Latest Forecast for Mauna Kea Observatories
10 AM HST (2000 UTC) Wednesday 16 May 2018

Warnings
None

Cloud Cover and Precipitation Forecast
There is a moderate risk for fog as humidity jumps into the 60-80% range for the night; precipitation is unlikely. A band of scattered/broken high clouds will continue to pass overhead and especially to the south of the Big Island throughout the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4.5 C this afternoon and 0 C for the night. Winds will be from the SW at 5-15 mph, with seeing near 0.7-0.8 arcseconds. Precipitable water is expected to linger near or exceed 4 mm for the night.

Graphical Summary

Discussion
Weak instability building passing through the area is expected to erode the inversion and lift the low-level cloud flow toward 12-13 thousand feet, which may increase summit-level humidity toward 60-80% and raise the risk for periods of fog at the summit over the next 2 nights; precipitation is unlikely. The atmosphere is set to quickly stabilize and dry out, allowing the inversion to restrengthen near 8 thousand feet through Friday, ensuring the summit steers free of fog for the weekend. Extensive daytime clouds are possible over the next 2 days, then will taper for Friday and over the weekend.

A band of scattered/broken high clouds will continue to pass overhead and especially along the southern half of the Big Island through the night. This band will eventually shift eastward through tomorrow, leaving isolated patches of high clouds in the area for tomorrow night. Clear skies will prevail for Friday night, but there is a possibility that thin strands of high clouds will fill in from the west for Saturday and Sunday night.

Precipitable is expected to linger near or exceed 4 mm for the next 2 nights, briefly slip below 1 mm for Friday night, then increase back to 2-3 mm for the following 2 nights.

While relatively light/calm flow will prevail in the free atmosphere probably throughout the forecast period, an elevated inversion and a risk for moisture at summit-level may result in some degradation and/or variability in seeing over the next 2 nights. Dry and stable conditions are expected to return to the summit, allowing seeing to settle in near 0.5-0.55 arcseconds for Friday night and over the weekend.

The mid-level ridge will continue to breakdown, while a trough passes to the far north and a patch of relatively deep low-level moisture fills into the area over the next 36-48 hours. While instability associated with the trough will remain off to the north, a short-wave trough spawned by this larger trough may briefly pass over the area during this period. This short-wave combined with the patch of moisture may help weaken/erode the inversion and allow for periods of high humidity and fog over the next 2 nights; precipitation is unlikely. High clouds flowing in from the west via a rather broad (but relatively weak) sub-tropical jet, may also contribute to periods of extensive cloud cover mainly for tonight. The bulk of these clouds will slide off toward the east with the STJ, opening up skies for that night and especially Friday night. The atmosphere is also set to stabilize as the short-wave slips eastward and a new mid-level ridge builds in from the west on Friday. The ridge will help reinstill large-scale subsidence in the area, which will rebuild a well-defined inversion near 8 thousand feet and ensure the summit steers free of fog/precipitation over the weekend.

Astronomical Observing Quality Guidance
05 pm HST Wednesday 16 May (03 UTC Thursday 17 May)CN2
08 pm HST Wednesday 16 May (06 UTC Thursday 17 May)CN2
11 pm HST Wednesday 16 May (09 UTC Thursday 17 May)CN2
02 am HST Thursday 17 May (12 UTC Thursday 17 May)CN2
05 am HST Thursday 17 May (15 UTC Thursday 17 May)CN2
08 am HST Thursday 17 May (18 UTC Thursday 17 May)CN2
Latest WRF CN2 Profiles Animation | Collage


5-day Forecast
Summary

Graphical Trend
02 pm HST
Wed 16 May
08 pm HST
Wed 16 May
02 am HST
Thu 17 May
02 pm HST
Thu 17 May
08 pm HST
Thu 17 May
02 am HST
Fri 18 May
02 pm HST
Fri 18 May
08 pm HST
Fri 18 May
02 am HST
Sat 19 May
02 pm HST
Sat 19 May
02 am HST
Sun 20 May
02 pm HST
Sun 20 May
02 am HST
Mon 21 May
00 UTC
Thu 17 May
06 UTC
Thu 17 May
12 UTC
Thu 17 May
00 UTC
Fri 18 May
06 UTC
Fri 18 May
12 UTC
Fri 18 May
00 UTC
Sat 19 May
06 UTC
Sat 19 May
12 UTC
Sat 19 May
00 UTC
Sun 20 May
12 UTC
Sun 20 May
00 UTC
Mon 21 May
12 UTC
Mon 21 May
Cloud Cover (%) 40 to 60 40 to 60 50 to 70 60 to 80 20 to 40 10 to 30 0 to 20 0 to 5 0 to 5 0 to 20 20 to 40 20 to 40 10 to 30
Cloud Height (km) above sea level 4-10 8-10 8-10 4-10 8-9 8-9 N/A N/A N/A 9.5-10 9-10 9-10 9-10
Chance for Fog/Precip (%) 40/10 60/15 55/15 75/20 60/15 40/10 10/0 0/0 0/0 0/0 0/0 0/0 0/0
PW (mm, summit upward) 4 to 8 4 to 6 4 to 6 4 to 8 4 to 6 3 to 5 1.5 to 2.5 0.7 to 1.1 0.7 to 1.1 1.5 to 2.5 2 to 3 2 to 4 2 to 4
Mean Seeing (arcsecs) N/A 0.75 ± 0.15 0.75 ± 0.15 N/A 0.8 ± 0.2 0.75 ± 0.15 N/A 0.55 ± 0.1 0.5 ± 0.1 N/A 0.55 ± 0.1 N/A 0.55 ± 0.1
Summit Temp (°C) 4.5 0 0 4 0 0 6 2 2 7.5 2.5 7.5 2.5
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)

SW/5 to 15

SSW/5 to 15

SW/5 to 15

W/5 to 15

NNW/5 to 15

N/0 to 10

NW/0 to 10

NNE/5 to 15

NE/5 to 15

SE/5 to 15

SE/5 to 15

S/5 to 15

WNW/5 to 15


 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
Wed.May.16/Thu.May.17
19:01
20:12
4:25
5:36
N/A
20:41
5
5 27.3
19 03
Thu.May.17/Fri.May.18
19:01
20:12
4:24
5:35
N/A
21:44
11
6 29.0
20 05
Fri.May.18/Sat.May.19
19:01
20:13
4:24
5:35
N/A
22:44
20
7 30.9
19 45
Sat.May.19/Sun.May.20
19:02
20:13
4:23
5:35
N/A
23:41
30
8 31.5
18 06
Sun.May.20/Mon.May.21
19:02
20:14
4:23
5:35
N/A
0:32
41
9 29.8
15 18


Forecast issued by: Ryan Lyman
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Next update at 5 PM HST Wednesday 16 May (0300 UTC Thursday 17 May) 2018.
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