Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
172 FXHW60 PHFO 312000 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 1000 AM HST Fri May 31 2024 .SYNOPSIS... Surface high pressure positioned far northeast of the state will maintain a tight enough pressure gradient across the region to support breezy to locally windy trade winds into early next week. Showers moving in on the trades will favor windward areas and likely become more prevalent during the nocturnal hours. An upper trough developing over the islands the next few days will be the impetus to increased shower frequency and areal coverage. && .DISCUSSION... The synoptic set up across the Central Pacific has changed very little these past few days. This being of a near 1030 mb high centered approximately 1,100 miles from Oahu with a pesky surface trough about 900 miles west of Kauai. These two features have created a tight enough pressure gradient between them to hold primarily breezy regional trade winds in place. Dry 12Z local soundings are evidence of a very dry air mass where precipitable water values are registering in the lower 10% percentile range for late May. Enhanced stability is also noted in both satellite, radar and soundings as the 5-6k foot subsidence inversion sustains a shallow somewhat dry boundary layer. The surface high will only subtly settle southward as the trough washes out through the day. This will slightly tighten the gradient over and upstream of the islands enough to provide a minor uptick in weekend trade wind magnitudes. The only other near term mentionable change will occur with increasing mid level moisture this weekend that will likely lead to more frequent windward showers that will expand across a larger area. Heights will lower over the state into the weekend. Extended NWP guidance is still a little out of phase with the location of the lowest heights or mid-upper level troughing over the islands. The emergence of the subtropical jet over or just south of the state this weekend may offer more argument to a wetter commencement of June, but this is very dependent upon its positioning. While there will be some eastern moisture advection, the mid levels of the atmosphere will remain fairly dry until Tuesday and this may weigh forecast philosophy toward more dry. Regardless of any minor model nuances, there continues to be moderate confidence that next week`s upper troughing scenario will increase areal shower coverage and intensity...especially going into the middle of next week due to increased lower and mid layer moisture. Trades may become disrupted late next week in response to the passage of a surface trough/front to the north. && .AVIATION... Breezy to locally strong easterly trade winds are expected to increase slightly this weekend, as a surface high moves closer to the state from the northeast. Clouds and showers will continue to favor windward slopes with periodic MVFR conditions possible, especially during the overnight to early morning hours when an uptick in shower activity is expected. Otherwise and elsewhere, VFR conditions will prevail. Winds of 20 to 30 kts are expected to persist below an inversion between 5000 and 7000 feet. These conditions justify continuance of AIRMET Tango for tempo moderate low-level turbulence south through west of terrain. In addition, strong jet stream winds passing over Hawaii today through Friday warrant AIRMET Tango for tempo moderate turbulence aloft between FL300-FL400. AIRMET Sierra for mountain obscuration is cancelled for all areas, as shower activity has decreased and ceilings have improved. && .MARINE... A 1030 mb surface high centered 1100 nm NE of the islands will change little as it sags slowly S over the next couple of days, resulting in a slight increase in ENE trade wind speeds. Fresh to strong trade winds will prevail into early next week, and an existing Small Craft Advisory (SCA) for the windier zones around Maui and the Big Island is slated to expand to most zones tonight. Long-range guidance indicates a trend toward diminishing and veering winds around the middle of next week as the ridge to the N is weakened by a passing front. There will be an extended run of elevated surf along S facing shores over the next week (and likely longer), with the largest of the swells likely to produce surf near High Surf Advisory (HSA) levels of 10 feet. The first of several pulses of long- period SSW swell will gradually build today, and peak near HSA heights over the weekend. Latest buoy observations indicate modest energy increases in the longer periods, resulting in inconsistent and small sets. Additional pulses of swell arriving early next week will persist through the middle of the week, with the potential for a larger swell by the following weekend. A strong, slow-moving low in the distant NW Pacific has generated a small NW swell that will arrive Monday and Tuesday, with the potential for a small follow-up NW swell generated by the extra- tropical version of former west Pacific Typhoon Ewiniar. Trade winds will continue to generate short-period wind waves that will support choppy surf along exposed E facing shores. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory from 6 PM this evening to 6 PM HST Saturday for Kauai Northwest Waters-Kauai Leeward Waters-Kauai Channel- Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters. Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...Blood AVIATION...TS MARINE...Birchard