Vapor channel from tropics

Warm front advances
The northeastern frontal map for 12Z Thu. 24 July, 2008 (left) shows that the stationary front that was over southern New England on Monday is being pushed northward by southerly winds circulating in a counterclockwise direction around a low pressure system over Lake Ontario. Cloudy conditions. Dewpoints in the high 60's almost everywhere in New England indicate humid conditions. Overcast conditions keep temperatures int he 70's all day.

Transport of humidity
The main weather story today (and all of this week!) is the rain and humidity. This is largely because of a channel of water vapor (i.e. humidity) that has opened up between New England and the Gulf of Mexico. This occurs along the narrow band of clouds ahead of a cold front along the Atlantic coast in the image at left (valid 12Z Thu. 24 July, 2008). A cold front extends southward from the low pressure system over southeastern Ontario all the way to the Gulf of Mexico. Red arrows in the image at left show the direction of the warm, humid flow along the Atlantic coast of the U.S. ahead of the cold front. This flow is pushing the warm humid air northward. The cold front is pushed eastward by a surge of cooler, drier westerly winds behind the front (blue arrows).

Warm conveyer belt
This is pretty typical of a mature phase of mid-latitude cyclone development shown at left (see Jetstream section of cyclone model for details). The image shows a warm, humid conveyer belt (red arrows) overrunning cooler air to the north where the warm flow meets the warm front. This rising motion leads to large amounts of precipitation ahead of the warm front. Note that the occluded front (purple at left) does not appear on todays map. Occluded fronts are rare in the summertime because weather systems evolve more slowly, with fronts moving much more slowly. Your text discusses this in more detail in Chapter 10.

Vapor channel in satellite imagery
The vapor channel is easy to see in satellite animations. Click here to see an animated loop of the warm conveyor belt (or vapor channel) discussed above. The cold front can be discerned along the border between clear and cloudy skies in the eastern half of the U.S. The thunderstorms and clouds along the eastern seaboard ahead of the front all moves northward in the fast moving southerly flow ahead of the front. Note also Tropical Storm Dolly along the Gulf coast of Texas/Mexico.