Weather-permitting, a 23-story-tall United Launch Alliance Delta IV Heavy rocket is poised for liftoff Monday with a top-secret reconnaissance payload. (United Launch Alliance photo)

Pre-launch preparations for a Delta IV Heavy rocket’s Monday departure progressed through the weekend at Vandenberg Air Force Base as weather continues to cast a cloud over blastoff — and viewing — plans.

Liftoff of the space booster built by United Launch Alliance is targeting 1:46 p.m. Monday from Space Launch Complex-6 on South Base. Depending upon atmospheric conditions, the rumble and rattle of the departure could reverberate around Santa Barbara County.

Since the rocket will carry clandestine cargo, the launch window remains top secret. However, officials earlier said the launch will not occur after 3:57 p.m.

The rocket, standing 23 stories tall, will carry a spacecraft for the National Reconnaissance Office in a mission dubbed NROL-82. It is the first NRO mission in more than two years.

“Everything is progressing toward the ULA Delta IV Heavy launch carrying the NROL-82 mission for the National Reconnaissance Office,” ULA representatives said Sunday.

On Sunday, a slightly improved forecast calls for a 60 percent likelihood that conditions will interfere with Monday’s launch attempt. Key concerns are surface winds, cumulus clouds, thick clouds and precipitation. Friday’s forecast had a 70 percent chance of violating launch rules.

A similar outlook exists for a 24-hour postponement, still forecasting a 60 percent possibility of delay, with surface winds and cumulus clouds among top worries.

Entrance to Vandenberg is restricted to only people with regular access, such as base employees, military members, retirees or special guests. Spectators typically gather around the Central Coast for a chance to see the flight as it heads in a southerly direction en route to carrying its cargo into space.

Rocket launch visibility

If the United Launch Alliance Delta IV Heavy rocket lifts off as planned at 1:46 p.m. Monday, it should be visible in from Santa Barbara to San Luis Obispo within 90 seconds. (United Launch Alliance illustration)

Unlike many of the rocket launch pads at Vandenberg, SLC-6, once eyed for the West Coast space shuttle program, sits tucked in a valley and isn’t visible from sites off base.

Would-be spectators flock to several vantage points around the Lompoc Valley to see the rocket’s flight once it rises above the hills — assuming clouds accommodate those hoping to catch more than a quick glimpse.

Those sites include West Ocean Avenue, the peak of Harris Grade Road and at the western edge of Vandenberg Village near the intersection of Moonglow and Stardust roads.

Due to the launch, Jalama Beach County Park, just south of Vandenberg, will be evacuated from 7 a.m. to 4 p.m. Monday, according to county officials.

The status of Surf Beach, at the western end of Ocean Avenue, remained unclear as VAFB representatives did not respond to an inquiry about whether it would remain open.

ULA will provide a live broadcast, starting at 1:26 p.m. Monday, of the final minutes before launch plus text updates, starting at 4 a.m., of countdown milestones. Clicking here for the webcast and updates.

At the request of NRO, the broadcast will conclude after the payload fairing has been jettisoned, less than 10 minutes into flight.

Noozhawk North County editor Janene Scully can be reached at Follow Noozhawk on Twitter: @noozhawk, @NoozhawkNews and @NoozhawkBiz. Connect with Noozhawk on Facebook.