Bow echo? Damaging wind, hail, flash flood risk tonight

This is going to be an interesting evening of weather across Minnesota.

All signs point to another severe outbreak tonight. The bulk of guidance products favor storms firing around Fargo, N.D., late this afternoon and racing down the Interstate 94 corridor toward the Twin Cities tonight. A few models take the storm core a little farther south along the Minnesota River valley. Some take the storm core north of Minneapolis-St. Paul.

The National Oceanic and Atmospheric Administration paints an enhanced severe risk zone bracketing the I-94 corridor from Fargo to the Twin Cities tonight.

I-94 express?

Many models favor storms firing in the eastern Dakotas and western Minnesota late this afternoon and evening.

There is still uncertainty as to the precise track of the likely eventual storm core. Will it track along and north of the I-94 corridor into the Twin Cities or farther south favoring the Minnesota River Valley? We'll have to see where the maximum "instability gradient" sets up tonight. Alas, there is still some mystery in weather.

NOAA's North American Mesoscale Forecast System 3 km resolution model paints a blotchy storm cluster near the Twin Cities around midnight, then sends a nasty looking bow echo through Wisconsin toward Milwaukee by Wednesday morning.

Flash flood threat

In addition to the usual widespread damaging wind and large hail threats, there is a threat for flash flooding tonight. If storms stall in any one spot, a quick 2 to 3 inches of rain will fall.

NOAA's NAM model paints the heaviest rainfall zones through Brainerd and central Minnesota just north of the bulk of the Twin Cities.

Elevated threat level

These are the days when we preach severe weather "situational awareness." Know that storms are likely tonight and have a plan. The Twin Cities National Weather Service office has some pretty attention-grabbing language in today's forecast discussion. I've pulled a few clips.

This clip cites a similar pattern that produced a widespread damaging wind event in June, 2010.

Mesoscale models and global models are somewhat at odds with each

other, and despite being about 12 hours away convective evolution

is still far from certain. Looking at the CIPS analog guidance

from St Louis University, today's top analog is June 26, 2010

which featured fairly widespread wind damage and several tornadoes

across southern MN from a QLCS.

Here, a discussion of model variability in the precise location of the likely bow echo (QLCS) tonight.

Indeed, the NAM and GFS QPF fields would hint at a

QLCS tracking along or just south of the I-94 corridor. CAMs such

as the NAMnest, WRF-ARW, and NSSL WRF, in addition to the ECMWF

are a bit different in that they spark supercells in the vicinity

of the ND/SD/MN state borders similar to the NAM and GFS in the

5-7 pm time frame, except they dive that activity SSE across

western MN and along the MN River this evening. It will all depend

where the surface warm front ends up with storms very likely

tracking along the instability gradient. My opinion would be to

follow history and a wider synoptic scale conceptual model similar

to the NAM/GFS at this range, except just a little further south

with an expected convection-augmented boundary. This would put

the greatest threat zone from Alexandria/Morris to the western and

southern Twin Cities metro, toward Red Wing this evening.

Bottom line? Stay alert for watches and warnings tonight. Storms may congeal into a fast moving bow echo packing damaging winds moving parallel to the I-94 corridor tonight.

A few tornadoes (including gustnadoes, which are small tornadoes that form along the gust front), damaging straight-line winds, large hail to baseball-sized and flash flooding may occur tonight. The most likely time for storms in the Twin Cities appears to be between 10 p.m. and 2 a.m.