This picture, from a current (as of this writing) accuweather forecast page, is an excellent illustration of what happens here in the upper Midwest the spring.

The overall pattern of movement of air masses at the continental scale is west to east, with extra moist and extra warm air secondarily moving north from the Gulf of Mexico, and cool and usually dry air coming form the Northwest (not shown here but note the “blizzard” part) and with dry Pacific/Rocky Mountain air coming from the west. The main energy flow to keep an eye on is that coming from the Gulf.

There is a rule of thumb you can apply here….

If you see continental above-sea level areas forming cup-shaped or lunate regions pointing at the equator, then you may be looking at a heat trap, where water masses will become relatively warmer than expected given lattitude and time of year. The Gulf of Mexico (given the way Florida encloses it) is one such example. At an equivilant latitude in the Pacific or Atlantic warmth accumulated on the sea surface has multiple ways to disperse northerly (for the northern hemisphere; southerly for the southern hemisphere mirror image). But with the continent in the way, the heat tends to accumulate more and affect areal conditions by adding heat and moisture to winds moving poleward.

These warm areas become the source of what you see in this illustration: Severe storms. A bit later in the year, the extra warm water of the gulf may either lead to the formation of hurricanes or strengthen hurricanes that meander into the gulf.

The big problem in this particular graphic, at this time and through the weekend, is the region at the Minnesota/North Dakota border, where the Red River runs north in the channel of the once south-flowing Warren River. (See this post for a more detailed description of this area.) Epic flooding will occur here starting Thursday or Friday, from sudden snow melt of a thicker than average snow pack combined with the storm rains you see here now and over the next few days.

By the way, if you happen to live in the Fargo region, they are looking for volunteers for sandbagging. I just hear the schools are closing down in order to provide the labor for flood preparation.

Have you read the breakthrough novel of the year? When you are done with that, try:

In Search of Sungudogo by Greg Laden, now in Kindle or Paperback
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