Now that spring has officially started (3:29 am Monday was the time of the vernal equinox) it looks like we will move out of the spring-like weather we had for the last couple of weeks of winter. The storm track will move back down from the north and bring valley rain with high elevation snowfall in the mountains. Tuesday snow levels will range from 7,500-8,000’ on Tuesday and will then lower to about 6,500’ on Wednesday. Snowfall amounts will of course largely depend on elevation, but will range from just trace amounts at lake level to 1-2 feet on the crests above 8,000’.
Thursday will bring a break in the action, and then another storm system moves in late Friday into Saturday, dropping snow levels below lake level with valley rain or snow.
A couple of days ago, a question was posed wondering if the barometric pressure was more influenced by the location of the low or the coldness of it. Either can be the case. Typically, the deeper the low, the colder the air is in the center… although there are exceptions… after all, hurricanes aren’t exactly known for cold temperatures, but have extremely low pressure in their centers. But winter lows around here are typically cold-core lows, and as a rule they get colder the deeper they are.
But the path of the low also has a lot to do with our barometric pressure. If the low hits us square on then our pressure will generally drop more than if the storm just gives us a glancing blow. But keep in mind… the lows that you see on the satellite pictures aren’t necessarily the lows that the barometer “feels”. The lows you see spinning in from the ocean are mid and upper-level lows, and typically control the circulation up near the 500 millibar level… about 18,000 feet above sea level. But the barometer measures high and low pressure at ground level, and with our winter storms, the surface lows generally come in ahead of the upper-level lows. In fact, the coldest storms usually have a deep upper-level low positioned right above a strong surface high pressure center.
So how can you tell just how cold a storm is likely to be? Let’s review: I mentioned that the coldest storms have a very deep upper-level low coupled with a strong surface high pressure right underneath. If you think of that for a moment, it shows one way to measure how cold a storm will be by looking at the height distance between the 1,000 millibar (mb) and 500 mb surfaces. If you have a strong surface high, you have to go higher above the ground before you reach the 1,000 mb level, and a deep upper level low will cause the 500 mb surface to drop to a lower elevation. The shorter the distance between those two surfaces… the colder the storm is. That 1,000-500 mb thickness is a very important forecasting tool, and is used to help determine snow levels. It’s not a perfect parameter, but is another tool in our satchel to forecast when a storm might turn over from rain to snow.