Latest Forecast for Maunakea Observatories
7 AM HST (1700 UTC) Friday 05 April 2024

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while light cirrus passes along the northern and southern skies during the evening and morning hours, respectively.

Summary of Key Meteorological Variables
Summit temperatures will be near 6 C this afternoon, 0.5 C this evening and 0 C tomorrow morning. Winds will be light and variable, with seeing near 0.5-0.55 arcseconds for the night. Precipitable water is expected to be in the 0.8-1 mm range through the night.

Discussion
A distinct tradewind inversion will continue to cap low-level moisture primarily near 8-9 thousand feet and ensure a dry/stable summit-level air mass through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

There is a possibility for light cirrus passing along the northern skies for this evening, while a band of scattered high clouds to the south creeps closer to the Big Island overnight. The former is expected to clear out leaving mostly clear skies along the northern skies for the remainder of the forecast period, but the band to the south may occasionally scrape the Big Island over the weekend and into next week.

Precipitable water is expected to linger near 0.8-1 mm for tonight, increase to 1.5 mm for Saturday then settle back in around 1-1.25 mm for the remainder of the forecast period.

Relatively calm/stable skies, combined with laminar westerly flow in the free atmosphere and light winds at the summit should allow seeing to settle in near 0.5-0.55 arcseconds for much of tonight. However, upper-level westerly winds (between 20-30K feet above sea level) is set to strengthen, resulting in an increase in shear which could degrade seeing toward 0.6 arcseconds over the weekend and into next week. There is also a possibility that summit-level winds will increase, enough to stir up boundary layer turbulence and further degrade seeing late Monday night and Tuesday night.

The upper atmosphere will maintain a more spring-like pattern with the establishment of the tropical upper-tropospheric trough (TUTT) extending out from the west coast toward the west of the state and out to Guam, while the westerly jet retracts northward probably well into next week. Fortunately, embedded short-wave troughs/lows in the TUTT will remain rather indistinct, limiting upper-level instability in the area during this time. This will allow steady/strong subsidence associated with the surface/mid-level ridge to the NE to prevail in the area, which will help maintain a well-defined inversion primarily near 8-9 thousand feet and ensure a dry/stable summit air mass through the next 5 nights. A persistent sub-tropical jet (STJ) will settle in over or just south of the Big Island, but will remain relatively shallow, resulting in light/moderate shear in the free atmosphere probably for much of tonight. This should allow seeing to linger near 0.5-0.55 arcseconds during that time, but there is a possibility for some minor degradation in seeing (toward 0.6 arcseconds) as the STJ strengthens (mainly between 20-30K feet above sea leave) over the weekend. THere is also a chance that the STJ will gain access to light cirrus flowing out of the tropics, though the bulk of these clouds will pass within the core of the jet and just south of the Big Island over the weekend and into the early part of next week. While thin cirrus could still pass over the southern half of the Big Island, cloud cover is not expected to exceed 25% for any prolong period during this time. Latest model runs suggest that the STJ will deepen and creep northward early next week. This could result in an increase in summit-level winds, enough to stir up boundary layer turbulence and may further degrade seeing for Monday and Tuesday night.