Heavy rain will develop over the western end of the state tonight through Thursday potentially resulting in flash flooding. Light to moderate trades return for the weekend.
Stable and light ESE flow prevails beneath an inversion averaging about 7.5kft over the islands at press time this morning. Winds will continue veering through today becoming SSE by this evening. This is occurring in response to an approaching mid-level trough presently approaching Kauai from the NW. At this time, a frontal wave is developing in advance of this trough while frequent thunderstorms are noted in the base of the trough where 700-500mb lapse rates are firmly in excess of 7C/KM and 500mb are around -14C. As today progresses, cyclogenesis will focus near 30N/160W, or about 550 miles due north of Kauai as additional energy focuses in the base of the trough. In the near term, emerging sea breezes against a background of light southerlies will tend to focus afternoon clouds and showers over northern portions of the islands where scattered coverage is anticipated during peak heating.
By tonight, attention turns toward the northward progress of the tropical disturbance near 15N/162W, or about 600 miles SSW of Kauai. This disturbance is embedded within a broader tropical airmass characterized by surface dewpoints well into the 70s, PWAT values in excess of 2", and is surrounded by shower activity. Pockets of thunderstorms are also noted near its core where conditional instability is is apparently sufficient for moderately intense updrafts in spite of warm 500mb temperatures around -4C and sub 6C/km lapse rates. Southerly flow in advance of the developing extratropical cyclone north of the area will draw this feature north and shear it across the islands tonight into Thursday. The presence of thunderstorms at this time increases confidence in their inclusion in the official forecast seeing as how instability will only increase with northward extent as 500mb temps over Kauai & Oahu will turn at least 2-4C colder than what the system is currently experiencing. In addition, fact that convection is already fairly widespread south of the islands suggests the incoming atmosphere will be primed for shower and isolated thunderstorm activity in spite of the resident hostile stable/dry environment which should in turn be displaced rather easily.
Hi-res guidance suggests showers, some heavy, will become increasingly widespread between sunset and midnight tonight. At the same time, the digging trough will result in outstanding right entrance jet dynamics parked over Kauai late tonight through much of Thursday. At the least, this will favor steady, broad ascent from Maui County through Kauai in maintenance of increasing shower coverage as moisture steadily deepens. However, while differing on exact location, the EC and GFS both allow a band of particularly intense rainfall to develop along this developing jet-front feature, though the front will be washed out at the surface subsequently limiting low-level convergence. Nonetheless, any corridor of enhanced confluent flow around Kauai or Oahu should be more than sufficient to trigger banding of deep convection, heavy rain, and isolated thunderstorms. The heaviest rain is anticipated from around sunrise Thursday through Thursday evening focused mainly over Kauai and Oahu. At this time, it appears that once the increasingly strung out tropical disturbance, or remnant thereof, advances through the area, shower coverage should decrease considerably Thursday night into Friday. With that said, right entrance dynamics will persist over the western end of the state through at least Friday as the upper trough closes off and stays nearly stationary NW of Kauai. This should sufficient for showers to capitalize on rich low-level moisture and produce pockets of heavy showers Thursday night into early Friday night, especially over leeward zones.
A Flood Watch has been issued for tonight through Thursday for Kauai and Oahu. Given the potential for flow-parallel bands of convection to develop, there is concern for training of heavy showers. Rain rates could also be intense at times given the combination of forcing and deep tropical moisture. Subsequent shifts will evaluate the potential for an expansion to Maui County pending additional data.
Mid-level ridging becomes increasingly prominent beginning Friday night with light to moderate trades and increasing stability making a push into the area for the weekend. Should trades remain sufficiently light, then afternoon sea breezes will still have plenty of moisture to work with for afternoon showers over island interiors this weekend potentially leading to pockets of nuisance flooding during peak heating.
Light southerly winds expected through the forecast period due to a nearby surface trough. Increasing SHRA and a few isol TSTMs are possible through Thursday as the surface trough gets closer. IFR/MVFR conds likely within heavier SHRA or TSTMs. VFR prevails elsewhere.
No AIRMETs are in effect.
A low pressure system will drop into the Hawaii region from the north and become cut off from the westerly steering flow and linger northwest of Kauai through Thursday. Southerly to southeasterly surface winds will draw up tropical moisture over the islands under the cold upper low, producing periods enhanced showers, heavy at times, with the potential for a few thunderstorms near Niihau, Kauai and Oahu during this time. A passing low level trough will then drift from east to west through the islands from Sunday into the first half of next week, potentially extending these unstable wet weather trends.
Long period south swell energy will begin to fade over the next few days with overlapping tiny long period swell energy keeping surf heights elevated along south facing shores through the week. Another small long period south swell will fill in this weekend, bumping surf back up to just a few feet below advisory levels.
Small to moderate surf along north and west facing shores remains in the forecast through the rest of the week as swell energy is reinforced by the slowly moving surface low northwest of the Hawaiian Islands. A larger moderate swell could arrive early next week, bringing north shore surf to just below advisory thresholds. Surf along east facing shores will remain small as local winds remain fairly weak.
Flood Watch from this evening through Thursday afternoon for Niihau-Kauai-Oahu.
