Major Flood Threat, Including Some Severe Thunderstorms, Targets the Plains Friday Into the Weekend (FORECAST)

Jon Erdman
Published: April 14,2016
A stalled storm system is likely to produce several days of flooding rainfall over a broad swath of the Plains, in addition to a threat of severe thunderstorms from Nebraska to Texas.
In some parts of the High Plains, rainfall totals over a 4-5 day period from Friday into early next week could approach all-time monthly records for April.
When winds at jet-stream level don't simply flow generally west to east (in the northern hemisphere), but take large north-south (or south-north) meanders, 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 diving southeast from the Pacific Northwest will 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 will be in place from Texas as far north as the southern Canadian prairie.

The Flood Threat

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).
As mentioned earlier, there remains some uncertainty regarding how slow the trapped upper low moves. In general, the slower it moves, the greater the potential for heavy rain over the same areas over multiple days.
The map below is a general view of the area where we're expecting the heaviest rain into early next week. Keep in mind this forecast will change as we get nearer to the event, and that 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

In general, many areas of the Plains from Texas to parts of South Dakota could pick up at least 3 inches of total rainfall through Tuesday night, a significant multi-day rain event for the High Plains.
Parts of western Nebraska and western Kansas could tally 5-8 inches of total rainfall over a 4-5 day period, on par with existing April, even May monthly precipitation records in North Platte, Nebraska:

North Platte, Nebraska, Record Monthly Precipitation (Data: NWS)

	Monthly Precipitation Record (inches)
April	7.10 (1915)
May	8.01 (1962)

This potential multi-day rainfall would be roughly equivalent to the combined average April-May precipitation total in North Platte.

In addition, slow-moving clusters of t-storms will rumble across Texas from Sunday morning through at least Monday, if not Tuesday, giving rise to a threat of flash flooding, there.
(FORECASTS: Dallas | Austin | San Antonio | Houston)
There will also be a threat of heavy, wet snow in parts of the High Plains and Rockies, with over a foot possible in some areas.
(FORECAST: An April Snowstorm Ahead)

Severe Threat Starts Friday

This closed, lumbering low finally pushes some of its upper-level energy and cold air aloft into the High Plains on Friday.
Thunderstorms should be rather numerous Friday afternoon and evening in the High Plains from parts of the Dakotas to the rolling plains of West Texas.
These will likely include supercells with large, possibly accumulating hail, damaging winds, and perhaps a tornado or two.

Friday's Severe Threat

The thunderstorm threat may not move much farther east this weekend, and could linger into at least early next week in parts of the southern Plains.
A more concentrated severe threat should be in play once the majority of the upper low's energy finally shifts into the Plains, but that timing is still uncertain.
Each day will likely feature at least some scattered severe thunderstorms, but the outlook is further complicated due to the widespread nature of clouds and rain, which may hold down instability in some parts of the Plains.
(MAPS: 7-day National Rain/Snow Forecast)

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

Yet Another Bizarre Flip-Flop

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 will take 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)