A flash flooding threat persists for Kauai this morning and Oahu through this afternoon with comparatively few showers expected elsewhere. Light to moderate trades return this weekend.
Widely scattered moderate to locally heavy showers are ongoing from Kauai to Oahu with tendency for increased organization in the Kauai Channel beneath mature right entrance dynamics. Heavy rain rates of 1 to locally 2 inches per hour have been quite isolated thus far, but are beginning to align within the flow-parallel band that has formed in the channel. Cloud top cooling associated with this band is associated with a modest increase in thunderstorm activity and suggests the intensification process will continue in the near term, though it is clear at this point that organization will continue to struggle due to the lack of a surface-based forcing mechanism. Regardless, eastward progress of this band will critical for this morning's Oahu forecast and subsequent flash flooding potential. Though not particularly accurate in their depiction of the position of the primary band of heavy rain, the guidance is generally in agreement that once formed eastward progress will be minimal. While convection may inherently develop a tendency to propagate eastward with time, the greatest organization is expected to remain in the Kauai Channel. With that said, a secondary surge of deep moisture is still poised to lift across Oahu during the morning hours and the modest ascent field away from the primary band may still be sufficient to maintain elevated rain rates for at least the next several hours.
To the southwest, Channel 10/low-level water vapor imagery depicts a plume of dry air lifting toward Kauai within a corridor of backing mid-level southerlies. The arrival of this dry air intrusion will bring a quick return to stability and will signal the end of any flooding threat for Kauai upon its arrival later this morning. However, the loss of a westerly component to the mid-level flow will delay the arrival of this dry air for Oahu where periods of showers, some heavy, will continue through much of the day. The lead edge of this dry air is also enhancing mid-level convergence and helping to organize convection upstream of Oahu. The Flood Watch remains in place, though it is increasingly tenuous for Kauai which may be dropped later this morning.
Tropical moisture characterized by PWATs around 2" will remain in place into the first half of the weekend, but the wave of deep saturation that spread over the islands last night into this morning will be long gone to the north by then. This leaves the islands in an area of very broad, weak ascent east of lingering right entrance dynamics likely without sufficient forcing to generate meaningful shower activity. While isolated nuisance flooding can't be ruled out late this evening into Friday, mainly from Oahu through Maui County, it is unlikely to warrant an extension in time or space to the Flood Watch, especially considering this event's underwhelming performance thus far.
By Friday, the gradient may be sufficiently light to support sea breeze development and pockets of heavy downpours on any/all islands during the afternoon and then Maui and the Big Island again over the weekend. Returning moderate trades during this time will eventually limit afternoon convergence over Molokai, Oahu, and Kauai favoring a return to a modest coverage of clouds and showers over windward and mauka zones as early as Saturday. Dewpoints return to more typical values in the 60s Saturday night into Sunday.
Light southerly winds will increase in speed through the day today as a surface trough edges over the western end of the state. Heavy showers and isolated thunderstorms will continue to form in the vicinity of this surface trough as an area of enhanced moisture moves up from the south and instability increases aloft. The main band of heavy showers with a few isolated thunderstorms is currently over the Kauai Channel and is expected to gradually edge further eastward and impact Oahu later this morning. MVFR conditions are likely and IFR conditions possible within heavier showers and thunderstorms. VFR conditions will prevail elsewhere.
AIRMET Sierra is in effect for tempo mountain obscuration across all of Kauai and Oahu and south through west facing portions of Molokai and Lanai. This AIRMET, and possibly one for IFR conditions, will be needed for portions of the island chain through the day.
AIRMET Tango is in effect for low level turbulence downwind of terrain (west through north of island mountains in southerly flow) and is expected to be needed through the day.
Gentle to moderate south to southeast winds will continue through Friday as a low pressure system deepens north to northwest of Hawaii. Southerly to southeasterly surface winds will continue to 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.
South swell will maintain into next week with pulses of long period reinforcement arriving Thursday and Saturday. Surf will remain a few feet 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 north to northwest of the Hawaiian Islands. A larger swell will arrive around Tuesday, boosting 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 6 PM HST this afternoon for Niihau, Kauai, and Oahu.
