It looks like April will start off on a cool and unsettled note for its first week. Expect a hard freeze Friday morning in all western Nevada valleys, with sunny skies and a warmer Friday afternoon in the 60s. A few clouds on Saturday (again in the 60s) will proceed a cold storm system on Easter Sunday giving the valleys a slight chance of rain or snow as temperatures will struggle to get out of the 40s. Temperatures remain in the low to mid 50s through as another storm system gives us one more chance of rain and snow Tuesday.

500 mb chart, courtesy NOAA

500 mb chart, courtesy NOAA

Yesterday, I mentioned when you map a surface pressure feature, you make an isobar map. In other words, the lines on the map represent lines of equal pressure. But when you map an upper level high or low, instead of measuring the actual pressure at certain elevations, you measure the height to whatever pressure level you are interested in. For instance, if you are looking at trying to find the highs and lows at the 500 millibar level, you send up weather balloons at various places around the globe. If one of them reaches that pressure at an altitude of 5300 meters… then that spot could be considered to be in an upper level low compared to another place that needs 5800 meters to reach the same pressure.

In order to map the highs and the lows, it works just like making that topographic map. Lines of equal heights (iso-heights) are drawn through the data points, with the peaks (or ridges) being the highs, and the valleys (or troughs) being the lows. It’s a very important map for weather prediction. This works much better for upper-level features.