Maunakea Weather Center

Maunakea Observatories Forecast
5 PM HST Friday 16 October (0300 UTC Saturday 17 October) 2026
Warning(s)
None

Cloud Cover and Fog/Precipitation Forecast
There is a small risk for periods of fog/high humidity mainly around sunset; precipitation is not expected and skies will remain clear.

Summary of Key Meteorological Variables
Summit temperatures will be near 3.5 C this evening and 3 C tomorrow morning. Winds will be from the NNE at 5-15 mph, with seeing near 0.6-0.65 arcseconds for the night. Precipitable water is expected to start out near 2 mm, but should settle in the 1-1.5 mm range for most of the night.
Discussion
While the tradewind inversion will remain rather elevated but quite distinct near 9-10 thousand feet over much of the forecast period, there is a chance that it will peridically lift toward 11-12 thousand feet mainly for this evening and again between Sunday afternoon and Monday evening. This could allow for short-lived fog/high humidity at the summit during that time. Dry/stable conditions are expected for Saturday and Tuesday night. Precipitation is not expected throughout the forecast period. Some afternoon clouds are possible mainly for Sunday and Monday, but should be fairly minimal and short-lived for the remainder of the forecast period.

Skies overhead will remain predominately clear through the next 5 nights, but there is a chance for cirrus passing along the southern skies for Sunday night.

Precipitable water is expected linger primarily in the lower half of the 1-2 mm range throughout much of the forecast period, through some spikes are possible mainly for this evening and again between Sunday evening and midnight Monday.

Although light easterly flow will persist near summit-level, shear is set to gradually increase over the next 4 nights. As a result seeing will gradually trend somewhat good values to more average-like (or slightly worse) over the next 4 nights. There is also a possibility that an elevated inversion may contribute to some partial/periodic degradation in seeing mainly for this evening and again for Sunday night as well as Monday evening. A more defined inversion, combined with laminar westerly flow in the free atmosphere should help seeing settle back in around 0.5 arcseconds for Tuesday night.

No change since the morning forecast...The surface ridge will sit steadily to the north of the state likely into next week, while a new mid-level ridge fills in from the west and now continues to progress eastward over the weekend. This will allow large-scale subsidence to persist in the area, which will help maintain a fairly well-defined inversion near 9-10 thousand feet over much of the forecast period. However, a rather distinct tropical upper-tropospheric trough feature is set to rapidly take shape to the north of the state over the weekend, which is a bit unusual for this time of the year. An initial pocket of upper-level diffluence associated with an embedded short-wave could enhance the low-level cloud field mainly around sunset today. This will could weaken/elevated the inversion and increase the risk for fog/high humidity at the summit during that time. This pocket of diffluence will likely sag off toward the SE as the mid-level ridge fills in from the west, rebuilding the invesion and ensuring dry/stable conditions again for Saturday night. However, there is a possibility that the TUTT will enhance the incoming low-level cloud field as it reaches its peak between Sunday afternoon and midnight Monday. This could result in a slightly weaker/elevated and could raise the stakes on fog/high humidity during that time. The risk of moisture at the summit (for Friday evening and this second period) is significantly smaller than previous runs, mainly due to the eastward advancement of the ridge. Regardless, the TUTT is expected to weaken and/or shift off to the SE on Tuesday, which should help to stabilize the air mass and rebuild the inversion for that night. Its also worth noting that the development/strengthening of the TUTT will also result in a rather strong sub-tropical jet building overhead during the next 4 nights. This will contribute to a steady increase in free atmospheric shear, that could negatively impact seeing (toward more average-like values or slightly worse) during that time.
5 Day Forecast Summary (Graphical Trend)
HST Cloud Fog/Precip Temp Wind Seeing PW
Cover (%) Height (km) Probability (%) (Celsius) (Dir/MPH) (Arcseconds) (mm)
Fri Oct 16 - 8 PM0-15Clear25 / 03.5NNE/5-150.525-0.7751-2
Sat Oct 17 - 2 AM0-15Clear10 / 03.5NNE/5-150.45-0.651-1.5
2 PM0-20Clear10 / 08NNE/5-15NaN1-2
8 PM0-10Clear0 / 03NE/0-100.4-0.61-1.5
Sun Oct 18 - 2 AM0-10Clear0 / 02.5E/0-100.4-0.61-1.5
2 PM20-404-4.2525 / 07ENE/0-10NaN2-4
8 PM0-15Clear25 / 02.5ENE/5-150.525-0.7751-1.5
Mon Oct 19 - 2 AM0-20Clear30 / 02.5E/0-100.5-0.70.8-1.2
2 PM20-404-4.2530 / 05.5ESE/0-10NaN2-4
Tue Oct 20 - 2 AM0-10Clear15 / 01ESE/0-100.425-0.6751-1.5
2 PM0-20Clear10 / 05ESE/0-10NaN2-4
Wed Oct 21 - 2 AM0-10Clear0 / 00.5ESE/0-100.375-0.6251-2
2 PM0-20Clear10 / 05.5N/0-10NaN2-4
Rise and Set times for the Sun and Moon
Night (HST) Sun Set Twilight End Twilight Beg Sun Rise Moon Rise Moon Set Illumination (%) RA DEC
Fri Oct 16 - Sat Oct 17 18:06 19:11 5:04 6:08 N/A 22:50 37 18 42.7 -27 00
Sat Oct 17 - Sun Oct 18 18:06 19:10 5:04 6:09 N/A 23:43 47 19 34.4 -24 51
Sun Oct 18 - Mon Oct 19 18:05 19:10 5:04 6:09 N/A 0:37 56 20 24.5 -21 35
Mon Oct 19 - Tue Oct 20 18:04 19:09 5:05 6:09 N/A 1:29 66 21 12.9 -17 22
Tue Oct 20 - Wed Oct 21 18:04 19:08 5:05 6:10 N/A 2:22 75 22 00.2 -12 19
Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Monday 19 October 2026.
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