Thursday, August 14, 2014

Too Much Rain

Spectacular flooding rains occurred in several parts of the country in the past few days. On Friday night and early Saturday morning two rounds of heavy thunderstorms hit Kearney, NE. CoCoRaHS observers in and around Kearney reported from, 2.35 to 3.89 inches of rain for the 24 hours ending Saturday morning. Most of it fell in a span of a little more than three hours from 11:00 p.m. to 2:00 a.m.

Radar reflectivity for 11:14 p.m. CDT August 9 (left) and 1:18 a.m. August 10 showing storms approaching
Kearney, NE (inside white circle)

The rain caused flash flooding as you might expect as the amount of water exceeded the capacity of storm sewers to drain it away. You probably have seen this video - it's making the rounds on the Internet, but just in case you haven't I've included it here. A security camera at Good Samaritan Hospital in Kearney captured flood waters bursting through windows on the lower level of the hospital flooding the cafeteria and other areas.

Surface weather map for 8:00 EDT on Monday, August 11, 2014
On Monday, August 11 a nearly stationary low pressure wave along a front in the Midwest helped generate thunderstorms that dumped more 4 to 6 inches of rain across southeastern Michigan in a matter of four hours. One of the worst hit areas was the Detroit Metro area. The Detroit Airport received 4.57 inches of rain, setting a new daily record. The old record was 2.06 inches in 1964. This was also just 0.14 inches short of the all-time daily rainfall records. Daily rainfall records were also set at Flint and Saginaw. The flash flooding from these thunderstorms quickly inundated roads and highways. There were 17 CoCoRaHS reports of rain 4 inches or more with the highest amount 6.31 inches at MI-WY-7 Dearborn 3.5 NE. The water was so deep in some areas that the Michigan State Police sent divers to check flooded freeways in the Detroit area to make sure no one was trapped in vehicles. More information on this event, including many photos, can be seen on the August 11, 2014 Historic Rainfall Event page on the NWS Detroit/Pontiac web site.

Then, Tuesday through early Wednesday morning very heavy to extreme rainfall occurred from the mid-Atlantic into southern New England. This rain resulted from the same system that affected Michigan on Monday. The cold front extended along the Appalachians Tuesday evening, with warm, very moist air feeding into it from the Atlantic and the surface low moving east through northern Virginia.

Surface map for 2:00 a.m. EDT, August 13, 2014
In the upper atmosphere, an unseasonably strong trough extended south from the eastern Great Lakes. This trough produced strong upward motion of the air ahead of it, and all of these components came together to produce inches and inches of rain. This water vapor satellite image shows setup Tuesday night night.

Water vapor satellite image.  Green arrows indicate the wind flow. The divergent wind flow from the mid-Atlantic up through New England helped produce the strong lift needed to generate the heavy rain.

Here is the 500 millibar chart for about the same time;

500 millibar chart for 2:00 a.m. EDT August 13, 2014

In the Baltimore-Washington area flash flooding snarled traffic and caused numerous other problems. Rainfall amounts were in the 3 to 5 inch range around Washington DC, but in Maryland amounts were higher. Several Maryland CoCoRaHS observers reported more the 7 inches of rain,and the highest amount reported was 10.32 inches near Greenhaven in Anne Arundel County.

The band of heavy rain extended from Baltimore-Washington northeast across southern New Jersey and across western Long Island. For more information on the rain in New Jersey, see this special report by the New Jersey State Climate Office. The band was quite narrow, and amounts rapidly fell off as you went north and west from the rain band.

Radar estimated precipitation from the radar at Upton, NY from about midnight on August 12 to noon EDT on August 13.

Long Island showing location of Islip
and CoCoRaHS precip reports
The 24-hour rainfall at the Islip Macarthur Airport on Long Island was 13.57inches as of 9:30 a.m. this morning, and is a new 24-hour New York state precipitation record. The previous record was 11.60 inches at Tannersville, NY on August 27-28 during Hurricane Irene. The daily record was also set at Islip, with 13.51 inches. This amount obliterated the old record of 0.91 inch. The total for the month (so far) for Islip is 14.03 inches, a new record.  The old record was 13.78 inches in August 1990.

On Wednesday the heavy rain shifted further northeast into southeast Maine, where upwards of 6 inches of rain accumulated by this morning.

CoCoRaHS map for Cumberland County, Maine for August 14, 2014.

If you would like some quick summary information on flooding and its causes and costs,Weather Underground has a nice "poster type" web page on floods.

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