280 FXHW60 PHFO 100213 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 413 PM HST Wed Oct 9 2019 .SYNOPSIS... Winds will strengthen and veer southeast on Thursday in response to a low forming northwest of the islands and building high pressure to the east. Deep tropical moisture moving onto the Big Island this afternoon will spread westward tonight and bring a chance of heavy rain and thunderstorms through Thursday night. Trade winds will build in Friday into next week and bring drier and more stable weather. && .DISCUSSION... A slow-moving cold front is located about 650 miles northwest of Kauai with a ridge axis about 250 miles north of Kauai. The parent high associated with the ridge is located far to the northeast is driving moderate winds, just south of due east this afternoon. Water vapor imagery shows an upper low about 1300 miles northwest of Hawaii digging southward. Current visible imagery over the state shows partly cloudy skies over the smaller islands. On the Big Island, an area of deep tropical moisture moved in from the east earlier this afternoon bringing moderate to occasionally heavy showers along with a few embedded thunderstorms. The upper low is expected to induce a surface low along the front tonight while high pressure east of the state begins to strengthen. As a result, winds will strengthen from the southeast tonight and spread the tropical moisture over the rest of the state through Thursday night. Current MIMIC layered precipitable water (PW) imagery shows values of 2.5 inches just east of the Big Island moving towards our area. Expect the rain to continue across the windward and southeast sections of the Big Island through the night. Models indicate a decently strong convergence band setting up over the state by Thursday (along the eastern flank of the earlier mentioned developing surface low). Thus, heavy rain and thunderstorms will be possible for the eastern end of the state tonight, and for most of the state Thursday and Thursday night. Southeast winds will focus rain over windward and southeast facing slopes, however, with winds running parallel to the island chain tomorrow (and thus weakening the winds) leeward/interior portions of the islands could also see rainfall increase during the afternoon tomorrow. Leeward plume showers could also anchor to any of the smaller islands. For now, we are not issuing a Flood Watch due to mitigating factors such as the upper ridge over Hawaii and with some of the high-resolution models (ARW and NAMNEST) indicating the rainfall deflected north and south of the island chain. We still may consider a watch on Thursday depending upon how features set up through the overnight. Improving conditions are expected on Friday and Friday night as the southeast winds gradually backs to the east, or trade winds, which will push the moisture band westward. Lingering heavy showers will remain over the western end of the state during the day Friday. The tropical airmass is expected to clear Kauai by Friday evening. Locally breezy trades will persist through the weekend and into next week with drier and more stable air tracking in. && .AVIATION... Expect light to moderate east southeast winds through this forecast period. The best chance for rain through the forecast period, with the exception of the Big Island, is windward mauka areas but it should be just scattered showers with passing MVFR conditions. The Big Island showers have already begun and it looks as if this activity will not end through this forecast period. Expect showers which could be heavy at times with a possible thunderstorm. The rest of the area should see a pretty nice day and VFR conditions except for a chance for interior island showers during the afternoon. As the tropical moisture moves in from the southeast to the northwest, each island should get a good chance for rain during this event. We will monitor and issue AIRMETs if necessary. There are no AIRMETS in effect or expected for this forecast period. && .MARINE... A high pressure ridge north of the state continues to strengthen, which will bring an increase in the winds over the coastal waters. As a result, a Small Craft Advisory (SCA) has been posted for the waters around the Big Island including the Alenuihaha Channel. Later tonight and into Thursday, a low pressure system will be developing northwest of the area in conjunction with a strengthening of the high pressure area far to the northeast. This will result in a veering of the surface winds to southeasterlies along with a tightening of the pressure gradient which will further increase the wind speeds. The higher wind speeds will primarily affect the windward waters off of the Big Island and Maui County, and to a lesser extent Oahu. This shift will be short-lived as the trades return on Friday and persist through the weekend. The resumption of trades should result in a shift in SCA areas to include more of the channels. In the short range, not much is going on with the swells and surf. The existing north swell is decreasing across the area, but a new, long period swell is expected to arrive from the north-northwest Thursday night. This swell will be small with surf well below the advisory level. A west swell, generated by Tropical Cyclone Hagibis in the western North Pacific, may reach exposed coasts early next week. Surf along east facing shores will increase during the second half of the week as east to southeast winds increase. Small south- southwest swells will keep the surf from going flat along south facing shores through the weekend. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 PM HST Friday for Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...Foster AVIATION...Chevalier MARINE...Kodama