A Few Forecasting Rules of Thumb

A Few Forecasting "Rules of Thumb," or "Translating the Chart Numbers"
FORECAST QUESTION	USE OF FORECAST CHART
Cloudy or clear?	On the 700-mb chart forecast chart, the 70 percent relative humidity line usually encloses areas that are likely to have clouds.
Will it rain?	(a) On the 700-mb forecast chart, the 90 percent relative humidity line often encloses areas where precipitation is likely. If upward velocities are present, the chance of measurable precipitation is enhanced. (b) Along the west coast of North America, precipitation is much more likely north of the 5640-m height contour on the 500-mb forecast chart.
Will it rain or snow?	On the 850-mb forecast chart, snow is likely north of the -5°C (23°F) isotherm, whereas rain is likely south of this line.
Will the surface low intensify?	For the storm to intensify (deepen), an area of upper-level divergence must be over the cyclonic storm. On a 500-mb forecast chart that shows vorticity, look for a vorticity maximum (vort max) and remember from Chapter 13, p. 359, that to the east of an area of positive vorticity we usually find upper-level divergence, upward air motions, and cyclonic storm development.