Spring Storm for the textbooks
A very dynamic, multi-impact spring storm is currently taking shape in the middle of the country. A strong, closed upper low has slid southeastward into western New Mexico, as shown on this upper level (500mb chart):
As the strong spring jet stream dives around this upper low, warm air advection has already sparked a corridor of incredible rain/storms stretching from Oklahoma through Ohio. Chicago is currently north of this boundary, and is experiencing strong easterly flow as a dynamic surface low forms over the Texas Oklahoma border:
Zooming in on the upper Midwest shows the incredible temperature contrast north and south of the strong boundary that defines a SW->NE occluded front stretching across almost the entire CONUS. The strong easterly winds north of the boundary are pulling very cold air off the Great Lakes which is helping develop a cold column of air stretching southwestward toward the surface low, and keeping temperatures in the Chicago area in the 40s and feeling just generally miserable:
So what happens?
As the upper low slogs eastward, the surface low will draw north-northeastward, increasing in intensity, drawing ever-colder air off the Great Lakes and sending a plume of moisture directly into Chicago as the warm-cold boundary lifts northward:
This is actually a beautiful, textbook-like example of a strong continental spring storm with rapid cyclogenesis in the lee of the Rockies drawing moisture from the Gulf and taking advantage of extreme N-S temperature contrasts to wring out insane amounts of moisture over the central and eastern US.
Because this system is incredibly dynamic, the surface low will slowly creep north-northeastward until the upper low catches up and the system becomes vertically stacked (vertically-stacked is the last, mature, stage of a textbook storm). This should happen sometime late Sunday, at which point the storm system will look like an aesthetic beauty:
As you can see, by this time, the system will be past maturity, with a well-defined warm front stretching across Lake Michigan and a very strong cold front extending southward through Illinois to the Gulf. Whether the warm air makes it to Chicago tomorrow is still debatable. Either way, the low will then begin to fill in as the upper-low and upper-level support outruns the surface feature. The rain will taper off Monday as the cold air behind the low filters over the region. Tuesday should bring a rapid warm-up.
Give me specifics, Mick:
The models are all pointing to somewhere between 2 and 4 inches of rain in the Chicago area between now and Monday AM:
The wind will also be a problem. Strong northeasterly winds will blow most of the night into Sunday as the dynamic boundary shifts northward. Sunday afternoon may see calmer southerly winds before a dramatic turn to westerly winds overnight Sunday. The strongest rain should occur this evening - overnight - Sunday AM and then again Sunday evening with the passage of the cold front.
Temperatures may spike into the 70s tomorrow in the late afternoon before the passage cold front, but this is less of a sure thing, and we need to wait for (one?) more model solution showing this scenario before confidence.
As usual, stay tuned. The heaviest rain should begin sometime around 7pm tonight.