Tuesday, August 30, 2016

Beautiful Week and Weekend Ahead!

Temperatures and dew points have been quite high over the past few days. That will completely change by the time we get to Wednesday afternoon. A strong Canadian high pressure system will anchor itself north of Lake Superior beginning Wednesday. It won't move much at all through Saturday, which will keep our pattern very quiet. It will also allow cooler, less humid air to filter into the Midwest and Great Lakes.
Temperatures won't exceed 80° until potentially Sunday afternoon when our winds start to turn southerly again. Humidity values will be low with dew points in the upper 40's and 50's, which will make it feel much more fall-like as we enter September on Thursday. Clouds will be few and far between Thursday, Friday and Saturday. The next chance of rain arrives just in time for Labor Day Monday with an approaching cold front, but that system is still too far out to pinpoint any details at this time.

Rainfall Roundup

Here's the latest list of 24 hour rain totals from the National Weather Service in Chicago:

Paw Paw (Lee)................................2.58
Cary (McHenry)...............................2.08
Algonquin 1N (McHenry).......................1.58
Amboy (Lee)..................................1.4
De Kalb (De Kalb)............................1.43
Genoa (De Kalb)..............................1.42
Amboy (Lee)..................................1.40
Dixon 3NNW (Lee).............................1.38
Rockford 1NW (Winnebago).....................1.23
Belvidere (Boone)............................1.12
Steward (Lee)................................1.01
Ashton (Lee).................................0.97
Rochelle (Ogle)..............................0.88
De Kalb (De Kalb)............................0.87
Dixon 2SW (Lee)..............................0.80
1 Rockford (Winnebago).........................0.81
Dixon (Lee)..................................0.77
Rockford 3NE (Winnebago).....................0.70
Rochelle (Ogle)..............................0.52
Cortland (De Kalb)...........................0.37
Harvard (McHenry)............................0.20
Byron 3N (Ogle)..............................0.18
Capron (Boone)...............................0.16
Mchenry (McHenry)............................0.12
Woodstock 4SW (McHenry)......................0.10
Woodstock 1SSW (McHenry).....................0.07
Woodstock 5nw (McHenry)......................0.07
Roscoe 2ESE (Winnebago)......................0.02
Roscoe 2se (Winnebago).......................0.02

Not Done with the Rain Yet..

....more storms possible this afternoon.

It was a soggy start to the work week with heavy rain producing storms that developed around 1pm yesterday afternoon. They slowly moved NNE across northern Illinois and southern Wisconsin, dumping inches of rain.

Storms cleared the area around 9pm but then overnight, ahead of moisture and warm air advection, more storms developed dropping more rain through southern portions of the Stateline area. We can expect some dry time today, with more storms possible after about 1pm. These will form ahead of a mid-level disturbance, and will linger past sunset. The coverage of the heaviest rain will be just south of I-80 and I-88, closer to a stalled boundary. However, we will still see periods of heavy rain this afternoon.

As we've mentioned, the localized flash flooding is due to heavy rain producing thunderstorms that are very slow moving. A few components have come together to allow for this to happen.

First, a ridge situated in the southeast and extending up into the Ohio River Valley, is actually directing the flow in the atmosphere, because of this southwest flow is drawing up quite a bit of moisture and warmth, something meteorologists call warm air advection, and moisture advection, right over northern Illinois. Our dew points are again in the low 70's and the potential moisture in the atmosphere is approaching 1.5" again today, a lot like yesterday.

Our upper level winds, or jet stream winds, are also known as our steering winds. They will typically help to drive high and low pressure systems, cold fronts, and even storms. Currently our jet winds are zonal, meaning flowing from west to east. But these typically fast moving winds are actually rather weak, so instead of these storms marching quickly east, they have been very slow. When you have high amounts of moisture and heavy rain producing storms that move slowly, they are going to dump a lot of rain over certain areas of the region.

Day 1 Excessive Rainfall Forecast
This is basically what happened yesterday, and we still have the potential this afternoon as well. The greatest coverage of heavy rain looks to be just to the south of the area, as mentioned. This area is also highlighted for excessive rainfall by the Weather Prediction Center.

Monday, August 29, 2016

Heavy Rainfall & Fall

Plenty of rain fell Monday across the Stateline with many locations recorded upwards of 2-4" of rain. Some of those locations include Dixon, Sterling, Rockford, Belvidere, Genoa, and Kirkland. Many other areas saw moderate to heavy rain, but the rain totals weren't as extreme. Flash flooding was also reported in downtown Rockford, Dixon, and Belvidere as a result of the intensity of the rain. The reasoning behind the storms dumping copious amounts of rain is the fact that we had ample heat and moisture available. Precipitable water is a tool we use to analyze how much moisture is in the atmosphere. When those values exceed 1.5", heavy rain is a distinct possibility. Monday we had totals of 2". As a result, the atmosphere was prime for heavy rain. In addition, the storms didn't move much at all due to a lack of upper-level winds. There was nothing in place to push the storms along, which led to flooding in portions of the region.
A cold front will swing through Tuesday evening. Ahead of that will again exist an air mass supportive of isolated thunderstorms and heavy rainfall. Once that front passes Tuesday evening, much cooler and drier air will filter into the area. Temperatures by Wednesday will be in the 70's for highs, and likely won't exceed 80° until possibly Sunday. Overnight lows will return to the 50's, and humidity values will be very low, making it feel more like fall as the calendar flips to September later this week!

More Heavy Rain Possible

Over the weekend multiple showers and storms brought heavy rain to the Stateline, mainly in Whiteside and Lee counties where we had Flash Flood Watches and Warnings. We aren't done with the heavy rain however, slow moving storms today will once again bring a threat for high rain totals and localized flash flooding.

While the atmosphere lacks distinct forcing, storms are still probable across the area because of a few factors. High pressure this morning is centered near the central Great Lakes, but will move east by mid morning. As this happens, return flow on the backside of the system will bring increasing humidity across the area.

Already modest dew points in the 60's will rise into the low 70's with southerly winds in place. With sunshine and these increasing dew points, showers and storms will then develop. Because there is not a lot of forcing or lift in the atmosphere it is harder to time out when and where the storms will develop. They will however be likely today across northern Illinois and northwest Indiana. The threat for severe storms is low but a few stronger storms can't be ruled out that will produce gusty winds. The threat we are tracking today is heavy rain producing storms and even localized flash flooding. In the low levels of the atmosphere there is a lot of moisture to work with, something referred to as PWAT values, which will be rising to 1.5" to 2". We like to use the analogy that if we think of the atmosphere as a sponge, and we were to squeeze out the available moisture or PWAT we could see up to 2" of rain fall.

Also in the low levels of the atmosphere the 'flow' or wind speed is rather slow, meaning the storms we see today will be slow movers. With these two factors, high rain totals and localized flash flooding is possible.

Storms will be diurnally driven, meaning with the heating of the sun storms will be able to be sustained, but once the sunsets the storms essentially lose the energy and will faded away. However, a couple isolated storms will linger into the overnight.

Another day with storms for Tuesday with an approaching cold front, this will bring scattered showers and storms in the area. A shortwave trough or small low pressure system in Canada will move eastward, but it's trailing cold front will sink south through the area tomorrow firing up scattered showers and storms. Once again the severe threat is fairly low, but a few stronger storms will bring gusty winds and heavy downpours.

Good news is, the weather turns quite Wednesday into the rest of the week.

Sunday, August 28, 2016

Flash Flood Watch and Warning

Certainly an active night for our southern counties Whiteside and Lee this evening as thunderstorms continue to form over the same areas.  A Flash Flood Warning is in place for Whiteside County until 11:15 this evening and a Flood Advisory is in effect until 8:45 pm for Lee County.  More than 2 inches of rain has fallen in a number of areas that were already saturated from rains on Saturday.  Water has been reported to be completely covering Highway 30 near Sterling this evening, and many other roads will be impacted as well. 

Have a plan in place especially if you live in low lying areas to be able to exit to higher ground as needed and if you should come across a water covered road, do not attempt to cross it.  Even small depths of water can be very powerful. Turn around, don't drown. 

The storms should begin to diminish later tonight by midnight.  We will again have chances of rain and thunderstorms Monday and Tuesday before a cold front moves through, drying us out at least until the weekend.  

Saturday, August 27, 2016

We've been in a very cyclical pattern all summer, with generally warm and stormy weather interrupted regularly by storm systems that break the heat and humidity - at least for a few days!   The storm track has been in southern Canada for some time which is typical for this time of the year.  It will start to shift further south in the coming weeks and months as we head into fall. 

The pattern will be the same this week with warmer than average temperatures that will be broken on late Tuesday or early Wednesday with our next storm system located in southern Canada.  A cold front associated with the storm will push through bringing in a chance for rain and thunderstorms, followed by clearing skies, cooler temps and thankfully lower humidity.  The month of September is predicted to be warmer than average so we should expect increased temperatures to return, but at least we are getting some nice breaks.  We all know what is eventually coming - so lets enjoy the warmth while we have it!