Current Conditions
Temp2.6 C
RH27 %
WindENE 4 mph
RoadOpen (4x4)
Mauna Kea Observatories Forecast
5 PM HST Thursday 19 July (0300 UTC Friday 20 July) 2018
Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, while mid/upper-level clouds continue to creep in from the north and south through the night.
Summary of Key Meteorological Variables
Summit temperatures will be near 3 C, with winds from the NE at 5-15 mph and seeing near 0.55-0.6 arcseconds. Precipitable water is expected to be in the 3.5-4.5 mm range for the night.
The tradewind inversion will continue to cap low-level moisture near 6-7 thousand feet and ensure the summit remains dry and stable through at Sunday morning. There is a chance that a patch of relatively unstable tropical low-level clouds will fill into the area and lift the inversion toward 13-14 thousand feet, increasing the risk for fog, high humidity and light rain at the summit for Sunday and perhaps Monday night. Daytime clouds will be minimal and short-lived through Sunday, but could turn extensive for the early part of next week.

Mid/upper-level clouds passing to the south and north are expected to slowly creep toward the summit through tonight. The southern portion will eventually spread further northward and merge with the residual clouds in the area, contributing to extensive cloud cover, if not overcast skies for at least Friday and Saturday night. While there is a good chance that these clouds will shift subtly northward, perhaps opening up skies to the south, periods of extensive cloud may continue into the early part of next week.

Precipitable water is expected to linger near or exceed 4 mm through the next 5 nights.

Relatively calm skies, with light/moderate shear/turbulence in the free atmosphere should allow for better than average seeing over the next 3 nights. However, there is a possibility for times of variability, particularly if/when die off completely for prolong periods, mainly for Friday night. An increase in low-level moisture and perhaps mid-level turbulence may contribute to some degradation or even poor seeing for Sunday and Monday night.

Little change since the morning forecast...The mid/low-level ridge will continue to sit to the north of the state, with an upper-level ridge and a tropical upper-tropospheric trough (TUTT) persisting to the SE and the NW, respectively, probably well into next week. Nonetheless, the ridge will negate the affects of the TUTT, promote strong/steady large-scale subsidence in the area, and maintain a well-defined tradewind inversion near 6-7 thousand feet through at least late Sunday morning. Models suggest that a patch of relatively unstable low-level clouds, likely a remanant tropical system, will move through the area and perhaps weaken/erode the inversion between late Sunday afternoon and Monday evening. The global model has been a little inconsistent with this outlook, being a little aggressive with this moisture/system early in the week, dismissing it over the last couple of days, then moving forward with it again over the last 3 runs. As a result, it warrants an increase on the risk for fog, high humidity and light rain at the summit for Sunday and Monday night. In the meantime, the TUTT and its southern counter part, the sub-tropical jet (STJ), are expected to dig into the SW over the next 24 hours. This will allow the STJ to gain access to more widespread/thicker clouds and stream it over the summit area later tonight and especially for the following 2 nights. This will likely contribute to periods of extensive cloud cover, if not overcast skies for Friday and Saturday night. There is a possibility that the upper-level ridge (to the SE) will buldge a bit northward, and subsequently shift the STJ and its stream of clouds northward on Sunday. This may help to open up skies a bit to the south for that night and early part of next week.
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)
Thu Jul 19 - 8 PM20-409.5-100 / 03NE/5-150.5-0.73.5-4.5
Fri Jul 20 - 2 AM30-509-100 / 03NE/5-150.45-0.653.5-4.5
2 PM50-707-90 / 09NNE/5-15NaN4-6
8 PM60-807-90 / 04NE/0-100.45-0.653.5-4.5
Sat Jul 21 - 2 AM80-1007-100 / 04NE/0-100.45-0.753.5-4.5
2 PM80-1007-100 / 09E/0-10NaN4-6
8 PM80-1007-100 / 04ENE/5-150.45-0.653.5-4.5
Sun Jul 22 - 2 AM70-907-100 / 04E/5-150.45-0.653.5-4.5
2 PM60-807-90 / 08.5ENE/5-15NaN4-6
Mon Jul 23 - 2 AM60-804-950 / 252E/5-150.6-14-6
2 PM70-904-975 / 405S/0-10NaN6-10
Tue Jul 24 - 2 AM50-707-940 / 102N/5-150.5-0.94-6
2 PM60-804-1050 / 107.5NNE/5-15NaN4-8
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
Thu Jul 19 - Fri Jul 20 19:13 20:25 4:32 5:44 N/A 0:44 56 14 13.5 -8 27
Fri Jul 20 - Sat Jul 21 19:13 20:24 4:32 5:44 N/A 1:25 66 15 02.6 -12 26
Sat Jul 21 - Sun Jul 22 19:12 20:24 4:33 5:45 N/A 2:06 75 15 52.1 -15 50
Sun Jul 22 - Mon Jul 23 19:12 20:24 4:33 5:45 N/A 2:49 83 16 42.3 -18 29
Mon Jul 23 - Tue Jul 24 19:12 20:23 4:34 5:45 N/A 3:35 89 17 33.1 -20 19
Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Friday 20 July 2018.
Additional Information
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