FORECAST DISCUSSION

Synopsis

23 Jan 2018 13:59:00 GMT: A ridge of high pressure north of the state will keep trade winds blowing through Thursday, with clouds and showers focused primarily over windward and mauka areas. The trades will diminish late in the work week as a front approaches from the northwest, and this should allow for a land and sea breeze pattern with convective shower development over the interior of the islands during the afternoon hours on Friday. Gentle trade winds are then expected to return over the upcoming weekend, with showers transitioning back into the more typical windward and mauka areas.

Discussion

23 Jan 2018 13:59:00 GMT: Currently at the surface, a west to east oriented ridge of high pressure located around 800 miles north of Kauai, is driving moderate trade winds across the island chain early this morning. Meanwhile, a cold front is located around 1200 miles west of Kauai. Aloft, water vapor imagery shows a southwest to northeast oriented upper level trough over the western islands. Infrared satellite imagery shows extensive cloud cover over windward sections of the Big Island and Maui County, with a broken band of low clouds associated with an old front extending over 1000 miles to the east-southeast of the state. Across the remaining islands, mostly cloudy skies are in place over windward areas, partly cloudy conditions in leeward locales. Radar imagery shows widespread showers moving into windward sections of the Big Island, with scattered to numerous showers moving into windward sections of the smaller islands. Main short term concern for today revolves around rain chances. Today through Thursday night, A ridge of high pressure will remain in place to the north and northeast of the state through the period, while a cold front approaches the islands from the northwest. Moderate trade winds are expected to continue through Wednesday, with the trades then trending lighter from west to east across the state Wednesday night through Thursday night. Meanwhile aloft, the upper level trough will shift into the central and eastern islands Tuesday through Wednesday, while a weak ridge aloft builds over the western end of the state. The upper level troughing will then lift back across the entire island chain Wednesday night through Thursday night. As for sensible weather details, the band of deeper moisture will remain over the entire island chain on today, then shift southeastward and over the eastern islands tonight through Wednesday. Showery trade wind weather is expected over windward areas of all islands today and tonight, with shower coverage diminishing across the smaller islands on Wednesday, while windward Big Island remains wet. Shower coverage should increase over windward sections of the smaller islands beginning late Wednesday night and continuing through Thursday, while wet conditions persist over windward Big Island. Meanwhile, a few showers may occasionally drift into leeward areas through the period, while isolated to scattered afternoon shower development is expected over leeward sections of the Big Island each day. Additionally, an isolated thunderstorm will be possible over the Big Island today as the atmosphere will remain unstable here. Some snow showers and freezing rain will be possible at the highest elevations of the Big Island today as well, but confidence in this occurring is too low to warrant the issuance of a Winter Weather Advisory at this time. Friday through Monday, Model solutions begin to show some subtle but important differences over the weekend, so forecast confidence is a bit lower than normal during this time frame. Both the GFS and ECMWF show a front stalling out a couple hundred miles northwest of Kauai on Friday, so confidence in a land and sea breeze dominant pattern is fairly high for Friday and Friday night. Solutions then diverge over the weekend into early next week, with the GFS keeping troughing in place over the islands, while the ECMWF shows ridging north of the islands being dominant with little in the way of troughing over the state. The GFS solution would suggest light winds with land and sea breezes dominant through the weekend, while the ECMWF solution would suggest light to moderate trade winds. Given the differences in the solutions, will take a blended forecast approach and show a gentle trade wind regime prevailing over the weekend into early next week. As for sensible weather details, expect a convective pattern Friday and Friday night, with a bit of a hybrid convective/trade wind pattern shower pattern over the weekend into early next week.

Aviation

23 Jan 2018 13:59:00 GMT: A large band of MVFR clouds and showers continues to stream into the eastern slopes of the Big Island this morning. Moisture with this band will drift northward spreading clouds and showers moving into windward Maui and Molokai this morning. An upper level disturbance aloft may produce enough lift for isolated thunderstorms near the Big Island and Maui this morning. Elsewhere mostly VFR conditions will prevail with showers focused along windward slopes of all islands. AIRMET Sierra is in effect for mountain obscuration across windward sections of the Big Island. These conditions are expected to continue this morning. This AIRMET may need to be expanded to include windward sections of Maui later this morning. AIRMET Tango remains in effect for moderate low level turbulence over and downwind of the terrain of all islands. The AIRMET is expected to remain in place through this afternoon. AIRMET ZULU is in effect for TEMPO light to moderate icing in clouds around the Big Island from 12000 to 20000 feet. This AIRMET may be canceled later this morning.

Marine

23 Jan 2018 13:59:00 GMT: Guidance remains in good agreement and depicts fresh to strong trade winds continuing through midweek as the ridge of high pressure strengthens. A combination of winds and seas will support small craft advisory conditions over most waters through Wednesday. The advisory has been extended for these waters. ECMWF and GFS both support a downward trend for the local winds through the second half of the week as a storm-force low passes north of Midway today, then tracks slowly northeastward to an area well north of Hawaii Wednesday through Friday. Light and variable (land/sea breeze conditions) winds can't be ruled out for the northwest islands as early as Thursday, then for the entire state by Friday, which could linger into the weekend. Largest model differences are shown Thursday, where the ECMWF depicts moderate to fresh trades holding and the GFS transitions quickly to more of a land/sea breeze setup. Differences are also noted over the upcoming weekend as the ECMWF keeps more of a light trade pattern in place (GFS holds with variable wind pattern). A blend of these solutions will be included in the package. The high surf advisory in place for east facing shores continues through 6 PM HST Tuesday. Observations and reports today reflected this with up to 8 ft faces. A combination of the fresh to strong onshore breezes associated with the trades locally and a background moderate easterly swell (10 sec) from a persistent large area of fresh to strong breezes upstream of the state will likely support advisory-level surf through Wednesday. For the second half of the week, guidance supports the solid easterly swell holding, despite the local winds trending down. The easterly swell will gradually trend down Friday through the weekend. For north and west facing shores, surf associated with the long- period, west-northwest (300-310 deg) swell that began filling in Sunday night will hold through at least the first half of the day before slowly trending down into midweek. The current advisory for north and west facing shores remains in place through 6 PM HST today. For southern shores, surf will remain up today due to an out-of- season, long-period, south (170-180 deg) swell that has filled in from recent activity from the southern Pacific. A new, long- period, background south-southwest (210-220 deg) Tasman swell, is forecast to fill in by Wednesday, then hold into Thursday, which should be enough to keep southern shores from going flat. A small west-northwest (300-310 deg) will be possible Thursday through Friday from the previously discussed gale- to storm-force low currently north-northwest of Midway near the date line. Although the bulk of the winds associated with this feature will focus energy away from the islands, the ECMWF and GFS support a window of time late today into Wednesday where a decent area of strong- to gale- force west-northwest winds are focused toward the islands within the 300-310 deg directional band. Provided its proximity to the state, shorter periods (13 sec) will accompany this small swell with small surf expected. A small northerly (000-010 deg) swell associated with a complex and broad gale-force low parked over the Gulf of Alaska can't be ruled out Thursday through the weekend. A recent ASCAT pass showed a decent sized area of strong- to gale-force northerly winds focused toward the islands earlier today (strongest northerlies near Aleutians). WAVEWATCH III is depicting 2 ft at 13 sec beginning Thursday, then becoming reinforced Friday into the weekend within the 12-14 sec band. Guidance typically handles the timing of this source well, but under predicts the peak. A small, long-period, west-northwest (300-310 deg) swell will also become a possibility late Friday through the weekend. The ECMWF and GFS both support a decent sized area of persistent westerly gales setting up off the coast of Japan tonight through Wednesday night. Small surf will result (largest on Kauai considering direction and shadowing impacts) over the weekend for north and west facing shores.

HFO Watches/Warnings/Advisories

23 Jan 2018 13:59:00 GMT: High Surf Advisory until 6 PM HST this evening for Niihau, Kauai Windward, Kauai Leeward, Waianae Coast, Oahu North Shore, Oahu Koolau, Olomana, Molokai, Maui Windward West, Maui Central Valley, Windward Haleakala, South Big Island, Big Island North and East. Small Craft Advisory until 6 PM HST Wednesday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters, Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Footer

23 Jan 2018 13:59:00 GMT: DISCUSSION...Jelsema AVIATION...Bohlin MARINE...Gibbs

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