I'm not a registered professional engineer either, nor did I stay in a Holiday Inn Select last night, but with a few assumptions detailed below and some calculations from Mark's Standard Handbook for Mechanical Engineers - it looks like you're good for ~100 MPH winds before starting to tip.
Details
- 3x4x6 concrete base is 72 cubic feet of concrete at 150 lb/ft3 so weighs 10,800 pounds
- At 9' tall and 8.5 feet wide, the sign is 76.5 square feet
- With the sign on an 8 foot tall pole, the center of the sign is ~12.5 feet above the top of the base
- Assuming from the pictures that the sign is surrounded by other closely spaced buildings at least the size of a house (more so that it's not on the wide open plains of Nebraska

)
- Assuming that the above mentioned concrete base is sitting on flat ground - not underground or with supporting material around it
- Assuming the flat side of the sign is parallel with the long side of the base (worst case scenario for tipping)
Calculations are where it first starts to tip, not when it actually tips over - that requires a lot more math

.
If anyone has any experience with this kind of thing and/or if anyone would like more details of the above analysis - let me know, I'm glad to share and learn more in the process. The analysis is just a simple sum of moments based on the wind load on the exposed sign and exposure factors from tables in the book.