Giving every opportunity for the storm clouds to blow clear after a four-hour hold, the United Launch Alliance Delta 4-Heavy rocket counts down Thursday afternoon at Cape Canaveral until the clocks had to be halted for the day.

Watch a video replay of how it all went down.

In the end, the rule governing disturbed weather and the attached anvil cloud rule remained NO GO and forced the Range to call a hold at T-minus 60 seconds. The Surface Elctrical Fields rule that had been RED did go GREEN, but that was not enough.

SCRUB! The persistent gloomy weather at Cape Canaveral this afternoon will keep the United Launch Alliance Delta 4-Heavy rocket from flying today to deliver a classified U.S. national security satellite into space.

A combination of rain, clouds and lightning meant it was unsafe to permit the triple-barrel rocket launch during its lengthy opportunity today. Weather rules went out of limits just before 11:30 a.m. this morning and remained that way through the rest of the countdown and a four-hour launch hold.

Liftoff of the NROL-37 mission is rescheduled for Saturday at 1:51 p.m. EDT (1751 GMT). Forecasters say there is a 60 percent chance of acceptable weather then.

Over the next couple of hours, the 235-foot-tall rocket will be safed and its 465,000 gallons of cryogenic fuel and oxidizer drained back into the launch pad storage spheres.

When the rocket does fly, it will carry a top-secret payload to orbit for the National Reconnaissance Office, the agency responsible for the design and operation of the country's fleet of spy satellites.

Experts believe the payload is an eavesdropping spacecraft that will be launched into a circular geosynchronous orbit 22,300 miles high to perform intelligence-gathering on terrorist networks and adversarial nations for U.S. warfighters and policy-makers.

HOLD FOR WEATHER. The countdown sequence has been stopped by the unacceptable weather conditions today.

T-minus 1 minute and counting. The three RS-68A main engines are ready for ignition.

T-minus 75 seconds. The liquid hydrogen fuel tank on the upper stage is being secured for launch.

T-minus 80 seconds. Upper stage liquid oxygen tank has been secured at flight level.

T-minus 1 minute, 50 seconds. The Common Booster Core liquid hydrogen tanks have reached flight level and pressure.

T-minus 2 minutes. The rocket's upper stage liquid oxygen tank is being secured.

T-minus 2 minutes, 30 seconds. The liquid oxygen tanks in the three Common Booster Cores are confirmed at the proper level and pressure for flight.

T-minus 3 minutes and counting. Ordnance devices aboard the vehicle are being armed.

T-minus 3 minutes, 25 seconds. The systems of the Delta 4-Heavy rocket have switched from ground-fed power to internal batteries for launch.

T-minus 3 minutes, 30 seconds. Replenishment of liquid hydrogen and liquid oxygen to the Common Booster Core first stage is being secured in preparation to pressurize the tanks for launch.

T-minus 4 minutes and counting! Now into the final phase of the countdown for liftoff by the Delta 4-Heavy rocket carrying the NROL-37 spacecraft from pad 37B at Cape Canaveral Air Force Station.

Resuming the countdown in one minute.

Permission to proceed has been granted by the ULA launch director and the National Reconnaissance Office mission director.

However, the weather remains NO GO as it has since about 11:30 a.m. EDT today. The team has been instructed to pick up the countdown to run through procedures before calling a hold prior to liftoff at T-minus 60 seconds.

The final readiness polls of the launch team and mission managers have been completed. No technical problems are being worked and all rocket, spacecraft and ground systems are "go" for liftoff at 5:58 p.m.

Now 12 minutes to the target launch time! The launch team will be polled in the next few minutes to confirm all systems are "go" to press onward for the 5:58 p.m. liftoff.

The RED weather rules currently are attached anvil clouds, disturbed weather and electrical potential.

Now entering the final 30 minutes until the Delta 4-Heavy rocket launch from Cape Canaveral at 5:58 p.m. EDT. Here's a look at some stats about today's mission. This will be: done

• The 374th Delta rocket launch since 1960
• The 32nd Delta 4 rocket mission since 2002
• The 9th Heavy configuration to fly
• The 48-49-50th main engine from RS-68 family used
• The 6-7-8th RS-68A main engine flown
• The 468th production RL10 engine to be launched
• The 35th RL10B-2 engine launched
• The 26th Delta 4 rocket launch from Cape Canaveral
• The 7th Heavy launch from Cape Canaveral
• The 10th classified Delta 4 for the NRO
• The 6th Heavy for the NRO
• The 44th NRO launch since acknowledging flights in 1996
• The 21st NRO launch from the Cape
• The 94th Evolved Expendable Launch Vehicle flight
• The 107th United Launch Alliance mission since 2006
• The 25th Delta 4 under the ULA banner
• The 4th ULA launch this year
• The 2nd launch of the Delta family in 2016

NEW LAUNCH TIME has been targeted for 5:58 p.m. EDT (2158 GMT). If the weather remains RED, the launch team will go through its procedures and resume the countdown before calling a hold.

The flight thru precip. rule has gone back to GO status.

While the rocket sits in this extended hold waiting to launch, the cryogenic fuels are replenished to keep tanks full. Each of the three Common Booster Cores and the upper stage use liquid hydrogen and liquid oxygen.

If you are looking for some hope, the launch weather team now predicts a 30 percent chance of GO conditions around 5:40 p.m. EDT (2140 GMT).

The RED weather rules currently are flight thru precipitation, anvil clouds, disturbed weather and electrical potential.

The countdown has been holding here at T-minus 4 minutes for exactly three hours now waiting for a break in the weather that has not come.

If launch does not occur today, it is believed that the next opportunity would come on Saturday afternoon.

While the weather wait continues, a reminder that today's Delta 4-Heavy launch period lasts another couple of hours.

After brief respite, multiple weather rules are RED again -- flight thru precipitation, anvil clouds, disturbed weather and electrical potential.

At this hour, the entire southern half of the Florida peninsula is shrouded by cloud cover. Weather remains NO GO and the countdown continues to hold at T-minus 4 minutes.

The launch director asks that the team remain poised. But a new launch time has not been established as officials wait for a hole in the weather.

Good news, bad news. Liftoff delayed beyond the 3:05 p.m. target. But down to just one weather rule violated now -- attached anvils.

Conditions have improved enough for the weather officer to clear the earlier Surface Electric Fields, lightning, cumulus cloud, disturbed weather and flight thru precipitation violations.

The launch team remains in a flight posture, ready to pick up the countdown at 3:01 p.m. for liftoff at 3:05 p.m. EDT (1905 GMT), if the weather conditions improve. So far, the weather remains out of limits for launch.

NEW LAUNCH TIME has been adjusted to 3:05 p.m. EDT (1905 GMT).

To recap, the day began with mostly clear conditions at Cape Canaveral this morning. But a stubborn frontal system that is nearly stationary over Central Florida began stirring up showers and storms as the morning continued.

By 11:30 a.m., launch commit criteria governing cumulus and anvil clouds over the pad went RED. And the situation went downhill from there with lightning and rain adding to the rule violations.

The odds of acceptable weather have gone from 40 percent at the start of the day, to 30 percent before fueling began and now stand at just 10 percent now.

The weather officer offered a glimmer of hope of possible clearing between 2:15 and 3:15 p.m. EDT.

The new target launch time is 2:55 p.m. EDT (1855 GMT), allowing the weather balloon data of high-altitude winds to be received and analyzed. No balloons could be deployed during the heavy portion of the storm that rolled through a little while ago.

NEW LAUNCH TIME set for 2:55 p.m. EDT when weather improvement is forecast for the Cape.

As expected, the current weather conditions won't permit an on-time launch at 1:59 p.m. EDT today. So the countdown will remain at this T-minus 4 minute mark and holding until the outlook improves. Today's launch window extends a few hours.

T-minus 4 minutes and holding. The countdown has entered the planned 15-minute hold leading toward the 1:59 p.m. EDT liftoff of the Delta 4-Heavy rocket.

Weather permitting, that is.

This pause is designed to give the launch team the opportunity to catch up on any work running behind schedule and verify all is in readiness for the final moments of the count. Also, a series of polls will be conducted during the hold to give approval to proceed with the launch.

The weather improvement trend is continuing, with the best shot for launch today likely coming between 2:15 and 3:15 p.m. EDT (1815-1915 GMT), the weather officer indicates. The odds of acceptable weather in the rest of the launch window have been reduced to just 10 percent.

The Phase 2 lightning warning for the Cape has been rescinded as the storm slowly begins to clear.

The three liquid oxygen tanks in the Common Booster Cores are conditioned for flight.

Launch minus 45 minutes! United Launch Alliance's Delta 4-Heavy is America's biggest unmanned rocket currently in service, capable of lofting the largest and heftiest cargos. Its most recent mission launched NASA's Orion capsule on the Exploration Flight Test in 2014.

The Heavy is created by taking three Common Booster Cores and strapping them together to form a triple-barrel rocket, and then adding an upper stage.

The vehicle stands 235 feet tall, is 53 feet wide, weighs 1.6 million pounds fully fueled and will unleash 2.1 million pounds of thrust at launch.

The rocket comes off the pad powered by three Aerojet Rocketdyne RS-68A engines, one apiece on the Common Booster Cores. The throttleable engine burns liquid hydrogen and liquid oxygen to produce 702,000 pounds of thrust each.

The Port and Starboard liquid rocket boosters stand more than 150 feet tall and the Center Core is over 175 feet with interstage permanently attached. Each booster measures 16.7 feet in diameter and is covered in orange insulating foam.

The cryogenic upper stage also burns liquid hydrogen and liquid oxygen to power its single Aerojet Rocketdyne RL10B-2 engine for 24,750 pounds of thrust. It features a cone-shaped carbon-carbon extendible nozzle that is 7 feet in diameter.

The stage carries 60,000 pounds of propellant and can operate for nearly 19 minutes in all. It features a larger diameter liquid hydrogen tank and a lengthened liquid oxygen tank from the stage used by the lower-power Delta 4-Medium rockets.

One hour till launch! Weather permitting, that is. Today's unclassified launch period stretches to 6:30 p.m. EDT (2230 GMT), offering a long time for the weather to clear.

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The flight hazard area has been established.

The flight slews and commanding tests for the vehicle steering systems are being performed. The three Common Booster Cores and upper stage engine steering checks are running through a pre-launch test pattern.

The vehicle is fully fueled! Loading of the upper stage liquid oxygen tank has been accomplished.

Now entering into the final 90 minutes of the countdown to launch of Delta 374 and the NROL-37 mission for the U.S. National Reconnaissance Office.

The 1.6-million-pound rocket stands fueled and ready for launch at 1:59 p.m. EDT today. The countdown remains on schedule.

For those keeping score at home, the weather rules that are RED right now: Surface Electric Fields, lightning, cumulus clouds, attached anvil clouds, disturbed weather and flight thru precipitation.

The first stage liquid oxygen loading just finished. The tank has been loaded with its supercold oxidizer that is chilled to Minus-298 degrees F. Topping will be completed as the count rolls on.

Loading of the upper stage liquid hydrogen tank has been accomplished as fueling proceeds today at Complex 37.

Two hours and counting!

The lightning rule now joins the cumulus and anvil rules in the RED column.

Fast-filling of the first stage liquid hydrogen tank was completed a little while ago. After post-filling checks and valve tests, the tank will be placed in topping mode. The launch team will confirm the propellant is conditioned for flight.

Odds of acceptable of weather for Delta 4-Heavy today stand at 30 percent favorable. But there is a lengthy launch opportunity to wait for a hole to open up.

Chilldown of the upper stage liquid oxygen system is complete for loading the rocket's tank with 4,500 gallons of supercold LOX. This is the last of the rocket's four cryogenic supplies to be filled in today's countdown to launch.

A check of the current weather shows the cumulus cloud and attached anvil cloud rules are RED as some storms drift over the Cape.

The "go" has been given for the upper stage liquid oxygen chilldown in advance of filling that tank.

The Delta 4 rocket's Common Booster Cores and upper stage are being loaded with supercold liquid oxygen and liquid hydrogen as the countdown rolls on.

Complex 37 has two giant sphere-shaped fuel tanks to store the cryogenic liquid oxygen and liquid hydrogen. The LOX tank holds 250,000 gallons and LH2 sphere about 850,000 gallons.

The cryogenics are fed from the storage tanks through pipelines to the pad. For the Common Booster Cores, the propellants are routed up to the launch table upon which the rocket sits. Tail service masts, the large box-like structures at the base of the vehicle, feed the oxygen and hydrogen to the booster via separate umbilicals.

The upper stage receives its cryos from the middle swing arm that extends from the Fixed Umbilical Tower to the front-side of the rocket.

CBC liquid oxygen tanking operation is switching from "slow-fill" to "fast-fill" mode.

As localized storms continue to push eastward, a Phase 1 lightning advisory for the entire Cape Canaveral Air Force Station has been issued. A higher-level Phase 2 warning could be posted for Complex 37 within a half-hour.

After chilldown of the upper stage liquid hydrogen system, the clear was given for loading the rocket's tank with 10,000 gallons. The launch team is actively filling the upper stage's liquid hydrogen tank with propellant for the RL10 engine.

The CBC liquid oxygen chilldown is complete. "Slow-fill" mode is beginning to load a small percentage of the tanks. The process then speeds up to the "fast-fill" mode until the tanks are nearly full.

A photo gallery of this morning's gantry rollback for today's Delta 4-Heavy rocket launch is posted here.

The launch team is preparing to start fueling the Delta 4-Heavy rocket's upper stage. The "go" has been given to start the chilldown conditioning of the upper stage liquid hydrogen system.

Countdown activities are tracking to the scheduled timeline today and the launch team is working no technical issues.

CBC liquid hydrogen tanking operation is switching from "slow-fill" to "fast-fill" mode.

And now the chilldown of the liquid oxygen systems on Delta's Common Booster Cores is starting. This preps the tanks and pumping to guard against shock when the supercold oxidizer begins flowing into the rocket a short time from now. The three boosters will be loaded with 120,000 gallons of supercold LOX.

Liquid hydrogen flow is confirmed. About 330,000 gallons of LH2 will fill the rocket's three boosters.

The cold gas chilldown conditioning of the liquid hydrogen system has been accomplished. Liquid hydrogen propellant will begin to flow into the Common Booster Cores in "slow-fill" mode. That is sped up to "fast-fill" after a small portion of the tanks are loaded.

Chilled to Minus-423 degrees Fahrenheit, the liquid hydrogen will be consumed by the three RS-68A main engines along with liquid oxygen during the first minutes of the launch.

A "go" has been given to start the cold gas chilldown conditioning of the liquid hydrogen system. This is the precursor to filling the vehicle with propellant.

T-minus 4 hours, 15 minutes and counting. The team is ready for cryogenic fueling as the countdown is underway for today's opportunity to launch the Delta 4-Heavy rocket with the NROL-37 satellite at 1:59 p.m. EDT.

The launch team is manning stations for the start of fueling operations.

T-minus 4 hours, 15 minutes and holding. The countdown has entered a 15-minute built-in hold, a pre-planned pause designed to give the team time to catch up on any work that could be running behind schedule. Once the clocks resume ticking, the main countdown for today's launch operation will begin.

At the launch pad, clearing of personnel is underway in preparation for the start of fueling operations this morning and liftoff at 1:59 p.m. EDT this afternoon.

Also, the powerup of the Delta 4 rocket's avionics has occurred.

In the pre-fueling weather briefing to mission managers from the Air Force launch weather officer Kathy Winters, this afternoon's conditions will be challenging due to frontal boundary lingering over Central Florida.

The odds of acceptable weather for today's launch have dropped to 30 percent.

The system is causing rainshowers and thunderstorms to develop over the area. A cluster of storms to the west of the Cape are pushing eastward already this morning.

The specific launch commit criteria in jeopardy of being violated today include weather rules governing cumulus clouds, lightning, anvil clouds and Surface Electric Fields.

The 330-foot tall mobile service tower has been retracted from the Delta 4-Heavy rocket at Cape Canaveral's pad 37B for today's launch that will place the NROL-37 payload into Earth orbit.

The wheeled structure moved along rail tracks to its launch position about the length of a football field away from the rocket. The 9-million pound tower shielded the Delta from the elements during the its stay on the pad, provided workers 360-degree access to the various areas on the vehicle and was used to attach the payload during the launch campaign. The tower is 90 feet wide and 40 feet deep.

Crews will spend the next couple of hours securing the complex for launch before leaving the danger area around the pad. All workers must be clear of the area for the start of hazardous operations in the countdown, which include fueling the Delta 4's three Common Booster Cores and the upper stage with supercold liquid oxygen and liquid hydrogen propellants.

Testing of communications links between the rocket and Air Force Eastern Range will occur after fueling is accomplished. Steering checks of the RS-68A engines and upper stage RL10B-2 powerplant are on tap in the last hour of the count.

A build-in hold is slated for T-minus 4 minutes, during which time teams will go through final polling to grant clearance to launch. The Delta 4 will transition to internal power as the count resumes, ordnance will be armed and the propellant tanks pressurized as clocks target the main engine ignition time at T-minus 7 seconds.

Liftoff remains scheduled for 1:59 p.m. EDT (1759 GMT).

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Tower rollback is underway.

Clearing its final readiness review and now simply waiting for countdown clocks to begin ticking, a Delta 4-Heavy rocket will haul a classified payload into space Thursday from Cape Canaveral.

Liftoff of the United Launch Alliance rocket is planned for 1:59 p.m. EDT (1759 GMT).

Although the duration of the day's launch window has not been publicly disclosed, officials previously said liftoff would happen before 6:30 p.m. EDT (2230 GMT).

We will have complete live play-by-play coverage of the count and launch on this page, as well as a webcast of liftoff.

A reminder that if you will be away from your computer but would like to receive countdown updates, sign up for our Twitter feed to get text messages on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)

Weather forecasters give a 40 percent chance of acceptable launch conditions due to thunderstorms developing during the window. Various cloud and lightning rules are threatened with violations.

The covert spacecraft riding atop the rocket is owned and operated by the National Reconnaissance Office, the government agency responsible for the country's fleet of spy satellites.

This launch is known as NROL-37.

It is the first of five national security space launches scheduled over the next four months, three aboard Delta 4 rockets and two on Atlas 5.

After the Heavy, the Navy's MUOS 5 communications satellite and the NROL-61 mission are planned on back-to-back Atlas 5 launches, then a pair of Delta 4s will launch the second set of Geosynchronous Space Situational Awareness Program, or GSSAP, satellites for U.S. Strategic Command followed by the Air Force's Wideband Global SATCOM 8 communications spacecraft at the end of September.

This will be the 26th Delta 4 rocket launch to occur at Cape Canaveral and the 7th in the Heavy configuration. It is the 32nd Delta 4 and 9th Heavy overall since 2002.

The triple-wide rocket was rolled from the Horizontal Integration Facility to the pad in early March to be hydraulically raised into the vertical position atop the launch table.

A full countdown dress rehearsal and fueling exercise occurred in mid-April.

The payload, already encapsulated in the rocket's 5-meter nose cone, was transported to the pad for mating with the rocket in mid-May.

Countdown activities will get underway at sunrise Thursday with retraction of the mobile service gantry. Fueling operations will start in the mid-morning.

PREVIEW: One of the largest satellites in the world will launch aboard America's biggest operational booster Thursday, riding that power to a listening post 22,300 miles above the planet for its clandestine eavesdropping mission, all indications suggest.

A United Launch Alliance Delta 4-Heavy rocket will fly from Cape Canaveral's Complex 37 at 1:59 p.m. EDT (1759 GMT). Although the duration of the day’s usable launch window has not been revealed, officials previously said liftoff would occur by 6:30 p.m. EDT (2230 GMT).

Read our preview story.

WEATHER: After tropical weather passes through Florida to start the week, meteorologists say there is a 40 percent chance of acceptable conditions for the United Launch Alliance Delta 4-Heavy rocket to fly Thursday from Cape Canaveral with a reconnaissance satellite payload for the U.S. government.

Read our full story.