This flight took off from BFI at 1730 Zulu and contact was lost 35000
feet over Meridan, ID at 1818 Zulu, what is the best explanation
why radar contact with this flight was lost?
These 8000 flight numbers look like equipment repositioning. Maybe Hand off of flight was punched in as different flight Number while enroute, SWA8553 for example.
Others may have more info on this.
N258WN.
Is this the registration # of the SouthWest Flight??? 
edit: and it is a dilivery to SWA from Boeing. back to original question, why the tracking probs.
Yes and yes.
VFR on top
Does that mean they flew the remainder of the delivery flight VFR?
Don’t think so!
Class A airspace IFR is required, not an option.
You cannot be VFR on top in Class A airspace.
Allen
I think its a problem with handoffs between ARTCC ‘centers’. The flights I have seen drop off are roughly on the borders. Also, one of my flights the other day did this around where Seattle and Oakland centers hand off.
And yes, would not be VFR On Top above FL180. On clear winter days, Horizon Airline pilots can frequently be heard requesting ‘VFR On Top Direct Rainier’.
This flight actually left our atmosphere and returned around Texas.
I noticed a few other flights flying eastbound over Salt Lake Center drop tracks in the same area.
DM
Good point.
Ahhh, no! VFR at FL350???