Forecasting "Rules of Thumb"

  • On the 700 mb forecast chart, the 70% relative humidity line usually encloses areas that are likely to have clouds

  • On the 700 mb forecast chart, the 90% relative humidity line often encloses areas where precipitation is likely. If upward velocities are present, the chance of measurable precipitation in enhanced.

  • On the 850 mb forecast chart, snow is likely north of the -5 degree Celsius isotherm, whereas rain is likely south of this line.

  • The 5400 1000-500 mb thickness line is also used during the cool season east of the Rockies to forecast rain/snow.

  • 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 that to the east of an area of positive vorticity we usually find upper-level divergence, upward air motions, and cyclonic storm motion

from Meteorology Today by C. D. Ahrens


You are in Calgary, Canada, 100 km east of the Rockies. The current wind is from the north. Looking at a prog for tomorrow, you see that the wind will be from the west. Will tomorrow's temperature be warmer or cooler than today? Explain.

What type of weather would you expect if the 700 hPa relative humidity is 95% and the 1000-500 hPa thickness is 5000 meters?