You may have seen my forecast on Fox 44 News at 10pm for the mountains, calling for undercast.
Undercast is NOT something you wear, let's just get that straight! Undercast is the opposite of overcast, which I'm sure you've heard of! Overcast is another term for a cloudy sky. Undercast is the term for a cloudy sky, UNDER the observer. How can that happen, you ask? Picture yourself on a mountain top, where that cloud deck is lower than the peak of the mountain top. Ohhhh. It all comes together now!
I'm forecasting an undercast sky for the mountain tops on Tuesday morning and early afternoon. Now, this is assuming the cloud cover doesn't break too much Monday night. A layer of moist air should get trapped underneath an inversion layer (a layer in the atmosphere where the temperature increases with height, just opposite of what typically happens). That inversion, or warm layer, is stable. It acts as a cap on the atmosphere. As moist air gets trapped below that inversion, you will have a stuck layer of cloud cover, and perhaps some drizzle. So, why undercast? Because that warm (inversion) looks to be located at about 1800'-3000' elevation...just under many of our mountain tops and mountainous areas.
Check out the photo to the left. This is the forecast sounding (a forecast of the vertical setup of the atmosphere). Remember the bottom of the picture is the ground, and the top is thousands of feet in the atmosphere. Now the bottom axis is temperature in Celsius, which slants slightly, up to the right with height on the image. The red line shows temperature. Notice how that red line begins to slant toward warmer temperatures above about 1800' elevation. That is the warm, inversion layer (surrounded in pink in the image). Finally, the green line on the sounding shows the dew point temperature. If the red and green line meet, then the air becomes saturated. You'll see the 2 lines very close below that trapping inversion, indicating where a layer of cloud will set up. The sounding shows this as the light gray striations horizontally on the image.
The photo, bottom left (click to enlarge), shows what undercast looks like from the top of Mount Washington (another day).