Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
993 FXHW60 PHFO 031859 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 859 AM HST Fri May 3 2024 .SYNOPSIS... Breezy trades will become slightly more unstable with pockets of heavier showers possible through the weekend, mainly windward and mauka. A drier moderate trade wind pattern then becomes established by the middle of next week. && .DISCUSSION... Regional satellite shows a closed upper low positioned just SE of 170N/30N. Modest convection, including a few thunderstorms, are noted within its core and beneath the cold pool aloft (-15C at 500mb) several hundred miles NW of the Hawaiian Islands. A shortwave trough wrapping around the southeast periphery of this low is currently enhancing trade wind showers where local moisture is the deepest, mainly over Windward Big Island and Maui. Otherwise, breezy trade winds have settled in and will take the edge off of afternoon heat compared the last several days as showers focus over windward and mauka zones in routine fashion. Trades will then acquire a decreasingly stable character as the aforementioned upper low opens up and shears east along the northern periphery of the subtropical jet. The inversion is modeled to weaken and lift by a few thousand feet which will increase the depth of trade wind showers as they advance across the area. This will facilitate slightly greater rain rates and increase leeward penetration of showers. Waves of forcing rotating around the low will be responsible for periodically increasing shower coverage Saturday night through early next week. During this time, the cold pool aloft will move directly overhead and is modeled to moderate to around -11C at 500mb. Lapse rates will still be adequate for pockets of deep convection to develop Sunday night through Monday night, mainly during the overnight periods. Isolated thunderstorms cannot be ruled out during this period, but confidence at this stage is too low to warrant explicit inclusion in the forecast. Meanwhile, a separate thunderstorm threat may manifest over the Big Island where a plume of steep lapse rates on the order of 7C/km is modeled to extend northeastward over the eastern end of the state. This will contribute to an increased probability of pockets of heavy showers and/or isolated thunderstorms Sunday and Monday afternoon, particularly as the upper jet provides maximum venting aloft on Sunday followed by the upper wave itself pivoting through on Monday. Confidence at this stage once again falls short of inclusion in the forecast but will be given consideration in future forecast cycles. Pressure falls over a broad swath of the Central Pacific will then ease the gradient leading to a drier moderate trade wind pattern by the middle of next week. && .AVIATION... Breezy easterly trades will continue today as high pressure builds in from the NE and pushes the surface trough NW of Kauai further away from the state. Moisture riding in on the trades will focus clouds and showers over windward and mauka areas, with the greatest shower activity during the late night and early morning hours. A few showers may spill over to leeward areas at times. VFR conditions will generally prevail, but MVFR conditions will be possible in any heavier showers that develop. AIRMET Sierra is in effect for tempo mountain obscuration for windward and mauka portions of the island chain. This AIRMET will continue for a few more hours this morning and may be needed again late tonight. AIRMET Tango is in effect for mod turb below 080 downwind of island terrain. Conditions are borderline, but should continue through the day. AMD NOT SKED in for Midway due to missing wind speed and direction && .MARINE... Latest ASCAT shows fresh to locally strong winds have arrived as a high pressure builds far north of the state. These stronger trades will continue through the weekend and drop a notch or two early next week as the high moves far northeast of the state. The Small Craft Advisory (SCA) for typical windy waters around Maui and the Big Island has been expanded to all waters through early Monday morning. Some area may drop out earlier as winds begin to ease due to the high shifting northeast. Tiny surf will persist along north and west facing shores through much of today. Forerunners of a long period northwest swell is expected to fill in this afternoon and peak Saturday, bringing small surf along north and west facing shores. Another small, long period northwest swell will fill in late Sunday, maintaining small surf along north and west facing shores into early next week. Small background south swells will continue into Saturday, providing tiny surf for south facing shores. Forerunners of a small, long period south swell may arrive Saturday afternoon with a slow onset through Sunday providing a boost in surf along south facing shores into early next week. Rough and choppy surf along east facing shores will bump up a notch Saturday and hold hold into early next week as strong trades hold. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Monday for all Hawaiian waters- && $$ DISCUSSION...JVC AVIATION...Farris MARINE...Vaughan