Flash flood watch through Thursday morning

Minnesota Monsoon

Our seemingly perpetual 'June monsoon' weather pattern continues this week.

Flash flood watches are flying now for all of southeast Minnesota and parts of Wisconsin and northern Iowa. Strong thunderstorms will develop overnight tonight, and produce heavy downpours through Wednesday.

FLASH FLOOD WATCH

NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN

1221 PM CDT TUE SEP 20 2016

...MULTIPLE ROUNDS OF HEAVY RAINFALL WILL BRING AN INCREASING RISK OF FLOODING THROUGH THURSDAY...

A BAND OF SHOWERS AND THUNDERSTORMS IS EXPECTED TO DEVELOP ACROSS SOUTHERN MINNESOTA AND PORTIONS OF WESTERN WISCONSIN BY LATE THIS EVENING. THESE STORMS HAVE A GOOD POSSIBILITY OF TRAINING AND PRODUCING 2 TO 4 INCHES IN LOCALIZED AREAS. AFTER A POSSIBLE BREAK WEDNESDAY MORNING...WIDESPREAD THUNDERSTORMS WILL REDEVELOP WEDNESDAY AFTERNOON AND NIGHT ACROSS THE ENTIRE WATCH AREA WITH THE POTENTIAL FOR VERY HEAVY RAIN RATES. A FRONT WILL SAG SOUTHEAST AND FOCUS THUNDERSTORM ACTIVITY FROM SOUTHERN MINNESOTA TO CENTRAL AND SOUTHERN WISCONSIN THURSDAY.

AMOUNTS ARE FORECAST TO RANGE FROM 2 TO 5 INCHES ACROSS SOUTH CENTRAL MINNESOTA TO PORTIONS OF WEST CENTRAL WISCONSIN ALONG AND SOUTH OF I-94. 2 TO 3 INCHES SHOULD FALL FURTHER NORTH ACROSS CENTRAL MINNESOTA AND THE REST OF WEST CENTRAL WISCONSIN. LOCALLY HIGHER AMOUNTS ARE LIKELY...PARTICULARLY ACROSS SOUTHERN

MINNESOTA.

CONFIDENCE IS FAIRLY HIGH. THE PATTERN FOR MIDWEEK CLOSELY MIMICS SEVERAL HIGH END RAINFALL EVENTS IN THE PAST...INCLUDING SEPTEMBER 23 2010 WHERE RAINFALL TOTALS OF 5 TO 8 INCHES FELL ACROSS ALL OF SOUTHERN MINNESOTA. MAJOR FLOODING RESULTED.

Anatomy of a flash flood

Flash floods are produced when individual thunder cells form and stall over the same areas. The best atmospheric mechanism for a flash flood? A stalled stationary front with tropical moisture riding over the top. Throw in a moisture injection from a dying Pacific hurricane for good measure. You can't draw it up any better on the maps for southern Minnesota.

4"+ rainfall totals?

NOAA's NAM model continues to paint widespread 2"+ totals with a relatively narrow band of 4"+ rainfall very close to the Twin Cities into Wisconsin. The big and still unanswerable question is precisely where that heaviest rain band will set up. Where it does, expect flash flood warnings Wednesday into Thursday morning.

Watersheds reacting

It's been a summer of high water across our region. In a rare occurrence, the Minnehaha Creek Watershed issued this today about plans to deal with the expected heavy rainfall. The MCWD is reducing the flow into Minnehaha Creek out of Lake Minnetonka to leave headroom for possible flooding. Lake Minnetonka levels may rise as a result.

In anticipation of the rain, MCWD has lowered the Gray’s Bay Dam’s  discharge of water into Minnehaha Creek to 150 cubic feet per second (cfs) to allow additional room in the creek for runoff from local streets. Depending on the duration and the amount of rain received it is unknown how the creek level and flow of Minnehaha Creek will be directly affected. Minnehaha Creek flows and height at Hiawatha Avenue can be viewed in real time on the USGS website.

The water level on the creek’s headwaters – Lake Minnetonka – is expected to rise several inches during this weather event. MCWD has been working hard to lower the lake level which spiked after heavy rains in August, while preventing flooding downstream. In an effort to draw down Lake Minnetonka to the desired level prior to ice-in, it’s likely the dam will be discharging water into the creek at a rate between 150 and 250 cfs for the rest of the season.  Lake Minnetonka lake levels can be viewed in real time on the USGS website.

No room for absorbtion

The Twin Cities has picked up 20-inches of rain since July 1st. Parts of southeast Minnesota have bagged 30+". That's a year's worth of rain, basically three hurricanes worth. There's just not much headroom in soils to absorb additional rainfall at this point. That's why rivers and creeks may rise faster than usual over the next 48 hours.

Bottom Line: Showers and thunderstorms will develop and increase in coverage overnight through Wednesday. Multiple rain waves will develop. The heaviest rains may occur Wednesday night in many areas. Overall rainfall totals of 1" to 3" will be common, with localized totals of 4" to 6'+ possible by Thursday morning. Commutes will be impacted.

Expect flash flood warning to be issued across the region Wednesday.