Rainfall: Metro skunked again

WUNIDS_map.jpg

NWS doppler storm total rainfall shows lack of rain near the metro with higher amounts south.

This is getting old.

Another is a series of rain systems moved through southern Minnesota last night. Again the drought areas most in need of rain got mostly skunked. We’re starting to see a pattern here, and it may not just be luck of the draw.

It would be tempting to say this one just “slid south” of the metro again, but there may be something more going on here. As I watched the radars last night when rain was approaching the metro, you could just see the rain evaporating as it approached the severe drought areas. The rain sustained itself much better in those areas south of the Minnesota River.

Surface reports and the rainfall image above indicate rainfall amounts overnight between .25″ and .50″ in southern Minnesota.

-Twin Cities .02″

-Red Wing .25″

-Zumbrota .31″

-Rochester .46″

-Blue Earth .47″

There is even a narrow swath of rainfall in excess of 1 inch from near Wabasha to Mondovi, Wisconsin evident on the storm total rainfall image above.

Why is this happening?

One reason may be a faulty moisture “feedback loop” near the surface. There is usually some moisture transfer into the lowest levels of the atmosphere from evaporation. In July we would normally expect to evaporate about .25″ per day from the soil into the lower atmosphere. This moisture is usually available for incoming rain systems to tap into to help rain that forms aloft survive the trip to ground level.

Topsoil in and near the metro this year is powder dry. With rainfall deficits of 7 to 8 inches this year in the metro, there just isn’t any available moisture in the topsoil to evaporate back into the low levels of the atmosphere to prevent rainfall from evaporating before it reaches ground level. This was evident as I watched the doppler radar loop as the solid rain area move from South Dakota into western Minnesota toward the Twin Cities. It was like watching the radar in Arizona as virga evaporates before hitting the desert floor.

As you moved south into southern Minnesota last night the rain was able to sustain the trip to ground level much easier where the soil moisture feedback loop is still intact.

It appears that old saying “the rich get richer” applies to drought conditions too.

PH

  • Noelle

    Would pollution have any effect on weather patterns near the metro area? I’ve often seen even severe weather tend to dip north or south of the metro.

  • Erik

    Paul

    You are the expert, but I’ve been thinking that for weeks. I somehow became rain-obsessed this summer and have watched the radar echoes fizzle out over the metro more than once.