After several warmer-than-average days in the Stateline, a storm system is on its way which will cool temperatures and bring with it the possibility for a few stronger thunderstorms.
Since entering the second half of May, all but one day has seen high temperatures above their daily normals. In addition to the warm weather, rain has been a frequent visitor with 4 days out of the past week having seen measurable rainfall. This includes a rainy May 18th on which nearly a half an inch of rain fell in Rockford. So far, 2021 has been a very tame year in terms of thunderstorm activity for the Stateline. That could soon change, however, with the possibility for a few strong thunderstorms followed by a significant cooldown later this week.
As of early Monday morning, the Stateline is cooling down after a backdoor cold front moved over from the east pulling in some colder air from the northeast. This front is tied to a storm system centered over western North Dakota. Also attached to this storm is a strong cold front draped across parts of the northcentral Plains with a potent cold air mass trailing behind. This air mass kept parts of the northern Plains in the 50’s on Saturday and Sunday while areas under the storm’s warm sector no more than a couple of hundred miles to the east made it into the 80’s. This storm system is made possible by a closed upper-level low pressure system centered over the Pacific Northwest with an associated positively tilted trough. As this system slowly progresses eastward, the Stateline will enter the storm’s warm sector on Monday allowing for temperatures to return to the 80’s. The 80’s will return on Tuesday as warm air keep flowing into the area from the south and southwest. The cold front will start to move through the Midwest and begin approaching the Stateline Tuesday afternoon and evening. As it approaches, rain showers and a few thunderstorms are likely to pop up just east of the frontal boundary. With ample moisture expected throughout the atmosphere and 400-500 J/kg of mean-layer CAPE, there is a chance that some of these thunderstorms could be on the stronger side. With a poor shear profile, the chance for tornadoes can be disregarded. However, heavy downpours and strong straight-line winds appear very possible and the chance for some hail cannot be ignored either with a sufficient amount of upward vertical motion in the mid-levels of the atmosphere provided by some synoptic forcing. As of Monday, the Storm Prediction center has placed the northwestern half of the Stateline under a marginal risk for severe weather with 5% severe wind and hail probabilities.
Following the passage of this cold front late overnight between Tuesday and Wednesday, cold air will begin to flow in from the northwest. It appears as though temperatures will likely squeak into the upper 70’s on Wednesday while the air mass still works its way in. Lots of sunshine will likely help to keep conditions unseasonably warm through Wednesday as high pressure builds in from the west. Come Thursday, temperatures will find it difficult to escape the 60’s and more rain is likely as another storm system passes through just to the south. Highs in the 60’s are likely on Friday as well with cold air continuing to move in from the northeast. High pressure building to the east will shift our winds to southerly for the start of the weekend bringing temperatures back up into the 70’s.