Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
818 FXHW60 PHFO 020617 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 817 PM HST Sat Jun 1 2024 .SYNOPSIS... Breezy trades continue through mid-week before weakening and potentially giving way to a land and sea breeze pattern. A passing upper trough may allow some showers to be locally heavy Sunday through Tuesday, especially over interior Big Island where there will be also be a chance for afternoon thunderstorms. && .DISCUSSION... Some changes to the forecast were made this evening to ensure more consistent sky and qpf parameters given the current PoP forecast throughout the forecast period. Not anticipating any significant updates with the morning package. High pressure remains far to the northeast of the islands, with a ridge north of 30N. The resultant pressure gradient over the islands is maintaining a breezy trade wind pattern. Satellite derived precipitable water shows a drier airmass has moved over the islands, even since the afternoon soundings. The most moisture remains near the Big Island, and to a lesser extent Maui. That is reflected in rainfall amounts over the last several hours with some windward locations having received over .5 inches in the last 6 hours. Anticipate showers to continue overnight, mainly over the windward slopes, of the Big Island and Maui. The remaining islands will have some clouds overhead, but the chance for showers will be significantly lower. A mid to upper level trough moving over the islands from the east overnight will lead to an increasing unstable airmass over the islands Sunday through Tuesday. The trough is expected to sharpen and pivot over the islands during this time, before lifting north and then east away from the region. The global models are in good agreement that this trough will bring colder temperatures to 500 mb. The ECMWF keeps temperatures around -10/-11C, while the GFS brings in even colder temperatures of -12/-13C. These temperatures are significantly colder than normal for June. At 700 mb there isn`t much different in the temperatures between these models with temperature getting to around 4/6C which is colder than normal but not as climatologically colder. Additional the coldest temperatures at 700 mb look to be focused near Kauai, while the colder 500 mb temperatures reach most of the islands. Thunderstorms were added with the afternoon package to the Big Island forecast for Monday and Tuesday afternoon, with the best chance being Tuesday afternoon. Leeward island plumes over the nearshore waters will also be candidates for nocturnal thunderstorm activity during this time, however confidence is lower so that is not currently reflected in the forecast. The second half of the week will see the a new mid-level ridge setting up far northwest of the islands, as a front north of the islands weakens to a trough. This will cause our winds to weaken and turn to the southeast. This could lead to some land and sea breezes over the islands. With a decrease in precipitable water and a more stable airmass overhead, expect a decrease in shower activity, with clouds and showers more likely during the afternoon and evening hours over interior and southeast facing portions of the islands. Today marks the start of hurricane season in the central North Pacific basin. It was 67 years ago today, in 1957, when the Weather Bureau Airport Station (which later became the National Weather Service Forecast Office) began Central Pacific Hurricane Center operations. Visit hurricanes.gov throughout the season, which continues through November 30, to stay informed. && .AVIATION... Breezy to strong trades will continue, as a surface high moves closer to the state from the northeast. Low cigs and SHRA will favor windward and mauka areas with periods of MVFR conds possible in any SHRA. VFR conds should prevail elsewhere. AIRMET Sierra is in effect for mtn obsc for windward Big Island and N thru SE sections of the smaller islands. AIRMET Tango remains in effect for moderate low- level turb S thru W of terrain, as well as for moderate turb between FL300-FL400. AMD NOT SKED for PMDY as equipment remains unavailable with no time table for return to service. && .MARINE... A 1029 mb surface high centered about 900 nm NE of the islands will stagnate through the remainder of this weekend. The high is expected to weaken slightly on Monday, but will still help to support fresh to locally strong ENE trade winds through the day Tuesday. A Small Craft Advisory (SCA) remains in effect for most zones through early Sunday and will likely need to be extended for the typically windier zones around Maui and the Big Island into early next week. On Wednesday, a frontal system approaching from the northwest will allow the high to weaken significantly, and lead to light and variable winds through the end of the week. An extended period of elevated surf along S facing shores is now underway, with numerous swell pulses expected, mainly from the SSW. A High Surf Advisory (HSA) is likely to be needed for the largest of the swells, which may not arrive until next weekend. However, there is potential for HSA-level surf before then as the storm track near New Zealand has been conducive for S swell production for Hawaii for the last week or so. Nearshore buoys S of the islands continue to indicate increased energy in the longer 15-17 second period bands this evening, equating to elevated surf that is below HSA levels. While the larger sets of waves remain inconsistent due to the long period, wave heights are definitely on the increase from yesterday. This swell likely peak early Sunday into Monday, then gradually ease Tuesday before additional long-period swells arrive. A strong, slow-moving low in the distant NW Pacific generated what will be a small NW swell when it arrives in the islands Monday and Tuesday (3 to 4 foot medium period swell from 320 direction), with the potential for a small follow- up NW swell later in the week. Trade winds will continue to generate short- period wind waves that will support choppy surf along exposed E facing shores the next couple of days. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Sunday for Kauai Northwest Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters-Maalaea Bay-Pailolo Channel- Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...M Ballard AVIATION...Walsh MARINE...Thomas