659 FXHW60 PHFO 190141 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 341 PM HST Mon Oct 18 2021 .SYNOPSIS... A drier, more stable airmass is expected over the islands through mid week along with weakening trade winds. Trades are expected to strengthen again late in the week, along with an uptick in shower activity. && .DISCUSSION... Little change in the forecast philosophy this afternoon, and the forecast remains on track. The morning ASCAT pass showed 23 knots still in the Alenuihaha Channel. The models and our forecast expected winds to be on the decline a bit today, so feel this was to be expected. Additionally, the high clouds have largely left the islands as expected. The high pressure ridge north of the islands continues to edge closer as the front further north sinks southward. This will continue the gradual decline in trade winds through Wednesday. A new high building further north of the islands during the second half of the week will help trades to gradually increase towards the end of the week and into the weekend. The upper level low to the west that was responsible for ushering in the high clouds has, as expected, moved to the west. Expecting this motion to continue, and that upper level feature to no longer be an impact to our weather. Another upper low/trough developing north of the islands and sinking southward over the islands during the latter part of the week will likely help to boost shower activity, but how much remains to be seen. Satellite precipitable water immediately upstream of the islands supports the short term drying trend we expect. The GFS and ECMWF both bring an increase in precipitable water (PW) with this upper level feature, but the GFS is wetter than the ECMWF. The forecast does reflect an uptick in showers during the second half of the week, but will have to watch future runs to determine how much of an impact this will have on our local weather. && .AVIATION... As a surface ridge located north of the state moves southward and weakens through Wednesday, trade winds will gradually weaken as well. For now, AIRMET Tango remains in effect for moderate low- level turbulence over and immediately south through west of the mountains of all islands. However, this AIRMET may be cancelled later this afternoon or evening given the waning surface winds. By tomorrow through mid-week, localized land/sea breezes are possible with as background flow becomes rather weak. Mid-afternoon satellite and radar imagery show relatively few areas of low clouds and showers across the state, with isolated showers and low clouds favoring windward areas and their adjacent waters. Cloud coverage has also increased across the Kona slopes of the Big Island, though ceilings remain in the VFR category. Although isolated MVFR conditions are possible throughout the period in any heavier showers that do develop, VFR conditions are expected to prevail state-wide. && .MARINE... Trade winds are already beginning to diminish, with only a few 20 to 25-knot wind barbs left in the Alenuihaha channel with the morning ASCAT pass. Given the decreasing trend indicated by the hi-res models, the Small Craft Advisory (SCA) has been dropped for all remaining waters. The moderate trades will gradually trend down over the next few days, due to a series of fronts from the far northeast Pacific weakening the ridge north of the islands. These fronts will reach the northern offshore waters Wednesday and Thursday, though both are expected to fall apart before reaching the Hawaiian Islands. High pressure reestablishes north of the islands Thursday through Friday, bringing a return of locally strong trades, favoring the typically windy waters around Maui County and waters south of the Big Island. As the trades ease, the surf along the east facing shores will lower a little each day as well. The boost in the trades on Friday will lead to increasing surf along the east facing shores over the upcoming weekend. A series of north-northwest to north swells are expected to move through the coastal waters this week. A small pulse that arrived earlier today will be reinforced by a slightly larger north- northwest swell Tuesday night through Wednesday. An even larger north swell follows Thursday night and Friday, with surf likely to reach advisory level for north facing shores, before slowly lowering over the weekend. In addition, Kahului Harbor and other north facing harbors will be susceptible to harbor surges from this north swell. Small surf along the south facing shores will get a bit of a boost beginning early Tuesday as a small, but long (14 to 16 second period) south swell due in tonight. This swell will linger through Wednesday before tapering off through the end of the week. A similar swell could arrive over the upcoming weekend. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 PM HST this evening for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...M Ballard AVIATION...Vaughan MARINE...TS