Model simulation of cloud cover at 6 p.m. (EDT) today shortly before the transit of Venus commences. Color shades indicate percent of cloud cover with green and blues representing mostly cloudy skies and pinks and oranges partly cloudy skies. Note: this is a simulation and will not perfectly match reality. (NOAA)

For skywatchers in the U.S., clouds may well be an issue for substantial parts of the country, including the Washington, D.C. region. NOAA’s Rapid Refresh model shows about half the country shrouded in at least partial cloud cover at 6 p.m. EDT (the transit begins at 6:04 p.m. EDT).

Links: Transit of Venus 2012: the last trip across the sun for 105 years | As Venus crosses the sun, a rare treat for skywatchers

Here’s a general idea of where some of the best and worst viewing will be:

Best viewing (clear skies likely):

* Southern California and the Southwest U.S.

* The western Great Lakes

* The Midwest

Cities which look to be in good shape include Los Angeles, Phoenix, Chicago, Milwaukee, Minneapolis, St. Louis, and Kansas City

Worst viewing (cloudy skies likely):

* Pacific Northwest

* Northern Rockies

* Southeast

* Coastal New England

Cities which will probably be cloudy and unable to observe the transit include: Seattle, Portland (Or.), Boise, Atlanta, Charleston, Miami, Portland (Me), Boston and New York City.

Weather forecast map at 8 p.m. EDT showing front sprawled across south, and cold front in the northern Rockies. (National Weather Service)

* A cold front moving through the Pacific Northwest and Northern Rockies

* A stationary front draped across the south from Texas to the Carolina coast

* Upper level low pressure over New England

Note: this is a rough overview and local conditions may vary. Check your local conditions and forecast for additional detail.

Link: U.S. satellite image and loop

Washington, D.C. area viewing

The Washington, D.C. metro region and the mid-Atlantic sit between the upper level low over New England and the front in the Southeast. Consequently, skies will be mostly cloudy meaning viewing prospects are murky, but not hopeless - a few breaks are possible. Stay tuned to our Twitter feed and Facebook page for updates.

WJLA has a nice compilation of organizations hosting viewing parties in the D.C. area which include the National Air and Space Museum (the District), the University of Maryland Observatory (College Park), the National Science Foundation (Arlington), and the NASA Goddard Visitor Center (Greenbelt). Most of these events start between 5 and 6 p.m. and continue until 7:30-8:00 p.m. when the sun sinks behind the trees.