(1) According to the image, the coordinate of point A is (10,30).
Therefore, the analytical formula of straight line OA is v=3t,
When t=4, the coordinate of point D is (4, 12).
∴OT=4,TD= 12，
∴s= 12×4× 12=24(km)；
(2) when 0≤t≤ 10, OT=t, TD=3t (as shown in figure 1),
∴S= 12？ t？ 3t=32t2，
When 10 < t ≤ 20, at this time, OT=t, AD=ET=t- 10, TD=30 (as shown in Figure 2),
∴S=S△AOE+S rectangular ADTE =12×10× 30+30 (t-10) = 30t-150,
When 20 < t ≤ 35, the coordinates of ∵B and c are (20, 30) and (35, 0) respectively.
The analytical formula of line BC is v=-2t+70.
∴D point coordinates are (t, -2t+70),
∴TC=35-t, TD=-2t+70 (as shown in Figure 3),
∴S=S trapezoidal OABC-s △ DCT =12 (10+35) × 30-12 (35-t) (-2t+70) =-(35-t) 2+677.
(3) The maximum value of s1= 32t2 (0 ≤ t≤10) is 150≤650,
S2=30t- 150=650，
It's impossible,
When t=35, S=-(35-35)2+675=675(km), and 450 < 650 < 675,
Therefore, Qingbaijiang City will be affected, and the impact time t should be between 20 hours and 35 hours.
From -(35-t)2+675=650, we can get t=30 or t=40.
So it will invade Qingbaijiang city 30 hours after the thunderstorm.