Wednesday, September 7, 2016

Tropical Airmass Leads to Flash Flood Threat

If you've stepped outside the past couple of days, you can actually feel the air. Humidity is at a high across the Stateline with dew points in the low to mid 70's across the area. This is not only an oppressive air mass, but a tropical one at that.

The reason for that is the placement of the jet stream. Upstream in the jet, it's actually tapping into the moisture from remnants of former Hurricane Newton. With the jet stream flowing from southwest to northeast, it's spreading the high levels of moisture into the Stateline.

That jet then takes more of a zonal turn, meaning flowing from west to east, over the Midwest which will help set the stage for heavy rain concerns.

Locally a stalled boundary is set up over northern Iowa and southern Wisconsin, which has allowed for most of the heavy rain producing storms to stay to the north of the area overnight. But that boundary is slowly moving to the east south east, and as that happens showers and storms will expand in coverage over the area.

Not only will the storms be producing heavy rain, but they will also be slow moving as well and training over certain areas. This will bring even higher rain totals for some as storms move slowly over the area.

The Weather Prediction Center has out looked the area over northern Illinois and southern Wisconsin with a Moderate risk of excessive rainfall. In terms of severe weather, there is a small threat for strong gusty winds, especially late afternoon and evening.

The slow moving nature of the storms comes from the perpendicular motion of the stationary boundary and jet winds. the winds are moving west to east and the front is sinking south, allowing for slow moving storms to drop high rain totals.

In anticipation of these heavy rain producing storms, there is a Flash Flood Watch (bright green) for our western counties until 1am Thursday, and an Areal Flood Watch(darker green) for most of the Stateline until 4am.

While there are scattered showers and storms this morning, we will expect a lull in coverage before more storms with greater coverage arrive in the early evening/late afternoon (around 4pm).

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