April will certainly start off a lot cooler than March finished. While a very early Thursday morning shower of rain or snow isn’t impossible, the skies should clear out as you go through the day. Temperatures will struggle to reach the mid-50s in the valleys of western Nevada Thursday before rebounding to the mid-60s Friday and Saturday. But as the skies clear off Thursday night, expect a hard freeze Friday morning…dropping well into the 20s, especially as you get away from downtown Reno.  Another chilly storm system will bring a slight chance of rain or snow to the region on Easter Sunday.

Yesterday I mentioned there are two ways to measure high and low pressure systems. To map them at the surface, you simply measure the pressure at sea level (or measure the pressure at ground level above the surface, and correct to sea level) at various points on the map. Plot the pressures in millibars, and connect lines of equal pressures, much like you would when drawing a topographic map. Those lines are called isobars, and they define the surface highs and lows.

When mapping upper-air features, you use some of the same techniques, but you are actually measuring something different. Tomorrow, I’ll tell you how to map those 500 mb lows.