Benign weather continues into Thursday. A Kona low then develops west of the islands bringing widespread rain and potential flash flooding Thursday evening through the weekend.
Elongated surface troughing extending from a low in the NW Pacific serves as the focus for weak surface low development around 170W west of Kauai this afternoon. This low is forming within a corridor of very subtle height falls and is characterized by a very light wind field. In advance of the low, deep layer subsidence prevails over the islands in maintenance of benign weather tonight into Thursday. Mid-level forcing steadily increases Thursday into Saturday allowing aforementioned low pressure strengthen and increasingly deep SW flow to deliver waves of moderate to heavy rainfall to the islands heading into the weekend.
Thursday night. Visible satellite indicates ample upstream moisture in the form of a high coverage of low clouds and a few thunderstorms well west of Kauai. This will be the initial moisture push into the forecast area late Thursday as forcing steadily increases and advances eastward. Soundings remain fairly saturated from moisture left in place by the last event and PWATs on this morning's Hilo sounding were in excess of 2". The initial slug of moisture also arrives coincident with a moderate increase in conditional instability in the column. These factors increase confidence in model depictions of deep convection with relatively high rain rates. Low-level convergence is sorely lacking during this time, but strengthening SW aloft will provide a background of widespread weak ascent on Thursday night suggesting disorganized but potentially widespread convection.
Friday. Mid-level dry air sweeps across the far western end of the state leaving the deepest moisture over Oahu through Maui, the eastern extent of the moisture band further displaced from evolving forcing aloft. For this reason, prefer solutions with the heavier rainfall on Friday over Maui and Oahu as opposed to the Big Island.
Friday night through Sunday. This period represents the peak of the event as forcing strengthens considerably during this period and multiple embedded shortwaves rotate around the parent trough and across the state. The rather weak area of upstream low pressure will begin to advance northeast and strengthen which will aid in the development of enhanced surface convergence and better potential for organized heavy rainfall somewhere in the central portion of the state Friday night into Saturday. Large scale forcing and moderate right entrance jet support will peak late Saturday into Sunday leading to the greatest potential for heavy rainfall. Rain rates may be particularly intense during this time. However, the upper wave digs sufficiently deep that it will usher the heavy rain band south of the Big Island by Sunday night. Thus, the peak rainfall potential will coincide with an increasingly progressive heavy rain band.
The initially weak nature of the low and subtle surface convergence features lend themselves to lower than normal forecast confidence. A Flood Watch has been issued for Oahu, Maui County, and the Big Island with the highest confidence over Oahu and Maui County at this time. Flash flood potential for Kauai is non-zero, but low confidence does not support Watch issuance there at this time. Note that saturated soils will favor increased flooding potential even though overall rainfall will not match the previous system. Finally, wind is of no concern this time around. Wind may peak in the 10-15kt range except with some higher gusts possible within deep convection. Given uncertainty surrounding this event, took a probabilistic approach to the rainfall forecast, focusing on 50th percentile values early and late and 75th percentile values during the event peak Friday night into early Sunday.
Trades return late Monday or Monday night. The lingering moisture band remains in place during this time and may provide a focus for enhanced trade wind showers over Windward Big Island during the early to middle portion of next week.
Light, land-sea breezes will continue through the evening, with clouds expected to build further as another system approaches the islands. This system will bring a more widespread threat of showers starting Thursday afternoon, while also orienting winds more southerly. Until then, expect mostly VFR conditions, outside of a stray shower during the evening and overnight hours for windward sites.
AIRMET Tango is in effect for moderate turbulence for FL 200-350, due to the oncoming trough. These conditions are expected to persist through tomorrow.
Light to locally moderate east to southeast winds will slowly strengthen to moderate speeds and veer south by Thursday as a ridge strengthens northeast of the state and a broad area of low pressure develops west of the state. By Friday, the low will strengthen over or near the far northwest offshore waters before lifting northeast on Saturday. Winds will shift out of the south southwest during this time at moderate speeds. Winds will become light and variable as the low shifts northeast of state with a trailing trough draped over the state. A strong high will build north of the state Sunday allowing moderate to locally strong northeast winds to return during the first half of next week.
Near shore buoys show the current moderate, medium period north swell has peaked and is trending down. Thus the High Surf Advisory has been cancelled for north facing shores of Niihau, Kauai, Oahu, Molokai and Maui. Surf along north facing shores will continue to gradually decline through Friday as the swell shifts out of the north and north- northeast. A small reinforcing pulse out of the north is expect Saturday. A new small to moderate long period west- northwest swell is expected to arrive late Saturday and hold through early next week.
Surf along south shores will see moderate surf just below High Surf Advisory (HSA) criteria as a long period south swell peaks tonight and holds through Thursday. The next small, long period south swell will fill in Saturday and peak Sunday, providing a small boost in surf along south facing shores before fading into early next week. Surf along east facing shores will remain well below average due to the lack of trades locally and upstream through the weekend. The exception will be for east facing shores exposed to north- northeast swells Thursday through the weekend.
Flood Watch from Thursday evening through Sunday afternoon for Waianae Coast-Oahu North Shore-Olomana-Central Oahu-Waianae Mountains-Lanai Mauka-Kahoolawe-Maui Windward West-Maui Leeward West-Haleakala Summit-Kona-Kohala-Big Island Interior-East Honolulu-Honolulu Metro-Ewa Plain-Koolau Windward-Koolau Leeward- Molokai-Lanai Windward-Lanai Leeward-Lanai South-Maui Central Valley North-Maui Central Valley South-Windward Haleakala- Kipahulu-South Maui/Upcountry-South Haleakala-Big Island South- Big Island Southeast-Big Island East-Big Island North.
Wind Advisory until 6 AM HST Thursday for Big Island Summits.
