First, the peak being 2-5 weeks away is based on an assumption the virus begane to spread on some date.

Spread rate is comprised of a couple factors - generation time and basic reproduction number (aka r zero or r naught). Generation cycle time ive seen quote for c19 is around 5 days, r zero is typically in the 2 to 4 range. R zero at 3 means it grown as 3 exponent #cycle. 3, 9, 27, 81, etc. The 15th cycle would result in 1.5M cases, 20th cycle (~100 days) is 3.5B cases.....

Peak cycle numbers reported is also a strong function of #tests deployed. Then, you get into test reliability (#false positives/negatives).

Side note, once an infection reaches a certain %of population, r zero drops because folks exposed have been previously exposed/infected. Then it begins to burn out.

Gotta run for dinner. Hope that helps. Its just a quick rather than precise explanation.