Current Conditions
Temp6.1 C
RH32 %
WindWNW 4 mph
RoadOpen (4x4)
Mauna Kea Observatories Forecast
5 PM HST Tuesday 24 April (0300 UTC Wednesday 25 April) 2018
Warning(s)
None
Cloud Cover and Fog/Precipitation Forecast
The summit will remain dry and stable, but banding high clouds are set to spread in from the south and scrape the southern half of the Big Island through the night.
Summary of Key Meteorological Variables
Summit temperatures will be near 1 C this evening and 0.5 C tomorrow morning. Winds will be from the NNW at 5-15 mph, with seeing near 0.65-0.7 arcseconds. Precipitable water is expected to be in the 1.5-2 mm range for the night.
Discussion
A fairly well-defined tradewind inversion will continue to cap low-level moisture at or below 10 thousand feet and ensure the summit remains dry and stable through at least Thursday evening. Building instability combined with moisture filling into the area could erode the inversion through Friday, and increase the risk for fog, high humidity, ice, rain/flurries and perhaps convection in the area for that night and Saturday night. Daytime clouds will be minimal and short-lived through Thursday, but could pick up for Friday and turn extensive over the weekend.

A narrow band of high clouds are set to spread in from the south and scrape the southern half of the Big Island through the night. This band will become more widespread and advance northward on Wednesday, likely contributing to extensive cloud cover for that night and perhaps overcast skies as these clouds thicken through Thursday night and especially the following 2 nights.

Precipitable water is expected to linger near 1.5-2 mm for the next 2 nights, increase into the 3-4 mm range for Thursday night and likely jump to 10 mm for the following 2 nights.

Increasing turbulence in the free atmosphere will likely contribute to slightly poorer than average seeing over the next 2 nights. Boundary layer turbulence may further degrade seeing for Thursday night and an influx of moisture/instability may result in inoperable conditions and bad seeing for the following 2 nights.

No change since the morning forecast...The eastern tip of the mid-level ridge to the west will continue to sit to the north of the state and promote large-scale subsidence in the area until a trough building into the north contributes to its decay around Friday morning. Nonetheless, this subsidence will help maintain a fairly well-defined inversion ear 9-10 thousand feet and ensure the summit remains dry and stable through at least Thursday evening. Unfortunately, the sub-tropical jet is expected to fill in from the south and strengthen, likely bringing high clouds and turbulence overhead for the next several nights. The turbulence will likely contribute to poorer than average seeing, while cloud cover will gradually increase as the high clouds become more widespread and advance northward over the next 2 nights. Conditions and skies are expected to take a turn for the worse as the developing trough draws thicker clouds in from the south, destabilizes the atmosphere and strengthens summit-level winds through Thursday night and into Friday. Eventually, this trough will deposit a rather strong late-season low to the north of the state, which is expected to further destabilize the air mass, erode the inversion and probably saturate the air mass between late Friday afternoon and through much of the weekend. This may result in mostly inoperable conditions, as extensive fog, ice, high humidity and flurries/rain plague the summit for Friday and Saturday night, at least. There is also a possibility that convection will develop in the area, which could deposit heavier snow at the summit, and models suggest that these conditions may persist as another short-wave trough digs in from the NW during the early part of next week. Now it should be noted that the latest model runs has shifted the development of the low slightly eastward, thus also pushing the bulk of its moisture eastward, perhaps sparing the summit of wet/conditions later in the weekend. While this is not too surprising for a number of reasons (quite late in the season, far out in the forecast, etc.) and that this could be an outlier run, I did reduce the risk for moisture at the summit a tad for the weekend...will wait till tomorrow to see if this scenario becomes more consistent.
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 Apr 24 - 8 PM30-509-100 / 01NNW/5-150.5-0.81.5-2
Wed Apr 25 - 2 AM40-609-100 / 00.5NNW/5-150.55-0.851.5-2
2 PM40-609-100 / 06WNW/5-15NaN1.5-2.5
8 PM50-709-100 / 01WNW/10-200.6-0.91.5-2
Thu Apr 26 - 2 AM60-808.5-100 / 01WNW/10-200.6-0.81.5-2
2 PM80-1008.5-100 / 06W/15-30NaN2-3
8 PM80-1008-100 / 01W/15-300.6-13-4
Fri Apr 27 - 2 AM80-1007-1010 / 00.5WSW/15-300.6-1.13-4
2 PM80-1004-1040 / 103SW/25-40NaN6-10
Sat Apr 28 - 2 AM80-1004-1075 / 50-1.5W/30-450.8-1.68-12
2 PM80-1004-1080 / 601.5WSW/30-45NaN6-10
Sun Apr 29 - 2 AM80-1004-1080 / 60-2WSW/40-600.8-1.66-10
2 PM80-1004-1080 / 601WSW/35-50NaN6-10
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 Apr 24 - Wed Apr 25 18:52 20:00 4:40 5:47 N/A 3:19 76 10 38.1 10 34
Wed Apr 25 - Thu Apr 26 18:52 20:00 4:39 5:47 N/A 4:02 85 11 31.4 6 09
Thu Apr 26 - Fri Apr 27 18:53 20:01 4:38 5:46 N/A 4:44 92 12 23.1 1 27
Fri Apr 27 - Sat Apr 28 18:53 20:01 4:37 5:45 N/A 5:24 97 13 14.0 -3 17
Sat Apr 28 - Sun Apr 29 18:53 20:02 4:37 5:45 17:46 6:05 99 14 04.4 -7 48
Forecast Issued by: Ryan Lyman
Next update at 10 AM HST (2000 UTC) Wednesday 25 April 2018.
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