The image at left shows the frequency of Atlantic Hurricanes over the last 100 years as described by the climatology at the NHC Homepage. It shows that while the hurricane is well underway in June and July (the season officially begins June 1), the season really spins up in August and peaks in mid-September. So we can expect an increase in tropical storm activity and intensity throughout August. This peak is due to the fact that ocean waters actually reach their peak temperatures in September (dry land reaches peak temperatures in late July).

This is also accompanied by a slight change in the track of the storms. The July climatology shows tropical storms developing over the West Indies and tracking either to the Gulf of Mexico or along the coast of the Southeastern States. Note that hurricanes Dolly and Cristobal both followed these paths, respectively, in the past week.The August climatology shows an over all intensification of tropical cyclone activity, but also a new track towards Florida not visible in July. As well, storms appear to be more likely to veer northward in the Gulf of Mexico, unlike July storms like Dolly which hook westward towards the coast of Southern Texas and Mexico.

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.

Hurricane/Tropical Storm Dolly

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.

Steering flow

Clicking on the image at left will show a 24-hr animation of the stationary front over New England on Monday 21 July, 2008. It is being distorted by small mesoscale disturbances (or low pressure systems) that advect parts of the front northward and southward as they rapidly move eastward. They also trigger local showers as they pass through a given area. These mesoscale systems are in fact smaller than the large synoptic-scale systems we usually track in the other seasons. Recent animations of New England station plots and fronts can be found and the HPC surface analysis page.

These disturbances are steered over New England by the upper flow. Clicking on the image of left for 8 AM EDT Sunday 20 January 2008 shows the jet stream on a 500 mb map dipping south of the Great Lakes and across New England roughly above the stationary front described above. It is this westerly flow that guides system eastward across New England.

The 24 hr precipitation field for 8 AM Sunday through 8 AM Monday reflects the path of small scale distirbances in the steering flow. Precipitation fall in the northern U.S. follows the same curved pattern around the Great Lakes and into New England as the systems guided by the jet stream. These maps (and more) can by found at the Daily Weather Maps webpage of the HPC.

Stationary Front over New England

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!