From the ABC 7 Weather team

A look at the set up for the massive flooding in the D.C. area

September 9, 2011 - 10:10 AM
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Look at some of the things that were going on in the atmosphere to cause such heavy rain.


Flooding on Route 50 in Fairfax from Twitter user @alliecat827

The massive flooding over the past 4 days in the D.C. area has been the ultimate result of a decaying tropical system to the west, a cold front to the northwest, a blocking high to the north and Hurricane Katia churning off the east coast. Here's a more in depth of the set up that resulted in such heavy rain across the D.C. area.


700mb Winds Wednesday Morning
Above is the 700mb winds and relative humidity, showing the remnant area of low pressure from Lee over the southern Ohio Valley. This helped ensure a southerly flow over the Mid Atlantic and a perfect fetch of tropical moisture from the Atlantic Ocean. You can see the green shaded area stretching from of the GA and SC coast north into the D.C. area. With a weak high to the north and an area of high pressure to the west, this low has been basically stationary over the past 4 days. Keep note that the main jet over the past few days has also been displaced well to the north so this low was cut off from the main flow in the atmosphere.
500mb Vorticity

Vorticity plots show areas of atmospheric lift which you can find by looking for areas of positive vorticity advection. In this picture, you would be looking for the areas of yellow, red and orange and see which direction they are going over time. This image showed one of the waves of energy spinning in a counter-clockwise manner around the low. You can see the small area of vorticity over the Richmond area which over time moves north, hence putting the D.C. area under positive vorticity advection. This was part of the wave of energy that helped bring heavy rain to the area Wednesday afternoon.


Surface Analysis Wednesday Morning
Above is the surface analysis from Wednesday which shows the surface low pressure to the west over West Virginia and a stationary trough extending from Richmond north through Southern Maryland, Baltimore and eventually Pennsylvania and New Jersey. That was a well drawn boundary, as often the areas of heavy rain will track along these boundaries, which is exactly what happened Wednesday. Heavy rain fell from Southern Maryland to Baltimore in our area and helped produce the massive flooding you've seen from videos in Ellicott City, MD.

Here is the balloon sounding from Wednesday morning, which is important to look at to see the amount of saturation, instability and wind direction. The red line is the temperature and green line is the dew point and you can see the different levels to the left with 1000mb closer to the surface and 300mb in the upper portion of the troposphere. This sounding is very saturated all the way up in the atmosphere with both lines so close together. This will help make for high precipitable water values which could lead to heavy rainfall. Next check out the winds. They are pretty much unidirectional, which means once rain or storms form, it will move in one direction, and if a line forms, numerous storms will begin to train over the same area.


Precipitable Water Values

This is from the SPC Mesoanalysis page from early Wednesday afternoon, showing the incredible precipitable water values over the east coast from North Carolina to the D.C. area. This basically is an indicator of available moisture in the troposphere over a given area. As you already saw the wind direction above, it is important to note that high moisture content was right along the area of surface convergence and storms will tend to build then train over the same area. An absolutely terrible set up for flooding.


Water Vapor Imagery Wednesday Morning

The water vapor imagery clearly shows the low to the west of the region and the plume of moisture streaming directly over the D.C. area with a fetch coming directly from the Atlantic. The bright blue and green colors denote the very high cold cloud tops around -60 degrees C. When you see cooling cloud tops you know you'll be in big trouble over the next few hours and heavy rain will become more likely. Here was a mesoscale discussion from the Storm Predicition Center yesterday concerning heavy rainfall for the area.


Visible Satellite from Wednesday late morning
Finally we get to the end of this montage of weather material with the visible satellite from Wednesday morning. This is one of those pictures you do not want to see as a meteorologist, as storms are lined up all the way into the Atlantic Ocean and are moving northward along the same areas. This will show you the potential for training thunderstorms and flash flooding right away. Unfortunately just that happened.


So the forecast was for heavy rainfall, but the exact placement was tough and timing was even tougher. When these situations arise, we often rely on nowcasting, looking for new material anywhere we can get it to have a better understanding of what is happening, where it is headed, and when it will end. As the sunshine finally breaks into the D.C. area, we as meteorlogists are finally breathing a bit of a sigh of relief and are wishing everyone in the D.C. area a great weekend to finally dry out.
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