Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
581 FXHW60 PHFO 010157 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 357 PM HST Fri May 31 2024 .SYNOPSIS... Surface high pressure positioned far northeast of Hawaii will maintain a tight pressure gradient across Central Pacific. This scenario will generally support breezy to locally windy trade winds through early next week. Showers moving in on trade flow will favor windward areas and likely become more prevalent during the nocturnal hours. An upper trough developing over the islands this weekend will be the impetus to increased shower frequency and areal rain 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 1,000 miles from Oahu with a pesky surface trough about 1,100 miles west of Kauai. The tight pressure gradient between these two features will hold primarily breezy regional trade winds in place over the next several days. The surface high will gradually settle south as the trough continues to weaken and wash out by early Saturday morning. The high`s subtle shift to the south will slightly tighten the pressure gradient over and upstream of the islands enough to provide a minor uptick in weekend trade wind magnitudes. Recent local soundings have been evidence to the resident dry air mass. This morning`s precipitable water values registered in the lower 10% percentile range for late May climatology. Enhanced stability is also noted in satellite, radar and afternoon soundings as a 6 to 8k ft foot subsidence inversion pins down a relatively dry boundary layer. The only other near term mentionable change will occur with increasing mid level moisture this weekend. This will likely lead to more frequent windward showers that will expand across a larger area. Better organized windward showers will hold on and occasional spill over into leeward locales. Heights will lower over the state into the weekend. The extended NWP guidance is still in disagreement on how to handle these height falls...whether it be from broad trough passage north of the state (EC) or take the form of a weak low settling in over the islands (GFS). 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 latitudinal position. 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 the increased lower and mid layer moisture. Trades may become disrupted late next week in response to the passage of a surface trough to the north. This will place the islands in more of a east-southeast flow pattern. If this does occur, the end of week weather pattern will transition to weaker winds with higher humidity. && .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 may be needed for mountain obscuration later tonight as shower activity picks up. && .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 has been expanded to include most zones. 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 tonight, and peak near HSA heights over the weekend. Latest buoy observations indicate continued modest energy increases in the longer periods, resulting in inconsistent and small sets at the moment. 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 until 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