Rainy Saturday I (warm front, and low pressure system

A rainy Saturday. A good day to stay inside and work on your weather studies.

The map at left for 15Z (11 AM) shows us why. We have a low pressure system currently situated just north of the Great Lakes and moving in from the west. A warm front extends eastward and across New England. This is bringing in steady rain of moderate intensity extending over a broad area from New York State to Atlantic Canada. This type of rain is called stratiform precipitation because it falls from a low-level, flat layer of stratus cloud. Rain like this usually lasts about a day because it is so widespread. It takes time for a system this large to move through an area.

As studies in Chapter 1, the wind circulation around this low (red arrows) is counterclockwise and inward. East of the low (i.e. New England and eastern Canada), this circulation causes southerly winds (FROM the south) that bring in a moist tropical air mass from the south behind the warm front. You'll notice that even though it is cloudy outside, it is still pretty warm.

You can find more maps like this at the NWS-HPC website.

Temperature Inversions

In Chapter 2, we learn that the temperature in the troposphere generally decreases with height at a rate of about 6.5 degrees C/km. These are average conditions, and in fact the temperature profile of the atmosphere varies considerably. In the case of cool, polar airmasses like the one currently over Vermont, there are small layers of the troposphere where the temperature actually increases with height. These are called temperature inversions.
The temperature profile chart at
right (the red line measures temperature) is taken this morning at 12Z (8AM) over Maniwaki, Quebec, and represents the sounding station closest to Vermont from the the North. You will note here that the temperature between 925 and 850 mb (about 2500 to 4500 ft) actually rises from about 6 to 10 degrees C (about 43 to 50 degrees F). On days like this, you'll find that the temperature is actually warmer at the top of a mountain than at its foot!

Charts like this that plot temperature against heigh are called STUVE diagrams. You can accesscurrent charts like this one at the LSC-MET webpage for upper-air data.

High pressure system over North America II

The image at left provides some additional information about the high pressure system discussed below, adding station plots and radar echoes to the analysis. Wind barbs confirm the circulation of wind around the high pressure center over the Great Lakes. In particular, the wind barb over Portland, Maine confirms a northwesterly flow (i.e. wind coming FROM the northwest) over New England. Remember that wind barbs point in the direction the wind is coming from. They point INTO the wind, whereas the arrows in the post below point in the opposite direction (WITH the wind).

It should be no surprise that it is so cool and clear today in Vermont. It's common knowledge that northerly winds bring in cool, dry air (i.e. continental polar air) from Canada.

The image above is updated hourly the Unisys website. It too can be accessed on the course website.

High pressure system over North America

The image at left represents a depiction of pressure contours, fronts, high and low pressure systems, and IR satellite imagery from the National Weather Service. It is valid at 8AM (12 UTC) this morning. The continent is dominated by a massive high pressure system that extends into New England. Note the large area of clear skies associated with this system. A band of cloud associated with a long cold front that extends from the Labrador sea to the southeastern U.S. and is moving out to sea. Arrows indicate the direction of winds circulating around the high pressure system. Note how over New England, northwesterly winds (winds blowing FROM the northwest) push the band of cloud out to sea.

Note that this image is updated and archived every six hours at the NWS/HPC Surface analysis archive and can be accessed on the course weatherpage.