So some good buzz here at the ole’ weather center today as we’re talking about low-elevation snow chances for Friday.
So here’s what we are primarily looking at in terms of will it be cold enough to drop the snow level to the valley floor:
The old standard in weather to determine what the cut-off is between rain and snow is called the “540 Line”. This references an imaginary “line” of pressure in the atmosphere at 540millibar (mb) thickness which is showing 5400 meters of air between the surface (1000mb) and 500mb, which is roughly the middle of the atmosphere. The thickness of the air above you determines if its cold or warm. It’s a good indicator of what temperature your airmass will be when it moves overhead by determining how thick the atmosphere is. The lower the thickness line, the better chances you have to move colder air closer to the surface which tends to be good for generating low snow levels.
At Mississippi State U. (where I went to weather school), we got scolded for relying too heavily on the 540 line as a strict cut off for where the rain/snow line will be. Here’s why it a guide and not Bible for where snow will fall: In the picture above, that is about 4p on Friday on the NAM model. The 540 line is dipping south near Sacramento. So if we followed the 540 rule, everything falling in the Northwest would be straight snow. We know that won’t be the case, but its a sign that there will be a VERY cold pool of air that is capable of generating snow to a very low level.
The more interesting “line” is the one closer to the Low itself, the 528 mb line. It runs from Northern Idaho south to just above the Oregon border into Washington. Traditionally, that’s the one that you watch more intently because it is that much lower in height, as usually everything that can fall near that line will be snow. Again, there are other factors that will come into play (topography, local effects, etc.) but that 528 line we’ll be watching very closely to see if it can slide across Western Oregon late Friday night into Saturday. If it can, maybe we can mix some wet snow down to the valley floor. I think it’ll be close, now we just need to have some moisture left in the air too. Stay tuned …
Chief Meteorologist- Justin Stapleton
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