Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
698 FXHW60 PHFO 031926 CCA 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... Consolidating high pressure far north-northeast of the state will produce fresh to strong trade winds across Hawaiian waters for the next several days. These stronger trades may drop slightly early next week as the high begins to move farther to the northeast. An advanced scatterometer (ASCAT) pass last night showed Small Craft Advisory (SCA) level winds across most coastal zones already, and these will continue through the weekend. As a result, a SCA remains in effect for all Hawaiian waters through early Monday morning. Tiny surf will persist along north and west facing shores through much of today. Forerunners of a long period northwest swell are expected to fill in later 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 increase slightly on Saturday and hold into early next week as strong trades persist. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Monday for all Hawaiian waters- && $$ DISCUSSION...JVC AVIATION...Farris MARINE...Vaughan