Fronts are usually analyzed at the leading edge of cold air. The past few days have seen a stationary front stalled over New England leading, with a moist polar air mass lying to the north and a moist tropical air mass lying to the south . High humidity in these air masses combined with the front itself has given us lots of rain over the past few days.
The image at left shows temperatures taken at 3PM Monday, July 21 2008. Northern New England and Southern Quebec are in a uniform polar air mass where most temperatures (with a few exceptions in the Burlington VT area) lie in the 70's. South of the front, temperatures are more tropical (generally in the 80's).
The station analysis (click on image below for better view) for 18Z (2 PM EDT) on the same date shows dewpoints (green numbers) in the 60's almost everywhere in the northeast, indicating an abundant supply of water vapor. These high humidity conditions make for sticky, uncomfortable weather everywhere. The nightime temperatures are not likely to sink below the dewpoint, meaning warm, clammy nights where you constantly have to flip your pillow to get the cold side (grrrr ... ).
A stationary front extends along the temperature gradient (the area of contrasting temperatures) across New York State and Connecticut. The strong temperature gradient is actually north of the front. Very weak lows associated with cloud and thundershowers are shown over Massachusetts, southern Ontario, and southern Michigan. Weak winds around these systems blow across the stationary front, leading to warm and cold advection. These weak systems are typical of the summer, when extratropical cyclone development is not as well defined as in the winter, making these systems difficult to track. Tropical cyclones, as we will see this week, are much more frequent and obvious at this time of year!
The image at left shows the current Atlantic tropical storm outlook. Tropical Storm Dolly made landfall along the south Texas coast as a Category 1 Hurricane but has since been downgraded to a tropical storm. The clouds in the yellow circle represent a tropical depression (weaker than a tropical storm) that is slowly moving westward across the Atlantic. This could develop into a tropical storm or hurricane over the weekend, but it is too early to tell. You can find updates to this graphic along with a brief forecast discussion at the National Hurricane Center's Webpage.
The second image shows the swath of tropical storm and hurricane force winds (winds greater than 38 mph and 64 mph, respectively) associated with Dolly. The National Hurricane Center has introduced this map because it shows the real effects of the storm. Dolly picks up energy shortly before it hits the coast and attains hurricane status. Dolly then loses much of its "punch" as it hits land. This is typical of tropical storms. Increased friction over land puts a break on the winds, and the storm gets cut off from the warm waters that give them their energy. This image is also updated at the NHC webpage.
The final image is the station map of the south-central U.S. taken 18Z Wed 23 July, 2008 as Dolly made landfall. This map illustrates how small the storm actually is. There is no station plot actually showing hurricane force winds. This is because the actual region of hurricane force winds is very small, but also because there is little data in the eyewall of a hurricane (hurricane force winds tend to topple wind towers over land, and ocean going ships avoid hurricane centers). Most data from hurricane centers is taken by dropsondes every six hours (dropsondes are like rawinsondes except that they are dropped from reconnaissance aircraft flying out of the NHC headquarters in Miami). You can find regional maps and loops like this one at the HPC North American Surface Analysis webpage.
