Latest Forecast for Mauna Kea Observatories
5 PM HST Friday 22 February (0300 UTC Saturday 23 February) 2019

Warnings
Fog/ice and high humidity
Chance for flurries and afternoon convection

Cloud Cover and Precipitation Forecast
There is a very high risk for fog, ice, high humidity and perhaps light flurries through the night. Patches of mid/low-level clouds will also linger in the area, particularly along the eastern skies throughout the night.

Summary of Key Meteorological Variables
Summit temperatures will be near -2.5 C this evening and -3 C tomorrow morning. Winds will be from the WSW at 20-35 mph, while seeing and precipitable water are expected to exceed 1 arcsecond and 4 mm, respectively, through the night.

Graphical Summary

Discussion
Persistent instability and low-level moisture will keep the tradewind inversion rather weak/indistinct and will likely allow for more fog, ice, high humidity and perhaps light flurries at the summit probably into early Sunday evening. There is a good chance that the inversion will begin to rebuild diminishing the risk for moisture at the summit through Sunday night and especially Monday night. However, this risk may increase again as the inversion may briefly breakdown again in response to a passage of moisture/instability on Tuesday night. Extensive daytime clouds are expected to plague the summit and there is a possibility for convection along the Big Island slopes through the weekend. Both should taper for Monday, but could pick up again on Tuesday and Wednesday.

While skies overhead will remain predominately clear of organized high clouds throughout the forecast period, there is a possibility for patches of mid/low-level clouds in the area and perhaps lingering cirrus exhaust should afternoon convection develop along the slopes through Sunday evening. More patches of thick mid-level clouds may develop over and/or fill in from the west and south for Tuesday night.

Precipitable water is expected to exceed 4 mm probably into early Sunday evening, but could slip toward 1 mm by sunrise Monday, then increase back to 2-3 mm through that night and perhaps 3-4 mm for Tuesday night.

An elevated/weak inversion combined with steady boundary layer turbulence will contribute to poor/bad seeing through Sunday night. There is a possibility for improving summit-level conditions, which could allow seeing to settle back in near average-like values as moderate turbulence prevails in the free atmosphere for Monday night. However, there is a very good chance that another influx of moisture/instability and/or increase in boundary layer turbulence will significantly degrade seeing for Tuesday night.

Little change since the morning forecast...A very broad/persistent mid/upper-level trough to the west will gradually progress eastward and destabilize the air mass, while a band of low-level moisture lingers over or just east of the Big Island through Sunday afternoon/evening. These features will help keep the inversion rather weak/indistinct and the air mass below 15 thousand feet fairly saturated, likely allowing fog, ice, high humidity and perhaps periods of light flurries to plague the summit probably into early Sunday evening. In addition, daytime heating may also fire off afternoon convection along the Big Island slopes, especially as the trough closes in on the area as the weekend progresses. Fortunately, the trough and band of moisture are set to abruptly shift off toward the east through Sunday night, which may allow the far eastern fringe of a low-level ridge to briefly build in overhead for the early part of next week. Subsidence associated with the ridge, combined with the departure of the moisture should help rebuild the inversion and virtually ensure dry/stable conditions at the summit for much of Monday and into Tuesday. Winds are also set to finally subside as the westerly jet flattens out and retreats northward for Monday night. However, these conditions could be short-lived as latest model runs suggest that another short-wave trough may help reinvigorate the large-scale trough to the north of the state by Tuesday (previous runs had this trough developing into a low further off toward the east). This could result in a sharp spike in summit-level winds, but may also help develop a frontal band and destabilize the air mass over the Big Island around Tuesday night. While this could result in another round of moisture at the summit, the low is projected to rapidly shift off toward the east, which could help stabilize/dry out the air mass through Wednesday (winds may still remain an issue through most of next week though).

Astronomical Observing Quality Guidance
Seeing/CN2 Images are not available today. Sorry for the inconvenience.
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
08 pm HST
Fri 22 Feb
02 am HST
Sat 23 Feb
02 pm HST
Sat 23 Feb
08 pm HST
Sat 23 Feb
02 am HST
Sun 24 Feb
02 pm HST
Sun 24 Feb
08 pm HST
Sun 24 Feb
02 am HST
Mon 25 Feb
02 pm HST
Mon 25 Feb
02 am HST
Tue 26 Feb
02 pm HST
Tue 26 Feb
02 am HST
Wed 27 Feb
02 pm HST
Wed 27 Feb
06 UTC
Sat 23 Feb
12 UTC
Sat 23 Feb
00 UTC
Sun 24 Feb
06 UTC
Sun 24 Feb
12 UTC
Sun 24 Feb
00 UTC
Mon 25 Feb
06 UTC
Mon 25 Feb
12 UTC
Mon 25 Feb
00 UTC
Tue 26 Feb
12 UTC
Tue 26 Feb
00 UTC
Wed 27 Feb
12 UTC
Wed 27 Feb
00 UTC
Thu 28 Feb
Cloud Cover (%) 20 to 40 20 to 40 80 to 100 20 to 40 30 to 50 80 to 100 10 to 30 0 to 20 20 to 40 0 to 20 60 to 80 20 to 40 0 to 20
Cloud Height (km) above sea level 4-5 4-5 4-8 4-5 4-6 4-8 4-4.5 N/A 4-4.25 N/A 4-5 4-5 4-5
Chance for Fog/Precip (%) 95/50 95/50 95/85 95/75 95/75 90/60 65/30 40/10 25/5 10/5 60/20 65/30 60/15
PW (mm, summit upward) 4 to 8 4 to 8 6 to 10 4 to 8 4 to 8 4 to 8 2 to 4 1 to 2 1 to 2 2 to 3 3 to 6 3 to 4 3 to 6
Mean Seeing (arcsecs) 1.2 ± 0.4 1.2 ± 0.4 N/A 1.2 ± 0.4 1.3 ± 0.4 N/A 1.0 ± 0.3 0.9 ± 0.2 N/A 0.7 ± 0.2 N/A 1.5 ± 0.5 N/A
Summit Temp (°C) -2.5 -3 -1 -3.5 -4 -2 -5 -5 1 -3 -1 -7 -2
Wind Dir/Speed (mph)
    Summit (615 hPa)
    19,000 ft (500 hPa)
    24,500 ft (400 hPa)
    31,000 ft (300 hPa)
    35,000 ft (250 hPa)
    40,000 ft (200 hPa)
    46,000 ft (150 hPa)
    54,000 ft (100 hPa)

WSW/20 to 35

WSW/20 to 35

WSW/20 to 35

WSW/20 to 35

WSW/20 to 35

W/20 to 35

W/20 to 35

N/20 to 35

NNW/15 to 30

WNW/10 to 20

WSW/20 to 35

W/30 to 45

W/40 to 60


 Rise and Set times for the Sun and Moon
Night (HST)
Sun Set
Twilight End
Twilight Beg
Sun Rise
Moon Rise
Moon Set
Illum (%)
RA
Dec
Fri.Feb.22/Sat.Feb.23
18:33
19:38
5:32
6:37
21:50
N/A
80
13 49.2
-5 46
Sat.Feb.23/Sun.Feb.24
18:34
19:38
5:32
6:36
22:48
N/A
70
14 41.9
-10 33
Sun.Feb.24/Mon.Feb.25
18:34
19:39
5:31
6:36
23:44
N/A
60
15 34.2
-14 37
Mon.Feb.25/Tue.Feb.26
18:35
19:39
5:30
6:35
0:39
N/A
50
16 26.3
-17 50
Tue.Feb.26/Wed.Feb.27
18:35
19:39
5:30
6:34
1:33
N/A
40
17 18.3
-20 07


Forecast issued by: Ryan Lyman
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Next update at 10 AM HST (2000 UTC) Monday 25 February 2019.
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