Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
639 FXHW60 PHFO 020109 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 309 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... Relatively dry conditions persist as visible imagery depicts breezy and stable trades delivering limited showers to windward and mauka zones this afternoon. A persistence-based forecast then carries into Sunday with little meaningful change in the pattern for most of the island chain. Conditions will trend toward increasing mid-level instability Sunday through Tuesday as an upper trough positioned north and east of the islands sharpens and pivots through the area during the next couple of days. Global models indicate this feature will contain a thermal trough characterized by 500mb temperatures around -10 to -12C as it moves overhead during the early portion of the week. At the same time, a conditionally unstable airmass presently aligned near and south of 15N/140W will be advected toward the islands as mid-level flow veers to southeasterly. This will set the stage for ample mid- level instability featuring 700-500mb lapse rates easily in excess of 7C/km to reside over the state no earlier than Sunday afternoon. This may allow for some additional clouds and showers to develop over interior slopes of the Big Island during peak heating tomorrow, but otherwise any mid-level instability will be inaccessible to trade wind showers courtesy of the existing inversion. Furthermore, even as instability increases Monday and Tuesday, continued breezy trades will make inland convergence difficult to come by with the exception of the Big Island. Thus, the smaller islands may see the early week period manifest as nothing more than a few heavier showers embedded within the trades. For interior portions of the Big Island, isolated thunderstorms have been added to the forecast on Monday and Tuesday afternoons. The best opportunity for thunder will be Tuesday afternoon when the trough axis will be advancing through the forecast area. Leeward island plumes over the nearshore waters will also be candidates for nocturnal thunderstorm activity. The second half of the week will feature a developing mid-level ridge centered along 30N well west of the islands which, in tandem with the occasional mid-latitude trough encroaching on the forecast area, will cause the trade wind belt to contract and migrate south of the islands Thursday into the weekend. This in turn favors warmer conditions and potential for an emerging land and sea breeze pattern. 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 clouds and showers will favor windward and mauka areas with periods of MVFR conditions possible in heavier showers, especially during the overnight and early morning hours. VFR will prevail elsewhere. AIRMET remains in effect for moderate low-level turb S thru W of terrain, as well as for moderate turb between FL300-FL400. && .MARINE... A 1029 mb surface high centered about 1000 nm NE of the islands will change little as it sags slowly S through Sunday. The high will weaken slightly on Monday, but the local low-level pressure gradient will remain sufficiently tight to support fresh to locally strong ENE trade winds into Tuesday. A Small Craft Advisory (SCA) in effect for most zones through early Sunday will likely need to be extended for at least a few zones. On Wednesday, the high will weaken significantly, and the associated ridge will move over the islands. This will lead to light and variable winds through the end of the week as a front passes unseasonably close to the area. 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. NDBC buoys 51003 and 51004 S of the islands indicate increased energy in the longer 15-19 second period bands this afternoon, 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 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, 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...JVC AVIATION...TS MARINE...Birchard