Tropospheric Scatter
TROPOSPHERIC SCATTER (TrS): This almost ever-present condition brings in distant fluttery signals beyond normal line-of-sight.  
Indeed, the general class license test states "signals have a fluttering sound" "is a characteristic of HF scatter".  
Further, "HF signals in the skip zone are usually weak" because "only a small part of the signal energy is scattered into the skip zone".  
Scattering of the signals occurs when they come into contact with discontinuities in the troposphere.  
About the Troposphere:  The troposphere is the lowest portion of the atmosphere.  
Most of our weather takes place in the troposphere.  
It contains 80% of the atmosphere's mass and 99% of its water vapor.  
The troposphere begins at ground level, and its height varies from about 20 km near the equator to 17 km in the mid-latitudes to 9 km near the poles in summer.  
Its height is greater in summer than in winter.  
The upper boundary of the troposphere is a temperature inversion called the tropopause, which separates the troposphere from the stratosphere.  
The pressure of the atmosphere is greatest at the surface and decreases with height.  
Because the temperature of the troposphere also decreases with height (by approximately 6.5 degrees C per km) and saturation vapor pressure decreases with decreasing temperature, the water vapor content of the atmosphere decreases strongly with altitude. 
Methodology:  The troposphere has irregularities in temperature, pressure, and water vapor content due to stratification and turbulance.  
Tropospheric discontinuities can be small temperature or humidity variations, such as can be found around cloud layers, individual clouds, updrafts, downdrafts, the tropopause (the boundary between the troposphere and the stratosphere), small particles and droplets such as drizzle, mist, haze, dust, smoke, volcanic ash, etc., or even flocks of birds and large swarms of insects.  
It is believed that these irregularities and discontinuities, and their effects on electromagnetic wave refraction, are the basis for tropospheric scatter.
Utilization:  Troposcatter occurs when two stations both point their antennas at a common volume in the troposphere, and that volume of the troposphere redirects the signal directed into it by one station towards the receiving antenna of the second station.  
The useful range of troposcatter is roughly 100 to 700 km, and it most commonly useful from 144 MHz through 10 GHz.
Prediction:   Prediction of troposcatter is a complex issue. The best reference that I have found can be found here thanks to Roger Rehr, W3SZ. 
Sure, its a long shot, but we did mention persistence as a prerequisite.