Todd Gross' Cape Cod and The Islands Weather Page!

WARM ADVECTION - Fancy Term, Obvious results

Warm Advection. It is a term I harp on over and over again, and you will probably be bored of it by the time we are through. However, it produces 90% of the precipitation here in New England. Directly related to "Isentropic Lift", it is simply the warming of the atmosphere (relative to the atmosphere above, not relative to the ground) producing uplifting currents of air, which in turn can produce rain or snow. When coupled with "positive vorticity advection", another fancy term, but a very real producer of precipitation (twisting in the atmosphere that is increasing) - the two team up to give us snow. Many meteorologists on TV especially in OTHER cities nowadays do not use these raw upper level elements to forecast precipitation but instead rely on how much precipitation is actually churned out by the computer models.

Saturday evenings weather event is tied to warm advection , with a little bit of the "PVA" thrown in for good measure. On the chart left, if you click on it, you will see what looks a bit like a surface weather map, but is actually a map from 5,000 ft. above. The area where the winds (following along the pressure, or "height" lines) are blowing across the temperature gradient (the colored lines) from warmer to colder is the warm advection area. . . I have circled it, and we are nearing the end of our event on Saturday evening. Now, we also usually get a last burst of precipitation right under the 850mb Low pressure system which is just about on top of us at that time. Many of the weaker systems that blew by earlier this week were MISSING this warm advection component. West of here, the mountains tend to do okay milking precipitation out of other situations, such as winds coming in from the west after the storm has moved by when upper air low pressure moves by, but the mountains rob us here along the coast of that precipitation, which is why warm advection is so key to MOST , not all, but MOST of our precipitation in Eastern New England.

RAIN SNOW LINE NAILED DOWN?

Nailing down the rain-snow line in a storm which hugs the coast can be a fruitless endeavor. I will share, technically some of the charts that I use to do that on the Cape. (CLICK ON THE CHART LEFT) Note, this is a very unusual storm coming up, one that is this close with winds off the water usually bring on rain. It is so cold aloft though , that the vertical profile of the atmosphere is showing a mix on the outer cape for a couple of hours near Noon on Sunday and that is it, with a few hour change to rain possible on Nantucket. The chart left (click on it) also shows the very "good" rain-snow line indicator for New England, called the 1000-850 mb thickness line. I studied this back in college in a special report in the late 70s, and it has also since become a standard rain-snow indicator in mid winter. Note that it DOES creep up and pass Nantucket for a brief time on Sunday early afternoon, but then it retreats back out to sea for the rest of the storm, meaning any rain will be short lived, we are about to be blasted!

Vertical profile of the atmosphere.. rain vs. snow

Here on the Cape it is a different weather world. Our winds blow off the water, and poof! what should have been snow, sometimes is rain. Even in the great Blizzard of 78' we managed in some parts to escape, and yet last winter we had nearly 100" of snow. What about this weekend. Well, one tool to use is called a "sounding" or vertical profile of the atmosphere. When look at it, you can see warm layers of air that may intrude. Here on the Cape though, it is the lowest layer that is usually indicative of the warm marine air. Let's see what it is "supposed" to look like during the storm this weekend. I used Chatham to try to get the warmest possible reading from the storm. If you CLICK ON the chart you will see that only the very lowest layer of the atmosphere, (you are looking at the atmosphere top to bottom on this chart, the red line is temperature) is above freezing. That means that if it snows hard enough, it should wipe out the warmth and bring us a wet snow. However, often when storms move closer than expected this changes and we become too warm anyway. This is a real toss-up at this point for the outer cape... as we are sure it is cold enough "aloft" for snow, the only wild card is right near the surface itself. To find out how to read soundings and weather maps off the links on this page, purchase WEATHERMAPS, 3rd edition by Peter Chaston available here.