Narrative:

We were on a visual approach to runway 25L on a very good VMC day (flight visibility exceeded 30 miles). Descending through 600 feet the tower controller cleared aircraft Y to cross runway 25L on taxiway F (from left to right) and to hold short of runway 25R. It seemed a little close to us; but aircraft Y did a nice job of expediting through our runway; and was clear of it by the time we descended through 400 feet. As we were over the approach lights - below 100 feet but not yet even with aircraft Y; the tower cleared aircraft Y aircraft to cross runway 25R. This meant aircraft Y was advancing the throttles - creating significant jet blast - as we were transitioning into our flare directly behind him; thanks to a careless ATC clearance.we quickly recognized what was about to happen; and the captain adjusted his glide path higher to target the final 1;000 feet of the touchdown zone. Even doing so; we were rocked fairly hard by aircraft Y's jet blast. I called the tower on the phone about 40 minutes later to discuss the incident. I could describe the response from the controller I spoke to; as a collective 'shrug'. He specifically said there was no prohibition to crossing a plane between the runways in a situation like that. And that they used to worry about it [years ago]. But they basically didn't worry about it now.this situation could have led to a far more dangerous outcome. Aircraft Y's crew could not see behind their jet; so they would have to assume it was clear behind them. And being heavily loaded; they would need higher power settings to taxi. If we were not alert nor anticipating the situation; aircraft Y's jet blast could have blown us off the runway or into a sideways yaw/roll at a time and altitude which would have put recovery in doubt.

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Original NASA ASRS Text

Title: B737 First Officer reported experiencing jet-blast in the flare from a heavy jet taxiing between the runways.

Narrative: We were on a Visual Approach to Runway 25L on a very good VMC day (flight visibility exceeded 30 miles). Descending through 600 feet the Tower Controller cleared Aircraft Y to cross Runway 25L on taxiway F (from left to right) and to hold short of Runway 25R. It seemed a little close to us; but Aircraft Y did a nice job of expediting through our runway; and was clear of it by the time we descended through 400 feet. As we were over the approach lights - below 100 feet but not yet even with Aircraft Y; the Tower cleared Aircraft Y aircraft to cross Runway 25R. This meant Aircraft Y was advancing the throttles - creating significant jet blast - as we were transitioning into our flare directly behind him; thanks to a careless ATC clearance.We quickly recognized what was about to happen; and the Captain adjusted his glide path higher to target the final 1;000 feet of the touchdown zone. Even doing so; we were rocked fairly hard by Aircraft Y's jet blast. I called the Tower on the phone about 40 minutes later to discuss the incident. I could describe the response from the controller I spoke to; as a collective 'shrug'. He specifically said there was no prohibition to crossing a plane between the runways in a situation like that. And that they used to worry about it [years ago]. But they basically didn't worry about it now.This situation could have led to a far more dangerous outcome. Aircraft Y's crew could not see behind their jet; so they would have to assume it was clear behind them. And being heavily loaded; they would need higher power settings to taxi. If we were not alert nor anticipating the situation; Aircraft Y's jet blast could have blown us off the runway or into a sideways yaw/roll at a time and altitude which would have put recovery in doubt.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.