Current Conditions
Temp4.8 C
RH64 %
WindWSW 24 mph
RoadOpen (4x4)
Mauna Kea Observatories Forecast
10 AM HST (2000 UTC) Thursday 21 September 2017
Warning(s)
Chance for fog/high humidity
Cloud Cover and Fog/Precipitation Forecast
There is a small risk for fog and high humidity throughout the night; precipitation is not expected. Broken mid-level clouds will continue to linger along the southern skies, while banding high clouds setting up to the SE of the Big Island creeps closer through the night.
Summary of Key Meteorological Variables
Summit temperatures will be near 6 C this afternoon and 2.5 C for the night. Winds will be from the WNW at 5-15 mph, with seeing near 0.55-0.6 arcseconds. Precipitable water is expected to be in the 1.5-2.5 mm range for the night.
Discussion
Although the tradewind inversion is set to restrengthen near 6-7 thousand feet, rather deep mid/low-level moisture persisting just south of the Big Island may occasionally stray into the area and allow for the development of fog and/or periods of high humidity over the next 2 nights; precipitation is unlikely. This moisture is expected to drift out of the area on Saturday, which will ensure the summit remains dry and stable for the remainder of the forecast period. Some afternoon clouds are possible over the next 2 days, but will become minimal and short-lived thereafter.

Broken mid-level clouds will continue to linger along the southern skies for the next 2 nights, while banding high clouds setting up along the SE skies gradully drift toward the Big Island through the night. The latter is expected to move over the summit area, contributing to periods of extensive cloud cover between late tomorrow morning and around Saturday afternoon/evening. Both sets of clouds are projected to quickly retreat southward on Saturday, leaving relatively clear skies for the remainder of the forecast period.

Precipitable water is expected to linger near 2 mm through the next 3 nights, though there is a possibility for some variability/spikes mainly over the next 2 nights. Precipitable water is set to settle in near 1 mm for Sunday and Monday night.

Despite increasing winds in upper air mass, predominately westerly flow will prevail in the free atmosphere, likely allowing seeing to linger near 0.6 arcseconds over the next 3 nights. Winds aloft are expected to diminish and switch to a more northerly and easterly direction over the following 2 nights, which should allow seeing to eventually to dip below 0.5 arcseconds, particularly for Monday night.

A persistent and rather deep upper-level low will to the northeast will allow mid/upper-level moisture to hang just south of the summit over the next 48 hours. While fortunately, the low will hardly have an impact on the stability of the atmosphere, this moisture may occasionally creep into the area and allow for more periods of fog and high humidity at the summit over the next 2 nights. Southwesterly flow along the southern flank of the low (aka the sub-tropical jet - STJ) is also expected to shuttle in banding high clouds overhead, contributing to periods of extensive cloud cover mainly between late tomorrow morning and around Saturday afternoon. Moderate/strong turbulence in the free atmosphere (and associated with the STJ) will also keep seeing near 0.6 arcseconds over the next 3 nights. The low is expected to subtly dip southward as the mid-level ridge broadens to the north over the weekend. This will not only push the moisture/clouds further off to the south, but further increase subsidence in the free atmosphere and ensure dry/stable summit-level conditions prevail for the remainder of the forecast period. Eventually the STJ will also shift off toward the SE, which will help diminish shear/turbulence in the free atmosphere and improve seeing, particularly as an upper-level ridge finally builds in from the west around the 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 Sep 21 - 2 PM40-604-760 / 206WNW/5-15NaN3-6
8 PM0-206-740 / 52.5NW/5-150.5-0.71.5-2.5
Fri Sep 22 - 2 AM10-309.5-1030 / 52.5WNW/5-150.45-0.651.5-2.5
2 PM40-609-1060 / 157.5WNW/10-20NaN3-6
8 PM60-808-1030 / 53NW/10-200.5-0.71.5-2.5
Sat Sep 23 - 2 AM60-808-1025 / 53NW/5-150.5-0.71.5-2.5
2 PM50-709-1010 / 08WNW/5-15NaN2-3
8 PM30-509.5-100 / 03WNW/5-150.45-0.651.5-2.5
Sun Sep 24 - 2 AM0-20Clear0 / 02WNW/10-200.55-0.751.25-1.75
2 PM0-20Clear0 / 07NNW/10-20NaN1-1.5
Mon Sep 25 - 2 AM0-5Clear0 / 02N/5-150.4-0.60.8-1.2
2 PM0-20Clear0 / 07.5NNW/5-15NaN1-1.5
Tue Sep 26 - 2 AM0-5Clear0 / 02.5NNE/5-150.35-0.550.8-1.2
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
Thu Sep 21 - Fri Sep 22 18:27 19:32 4:58 6:02 N/A 19:52 5 13 41.2 -5 59
Fri Sep 22 - Sat Sep 23 18:26 19:31 4:58 6:02 N/A 20:32 10 14 28.9 -9 48
Sat Sep 23 - Sun Sep 24 18:26 19:30 4:58 6:02 N/A 21:13 17 15 16.6 -13 10
Sun Sep 24 - Mon Sep 25 18:25 19:29 4:58 6:03 N/A 21:54 25 16 04.8 -15 56
Mon Sep 25 - Tue Sep 26 18:24 19:28 4:59 6:03 N/A 22:37 34 16 53.6 -18 01
Forecast Issued by: Ryan Lyman
Next update at 5 PM HST Thursday 21 September (0300 UTC Friday 22 September) 2017.
Additional Information
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