Current Conditions
Temp8.8 C
RH10 %
WindNNE 5 mph
RoadOpen (4x4)
Maunakea Observatories Forecast
730 AM HST (1730 UTC) Monday 20 September 2021
Warning(s)
None
Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, while scattered light cirrus will continue to fill in from the north, passing over and especially along the western skies through the night.
Summary of Key Meteorological Variables
Summit temperatures will be near 8.5 C this afternoon, 3.5 C this evening and 2.5 C tomorrow morning. Winds will be from the ENE at 5-15 mph, with seeing near 0.5-0.55 arcseconds. Precipitable water is expected to be in the 1-1.5 mm range for the night.
Discussion
A distinct tradewind inversion will continue to cap low-level moisture at or below 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.

Scattered high clouds will continue to spill in from the north, passing over and especially along the western skies for tonight. These clouds will push subtly westward, but more cirrus filling in from the south will begin to build along the eastern skies through tomorrow night and may creep westward on Wednesday. There is a chance that a narrow band of mid-level clouds will quickly pass over around early Thursday morning and patches of high clouds may pass over the summit area later that night. The bulk of the clouds will shift toward the western skies, leaving clear skies overhead and especially to the east for Friday night.

Precipitable water is expected to linger in the 1-1.5 mm range for tonight, slip near or just below 1 mm for Tuesday night, abruptly jump to 4+ mm for most of Wednesday night, then trend toward 2 mm for Thursday night and 1 mm for Friday night.

Relatively calm/stable skies with deep/light northerly flow in the free atmosphere will allow seeing to remain near 0.5-0.55 arcseconds for tonight. A subtle improvement in seeing is possible and winds further subside (and switch to a more southerly direction) for tomorow night. However, a patch of mid-level turbulence and moisture may degrade seeing toward 0.6-0.7 arcseconds for Wednesday night. An increase in winds is expected to stir up boundary layer turbulence and contribute to poor/bad seeing for Thursday and Friday night.

The mid/low-level ridge will continue to sit to the north of the state and promote steady/strong large-scale subsidence in the area through the week. This subsidence will easily maintain a well-defined tradewind inversion primarily near 8-9 thousand feet and ensure a dry/stable summit-level air mass throughout the forecast period. Upper-level flow is beginning to transition away from the summer-time set up with the decay of the persistent tropical upper-tropospheric trough (TUTT) and the associated sub-tropical jet. While this will help minimize widespread/weak instability in the area as well as free atmospheric turbulence, rather indistinct upper-level lows will continue to bring patchy/scattered high clouds to the area over the next 5 nights (CC should not exceed 50% during this time). Calm skies at and above the summit should allow for better than average seeing main for the next 2 nights, but there is a chance that an influx of mid-level moisture/turbulence will briefly pass through, subtly degrading seeing for Wednesday night. In addition, the mid-level wind gradient is expected to tighten as a new mid-level ridge builds in from the NW and a weak mid-level low passes to the south over the following 2-3 nights. This could result in an increase in summit-level winds, which will stir up boundary layer turbulence and contribute to poor/bad seeing for Thursday and Friday night.
WRF Astronomical Observing Quality Guidance
Cloud Cover and Precipitable Water Analyses
MK CN² Profiles
5 Day Forecast Summary (Graphical Trend)
HST Cloud Fog/Precip Temp Wind Seeing PW
Cover (%) Height (km) Probability (%) (Celsius) (Dir/MPH) (Arcseconds) (mm)
Mon Sep 20 - 2 PM20-409-100 / 08.5ENE/5-15NaN1-2
8 PM10-309-100 / 03.5ENE/5-150.45-0.651-1.5
Tue Sep 21 - 2 AM10-309-100 / 02.5ENE/5-150.4-0.61-1.5
2 PM10-309-100 / 07E/10-20NaN1-2
8 PM0-209.5-100 / 02E/5-150.4-0.60.9-1.1
Wed Sep 22 - 2 AM0-209.5-100 / 03E/5-150.35-0.550.8-1
2 PM20-408-90 / 09SSW/0-10NaN1-2
8 PM10-308-90 / 04SSE/0-100.5-0.72-4
Thu Sep 23 - 2 AM30-507-90 / 04.5SSE/0-100.5-0.83-5
2 PM20-408-100 / 010E/5-15NaN3-6
Fri Sep 24 - 2 AM0-209-100 / 05ENE/15-300.6-11.5-2.5
2 PM10-309-100 / 010ENE/20-35NaN1-2
Sat Sep 25 - 2 AM0-209.5-100 / 06E/25-400.8-1.60.8-1.2
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
Mon Sep 20 - Tue Sep 21 18:28 19:33 4:57 6:02 18:28 6:59 100 0 20.9 -2 57
Tue Sep 21 - Wed Sep 22 18:27 19:32 4:58 6:02 19:02 N/A 98 1 06.9 2 41
Wed Sep 22 - Thu Sep 23 18:26 19:31 4:58 6:02 19:36 N/A 94 1 52.5 8 05
Thu Sep 23 - Fri Sep 24 18:26 19:30 4:58 6:02 20:10 N/A 88 2 38.4 13 03
Fri Sep 24 - Sat Sep 25 18:25 19:29 4:58 6:03 20:47 N/A 82 3 25.4 17 25
Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Monday 20 September (0300 UTC Tuesday 21 September) 2021.
Additional Information
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