The long advertised remnants of tropical storm Christobal are set to swing northward across eastern Iowa Tuesday afternoon and evening bringing gusty winds and locally heavy rain. This is the satellite presentation of the depression Monday evening spinning steadily northward.
The official NHC track of the storm shows the center entering extreme southeast Iowa around 1:00 pm Tuesday afternoon.
Most of the operational models are a little bit west of that position. The EURO at 4:00 pm has the circulation center just southeast of Iowa City.
By 8:00 pm it has rocketed N towards Prairie Du Chien, Wisconsin with a pressure of 991mb
By then it is being absorbed by a deepening upper level low that is hot on its tail in SE Nebraska. Overall it's a complex interaction of energy but the trough is the feature that drew Christobal almost due north allowing it to be only the 3rd post tropical system ever to track through Iowa since 1886. This is a rare event indeed.
The rapid movement of the system will be a saving grace as torrential rains will fall just to the west of the circulation track. Eastern Iowa, especially NW of a line from Ottumwa to Iowa CIty to Monticello and McGregor seems favored for the heaviest totals which are likely to be in the 2-4" range. In spots, up to 2" could fall in an hours time. Amounts are likely to be substantially less southeast of this line, especially east of the Mississippi where some places in the dry slot may not even get 1/2 inch. The fast movement of the storm will keep the rain totals from getting overly excessive. Even so, flash flooding will still be a threat where the heaviest bands set up in eastern Iowa. A flash flood watch is in effect.
The heaviest rains will occur in a window of opportunity 2-5 hours long as the system rapidly sling shots northeast. Late morning to mid-afternoon is when things get underway south of I-80. North of there mid afternoon to early evening is the primary window.
The EURO shows this for rainfall...nothing more than a guide at this point. A little deviation in the track of 50 miles could make a significant difference in the location of the heavy rain axis east or west.
This is the excessive rain threat guidance for flash flooding Tuesday from the Weather Prediction Center.
As for wind, there will be some gusts that reach into the 30-40 mph range. A few spots could reach 50 mph.
There is significant shear with the system but CAPE is limited due to a lack of heating. However, if a few low topped supercells can get going a brief tornado spin up is possible with the low LCL's in place. The threat seems pretty low in most of my area at this time. This is the SPC risk for Tuesday.
After a break in the rain much of Tuesday night a secondary wave associated with the upper air low will whip scattered showers and storms back into the area late Tuesday night or Wednesday morning. These will be most likely in the northern half of my area, especially north of I-80, Strong winds and cooler air will push across the region from west to east during Tuesday afternoon.
Eventually, the whole mess departs and the week will come to a cooler, drier, and calmer conclusion! Enjoy today's rare event as I sit here in Maine chomping at the bit for a piece of the action.. It won"t happen with this storm but rest assured the day will come.Until then, I will be thinking of you all as you ride out what's left of Christobal. Roll weather...TS