In the long-term, the combined effects of the July 23 storm, which brought down a significant number of branches and uprooted a few trees, along with the continued presence of the emerald ash borer, which is causing damage to the many ash trees in Carol Stream, will affect flooding for the worse. Healthy ash trees weakened during the severe storm will become increasingly susceptible to ash borer damage, and other diseases due to stress.

Trees prevent runoff by slowing down the rate of rainfall that hits the ground and turns into runoff, and by taking up excess water through their roots and transpiring the water into the air through evaporation. (source) This effect is measured at hundreds of gallons per day, per tree; the thick canopy of trees in Carol Stream suggests that hundreds of trees transpirate removes many tens of thousands of gallons of water from the ground every day.

With hundreds of mature trees at risk of removal in the next few years, the combined effect suggests that this natural method of runoff control and floodwater reduction will be reduced over time, unless new trees are planted.

Even so, it will be 20-30 years until the full amount of stormwater and flooding detention benefits returns to the area.

Initial storm motions seem to be faster than yesterday, limiting the amount of time it rains at any given spot. Still, it may be a rainy next six hours.

The creek is at normal levels. Lake George is slightly higher than usual. In addition, the soil is largely saturated from yesterday’s rains.

Rainfall totals will be posted at 7:00 am tomorrow, or earlier if conditions permit.

A suspected microburst – a downward rush of waterlogged air that generates 90-110mph winds – struck the area around Thunderbird Trail and Gary Avenue last night and blasted outwards from that area. The exact area of damage is not clearly known at this time, but a 2′ diameter tree fell at the corner of Illini Drive and Arrowhead Trail. Large branches were ripped off trees in a line from Kuhn Road at Thunderbird Trail to Gary Avenue at Thunderbird Trail.

Down Thunderbird Trail, however, widespread tree damage was observed. Most trees lost three or four large branches, and some trees may have been uprooted and thrown onto houses. This type of damage is consistent with winds much greater than the 60-65 mph winds that I estimated at 12:30 AM.

The severe weather resulted in an area of subsidence behind the storms deflecting moisture to the south, around Carol Stream, after the power went out and the storms passed.

The threat for heavy rain is not over – the area is in a moderate risk for flash flooding. A post focusing on that aspect of tonight’s weather will be up later.

Factors working against flash flooding

• Fast storm motion–it may rain hard, but the storms can move away before causing flooding rains.
• Heaviest rain expected to be in a 50-mile-wide corridor (or less) between Chicago and Milwaukee

Final impacts depend on how much rain actually falls in the watershed. I’ll be posting rainfall totals as early as I can manage them through tomorrow morning.

The rain is expected to move into the area around midnight, lasting through the morning rush.

This graphic attempts to highlight the heavy rain threat in the next two days as depicted by the National Weather Service offices in the Quad Cities and Chicago. (Click the image to see a larger version.) It is important to note that rain will fall outside of these areas during the days specified; the discussions are attempting to place the axis of heaviest rain in the area in the next two days.

The NWS Chicago discussion in particular identifies areas north of I-80 as at-risk for heavy to excessive rain on Wednesday night/Thursday morning. By Friday morning, areas south of I-90 will be at the greatest risk for additional heavy to excessive rain. Chicago and most of the western suburbs fall in the zone where excessive rain may be possible during both the Wed/Thur and Thur/Fri periods.

This may be a particularly significant event, especially since soils are still moist-to-saturated from Saturday night’s rains. Stay tuned to the latest forecasts and up-to-date on the latest rainfall totals through Friday morning.

Forecast rain totals

Forecast rain totals for the next 3 days combined are currently in the neighborhood of 1-2″ (HPC). This amount does not take into account storms that training/backbuild over the same areas repeatedly; it is a broad-scale estimate of how much rain will fall in a large area. Higher amounts are to be expected in isolated locations.

What’s worse, the high-impact event rainfall event, accompanied by nearly constant lightning over almost a one-hour period, was completely missed by both computational models and forecasters. The HPC only estimated that less than two inches of rain would affect the area during the time period. No flash flood watches were issued prior to the event. There was no indication of a slight risk (or better) of flash flooding until the event had passed.

The entire rainfall episode can be stepped through on NEXRAD radar at this link. Precipitation totals for the event can be viewed here.

At 11:30p, strong thunderstorms begin forming along a subtle surface boundary along the Cook County-Lake County line. Where this boundary originated is somewhat of a mystery; it could have been from a storm that passed through Wisconsin earlier in the day.

By 1:00a, strong convection begins to fire to the east of Carol Stream, over the city of Chicago proper. Most likely, the mid-or-upper level system approaching from the west was beginning to draw tropical air over a lake breeze boundary. Rainfall rates within these storms exceeds 2″ per hour, and at the airport occasionally exceeds 8.0″ / hour (for one minute)! These types of rainfall rates are almost never found in the mid-latitudes where Chicago is located, and are certainly not a rate of rainfall to which the local terrain has evolved.

The first line of rain sweeps through Carol Stream at 2:00a, when other areas to the north and east have already received 4-5″ of rain. This line brings an astounding 2.36″ of rain in 40 minutes, for an hourly rate of 3.5″ / hr. A total of 2.92″ falls in Carol Stream through 9:00a, ultimately causing some nuisance flooding.

Later systems die out as they approach the area, something that generally occurs because air sinks in the area after a thunderstorm passes. (Thunderstorms form when air rises, condensing water vapor.) Only larger scale weather can induce further precipitation at that point (an approaching cold front, or a tropical system moving in from the south).

A late night (11:30p-1:00a) light show caused 3-4″ of rain to fall on Aurora and Naperville, but these storms were moving too quickly to generate the 7″ tallies of 7/23.

For summer storms in moist tropical air masses, the only way to be in touch with a rapidly-changing weather situation is to download an app like RadarScope for the iPhone, or by maintaining and reading your own rain gauge. Had the lake-effect breeze boundary been further west, across the Fox River, this could have been a major flood event for Carol Stream – essentially unforecasted, and largely taking place from midnight to 4:00a.

More concerning is that this convection is expected to repeat, with 1-3″ of rain falling this afternoon and tonight. This would bring the total rainfall amounts up to 3-6″, and may cause street closures. Higher amounts would bring worse damage.

Although we have not had meaningful rain for 16 days prior to tonight’s storms, the amounts noted are significantly more than the ground is able to handle at once, particularly with rainfall rates occasionally exceeding 3″ / hour (for durations of less than an hour). This will result in excessive runoff, and resultant flash flooding.

Precipitation totals will be posted here as the rain is measured.

Update 2:54a: Storm totals are as follows: Since 9:00a, 0.84″. 2:05a-2:45a, 2.36″. 24-hour storm total, 3.20″. This is (should be) sufficient to bring the creek above bankfull.

An additional 0.50″ is possible in the next hour. An additional 1″ is possible around 6:00a. The HPC is forecasting an additional 1-2″ this afternoon, but with low confidence, as storms may fire on the southern periphery of this current system, as opposed to in our area.

Update 9:51a: The 24 hour storm total as of 9:00a was 3.48″ of rain. This is not enough to cause flooding issues, although Lake George is full and the creek is a few inches above bankfull. Another couple of inches are expect before the end of the day, which would aggravate these conditions. Due to uncertainty regarding where the heaviest rain will fall, the HPC has downgraded today’s flash flooding possibilities to “low” from “moderate”.

Less than a half inch of rain is expected with the storms.

Today’s rains have brought over 2 inches of rain since about 11:00p last night, with a total of about 2.25″-2.50″ 2.35″ as the last batch of rain moves through. While no severe weather impacted the area, there were multiple instances of lights flashing and some strong outflow gusts preceding the storm.

This afternoon, storms will redevelop, although this activity is expected to remain mostly south of I-80. Nonetheless, an additional 0.50″ of rain is expected; if the models are too far south on the placement of this band of precipitation, up to 1.00″ of rain can be expected. Update 7:48p: At this point, no rain is expected until the main system arrives tonight.

Late tonight into early Friday morning, another round of thunderstorms is expected to roll across the area; it is too early to know how much rain will fall during this time period.

The main risk of heavy rain will come Friday night into Saturday morning, which is the time period the HPC expects there to be a slight risk of flash flooding. Again, prediction amounts are not reliably available at this time.

There is the distinct possibility that the storm total for Thursday, Friday and Saturday will be over 3.00″ (as we are already at about 2.50″ of rain). There is an outside chance the storm total will be over 5.00″ of rain. With very little time for the watershed to allow today’s rain to runoff into the DuPage River, moderate flooding is possible.

This blog will continue to be updated with the latest forecasts and rainfall amounts as they come in.

Update 3:14p: The latest QPF excessive rainfall forecast discussion points to widespread amounts of 1.00-2.00″ tonight, with isolated amounts of 2.00-3.00″ possible. This amount of rain in such a short period time would result in substantial rises on Klein Creek, but the rises should be less than the 7.28″ and 8.48″ amounts in July 2010 and September 2008, respectively.

This still does not take into account the broader threat of heavy rain on Friday night.

Update: 3:19p The latest area forecast discussion indicates that a Flash Flood Watch will be issued by the National Weather Service tonight. It will run through noon Friday. It is still too early to determine where the heaviest rain will fall on Friday night.