Current Conditions
Temp8.4 C
RH43 %
WindNE 22 mph
RoadClosed
Mauna Kea Observatories Forecast
9 AM HST (1900 UTC) Tuesday 17 September 2019
Warning(s)
None
Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, while there is a chance that patches of thin cirrus will periodically fill in from the NW and pass over/near the area through the night.
Summary of Key Meteorological Variables
Summit temperatures will be near 9 C this afternoon, 3.5 C this evening and 3 C tomorrow morning. Winds will be from the ENE at 10-20 mph for today, increasing to 15-25 mph for the night. Seeing will be near 0.8-0.9 arcseconds, while precipitable water is expected to exceed 4 mm through the night.
Discussion
The tradewind inversion will remain quite well-defined near 8 thousand feet, which will ensure the summit steers free of fog and precipitation and should help drop humidity into the 40-50% range for the next 2-3 nights. However, building instability may begin to enhance the incoming low-level cloud field, erode the inversion and thus increase the risk for periods of fog, ice, light rain/flurries and perhaps isolated convection in the area for Friday and Saturday night (maybe even as early as the ending of Thursday night). Some minor daytime clouds are possible over the next 3 days, then could turn extensive with the possibility for afternoon convection on Friday and over the weekend.

There is a chance that patches of thin cirrus will break off from the banding high clouds passing to the north and periodically fill into the summit area over the next 2 nights. The band of high clouds will sag southward on Thursday, contributing to extensive cloud cover for that night and Friday night. While there is a chance that the bulk of the high clouds will shift toward the SE skies during the latter night, thicker mid/summit-level clouds may drift in from the south and could blanket skies for much of the weekend.

Precipitable water is expected to exceed 4 mm for tonight, slip toward 3-3.5 mm for tomorrow night, then jump back to 4+ mm for Thursday, Friday and Saturday night.

A small increase in winds will likely stir up minor boundary layer turbulence and contribute to poor seeing over the next 2 nights. Winds are set to subside, which should allow seeing to dip toward 0.5-0.6 arcseconds as relatively calm skies prevail in the free atmosphere for Thursday night. Building instability and moisture will likely degrade and contribute to poor to perhaps bad seeing for the following 2 nights.

A rather tight upper-level ridge will continue to sit over the state, while the mid-level ridge redevelops to the NE over the next 2-3 days. This will allow strong/steady large-scale subsidence to prevail in the area, which will help maintain a well-defined inversion, keep summit-level humidity near 40-50% and ensure the summit steers free of fog/precipitation probably through Thursday night. Unfortuntely, stagnant/residual mid-level moisture in the area will keep PW near 3-4 mm (or more) during this time, while winds are set to pick up as a mid-level low passes to the south and helps tighten the wind gradient over the summit during the next 2 nights. The latter will likely stir up minor boundary layer turbulence and contribute to poor seeing during that time, despite the presence of an upper-level ridge in the area. There is a chance that seeing will improve as the mid-level low shifts westward and allow winds to subside on Thursday night. However, that will likely be short-lived as the upper-level ridge is expected to abruptly weaken and slide off toward the SW in response to an upper-level low developing to the NE and digging toward the state on Friday. This low could destabilize the air mass and bring rather deep/organized moisture to the area, which may erode the inversion and significantly increase the risk for fog, ice and light flurries at the summit for much of the weekend.
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 Sep 17 - 2 PM20-404-50 / 09ENE/10-20NaN4-6
8 PM0-209.5-100 / 03.5ENE/15-250.6-14-6
Wed Sep 18 - 2 AM10-309.5-100 / 03ENE/15-250.7-1.14-6
2 PM10-309.5-100 / 09.5ENE/15-25NaN4-6
8 PM0-209.5-100 / 04ENE/15-250.7-1.13-4
Thu Sep 19 - 2 AM20-409-100 / 03.5E/10-200.6-13-4
2 PM60-808-100 / 09N/5-15NaN4-6
8 PM60-808-100 / 04N/5-150.5-0.74-6
Fri Sep 20 - 2 AM70-907-1010 / 02.5NNW/5-150.4-0.76-10
2 PM80-1004-1090 / 603WSW/5-15NaN10-15
Sat Sep 21 - 2 AM80-1004-875 / 50-1NNE/5-150.7-1.38-12
2 PM80-1004-990 / 653E/5-15NaN8-12
Sun Sep 22 - 2 AM60-804-875 / 50-1E/0-100.7-1.38-12
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 Sep 17 - Wed Sep 18 18:32 19:36 4:57 6:01 20:47 N/A 84 2 46.3 10 57
Wed Sep 18 - Thu Sep 19 18:31 19:35 4:57 6:01 21:26 N/A 76 3 34.8 14 53
Thu Sep 19 - Fri Sep 20 18:30 19:34 4:57 6:01 22:08 N/A 67 4 25.8 18 12
Fri Sep 20 - Sat Sep 21 18:29 19:33 4:57 6:02 22:55 N/A 57 5 19.7 20 39
Sat Sep 21 - Sun Sep 22 18:28 19:32 4:58 6:02 23:47 N/A 46 6 16.3 22 03
Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Tuesday 17 September (0300 UTC Wednesday 18 September) 2019.
Additional Information
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