Current Conditions
Temp1.8 C
RH23 %
WindENE 10 mph
RoadOpen (4x4)
Mauna Kea Observatories Forecast
5 PM HST Thursday 18 January (0300 UTC Friday 19 January) 2018
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 0.5 C this evening and 0 C tomorrow morning. Winds will be from the ESE at 15-25 mph for the first half of the night, diminishing to 10-20 mph and switching to a more SE direction for the second half. Seeing may start out near 0.7 arcseconds, but should settle back in near 0.5-0.6 arcseconds as the night progresses. Precipitable water is expected to be in the 0.7-0.9 mm range for the night.
Discussion
A well-defined tradewind inversion will continue to cap low-level moisture at or below 7 thousand feet and ensure a dry/stable summit-level air mass through at least Saturday night. There is a chance that building instability will erode the inversion and increase the risk for fog, ice, high humidity and even light flurries at the summit for Sunday and Monday night. There is also a possibility for isolated convection along the eastern skies during that time. Daytime clouds will be minimal and short-lived through Saturday, but could turn extensive for Sunday and early part of next week.

Skies overhead will remain predominately clear for tonight. A stream of high clouds is expected to quickly move in from the west and pass just north and south of the summit area for Friday night, then exit eastward on Saturday, leaving clear skies again for that night. However, there is a possibility that patches of summit/low-level clouds will begin to develop over the eastern end of the Big Island on Sunday and could persist into Monday night.

Precipitable water is expected to linger neaer 0.8-0.9 mm for the next 2 nights, increase toward 1-1.5 mm through Saturday night, then jump to 4+ mm for the following 2 nights.

Minor boundary layer turbulence may contribute to slightly poorer than average seeing for this evening, but seeing should settle in near 0.5-0.6 arcseconds as calmer skies prevail for the second half of the night through Saturday evening. Building instability and upper-level turbulence may begin to degrade seeing early Sunday morning and perhaps contribute to poor/bad seeing for the following 2 nights.

Little change since the morning forecast...The fairly deep ridge will continue to slowly slide off toward the east over the next few days, but will still maintain steady/strong large-scale subsidence in the area through at least sunrise Sunday. This will subsidence will help sustain a well-defined inversion near 7-8 thousand feet and ensure a dry/stable summit-level air mass over the next 3 nights. It should also help keep PW primarily at or below 1 mm through at least late Saturday evening. There is a possibility that winds will stir up minor boundary layer turbulence as the ridge begins to slide off toward the NE over the next 12 hours. However, winds should subside as the upper-level portion of the ridge weakens and slides off toward the SE as the night progresses. There is possibility that a rather trough developing to the west will help briefly re-establish the sub-tropical jet (STJ) over the state during the early part of the weekend. The STJ could bring high clouds in from the southwest, while the westerly jet shuttles high clouds along the northern skies for Friday night. While the STJ is set to quickly shift off toward the east, the developing trough may begin to build overhead and destabilize the air mass during the second half of the weekend. This instability, combined with a persistent/residual cold front lingering in the area could help erode the inversion and increase the risk for moisture at the summit for Sunday and Monday night. There is also a chance that isolated convection will develop within this band, particularly along the eastern slopes of the Big Island between late Sunday afternoon and just after sunrise on Monday.
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 Jan 18 - 8 PM0-5Clear0 / 00.5ESE/15-250.55-0.750.7-0.9
Fri Jan 19 - 2 AM0-5Clear0 / 00SE/10-200.45-0.650.7-0.9
2 PM0-20Clear0 / 04.5SE/5-15NaN0.8-1.2
8 PM20-409-100 / 0-1SE/5-150.45-0.650.8-1
Sat Jan 20 - 2 AM40-609-100 / 0-1.5SE/5-150.45-0.650.8-1
2 PM20-409-100 / 04SE/0-10NaN0.8-1.2
8 PM0-20Clear0 / 0-1E/0-100.5-0.70.8-1
Sun Jan 21 - 2 AM0-10Clear0 / 0-2SE/5-150.5-0.81-1.5
2 PM60-804-775 / 251SSW/5-15NaN4-8
Mon Jan 22 - 2 AM40-604-880 / 40-2.5SSE/0-100.7-1.14-8
2 PM70-904-980 / 202SSE/0-10NaN6-10
Tue Jan 23 - 2 AM20-404-5.560 / 15-2SSW/0-100.6-14-8
2 PM60-804-660 / 152.5E/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
Fri Jan 19 - Sat Jan 20 18:16 19:23 5:43 6:50 N/A 20:47 10 22 35.1 -11 00
Sat Jan 20 - Sun Jan 21 18:16 19:24 5:43 6:50 N/A 21:38 16 23 22.4 -7 14
Sun Jan 21 - Mon Jan 22 18:17 19:24 5:43 6:50 N/A 22:29 24 0 09.8 -3 06
Mon Jan 22 - Tue Jan 23 18:18 19:25 5:43 6:50 N/A 23:22 33 0 58.0 1 16
Tue Jan 23 - Wed Jan 24 18:18 19:25 5:43 6:50 N/A 0:16 44 1 47.5 5 42
Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Friday 19 January 2018.
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