Air mass advection and fronts

The image at left from 21 Z (5 PM EDT) Tuesday 10 June 2008 will show how air mass advection and fronts around midlatitude cyclones can change the weather in New England.

A low pressure center can be seen in Quebec to the northwest of Vermont. Wind barbs indicate a counterclockwise and inward circulation around the low that is usually seen around a cyclone. A cold front extends soutwestward and a stationary front extends southeastward from the system center.

Three airmasses can be identified in the map. Over Vermont and most of New England, a marine tropical air mass predominates. It's this air mass that brought us near-record high temperatures between the weekend and Tuesday. It is bordered by the cold front to the west and a stationary front to the east, forming a wedge south of the low pressure center. Temperatures are in the 80's and 90's F and dewpoints are in the high 60's. Southerly winds in this air mass advect warm moist air northward in this region all the way from the Gulf of Mexico.

West and northwest of Vermont lies a continental polar air mass, with both temperatures and dewpoints in the 50's and 70's, the cooler temperatures residing to the north. It's this air mass that has moved in today, giving us cooler and drier conditions outside. Winds all have a westerly, with those over Canada having a more northerly component. They advect this airmass eastward. You probably noticed that it cooled off considerably yesterday evening as the front moved through with the storms yesterday.

Finally, over Maine and the Atlantic Ocean, a marine polar air mass had been destroying the hopes of anyone looking for a summer day at the beach. Temperatures and dewpoints along the coast of Maine and the adjacent ocean are all in the 40's and 50's, indicating the strong influence of cool Atlantic waters. It's these air masses that typically lead to ocean fog. Easterly and south easterly winds advect this cooler air onshore. These winds are enhanced by the afternoon seabreeze circulation.

The final feature to notice on the map is the dashed brown line, representing a trough. You can see this also in the isobar looping southward from the center of the low. This was associated with the line of thunderstorms that swept across northern Vermont and southern Quebec yesterday evening and knocked out our power.

You can access a 24 hr loop of New England fronts and weather station data at the NWS-HPC surface analysis website.