Discussion
A well-defined tradewind inversion will continue to cap low-level moisture near 8-9 thousand feet and ensure a dry/stable summit-level air mass through the next 4 nights. There is a chance that a pocket of mid-level moisture will pass through and raise the stakes on brief fog, ice and high humidity for Tuesday night; precipitation is unlikely. Daytime clouds will be minimal and short-lived through Monday, but could pick up for Tuesday and Wednesday.
Thin scattered high clouds will continue to drift in from the west, and pass along the southern skies for tonight. There is a chance that these clodus will shift northward by the end of the night, while more high clouds build in from the WNW and merge over/near the Big Island mainly for Saturday afternoon/evening. The bulk of these clouds will shift eastward as that night progresses, leaving predominately clear skies overhead by early Sunday morning. There is a chance for scattered high clouds persisting along the northern skies for Sunday night and may move to the eastern skies for Monday evening; clear skies will prevail for Tuesday night.
Precipitable water is expected to linger near 2 mm for tonight, then increase into the 3-4 mm range for the remainder of the forecast period, perhaps even to 4+ mm for Monday and Tuesday night.
Despite increasing upper-level winds, a relatively stable air mass, combined with laminar westerly flow should still allow for better than average seeing for tonight. However, stronger mid-level flow is expected to stir up boundary layer turbulence and contribute to poor seeing for Saturday, Sunday and perhaps well into Monday night. There is a good chance that the boundary layer turbulence will subside early Tuesday morning, but a possible influx of moisture will limit much improvement in seeing for that night.
Some changes have been made to the long-term fog forecast...The mid/surface ridge will continue to sit in the NE Pacific and promote steady/strong subsidence in the free atmosphere into early next week. This subsidence will easily offset the effects of the tropical upper-tropospheric trough (TUTT) to the NW, sustain a distinct inversion near 9 thousand feet and ensure a dry/stable summit-level air mass into ththe early part of next week. The TUTT will maintain its typical spring-time set up, extending from the NE Pacific and out toward Guam, while its southern counterpart, aka the sub-tropical jet (STJ), persists over or just south of the Big Island throughout the forecast period. The bulk of the instability associated with the TUTT will remain off toward the NW, which will allow a relatively stable to prevail in the free atmosphere during much of this time. However, the jet is expected to strengthen, and drift northward in response to an upper-level low developing to the NW later over the next 24-36 hours. Initially the jet will likely have very minimal impact on seeing or cloud cover for tonight, but models suggest that the STJ could deepen into the mid-levels, while a tight ridge develops to the SE beginning Saturday afternoon/evening. This could result in an increase in summit-level westerly winds, which may stir up light boundary layer turbulence and contribute to poor degradation in seeing over the remainder of the weekend and probably into Monday night. Winds are set to subside as the low continues to weaken and the STJ becomes more shallow early Tuesday morning. Latest model runs suggest that patches of mid-level moisture revolving around the low will pass through the area, perhaps weakening the inversion between late Tuesday morning and Wednesday afternoon. I suspect this is a bit overdone, but have nonetheless, increased the risk for fog, ice and high humidity at the summit for Tuesday night.
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