The key thing to look for to predict and undercast is a temperature inversion along with ample moisture. They are more frequent and predictable in the winter but with that being said they are still hard to predict. The main reason is around here (and most parts of the country) model soundings are all you can rely on, which aren't the most relyable sources but adequate for big picture stuff. If you are at a university or near a Met. office (NWS) that does soundings, you have a better chance at predicting them.
They can occur at all levels but in the White Mountains, they usually average between 2500 and 5000 feet. But occasionally they drop just low enought that we are the only peak above the clouds and other times they drop low enough that almost every summit in NH is exposed while valleys are covered. They vary greatly. More information on temperature inversions can be found here: http://en.wikipedia.org/wiki/Inversion_(meteorology) I was going to link to an NWS site that has a better explanation but the server is currently down. But you can also google "temperature inversions" and get some more sites.
As far as soundings go, model soundings can be found here: http://weather.niu.edu/machine/fcstsound.html or by using google and searching "model soundings". Most forecasters are taught to use Bufkit which is available for Berlin, NH (which is our closest model package) here: http://www.erh.noaa.gov/gyx/wxgraphix3.html. If you want other stations, you will have to google them and get those seperate packages. If you are looking for soundings from actual balloon launches at NWS, they are here: http://w1.spc.woc.noaa.gov/exper/soundings/. You are going to have to do some research on your own on how to read these soundings since it takes semesters worth of instruction to really know what you are looking for. But for undercast conditions, you want to focus on how to look for, identify, and read inversions.
Good luck and have fun.
Last edited by Knapper; 01-20-2011 at 05:18 PM.
Staff Meteorologist/Night Observer, KMWN (Mt Washington Obs., NH)