Undoubtably due to a tempearture inversion over the valley. It won't last, but they are more frequent during warm weather. The signal sort of skates in under the inversion, sort of a ducting effect. The real bummer is when your signal originates from *above* the inversion layer, then much of skips off the top of the layer, making the signal sound crappy down below...
copied this off the web for you info. It doesn't relate to radio signals, but explains the inversion.
A temperature inversion is a thin layer of the atmosphere where the decrease in temperature with height is much less than normal (or in extreme cases, the temperature increases with height). An inversion, also called a "stable" air layer, acts like a lid, keeping normal convective overturning of the atmosphere from penetrating through the inversion. This can cause several weather-related effects. One is the trapping of pollutants below the inversion, allowing them to build up. If the sky is very hazy, or is sunsets are very red, there is likely an inversion somewhere in the lower atmosphere. This happens more frequently in high pressure zones, where the gradual sinking of air in the high pressure dome typically causes an inversion to form at the base of a sinking layer of air. Another effect is making clouds spread out and take on a flattened appearance. Still another effect is to prevent thunderstorms from forming. Even in an air mass that is hot and humid in the lowest layers, thunderstorms will be prevented if an inversion is keeping this air from rising. The opposite of a temperature inversion is an unstable air layer.
another copy and paste related to radio signals...
Weather is an additional factor that affects the propagation of radio waves. In this section, we will explain how and to what extent the various weather phenomena affect wave propagation.
Wind, air temperature, and water content of the atmosphere can combine in many ways. Certain combinations can cause radio signals to be heard hundreds of miles beyond the ordinary range of radio communications. Conversely, a different combination of factors can cause such attenuation of the signal that it may not be heard even over a normally satisfactory path. Unfortunately, there are no hard and fast rules on the effects of weather on radio transmissions since the weather is extremely complex and subject to frequent change. We will, therefore, limit our discussion on the effects of weather on radio waves to general terms.
fmnostatic
> Not too long ago, KOSO actually moved their tower from Mount
> OSO west of Modesto, to a site at ground level just west of
> the city. They boosted their power to 50KW, but their
> coverage actually shrunk a bit. They now cover Modesto and
> Stockton with a stronger signal. As for Sacramento, I
> suppose it could be weather related. My trips up that way
> always have KOSO getting pretty weak in the southern Sac
> burbs. So, i guess maybe it is weather related--!!
>
>
> > Folks,
> > I don't know if this is a new frequency that comes out of
> > Modesto (Patterson is COL). This station is now just
> booming
> > in and my scan button catches it in most areas where it
> > never used to. I went to the Central Cal board here and
> saw
> > nothing major mentioned. Was there an upgrade? Is it the
> > weather (hate that excuse, wish it were more legitimate)?
> > Looked on Radio-Locator and its a 50kw directional on
> 93.1.
> > From the signal "map" it did not look at all directional.
> > What are the closest 93.1's? Thanks for the info!
> >
> > -Timmy
> >
>
