The Backdoor Cold Front

At the beginning of this week there was a good example of what is called a backdoor cold front in the Midwest. What was striking about this particular front was the contrast between the air behind the front and ahead of it.

A cold front is the leading edge of a mass of cold air that is cooler and/or drier than the air it is replacing, and usually marked by a wind shift. Typically in North America cold fronts move from northwest to southeast, from west to east, or north to south. The system moving through the central U.S. today is your "typical" cold front. The front is moving from northwest to southeast across the Plains and Midwest.

Surface map for 1:00 p.m. CDT June 15, 2016

A backdoor cold front is a cold front that moves to the west or southwest (from the east or northeast), typically in the Great Lakes or along the Atlantic seaboard. These are commonly occur in the spring along the Atlantic coast when colder air is pushed inland as high pressure builds over new England or the North Atlantic. These fronts can also move south along the eastern seaboard as well.

The cold front that pushed in the backdoor of the Midwest early this week was pretty impressive.

In the sequence of surface maps below you can see how the cold front went from an west-east orientation in the first map to a northwest-southeast orientation on the west end of the front. The cool, very dry air pushed from Michigan and Wisconsin across northeastern Illinois, eventually orienting on a line roughly fronm Des Moines, IA to St. Louis, MO to Evansville, IN.

Surface maps for 10:00 p.m. CDT Saturday, June 11 (top), 7:00 a.m. CDT Sunday, June 12 (middle) and 10:00 p.m. CDT Sunday, June 12

Dewpoints in the air behind the front reached the low 30s, not something you see very often in June in the Midwest. Southeast of the front, dewpoints were in the muggy upper 60s and low 70s. Here is a graph of the temperature and dewpoint at Chicago's O'Hare Airport prior to, during, and after the cold front passage. The front moved through the station between 6:00  and 7:00 a.m. CDT on June 12, and in the next hour the dewpoint dropped 16 degrees, and 24 degrees in the first two hours.

Below is an animation of dewpoint maps every three hours that shows the push of the drier air from northeast to southwest.

Loop of surface dew point maps from 10:00p.m. CDT Saturday, June 11 to 10:00 p.m. CDT Sunday, June 12

I watched with anticipation as the cold front pushed south and west during on Sunday, hoping for some of that dry air to clear out the humidity. Winds shifted to the northeast during the late afternoon, but it was almost sunset before the really dry air pushed in. It was, however, only a brief respite from the high dewpoints, and by the morning of June 13 dewpoints were beginning to climb back into the 60s as the muggy air mass to the south replaced the retreating cooler, drier air.

Torrents in Texas

If you watch the CoCoRaHs national precipitation map each day no doubt you have noticed that most of the highest amounts in the past two months - anywhere from 8-16 inches - have occurred in Texas. One of the locations impacted by the heavy rain (by no means the only one) is Houston. The Houston area has received an astounding amount of rain in the past 60 days, largely from two big events. One of these occurred in April, and the other within the past week. Now, any location receiving 12 to 16 inches of rain in 8, 12, or even 24 hours would experience some type of flooding. What is it about Houston that seems to make it susceptible to severe flooding?

One reason is geography. The landscape around Houston is criss-crossed by web of bayous. Bayous are slow-moving rivers or streams generally in flat, low-lying areas. There are often associated with marshes or wetlands and are a tributary to a larger body of water. Houston was founded the Buffalo Bayou, 52-mile long waterway that winds through Harris County, in 1836. The waterways are an integral part of the landscape supporting wildlife, recreational activities, and providing drainage.

Map of the bayous and drainage areas in Harris County, TX.
Credit: Harris County Flood Protection District.

Another reason - the Houston area is becoming a bowl. In the past 100 years the withdrawal of groundwater, oil, and gas has caused the land to sink. The situation is particularly critical in northwest Houston, where wells are tapping the groundwater to supply new residential areas. In the area northeast of two major reservoirs (Addicks and Barker) the land has dropped seven feet since 1906 and continues to do so.

Population increase, population density, and its attendant urbanization is another reason contributing to flood vulnerability. The natural landscape was once dominated by marshes, prairies, and wetlands which helped buffer floods. Much of that has now been replaced by impervious surfaces - buildings, roads, and other paved surfaces - that increase runoff.

The heaviest rain amounts during the heavy rain event of April 17-18 in the Houston area were found north and northwest of the city. The storms were slow-moving and often training over the same areas. The rainfall amounts were bad enough, but the rainfall rates were astounding. A gauge in Pattison, TX in Waller County which lies at the head of Cypress Creek measured 23.50 inches of rain in only 14.5 hours.

Quantitative Precipitation Estimates for the 24 hr. period ending the morning of April 17 (left) and April 18, 2016 (right).

Rainfall amounts (light blue) and flooding in Harris County for the April 17-19, 2016 storm.

The rain gauge at TX-MG-49 Magnolia 10.6 ENE
with 8.47 inches of rain on May 27.
Final total was 11.35". (via Facebook)

This was a particularly challenging storm for CoCoRaHS observers for several reasons. The heavy rainfall occurred overnight, and many were sleeping when their gauges started to overflow resulting in a loss of an actual measurement. Those who attempted to empty the gauge before it filled to capacity were thwarted by nearly continuous lightning which made it extremely dangerous to venture outside. It's not really possible to even estimate the amount of rain once the gauge overflows. The gauge holds from 11.3 to 12 inches (depending on if the tube and funnel are in place), so the most we know from the overflow gauges is that at least 11 inches of rain fell.










The most recent heavy rain event on May 26-27 repeated the April scenario from 6 weeks ago, with the heaviest rain north and northwest of Houston.

Quantitative Precipitation Estimates for the 24 hr. period ending the morning of May 27 (left) and May 28, 2016 (right).

Flood warnings are still in effect from this rain. CoCoRaHS observers in Waller County had the highest two-day rainfall amounts with amounts from 16 to 22 inches. The highest was reported at TX-WA-17 Brenham 9.9 N with a two-day total of 22.41 inches, and TX-WA-24 Brenham 0.7 E with 20.97 inches.

CoCoRaHS 48-hour precipitation amounts for the period ending the morning of May 28, 2016.
Only amounts in excess of 12 inches are shown.

The automated station (AWOS) at Brenham recorded 16.62 inches of rain on May 26, making it the wettest day in the city's history by more than six inches. The Brazos River in Texas surged to a record high 54.76 feet early this evening at Richmond, TX, northwest of Houston. The river is at  4.46 feet above the previous record (50.3 feet) set on October 21, 1994.

Hydrograph for the Brazos River as of 9:20 p.m. CDT June 1. The river reached a record 54.76 feet about 6:15 p.m.

The flooding from this latest storm has resulted in six fatalities and damage to hundreds of homes and buildings. Seven people lost their lives in the April flood. Most of these were in vehicles.

Not a river, but a flooded road in Fort Bend County (west of Harris County).
Note the swimming alligator in the lower right hand corner.
Credit: Fort Bend Sheriff Office via Twitter.

There's no rest for the weary, either. Thunderstorms have been frequent the past several days, and thunderstorms rolled through Harris County and surrounding areas today. The outlook for the next five days paints a wet picture for the Houston area.

Quantitative Precipitation Forecast (QPF) for the 5-day period ending 7:00 p.m. CDT Monday, June 6.

Flooding is the most frequent and dangerous natural hazard in the Houston area, but they are prepared to deal with it. The Harris County Flood Control District continuously monitors stream flow and precipitation to asses the flood potential. It also controls the releases from the reservoirs that hold the runoff from storms, maintains the infrastructure, and develops and implements flood damage reduction plans.You can learn more about the flooding issues in the Houston area on their web site  and view the real-time Harris County Flood Warning System at http://www.harriscountyfws.org/.

Screen capture of the Harris County Flood Warning System web page.