Dangerous Flood Threat Underway From Unusual April Multi-Day Heavy Rain Event (FORECAST)

Jon Erdman
Published: April 17,2016
A stalled storm system will continue to produce additional heavy rainfall over a broad swath of the Plains, triggering potentially serious flash flooding in parts of Texas, Oklahoma, Kansas and Nebraska.
In some parts of the High Plains, the average rainfall for April has been doubled in just a couple of days. This includes Dodge City, Kansas, which picked up 4.77 inches of rain Saturday into Sunday afternoon, more than doubling their April average of 1.82 inches. Northeast of Dodge City in Ellis County, a few roads were washed out by flooding.
(INTERACTIVE: Where the Heavy Rain is Now)
Now the dangerous flood threat is targeting locations farther to the South, particularly Texas. Flash flooding was reported Sunday morning in Clifton, Texas, causing several vehicles to stall out and several roads in central Bastrop county in Texas were closed on Sunday due to flash flooding.
(NEWS: Flooding, Severe Weather Impacts)
Below are more forecast details.

Dangerous Flood Threat Continues

Flood watches have been issued for periods of heavy rainfall for a large portions of the southern and central Plains from Texas northward into central Kansas, as well as an area in southern South Dakota. The heavy rain will cause roads, as well as streams and rivers to flood, especially in flood prone areas. Do not attempt to drive through flooded roadways should you encounter one.

Flood Alerts

Of particular concern are slow-moving clusters of t-storms moving across parts of Oklahoma and Texas through at least Monday night, if not Tuesday, giving rise to a high threat of flash flooding, there. This includes cities near and east of Interstate 35 such as Austin, Dallas-Fort Worth, Houston, Oklahoma City, San Antonio and points in between.
(FORECASTS: Oklahoma City | Dallas | Austin | San Antonio | Houston)
The map below is a general view of the area where we're expecting the heaviest rain into Tuesday. Some locations could pick up 5 to 8 inches of total rainfall. Keep in mind locally much heavier amounts may fall where bands of rain or thunderstorms stall for a few hours at a time.

Rainfall Outlook Through Tuesday Night

If that wasn't enough, a threat of some severe thunderstorms with large hail, damaging winds, and perhaps a few tornadoes will be in play in these areas.
(FORECAST: Severe Weather Threat Through Early Week)

Rainfall Totals So Far

Numerous reports of roads flooded in Texas and Kansas have already been received. Here is a list of notable rainfall totals by state, as of Sunday afternoon:

• Kansas: 5.1 inches in Atwood
• Nebraska: 5.08 inches near McCook
• Oklahoma: 7.0 inches in Frederick
• Texas: 4.25 inches near Bastrop, Texas

What's Causing the Heavy Rain?

Sometimes, winds at jet-stream level don't simply flow generally west to east (in the northern hemisphere), but rather take large north-south (or south-north) meanders. When this happens, weather systems producing rain or snow slow down.

Upper-level pattern forecast for Sunday, April 17, 2016, featuring an omega block in the East, trapped low in the Rockies, and deep moisture into the Plains states.

A southward dip in the jet stream over the Rockies has become trapped for a while from a so-called "omega block" of high pressure over the East and Great Lakes, responsible for a much-welcomed warm-up in those areas.
(MET 101: "Omega Block" Brings Welcomed Warm-Up)
With high pressure both to the northwest and northeast of the stuck low, there's nowhere for the closed low to go fast.
On the east side of that swirling, slow-moving low, deep moisture is in place in the central states.
In general, the slower the trapped upper low moves, the greater the potential for heavy rain over the same areas over multiple days.
This stagnant pattern with a deep plume of moisture in place is a prime setup for flooding rainfall, even despite parts of the Plains being rather dry, recently (more on that below).

Yet Another Bizarre Flip-Flop

Drought monitor animation from March 1 through April 5, 2016, illustrating the developing drought (light tan contour) in the Plains.

Over the past several weeks, a flash drought, one which develops quickly, has spread across parts of the Plains states.
Garden City, Kansas, hadn't seen any measurable rain or snow in over two months since Groundhog Day, until they picked up almost three-tenths of an inch of rain Monday.
The combination of this complete lack of precipitation with persistently warm and windy weather this spring has led to several large wildfires, including the largest fire on record in Kansas, hopping across the border from Oklahoma in late March.
(RECENT FIRES: Imagery of March Wildfire | Early April NW Oklahoma Fire)
Finally, a wetter pattern more typical of early spring has taken shape, however, its extremely stagnant nature will lead to extreme precipitation totals in the High Plains, as we mentioned earlier.
Thanks to this sluggish, blocked pattern, Garden City, Kansas, could pick up perhaps twice the average April monthly precipitation, potentially even topping the average wettest month of the year, June.

MORE: Largest Wildfire in Kansas History (March 2016)