Virtually all forecasts seen in the U.S. media, on the Weather Channel etc. are based on the National Weather Service computer model forecasts. Map derived from current observations, primarily radiosonde observations. Comparing an older 12 hour forecast with a more recent Analysis valid at the same time gives an indication of the recent model performance. Significant shortwaves can be found where there are X's surrounded by several vorticity contours. The 12-hour accumulation occurs in the 12 hours leading up to and ending at the valid time, so for the map above, it accumulates between 00Z Thu and 12Z Thu.
Hollow numbers are maximums of precip.
Large positive values sometimes occur in conjunction with heavy precip.