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We've still got a couple more days of solid September weather to enjoy but a hand-off is coming. Not only will we see a turn to below normal temperatures, wet weather is likely to accompany the change. As they say, all good things must come to an end.

The catalyst in this meteorological swing is a cut-off low that digs into the upper Midwest over the weekend. Models initially carve out an elongated trough (that aided with the injection of additional short wave energy), eventually formulates a cut-off low. Once this happens, the system is removed from the westerlies and is destined to become a slow mover. You can see the process unfold in this 500mb animation off the GFS. It begins Thursday and ends next Wednesday.

As I've outlined for several days, the precise track of the stacked circulation center is critical as the area north and west resides in a prolonged trowel or deformation zone. Looking at a projected water vapor loop you can see how initially moisture approaches from the NW and with time gets absorbed into the circulation of the deepening upper low. This accounts for wrap around rains over an extended period of time for areas north and west of the center. The position of the upper air low becomes very evident in the tight swirl of moisture in the animation below. It's something we often see in classic winter storms.

So what we know for sure is the system is going to be a slow mover, containing significant moisture (PWAT's of 1.6 inches), with a lengthy period of vertical velocity and lift. The combination is likely to make it a significant rain producer. The Weather Prediction Center has a marginal risk of excessive rainfall indicated for my far western counties Saturday night.

Sunday it's focused on my northern counties and Wisconsin.

All of this is predicated in the circulation tracking across southeast Iowa and central Illinois. What we don't know just yet is whether or not that track holds. A 50 mile shift north or south could make a big difference in how much of my area receives heavy rain. The latest guidance continues to point at the northern half of my area (especially spots north of I-80) as the region with the best chance of significant rain.

Last but not least, new guidance just in is not as robust with the upper air low and ejects it faster to the northeast. It also tracks the 500mb circulation further north over EC Iowa. That reduces the chances of significant rains in all but the far north due to the more northerly track and the more progressive nature of the system. The faster ejection of the upper air low diminishes the window where heavier rains can occur. Since this is a new trend, I would like to see a couple more runs of model guidance to establish that indeed this is a consistent signal.

My experience with cut-off lows is that they are very difficult to forecast and track until the circulation closes off at 500mb. That has not yet happened and is not expected to occur until sometime late Saturday. For that reason the degree of phasing and overall intensity of the system is still far from certain and I'm not comfortable pinpointing the axis of heavy rains. Having said that, here's what the latest guidance (hot off the press) is suggesting for rainfall totals. Overall, amounts are lighter and further northeast.



The NBM (national model blend)

The WPC (Weather Prediction Center guidance)

The bottom line is that cooler wetter weather is on the way later in the weekend. The emphasis remains on my northern counties for the heaviest rains in the Saturday night through Monday period. The cooler temperatures take hold Sunday and continue into Monday. Depending on the amount of cloud cover and precipitation highs could be relegated to the 60s, especially north of I-80 Sunday and Monday. 70s are likely south of I-80 where less rain and some broken cloud cover is a possibility.

The last thing I will say is that overall, confidence is moderate to high that a healthy rain producing system is on the table for much of the central Midwest starting the second half of the weekend. The biggest challenge remains the precise track and speed of the energy which dictates where and how heavy the rains will be. I'll give a nod to the area north of I-80 for the more substantial rainfall when it's all said and done by Tuesday of next week. Needless to say, we are facing a challenging forecast period ahead of us. Hopefully data becomes more consistent and reliable as we close in on the coming weekend.

With all that said, Happy Friday and roll weather...TS


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