155 FXHW60 PHFO 160635 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 835 PM HST Sun Mar 15 2020 .SYNOPSIS... A weak surface trough near Oahu this evening will continue to drift slowly westward as deeper tropical moisture is pulled northward over the state. Numerous showers, heavy at times, currently over Maui County and Oahu are moving northward with the deeper moisture into Kauai County this evening. Expect increasing rainfall trends across the state with moderate southeast to south winds over the next 24 hours as a kona low sets up just west of Kauai on Monday. All islands remain under an elevated threat for flash flooding through Tuesday. The latest guidance shows the kona low lingering west of the islands through Wednesday before weakening and drifting northward from Thursday and Friday. Periods of unsettled weather remain in the forecast through the end of this week. Easterly trade winds with drying trends will fill in across the region from Friday to Saturday as the low drifts further north away from the state. && .DISCUSSION... A weak low level trough near Oahu continues to drift westward as deep tropical moisture is drawn up from the south over all islands. Numerous showers over Maui County and Oahu are currently moving up from the south into Kauai County. Local radar also shows convergent plumes of moderate to heavy showers developing downstream or northwest of each island. On satellite imagery, a low pressure system roughly 750 miles west-northwest of Kauai will move into the Hawaii region and become a stationary kona low just 400 miles west of Kauai from Monday through Wednesday. Moderate to breezy southerly winds along the eastern flank of this kona storm will pull up deep tropical moisture levels across the state with Precipitable Water levels in the 1.5 to 2.0 inch range. At these higher moisture levels heavy convective showers can start with only a little dynamic forcing. A trough wrapping around this kona storm will produce a large band of heavy showers and thunderstorms. This large shower band will move from west to east across the islands from Monday into Tuesday. The forecast across the state keeps humid conditions in the forecast over the next several days in a south to southeast wind flow pattern. Expect active heavy rain showers, snowfall on the Big Island summits, thunderstorms, and elevated threats for flash flooding. The highest flooding threat will occur under this heavy rain band. A Flash Flood Watch remains in effect statewide through Tuesday afternoon. Predicting the movement of both the kona low and the large heavy shower band are always challenging as global models typically struggle with these types of low pressure systems. This model uncertainty will mainly affect the timing of weather impacts for each island. The overall threat for heavy rain and flash flooding remains elevated statewide with a high degree of confidence, especially over the western half of the state. That said, the latest American (GFS) and European (ECMWF) model runs have slowed the slow departure of this kona storm from Tuesday night into Wednesday. This change in movement may spread heavy showers and elevated flooding threats into the Wednesday forecast period. Stay tuned. Unsettled weather conditions are forecast linger into the second half of next week as the kona low weakens and slowly drifts northward away from the main Hawaiian Islands. Long range guidance hints at moderate easterly trade winds filling in over the region as the kona low departs with a return to a drier trade wind weather pattern from Friday into Saturday. Please continue to plan for heavy rain and thunderstorms developing through Tuesday and check for updates to the local island weather forecast over the next several days as the forecast impacts will likely evolve over time. && .AVIATION... A low approaching the state from the west will bring increasing clouds and showers to the islands tonight. The western smaller islands, mainly Kauai and Oahu, is expected to see the most widespread activity. Instability aloft will also bring the threat for embedded thunderstorms. MVFR conditions and brief IFR conditions will be possible with any passing storms due to lowered ceilings and decreased visibility. Southerly or south southeasterly flow is forecast to increase tonight and become breezy by Monday morning. AIRMET SIERRA remains in effect for Kauai, Oahu, Molokai, and Lanai above 1500 feet due to ceilings and lowered visibility in showers. Widespread mountain obscuration is likely later tonight as ceilings lower and the shower activity becomes more intense and extensive. AIRMET TANGO is in effect for moderate turbulence aloft between FL180 to FL300. The potential for elevated turbulence is expected to diminish through the night. && .MARINE... As the surface trough shifts westward and a kona low evolves to the west overnight into Monday, southeast winds will quickly ramp up into the fresh to strong category over the Hawaiian waters. Guidance depicts this pattern holding through the first half of the week, then trending down through the second half as the system lifts northward and the gradient relaxes. Seas will respond and become rough (Small Craft Advisory conditions) over waters exposed to southeast winds early this week. In addition to the winds and seas, widespread heavy rainfall and thunderstorms are expected, especially over the western end of the state. Surf along north and west facing shores will continue to lower through the first half of the week as the medium-period, north- northwest swell eases. Guidance depicts a rapidly intensifying low emerging off of Japan overnight into Monday, potentially reaching storm-force category late Monday through Tuesday as it quickly lifts north-northeastward toward the western Aleutians. Storm-force westerly winds driving seas into the 35 to 45 ft range focused at the local area out of the 300 to 320 degree range (2000 to 2500 nm away from Hawaii) are forecast through this period. WAVEWATCH III and ECMWF-Wave reflect this and depict a long-period, northwest swell arriving Friday and holding into the weekend. Although confidence remains low given the quick north-northeast track, heights could near advisory levels for exposed north and west facing shores at its peak late Friday into Saturday. Surf along south facing shores will trend up and become rough through the first half of the week due to the aforementioned strong south-southeast winds expected. Heights may near the south shore advisory level of 8 ft by Tuesday for Kauai. Similar conditions are possible on Oahu through this time. Otherwise, mainly small background south swells are anticipated through the second half of the week. For the long-term, although confidence remains low, guidance does show a 5-7 ft at 14-15 second south-southwest swell impacting Pago Pago, Samoa Thursday from Tropical Cyclone Gretel passing far to their south-southwest. If this materializes, a small, long-period south-southwest swell will become a possibility for Hawaii by next Monday (around the 23rd). Surf along east facing shores exposed to southeast winds will trend up and become rough through the first half of the week. This will especially be the case for Kauai, where heights may near the advisory level of 8 ft by Tuesday. Surf may trend up through the second half of the week for all east facing shores as moderate to fresh trades fill in. && .HFO WATCHES/WARNINGS/ADVISORIES... Flash Flood Watch through Tuesday afternoon for all Hawaiian Islands. Small Craft Advisory from midnight tonight to 6 AM HST Tuesday for Big Island Windward Waters-Big Island Leeward Waters. Small Craft Advisory from 6 AM Monday to 6 AM HST Tuesday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Maui County Windward Waters-Alenuihaha Channel. && $$ DISCUSSION...Bohlin AVIATION...JT MARINE...Gibbs