Current Conditions
Temp9.8 C
RH25 %
WindNNE 20 mph
RoadOpen (4x4)
Mauna Kea Observatories Forecast
9 AM AM HST (1900 UTC) Tuesday 21 May 2019
Warning(s)
None
Cloud Cover and Fog/Precipitation Forecast
The summit will remain clear, dry and stable through the night.
Summary of Key Meteorological Variables
Summit temperatures will be near 9 C this afternoon, 4 C this evening and 3.5 C tomorrow morning. Winds will be from the SE at 5-15 mph, with seeing near 0.4-0.5 arcseconds. Precipitable water is expected to be in the 2.5-3 mm range for the night.
Discussion
A fairly well-defined tradewind inversion will continue to cap low-level moisture at or below 8 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 patches of mid/upper-level clouds in the area for tonight. These clouds will dissipate and lift off toward the north through tomorrow, leaving predominately clear skies for the remainder of the forecast period.

Precipitable water is expected to gradually decline through the 2-3 mm range over the next 3 nights, then settle in near 1.5-2 mm for Friday and Saturday night.

Calm skies, a stable air mass and light winds at the summit should allow seeing to linger primarily near 0.4-0.5 arcseconds over the next 2 nights (some variability is possible should winds die off completely for a prolong period, mainly for tomorrow night). An increase in boundary layer turbulence could start to degrade seeing through Thursday night and contribute to poor seeing for the following 2 nights.

The mid/low-level ridge will remain squashed over the state by a rather large trough brewing to the NW, then will lift northward as the trough weakens beginning late Thursday. Nevertheless, the ridge will still promote steady/strong large-scale subsidence in the area, which will easily maintain a well-defined inversion near 8 thousand feet and ensure a dry/stable summit-level air mass through the week; the trough will have little, if any, direct impact on summit conditions. The southward supression of the ridge will keep summit winds relatively light, and combined with the absence of the sub-tropical jet aloft, should allow for calm skies and good/excellent seeing over the next 2 nights. However, summit winds are set to pick up again as the ridge rebounds northward and easterly flow along the southern flank of the ridge builds in overhead late Thursday and into the weekend. This could result in an increase in boundary layer turbulence, which will probably contribute to poorer than average seeing perhaps as early as Thursday night, but more likely the following 2 nights.
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 (%) (Celcius) (Dir/MPH) (Arcseconds) (mm)
Tue May 21 - 2 PM20-407-80 / 09SE/5-15NaN3-5
8 PM10-307-80 / 04SE/5-150.4-0.62.5-3
Wed May 22 - 2 AM0-207-80 / 03.5SE/5-150.35-0.552.5-3
2 PM0-207-80 / 09.5SE/5-15NaN2-4
8 PM0-10Clear0 / 04.5SE/0-100.3-0.52.25-2.75
Thu May 23 - 2 AM0-5Clear0 / 04.5ESE/0-100.3-0.62.25-2.75
2 PM0-20Clear0 / 010E/5-15NaN2-4
8 PM0-5Clear0 / 05E/10-200.5-0.72-2.5
Fri May 24 - 2 AM0-5Clear0 / 04.5ENE/10-200.5-0.82-2.5
2 PM0-20Clear0 / 09E/15-25NaN1.5-2.5
Sat May 25 - 2 AM0-5Clear0 / 03E/15-250.6-11.5-2
2 PM0-20Clear0 / 07.5ENE/15-25NaN1.5-2.5
Sun May 26 - 2 AM0-5Clear0 / 02.5NE/15-250.6-11.5-2
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
Tue May 21 - Wed May 22 19:03 20:14 4:23 5:34 21:42 N/A 86 19 02.4 -22 48
Wed May 22 - Thu May 23 19:03 20:15 4:22 5:34 22:32 N/A 79 19 56.0 -22 02
Thu May 23 - Fri May 24 19:03 20:15 4:22 5:34 23:19 N/A 70 20 47.7 -20 16
Fri May 24 - Sat May 25 19:04 20:16 4:21 5:34 0:01 N/A 61 21 37.0 -17 40
Sat May 25 - Sun May 26 19:04 20:17 4:21 5:33 0:41 N/A 52 22 24.4 -14 23
Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Tuesday 21 May (0300 UTC Wednesday 22 May) 2019.
Additional Information
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This message is also available at the MKWC road conditions page.
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