Moisture from former tropical storm presents flood threat from New Mexico to Great Lakes
Flash flood watches are stretching from the U.S.-Mexico border to the Great Lakes ahead of moisture streaming north from the remnants of Narda, formerly a tropical storm that's raining itself out over Sinaloa and Chihuahua, Mexico. The Weather Prediction Center has declared a "moderate risk" of flash flooding for a broad swath of central United States, where a widespread two to four inches may fall through Wednesday.
Some meteorologists say this weather pattern shares characteristics with a "predecessor rain event," which involves a vigorous low-level stream of tropical moisture interacting with a stalled cold front.
Imagine pressure washing a wall; all that water slides down the wall and pools at the bottom. That's sort of how these events work. Except, in this case, the cold front draped across the Midwest is the wall, and the stream of water is a river of moisture from Narda drawn north by atmospheric steering currents.
The axis of heaviest rainfall occurs just east of the front, on the warm side. Closest to the moisture source will be places such as Roswell, N.M. and Amarillo, Tex. Heavy rainfall could spell problems with flooding through early Wednesday, especially on more rural country roads, which in that region are frequently made of dirt or, on occasion, clay. Heavy rain was already falling there Tuesday morning.
After arcing through the Texas and Oklahoma panhandles, the railroad track of rain enters Kansas, where the flood risk is even higher Tuesday and Wednesday. Wichita, Topeka and even Kansas City will see high rainfall rates in any storms. North of the city, renewed river flooding is possible along the Missouri and its tributaries, which are already running several feet above flood stage.
The rain will increase in aerial coverage and intensity over Kansas during the afternoon hours on Tuesday.
Even parts of Nebraska, Iowa, Wisconsin and Michigan are in the target zone for heavy rainfall especially the second half of Tuesday into early Wednesday. Thunderstorms over Iowa will continue to produce locally heavy rainfall rates, though very much "off and on" in nature. Little movement is expected in shower and downpour activity, with moisture continuously overrunning the stalled front.
It's already been a wet year for residents of the Plains, who dealt with repeated rounds of historic flash flooding. Widespread flooding struck the Plains in March, forcing the National Weather Service in Grand Island, Nebraska. to flee their office. In May, more than a foot of rain fell near Tulsa within only a few days' time, forcing the Arkansas and Cimarron rivers into major flood stage.
Meteorologist Alex Lamers pointed out on Twitter that Topeka, Kansas City, Grand Island, and Des Moines all have had a top five wettest year to date.
Over some of these same areas, the Storm Prediction Center has also included a slight risk for severe thunderstorms on Tuesday, citing a risk of one or two tornadoes and isolated instances of damaging winds. This risk would be maximized if any glints of sunshine can badger their way through the overcast skies, enhancing instability. A corridor to watch is from Salina, Kansas. to Sioux City, Iowa. It's unlikely any cells will be able to break themselves off the stalled front enough to freely rotate.
Parts of Kansas and Oklahoma will see off-and-on showers through Thursday as more moisture swirls around the blocking high to the east. Finally, the pattern starts to break down late in the week, a surge of cool, dry air plunging south and east, while heat builds into the west.