Temperatures will continue to rise into the weekend with mostly clear conditions. Friday should bring sunny skies, light winds and high temperatures in the 70s. As we head into the weekend, the ridge of high pressure will slowly weaken as a low pressure center begins to move toward the Pacific Northwest coast. A sunny Saturday will give way to a partly cloudy Sunday, and afternoon winds will kick up both days, gusting to the 30 mph range. The low moves close enough on Monday to give us a slight chance of showers, although the latest model runs may keep the low too far offshore to get rain over the mountains.

rain-shadow-1

Yesterday I said that rain can form when air cools as it is moving upward. Conversely, when air moves from a higher elevation to a lower one, it compresses and heats up, and any condensed moisture (clouds) will also warm up and evaporate. At our latitude, the flow in general terms comes in from the west. It scoots up the west side of the Sierra, rising, cooling and ultimately raining and snowing. On our side…the lee (east) side, the air follows the slope downward, heating up and evaporating the clouds.

Obviously, it doesn’t necessarily mean that we won’t get any precipitation here. If the dynamics of the storm are stronger than the suppressive effect of the shadow we can still get some. But we will almost always get noticeably less here on the east side that they will get up in the mountains. Hence, we live in a desert caused by the rain shadow.

 

 

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