A Kona low will bring several rounds of moderate to heavy rainfall to the islands through the weekend resulting a risk of flash flooding.
Large scale forcing generally neutral at press time this afternoon with little change in the mid-level height field during the last 12 hours and weak subsidence aloft courtesy of the right exit of the resident upper jet. Little in the way of persistent convergence exists at the surface leads to a nebulous forecast scenario in which identifiable features supporting organization remain evasive. Weak isentropic ascent within SW moist mid-level flow has supported waves of elevated light/moderate stratiform rainfall across the state today. At the same time, southeast boundary layer flow wrapping around the Big Island has steered heavier surface-based showers over Kauai, Oahu, and the western portion of Maui County. Maui itself appears to be shadowed within this pattern of low-level SE flow. Rainfall amounts generally around a quarter of an inch for Oahu and one half to one inch for Kauai.
For tonight into Friday, there is atypically high uncertainty for such a short lead time. As upstream low pressure slowly evolves, boundary layer flow will veer to a more S/SW direction allowing boundary layer showers to spread over Maui this evening. Surface convergence then likely results in an increasingly organized band of moderate to heavy showers that could be a persistent feature through much or all of Friday. However, the weak nature of the surface low is causing models to contend with surface pressure falls resulting from downstream convection. This in turns leads to radically different low-level flow solutions during the next 24 hours in an environment in which identifying low-level convergence is crucial. The EC allows a secondary low to form downstream due to convective influences and attendant surface convergence supports persistent convection over Maui County as soon as 8pm HST this evening. The GFS maintains a more persistent and weak parent low and convergence takes more time to develop, but when it does it sets up over Kauai during Friday. The HRRR focuses convection directly over Oahu through tonight.
21z ASCAT showed well defined surface convergence about 320 miles west of Kauai which is evident on visible imagery as a meridionally oriented cloud band. On this seemingly critical corridor of developing low-level convergence, the EC is not only an outlier among the guidance but is far too fast advancing it eastward. Among all the guidance, including the HRRR, only the UKMet appears to handle the current situation adequately. As it happens, it is also the only model to not develop secondary low pressure centers due to convection. It has been, and remains, the preferred guidance for this Kona low through at least the next 24-48 hours.
Gentle ascent within moist SW flow will maintain shallow convective showers from Kauai through Maui tonight while convergence very slowly becomes established upstream. By early Friday, the UKMet indicates organizing convergence approaching Kauai supporting increased potential for organized heavy rainfall there during the day. By Friday night, large scale forcing begins to ramp up allowing heavy showers to become increasingly widespread from Kauai through Oahu and part or all of Maui County. The progressive nature of the forcing field means the uptick in intensity will coincide with increasing forward motion of the heavy rain band which helps mitigate a greater flash flooding threat. The UK then clears Kauai on Saturday night and Oahu by Sunday morning. Heavy rain then focuses over Maui County and the Big Island on Sunday, the Big Island being the last in line to receive heavy rainfall. By late Sunday, the band is likely ushered south the aggressively digging upper wave.
Breezy trades return on Monday or Monday night. With the remnant moisture band still in place over the eastern end of the state, enhanced showers are likely over Windward Big Island and/or Windward Maui Monday night into Tuesday.
Scattered shower activity has enveloped the state this afternoon, with persistent MVFR to IFR conditions expected for the period for Kauai in particular. Elsewhere, expect variable conditions, before showers become more widespread Friday afternoon. Winds will remain primarily SE/S across most sites, with bkn/ovc cigs expected.
AIRMET Sierra for mountain obscuration is in effect for Kauai and Oahu. This is expected to spread to other islands heading into Friday as shower activity becomes more widespread.
Light icing is also expected at FL 120-260, which is expected to remain in effect so long as this oncoming Kona low impacts the islands.
Light to moderate southeast winds will strengthen tonight as surface low pressure roughly 300 nm west of Kauai and an associated front stretching north of the state deepen. Moderate to locally fresh south to southwest winds will prevail Friday and Saturday, and periods of heavy showers and thunderstorms are expected. The low will lift to the north of the state on Sunday, likely allowing moderate northerly winds and lower chances for rainfall to develop around Kauai. On Monday, high pressure building north of the state will push a surface trough eastward over the islands. Fresh to strong northeast winds will require a Small Craft Advisory (SCA) over most waters, though variable winds could linger around the Big Island through the day. Fresh to potentially strong trade winds will persist on Tuesday.
A small, medium period north swell continues a slow decline. Nearshore PacIOOS buoys show the swell from 345-360 degrees at around 4 feet 12 seconds. The swell will drop slightly as it shifts out of the north-northeast Friday. A moderate reinforcing north-northeast swell is due Saturday and Sunday, and a small, overlapping west-northwest swell is expected to arrive late Saturday and hold into Monday. Early next week, a potentially larger pulse of medium period north-northeast swell could produce surf near the High Surf Advisory level and contribute to the need for a SCA.
Aside from areas exposed to wrapping north swell, surf along east facing shores will remain well below average through the weekend. As northeast trade winds develop early next week, rough surf will return to east facing shores, and some areas exposed to the north-northeast swell may approach the advisory level.
The south swell that produced surf around the High Surf Advisory level yesterday is on a gradual decline. The PacIOOS buoy off of Lanai shows that the swell is down to about 3 feet at 14 seconds this afternoon, with a further slow decline expected Friday. As this swell fades, a small south-southwest swell will arrive this weekend, then decline Monday. Smaller surf is expected along south facing shores through the remainder of the week.
Flood Watch from 6 PM HST this evening through Sunday afternoon for Hawaii.
