Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
541 FXHW60 PHFO 011904 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 904 AM 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... Morning visible imagery shows clouds and limited shower activity arriving over windward and mauka sections of the smaller islands courtesy of stable, breezy trade winds. Further east, a band of moisture is being deflected south of the Big Island, but with windward slopes grazing its northern edge, showers have been comparatively persistent there this morning. Activity will tend to mix upslope as the day progresses followed by returning showers over downslope areas of the Big Island tonight as another round of moisture embedded in the trades arrives. Conditions 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 during the early portion of the week. In general, continued breezy trades will make inland convergence, and therefore convection, hard to come by. Thus, the smaller islands may see this period manifest as nothing more than a few heavier showers embedded within the trades. Interior Big Island, however, is a higher confidence target for a forecast adjustment toward the inclusion of thunderstorms Monday and Tuesday afternoons, particularly Tuesday when upper divergence and mid-level forcing will be maximized. Leeward island plumes over the nearshore waters will also be candidates for nocturnal thunderstorm activity. Confidence remains quite low elsewhere. 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 are expected to increase slightly this weekend, 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 Sierra remains in effect for mtn obsc for windward Big Island as well as N thru SE sections of Maui. Conditions have improved over windward sections of Kauai through Molokai, so the AIRMET has been canceled for those islands. Conditions are expected to improve for Maui and Big Island later this morning. 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 1030 mb surface high centered about 1000 nm NE of the islands will change little as it sags slowly S through the weekend. Although the high will weaken slightly on Monday, the local low- level pressure gradient will remain sufficiently tight to support fresh to strong ENE trade winds into Tuesday. A Small Craft Advisory (SCA) in effect for most zones will likely need to be extended. 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. There will be an extended run of elevated surf along S facing shores, with numerous swell pulses expected. 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 S hemisphere strom track has been ideal for S swell production for Hawaii for the last week or so. The onslaught of surf is now beginning, with NDBC buoys 51003 and 51004 S of the islands indicating increasing energy in the longer 17-19 second bands. While the larger sets of waves remain inconsistent due to the long period, wave heights are definitely on the increase, and will likely continue to do so today and tonight. 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. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Sunday for Kauai Northwest Waters-Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters- Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters- Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...JVC/M Ballard AVIATION...TS MARINE...Birchard