Strengthening low pressure roughly 750 miles north of Kauai will produce increased south winds through Thursday and will draw deep tropical moisture over at least the western half of the island chain. The threat of heavy flooding rainfall and thunderstorms will be greatest tonight and Thursday over Kauai and Oahu, where a Flood Watch is in effect. Chances for heavy rain will diminish on Friday as the low weakens several hundred miles northwest of Kauai. The low will slowly fill Saturday and Sunday, though a few heavy showers may linger through the weekend under southeast to east winds.
The atmosphere is destabilizing this afternoon as deepening low pressure north of the state produces increased southerly winds. The main surface low is forming along a front roughly 750 miles north of Kauai, leading to light to moderate south to southeast winds and increasing humidity with dew points surging into the lower 70s over the islands this afternoon. Meanwhile, a sharp upper level trough is digging toward Kauai from the west, and a short wave rounding this upper level trough is rapidly destabilizing the atmosphere, especially over Kauai and Oahu. The southerly flow is also tapping moisture surrounding a tropical disturbance a couple hundred miles south of Kauai and drawing it northward. As of this writing, only scant rainfall has occurred over land today, but the leading edge of the heavy showers is pushing into the coastal waters south of Kauai and Oahu. Expect these showers to reach Kauai, and possibly Oahu, before sunset.
Heavy showers will develop across the western half of the island chain this evening and persist into Thursday, bringing a flood threat focused from Kauai to Oahu where a Flood Watch is in place. The aforementioned upper level shortwave will swing over Kauai tonight through Thursday afternoon as a jet stream entrance region along the eastern flank of the low provides moist ascent over the western half of the island chain. This will coincide with deep moisture, having precipitable water in excess of two inches, moving over the islands from the south. As a result, expect heavy showers and thunderstorms to move over Kauai and Oahu from the south this evening and maintain a flood threat through at least Thursday morning. The main concern in this flow would be heavy training showers over leeward areas that have a lower threshold for flood impacts, though the entirety of Kauai and Oahu are at risk of flooding rain. Maui County will be on the outer edge of the zone of concern for heavy showers, and even though it is not included in the Flood Watch, there is some chance of impacts, especially on Molokai and Lanai. The flood threat diminishes significantly on Big Island. Southeast slopes of the Puna and Kau Districts could experience a brief period of heavy rainfall, and spotty showers will be possible elsewhere on the island, mainly in the afternoon. While gusty downsloping winds to the north of terrain are typically a concern in this setup, the flow does not appear to be strong enough for gusts to reach the advisory level of 50 mph.
The low will stall and begin to gradually weaken roughly 800 miles northwest of Kauai on Friday. The jet stream on its eastern flank will retreat northwest of the islands. This will reduce upper level support for widespread heavy rainfall. However, light southeast winds will persist and rich but shallow moisture will remain pooled over the islands. Under these humid conditions with dew points hovering in the low to mid 70s, isolated pockets of heavy showers could still flare up, especially along terrain. At this time, an extension of the Flood Watch is not planned, though it is possible.
Guidance suggests light east to east-southeast will gradually push the high precipitable water air mass westward during the weekend as the low fills northwest of Kauai. Chances of heavy rainfall will diminish, though some isolated nuisance flooding will still be possible near terrain. In addition, this should bring some relief from the humidity as dew points slowly lower.
Guidance divergences somewhat early next week. The GFS clears out the low and ushers in a moderate trade wind flow, but the ECMWF maintains some lingering remnants of the low north of the state and keeps the trade wind flow very light.
Light southeast winds will increase in speed and become more southerly through Thursday due to a surface trough just west of Kauai County. Increasing SHRA and isolated TSTMs are forecast tonight and Thursday as an area of enhanced moisture and instability moves through. IFR/MVFR conds likely within heavier SHRA and TSTMs. VFR prevails elsewhere.
No AIRMETs are in effect.
Gentle to moderate south to southeast winds will continue over the next couple of days as a low pressure system develops north of Hawaii. Enhanced showers with the potential for a few thunderstorms are forecast for the western half of the state through Thursday. A surface trough will linger near the area through the weekend and early next week keeping potential for variable winds and unsettled weather.
South swell will maintain into next week with pulses of long period reinforcement arriving Thursday and Saturday. Surf will stay below advisory levels along south facing shores.
Small to moderate surf along north and west facing shores will continue through the rest of the week as swell energy is reinforced by the slowly moving surface low northwest of the Hawaiian Islands. A larger swell will arrive around Tuesday, bringing north and west shore surf to just below advisory thresholds. Surf along east facing shores will remain small as local winds remain fairly weak.
Flood Watch through Thursday afternoon for Niihau-Kauai Leeward- Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central Oahu-Waianae Mountains-Kauai North-Kauai East-Kauai South-East Honolulu-Honolulu Metro-Ewa Plain-Koolau Windward-Koolau Leeward.
Wind Advisory until 4 PM HST Thursday for Big Island Summits.
