007 FXHW60 PHFO 210157 CCA AFDHFO Area Forecast Discussion...CORRECTED National Weather Service Honolulu HI 357 PM HST Sun Sep 20 2020 .SYNOPSIS... A bit of an increase in shower activity is expected through tonight, mostly over windward areas, and mostly overnight and into the early morning hours. Drier air is expected Monday through midweek, which will limit shower coverage. Moderate trade winds are more likely to be locally breezy Wednesday and Thursday, with a bit of a downward trend next weekend. && .DISCUSSION... Have made some tweaks to the PoPs for tonight and tomorrow mostly near Kauai, since showers have persisted over the western end of the island chain today. Have also adjusted QPF values through the week to better reflect a blend of the models. For the short term, that involved a boost to Kauai QPF to better reflect the rainfall totals we have been seeing today. Little change to the winds. The morning ASCAT pass showed winds over the waters near the islands at 20 kt or lower, but winds did pick up a little this afternoon with the diurnal cycle. High pressure to the north of the islands will continue to bring moderate trades to the region. The high is expected to be disrupted by a front passing north of the islands over the next day or two, with a new high settling in behind the front. The new high will help to boost winds a little during the second half of the week, but then look for winds to weak as we head into the weekend. The front will introduce additional moisture upstream of the islands which will likely be carried in on the trades during the second half of the week. Water vapor continues to show the upper level low, now about 475 miles to the west-southwest. This feature has been moving to the east, and we have seen the high clouds over Kauai and Oahu this morning shift more towards Kauai. Additional high clouds are being pulled up over the southern end of the island chain. The global models remain in good agreement with an upper level trough/low to the northwest of the islands for the next 24 hours or so. They are also in good agreement with the development of a closed upper low north-northwest of the islands along the upper trough Tuesday night. The GFS and ECWMF keep the low/trough further to the northwest during the second half of the week, but bring it towards the islands as we head into the weekend. That is when the models diverge in their handling of the upper low, so will have to keep an eye on that over the next few days to see how the models handle it. Not expecting any significant weather from this upper low, but it could help to increase shower activity depending on how close it comes to the islands. && .AVIATION... Sea breezes have produced broken to overcast low clouds and a few showers over the southwest section of the Big Island of Hawaii. These clouds and showers will persist through this evening. A ridge far north of the area will maintain moderate to locally strong east winds over the area. These winds will push some showers over east sections of the islands. Isolated MVFR ceilings and visibilities are likely but the conditions are not expected to be widespread enough to warrant an AIRMET. && .MARINE... Moderate to locally strong trades will hold through tonight, then slightly weaken to fresh speeds as the high to our northeast weakens. Another high will build north of the state towards the middle of the week and will maintain locally fresh trades through around Thursday. Trade winds may decrease Friday into the weekend as the ridge of high pressure shifts slightly closer to the state. A Small Craft Advisory remains in effect through late tonight for the typically windy areas around Maui County and the Big Island. Winds will continue to remain near borderline advisory levels throughout the week for these windier areas. A small 3 foot north swell filled in during the day today and will likely hold through Monday morning then steadily decline into Tuesday. A larger 4 to 6 feet long-period north-northwest swell will begin to fill in on Wednesday and peak Wednesday night into Thursday for north and west facing exposures. Surf along east facing shores will remain small through much of the week, but northeast facing exposures should see some moderate surf from the north-northwest swell during the second half of the week. The current south-southwest swell looks to have peaked earlier today and will be on a slow decline over the next few days. A new smaller southwest swell should begin to fill in on Monday and will help keep surf heights near the summer average (3 to 5 feet) during the first half of this week. Another small south-southwest swell is possible Friday into Saturday. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Monday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...M Ballard AVIATION...Donaldson MARINE...Kino