Cloud Cover Forecast Maps for North America - New 2022 Aug. 18

For some past events, I have posted cloud cover forecast maps, 
but with time constraints, I will rarely do that in the future. 
Expedition organizers, mobile observers, and others are 
encouraged to use the Web sites I use, given below.

I find the best and most detailed models are the US GFS and 
European ECMWF forecasts, available from this Web site. Note 
that even though it has worldwide coverage, it uses North 
American Central Time, with Daylight time (CDT = UT - 5h) from 
2am of the 2nd Sunday of March to the 1st Sunday of November; 
at other times, it is Standard time (CST = UT - 6h). Also, at 
this web site, cloud cover is not given in the default parameters; 
you need to select "All" parameters and scroll down to expand 
"Clouds" and select "Cloud cover". I find the GFS forecasts are 
a little better than the ECMWF ones, and GSF updates every 6h 
while ECMWF only updates every 12h. For both, you can zoom in by 
selecting a State or Province, and even a county.

I find the easiest site to use, for the USA, is the National 
Weather Service (NWS) site. The main information you want is 
"Sky Cover". This site uses North American Eastern Time 
(EDT = UT - 4h) from 2am of the 2nd Sunday of March to the 
1st Sunday of November; at other times, it is Standard time 
(EST = UT - 5h). The default map is of the "lower 48" US 
States, but you can select "Go to Region" to select a map 
showing several States, or one with individual States. Near 
the bottom of the "Region" menu are options for Alaska, 
Hawaii, Puerto Rico, and some other US territories.

I find it generally too optimistic, but many astronomers 
prefer the Canadian Weather Forecast for astronomy, also 
called "Clear Dark Sky" and previously, "Clear Sky Clock". 
Most use it for individual sites, but it also has maps; I 
find the most useful to be the Sky Transparency maps that 
cover virtually all of Canada and the "lower 48" USA. The 
site uses Universal Time and is updated every 12h.

Sometimes, the type of cloud cover matters. Relatively bright 
stars can often be recorded through high (cirrus) clouds, but 
not more opaque low or middle clouds. With the GFS and ECMWF
forecasts, you can select middle or low clouds, to see if the 
cloud cover is only high clouds or not.  Also, if the relative 
humidity is high, use of dew shields and other measures, such 
as finding paved or less vegetated observing sites, may be 
needed to combat ground fog. It is also useful to look at wind 
forecasts.

David Dunham, 2022 August 18
e-mail:  dunham@starpower.net
cell phone:  301-526-5590