PHFO issues Area Forecast Discussion (AFD) valid 2019-11-27T23:50:00Z 115 FXHW60 PHFO 272350 AAA AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 150 PM HST Wed Nov 27 2019 .UPDATE... The Flash Flood Watch (FFA) for Kauai county was cancelled a short while ago. While a moist and somewhat unstable air mass will linger through the night, potentially bringing some brief heavy downpours, the threat of widespread flooding rain has diminished. && .SYNOPSIS... A cold front stalling just west of the islands will keep moisture and rainfall chances elevated over Kauai and Oahu through tonight, with southerly winds keeping a muggy air mass in place. Somewhat drier, light to moderate east-southeast winds over Maui and the Big Island will spread to all islands Thursday and Friday as the stalled front weakens and moves away. High pressure passing north of the islands this weekend will support strong and gusty trade winds that will bring low clouds and passing windward showers, although showers may be especially active around Sunday. && .DISCUSSION... Water vapor imagery shows an extremely amplified flow pattern aloft across the central N Pacific, characterized by a potent shortwave trough several hundred miles NE of Midway, high pressure NNE of Hawaii, and another potent trough along the W coast of the U.S. mainland. This configuration is drawing a plume of deep moisture northward over the western end of the island chain, with PWAT over 2 inches, and surface dew points in the lower 70's making it feel muggy. As a result, overnight lows at Lihue and Honolulu were only 76F. This deep moisture, combined with low-level convergence associated with a stalling and weakening front about 75 miles NW of Kauai, is expected to keep rainfall chances elevated over Kauai and Niihau in the short term, and to a lesser extent, Oahu. A Flash Flood Watch (FFA) remains posted for Kauai county through tonight for this reason. Meanwhile, a stable E-SE flow over Maui county and the Big Island is supporting a few windward showers, mainly over the Big Island's Puna and Kau districts. A limiting factor in the development of heavy rain is the loss of upper support as the aforementioned shortwave continues to lift NNE away from the islands. Recent radar trends support this notion, with very little in the way of precipitation currently observed. In fact, less than an inch of rain has fallen over all but one rain gage on Kauai (Kalaheo received 1.21" overnight) over the past 24 hours. If the front doesn't draw closer and bring an increase in precipitation, then the FFA will likely be cancelled later today, despite guidance indicating that the frontal boundary will continue to linger around Kauai through tonight. The front is expected to gradually weaken to a trough on Thursday as it drifts W and away from Kauai county. Light to moderate ESE trade winds are expected to spread westward to all islands Thursday through Friday, supported by a weak high to the ENE, with these winds delivering just a few windward showers. A stronger high passing N of the islands this weekend will bring an increase in trade wind speeds, while also backing them to a more typical ENE direction. Moisture associated with portions of the front currently well N of the area is expected to become embedded within the strengthening trade wind flow, bringing a period of increased low clouds and showers. The arrival of a diffuse but showery cloud band is now later than indicated by previous model runs; now expected to move through around Sunday morning. After the passage of this weak "backdoor" cold front, slightly cooler air is expected to briefly filter in from the NE Sunday into Monday, with dew points potentially dipping in to the lower 60's. Latest guidance indicates this will be short-lived, as developing low pressure W of the islands turns the flow to the ESE once again Tuesday and Wednesday. && .AVIATION... A cold front approaching from the west will bring some additional showers to Kauai and Niihau today, with a few showers possible near Oahu as well. BKN-OVC mid level clouds may produce light rime icing in layer 140-FL200 over Kauai and Oahu. Brief MVFR conditions with tempo mountain obscurations are possible in showers, but are not expected to be widespread enough for AIRMET Sierra. The cold front will stall out near Kauai today, then begin retreating westward Thursday. East through southeast slopes of the Big Island could see enhanced clouds and showers in a east to southeasterly wind pattern today. Elsewhere mostly VFR conditions are forecast through Thursday. && .MARINE... A moderate sized northwest swell will be moving through the area today. This will cause low end advisory level surf along north and west facing shores of Kauai and Oahu today. This swell will then lower gradually Thursday and Friday. A larger northerly swell is expected to fill in late Saturday and Saturday night, peak Sunday and Sunday night, then lower gradually gradually Monday and Monday night. Short period surf will continue to affect east facing shores through Saturday with larger surf expected Saturday night through Monday due to the north swell wrapping into exposed shorelines. No other significant swells are expected. A Small Craft Advisory is currently posted mainly for seas of around 10 feet. This will likely be lowered later today. Trade winds are expected to increase Friday and on through the weekend. This along with the new northerly swell, will likely cause Small Craft Advisory conditions over a vast majority of the coastal waters. High astronomical tides over the next few days may allow for some localized coastal flooding during the morning high tide cycle. A Special Weather Statement is posted with more details. && .HFO WATCHES/WARNINGS/ADVISORIES... High Surf Advisory until 6 PM HST this evening for North and West facing shores of Niihau, Kauai and Oahu. Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters- Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-Alenuihaha Channel-Big Island Windward Waters. && $$ DISCUSSION...Birchard AVIATION...TS MARINE...Burke