Passage of a cold front: Meteogram

The following shows a meteogram for Barre/Montpelier from 2 UTC 11 March and 2 UTC 12 March, 2009 (10 PM Tuesday to 10 PM Wednesday). Wind barb and cloud cover symbols show southerly winds and overcast conditions early in the period. The temperature (top line in top panel) stays relatively constant at 35 F for most of the evening. With sunrise at 11Z the temperature gradually climbs to a little over 40 degrees at 19Z.

At 20Z (4 PM EDT) the clouds begin to dissipate and the winds begin to shift towards the west as the occluded/cold front moves in. The dewpoint (lower line on upper panel) plummets as drier air moves in, although the temperature actually rises to a sharp peak of 49 F at 22 Z. After this, cold advection takes over and the temperature begins to fall rapidly.

The pressure trace is also consistent with the passage of a low pressure trough. Precipitation occurs between 5Z and 13 Z early in the morning as pressures drop. Pressure bottoms out at 20Z with the arrival of the front.

Finally, the weathercam image for Wednesday shows how dramatically the frontal passage changes sky conditions. For most of the day, a low deck of clouds moves rapidly from right to left (south to north) across the screen. At around 21Z, the front arrives and the clouds rapidly dissipate. They also begin to move into the page, indicating that the wind has shifted to the west. The setting sun can be seen briefly lighting Burke montain and the underside of the dissipating cloud. Nice sunset.

Passage of cold front: Local Maps

Local maps for 18Z (2PM) and 0Z (8PM) show the eastward passage of the occluded front over Vermont. It transforms into a cold front during this period. The winds shifts from southerly ahead of the front to westerly behind the front.

The second cold front approaching the U.S. from Canada weakens during this period. The dashed cold front symbol means that the front is dissipating. Winds behind this front are strong, ranging from 10 to 20 knots. As temperatures are also considerably colder in this region (in the 10's and 20's) than over New England (40's), we can expect cold advection by the westerly winds to cause temperatures to fall at least to these levels.

Passage of cold front: Overview

Wed. 11 March 2009 analysis shows that the weather maker for the northeast is a deep low pressure system (central pressure 976 mb) moving over Northern Quebec. It is following a track typical for Colorado cyclones (see red arrow). The system formed east of the Colorad0 rockies on Tuesday evening, moving rapidly northeastward across Lake Superior to it's current position. An occluded front (purple) extends southward from the system into New York State at this time. An additional cold front extends to the Great Lakes. The counterclockwise circulation around the low pressure system indicates a northwesterly circulation behind the fronts, suggesting that the front will bring in colder temperatures to the northeast.