday morning. A high surf advisory remains in effect for east facing shores. North facing shores will also see a boost to surf over the next few days, but surf heights will remain below advisory criteria there. Surf along the south facing shores will remain small through Monday, with a slight bump Tuesday as a new long period swell arrives. Another small northwest swell will arrive Tuesday as well.

The overnight ascat pass found solid areas of 25 knot winds across the typically windy marine zones south of the Big Island and in the Alenuihaha channel. Small craft advisory conditions will spread into most other marine zones later today as winds and seas build further, then continuing through Friday night and possibly into Saturday. Winds and swell will begin to taper off during the weekend.

Watches/Warnings/Advisories
High surf advisory until 6 am HST Saturday for east facing shores of Kauai Oahu Molokai Maui and Big Island.

Small craft advisory until 6 am HST Saturday for Maalaea Bay, Pailolo channel, Alenuihaha channel, Big Island leeward waters, Big Island southeast waters.

Small craft advisory from 6 am this morning to 6 am HST Saturday for Kauai northwest waters, Kauai windward waters, Kauai leeward waters, Kauai channel, Oahu windward waters, Kaiwi Channel, Maui county windward waters.


DISCUSSION/MARINE...JACOBSON

AVIATION...LAU