Current Conditions
Temp7.0 C
RH8 %
WindSSE 0 mph
RoadOpen
Mauna Kea Observatories Forecast
715 AM HST (1715 UTC) Thursday 18 September 2014
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 9.5 C this afternoon, 4.5 C this evening and 3.5 C tomorrow morning. Winds will be light and from the SSW, with seeing around 0.45-0.5 arcseconds. Precipitable water is expected to be in the 1.25-1.75 mm range for the first half of the night and 1.75-2.25 mm range for the second half.
Discussion
The tradewind inversion will continue to cap low-level moisture near 6 thousand feet and keep the mid/summit-level air mass dry and stable for tonight. There is a possibility that moisture will scrape by to the south, perhaps lifting the inversion toward 10-12 thousand feet and increasing the risk for short-lived fog at the summit for Friday and Saturday night. The inversion is slated to recover by Sunday afternoon, which will ensure normal dry/stable conditions for that night as well as Monday night. Daytime clouds will be minimal and short-lived for today, but there is a possibility that it will pick up tomorrow and Saturday, only to taper again for the remainder of the forecast period.

Skies overhead will remain predominately clear, but there is a possibility for isolated thin high clouds along the southern skies through Monday morning. These clouds are slated to dissipate and shift eastward early Monday afternoon, leaving clear skies for the early part of next week. Precipitable water will start out near 1.25 mm for this evening, but increase towrad 2 mm as the night progresses, likely peaking near 3 mm by tomorrow evening. It is expected to drop back to 2 mm by the end of that night and for Saturday night, then jump toward 3 mm again for Sunday and Monday night.

Minimal turbulence in the free atmosphere will allow for better than average seeing for tonight. There is a possibility that a patch of upper-level turbulence will drift overhead and briefly contribute to poorer than average seeing for tomorrow night. This turbulence is slated to subside as deep subsidence fills into the area, which should help to improve seeing over the remainder weekend (there is a possibility that seeing will dip toward 0.4 arcseconds for Sunday evening). However, increasing summit-level winds could stir up boundary layer turbulence and degrade seeing once again for Monday night.

The eastern flank of the mid-level ridge will continue to sit to the north of the Islands for another 12-24 hours, shift off toward the west as a trough passes to the far north, then move back toward its normal climitological position in the NE Pacific over the weekend and into next week. While it will remain close enough to the area to promote steady large-scale subsidence near the Big Island probably throughout the forecast period, the passing trough will try to drag moisture up from the tropics, which is expected to scrape the southern tip of the Big Island between late tomorrow morning and midnight Saturday. In addition, the tropical upper tropospheric trough, currently lingering to the northwest will get swallowed up by the trough and make a push over the summit area around tomorrow night. The instability associated with the TUTT will be insignificant, but the moisture to the south could help lift the inversion toward 10-12 thousand feet and increase the risk for short-lived fog and daytime clouds mainly for tomorrow and Saturday. The turbulence within the TUTT axis could negatively affect seeing for Friday night. Drier more stable conditions and clear/calm skies are expected to build back over the summit area as a deep ridge fills in behind the passing trough later in the weekend. There is a possibility that this deep subsidence and calm skies will help improve seeing toward 0.4 arcseconds mainly around Sunday evening. However, it looks like summit-level winds are slated to strengthen once the ridge settles in to the northeast of the Islands for the early part of next week. This will likely increase boundary layer turbulence and contribute to poor seeing during that time.
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 Sep 18 - 2 PM0-20Clear0 / 09.5SSW/0-10NaN1-2
8 PM0-10Clear0 / 04.5SSW/0-100.45-0.551.25-1.75
Fri Sep 19 - 2 AM0-10Clear0 / 03.5SW/0-100.4-0.51.75-2.25
2 PM0-20Clear50 / 108W/5-15NaN3-6
8 PM0-10Clear20 / 03.5NW/5-150.55-0.752-3
Sat Sep 20 - 2 AM0-10Clear25 / 03W/5-150.55-0.751.75-2.25
2 PM0-20Clear60 / 108WNW/5-15NaN2-4
8 PM0-10Clear25 / 03NW/5-150.45-0.551.75-2.25
Sun Sep 21 - 2 AM0-20Clear20 / 03NW/5-150.4-0.51.5-2
2 PM0-20Clear5 / 08.5N/5-15NaN2-3
Mon Sep 22 - 2 AM0-20Clear0 / 03NE/10-200.35-0.552-4
2 PM0-20Clear0 / 08ENE/15-30NaN2-4
Tue Sep 23 - 2 AM0-5Clear0 / 02.5E/15-300.5-0.92-4
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
Thu Sep 18 - Fri Sep 19 18:30 19:35 4:57 6:01 2:21 N/A 20 8 20.5 14 01
Fri Sep 19 - Sat Sep 20 18:29 19:34 4:57 6:02 3:10 N/A 13 9 07.2 11 19
Sat Sep 20 - Sun Sep 21 18:29 19:33 4:57 6:02 3:59 N/A 7 9 53.0 8 10
Sun Sep 21 - Mon Sep 22 18:28 19:32 4:58 6:02 4:48 17:00 3 10 38.1 4 41
Mon Sep 22 - Tue Sep 23 18:27 19:31 4:58 6:02 5:37 17:37 1 11 23.2 0 58
Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Thursday 18 September (0300 UTC Friday 19 September) 2014.
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