Area Forecast Discussion for Hawaii

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Issued: Oct 16, 2017 8:51 PM HST

Synopsis
High pressure north of Hawaii will keep a windy trade wind pattern in place through much of the work week, with the trades trending down slightly Friday and weakening further over the weekend. Clouds and showers will favor windward and mauka areas through the period, but showers will reach leeward areas from time to time as well due to the strength of the trades.

Discussion
Currently at the surface, a 1030 mb high centered 950 miles north of Honolulu, is driving windy trades across the island chain this evening. Infrared satellite imagery shows partly to mostly cloudy skies in place across the State, with cloud cover most prevalent over windward areas. Meanwhile, radar imagery shows scattered to numerous showers moving into windward areas, with showers occasionally reaching leeward areas as well. Main short term concern over the next few days revolves around the strong trades.

High pressure will remain anchored to the north of the State through mid-week, with another re-enforcing high building north of the islands on Wednesday. This high will then remain nearly stationary through Thursday, before shifting eastward and weakening Friday and over the weekend as a cold front approaches from the northwest. As a result, windy conditions are expected to continue across the entire State through Thursday night, with the trades easing a bit on Friday, and trending down further over the weekend.

Winds have dropped below advisory levels for much of the State this evening, so will cancel the Wind Advisory for most areas with the evening update. For the typically windy areas such as Lanai, Kahoolawe, as well as over and downwind of the Kohala mountains on the Big Island, we will extend the Wind Advisory through Tuesday evening. A Wind Advisory may also be needed for the central valley of Maui Tuesday afternoon, but will re-evaluate this for the morning forecast package. With the windy conditions persisting through much of the work week, the Wind Advisory may need to be extended and possibly expanded to cover other portions of the State right through Thursday night.

As for sensible weather details, clouds and showers will favor windward and mauka areas through the forecast period, with showers also reaching leeward areas from time to time due to the strength of the trades. Model solutions are indicating an increasing probability that deep tropical moisture will get drawn northward into or near the islands Sunday through early next week. As a result, will trend toward a wetter forecast and continue to monitor model trends and make adjustments as details become more clear.

Aviation
A strong high pressure center north of the Hawaiian Islands will produce windy northeasterly trade winds through Wednesday. Enough instability in the low levels will produce scattered shower activity with brief periods of MVFR conditions mainly over mountains and northeastern slopes of all islands. Otherwise VFR conditions will prevail.

AIRMET Tango for low level mechanical turbulence remains in effect over and south through west of all mountain areas due to windy conditions.

Marine
Very strong and gusty trade winds will continue through mid-week due to a strong surface high far north of the main Hawaiian Islands. This high will be replaced by another strong high on Wednesday, which will continue the gusty trades for an additional day or two. The strong to gale-force trade winds are also producing very rough and elevated seas around the islands. Gale warnings are posted for the Pailolo and Alenuihaha channels through Thursday afternoon, while a Small Craft Advisory remains in effect for the remaining Hawaiian waters through the same time. Trade winds speeds are expected to diminish slightly starting Friday as the high starts to weaken and begins to move toward the east.

Elevated, rough waves continue to produce dangerous conditions in surf zones along most east facing shores. Therefore, a High Surf Advisory remains in effect for these beaches through Wednesday afternoon. Since the trade winds will likely continue to be strong, this advisory will likely be extended from Wednesday night through Thursday.

A long-period south swell is forecast to peak some time later tonight or early Tuesday morning with a swell height of around 4 feet and a wave period near 17 seconds. A High Surf Advisory for the south facing shores of all islands remains in effect through Tuesday afternoon. This south swell will slowly diminish from Tuesday night through Wednesday. Small southwest and south- southwest swells arriving this weekend will produce small background surf along south facing shores.

The current north-northwest swell is expected to gradually subside through Wednesday. A long-period north-northwest swell arriving Thursday is expected to peak Friday. This swell will likely produce moderate-sized surf (below the High Surf Advisory threshold) along most north facing shores, as well as along some west facing shores of the smaller islands. A somewhat larger northwest swell is due in Sunday night and Monday of next week, with the surf potentially approaching High Surf Advisory levels along the north and some west facing shores of the smaller islands.

See the Oahu Surf Discussion (SRDHFO), which was updated earlier this afternoon, for more detailed information regarding the swell sources.

Fire weather
Strong trade winds coupled with a drier airmass pushing over the State will allow minimum relative humidity to drop into the 40 to 45 percent range during the afternoon hours over the next few days. These conditions will result in an increased potential for fires to ignite and spread quickly. The windy and relatively dry conditions will continue through the end of the work week before the trade winds weaken. The Keetch-Byram Drought Index, or KBDI, is at 572 today (and would need to be at 600 for Red Flag Warning criteria). Keep in mind that we will be close to these conditions each afternoon.

Watches/Warnings/Advisories
High Surf Advisory until 6 PM HST Tuesday for Niihau, Kauai Leeward, Oahu South Shore, Waianae Coast, Molokai Leeward, Lanai Makai, Kahoolawe, Maui Leeward West, Maui Central Valley, Leeward Haleakala, Kona, Kohala.

High Surf Advisory until 6 PM HST Wednesday for Kauai Windward, Oahu Koolau, Olomana, Molokai Windward, Maui Windward West, Windward Haleakala, South Big Island, Big Island North and East.

Wind Advisory until 10 PM HST Tuesday for Lanai, Kahoolawe, Big Island North and East, Kohala.

Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Maalaea Bay, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Gale Warning until 6 AM HST Thursday for Pailolo Channel, Alenuihaha Channel.


DISCUSSION/FIRE WEATHER...Jelsema

AVIATION...Bohlin

MARINE...Houston