Multiple days in the 90s this week, limited storm chances

 Written By: First Warn Weather Intern Patrick Murray

Strong high pressure overhead will lead to hot, humid and dry conditions for at least the first half of the work week. A Rex-blocking pattern has emerged, and is likely to stick around for the foreseeable future. With this high pressure overhead, we can expect temperatures to soar into the low to mid 90s with dewpoints approaching 70 degrees at times. This will combine to produce heat indices upwards of 105 degrees at times.

Despite daily heat indices potentially reaching 105 degrees, the National Weather Service has held off on any heat-related headlines as of now. This is likely thanks to the fact that only one or two days will really feature those heat indices approaching 105 degrees. As of writing this article, any heat headlines remain well north of the Stateline. We'll have to wait and see if any headlines are issued later for the forecast period.

Unfortunately, not much relief is in sight, as temperatures are expected to remain in the upper 80s/low 90s through the forecast period.

Today, expect temperatures to quickly climb into the low to mid 90s across the Stateline, with heat indices approaching 100-105 degrees. Temps will cool off into the low 70s/upper 60s overnight, providing minimal relief during this period.

Today, expect a high of around 92 degrees with mostly clear skies. A northwest wind at 5-15 mph will help keep things slightly cooler than if we had a south or even southwesterly wind. The next mentionable chance for showers/thunderstorms doesn't look to be until Saturday into Sunday, with a very slight chance for showers and thunderstorms Tuesday through Friday.

How evapotranspiration and mixing will impact the humidity this week

 A blocking high pressure will prevent much abundant moisture from reaching the area, but it will still feel quite humid as the next heat wave sets in this week. How can that be possible with such little rain over the last week? Where is that moisture coming from?

Since we live in the corn belt, a lot of our summertime humidity is actually released by the expansive corn fields. "Corn sweat" is another way to describe evapotranspiration, or the process of moisture evaporating from corn fields into the air, raising humidity levels. One acre of corn can release up to three to four thousand gallons of water into the air each day. All of that added moisture increases the dew point and the humidity which we then feel on hot summer days.

But there will be another feature playing into our upcoming heat wave that may limit the more oppressive humidity levels. That process is called atmospheric mixing. With the very dry ridge of high pressure overhead, there is a lot of very dry air above the surface. Some of that air can be mixed down toward the ground, lowering the humidity a bit. However, since dry air heats up faster than moist air, daily temperatures may rise a bit higher than they would with more abundant moisture.

Our dew points will likely range from the mid-60s to the low 70s most days, which would feel pretty humid especially paired with the incoming heat. Tuesday and Wednesday in particular will be very hot, with temperatures into the low/mid 90s. Wednesday may not quite mix out as much moisture as Tuesday, so I think the heat index may reach a little higher despite a similar temperature forecast. Point being, it will be another hot and humid stretch ahead, so be sure you are practicing heat safety tips including staying hydrated and using lots of sunscreen if you must be out for extended periods of time!

High pressure brings hot and dry weather through mid-week

 Blocking high pressure will result in very little rainfall potential across much of the Central Plains and Upper Midwest through at least the next 5-6 days. All of the gulf moisture will be trapped South of that high, producing typical summer-like showers and thunderstorms primarily South of the Ohio River.

The lack of gulf moisture will help with our heat and humidity, but it will not eliminate it entirely. Abnormally warm weather stretching all the way into Montana and North Dakota will gradually spread our way through next week, producing several 90-degree days. Monday through Thursday is currently forecast to see 4 such days in a row, with peak heat index values pushing the triple digits Tuesday and Wednesday. While not quite as hot as the last round of heat, be sure to stay hydrated and take breaks in the cool shade if you must spend extended time outdoors!

Summertime heat returns with limited rain chances ahead

 Yet another round of summertime heat will arrive next week, but it may not come with quite as much humidity as last time. The upper-level pattern will feature an expansive ridge of high pressure across the Central United States, bringing a broad region of heat. But the humidity will be kept at bay by a blocking pattern. The cutoff low stalling South across the gulf states will remain blocked, keeping us warm but relatively dry locally in the upper Midwest.

Aside from a very slim chance for a shower late Friday evening, we will remain dry through at least Wednesday morning. Temperatures will be on the rise, pushing into the low 90s starting Monday. Overnight lows are currently forecast to reach the mid to upper 60s, lacking more sufficient moisture and humidity that would bring oppressively warm nights.

Pleasantly warm festival weather this weekend before 90s return

Watching The Front:

Forecast models continue to show a frontal boundary lifting north towards I-80 this afternoon. If that front is able to push a little farther north than currently expected, it could provide enough lift for an isolated shower or thunderstorm locally. Otherwise, most will stay dry through the day with partly cloudy to mostly cloudy skies. Afternoon highs remain seasonable, landing in the low 80s this afternoon.  

Weekend Festivals:

For those heading to ByronFest or Tutty Baker Fest this weekend, the forecast cooperates. Rain chances remain low as high pressure settles into the region, bringing a mix of clouds and sun. While the weather stays largely dry, the heat will slowly begin to build back into norther Illinois. Afternoon highs are expected to reach the upper 80s each day, creating warm conditions for outdoor events. So while having a good time, drink plenty of fluids. 

Warm Up Continues:

Looking ahead to next week, summer heat will make a noticeable comeback as a strong ridge builds across the northern Great Plains and the Great Lakes. This will allow temperatures to climb up to the 90-degree mark Monday, with several consecutive days of 90-degree heat possible until Thursday.   
 

What's the difference between a tornado and severe thunderstorm warning?

 The National Weather Service issues many different type of warnings, including severe thunderstorm and tornado warnings. Both of which cause damage but have many differences in extent of damage. Most thunderstorms do not do much damage, but once you get closer to 60+ mph winds, larger limbs can be broken, and small trees can be uprooted. Above 80 mph is when structural damage is possible if not likely.

Once you get up toward 60-80 mph, that is equivalent to a weak tornado. An EF0 tornado ranges from 65-85 mph, while EF1 is when winds reach 86-110 mph. While the most destructive tornadoes do much more damage, those are very rare. Only around 5% of all tornadoes reach EF3 intensity or greater.

The damage paths of tornadoes vary greatly from straight line winds as well. A swath of straight-line winds can be around 5 miles wide, stretching across the core of the thunderstorm itself. Tornado damage may be confined to a much narrower area, as even larger tornadoes may only be a mile at their widest. That means the damage paths may be much more widespread from severe straight-line winds than tornadic storms.

Severe thunderstorm warnings from the National Weather Service can fall into one of three categories: Base, Considerable, or Destructive. A Base severe thunderstorm is for winds of 58+ mph and/or quarter sized hail. Considerable would indicate 70+ mph wind gusts and/or golf ball sized hail are possible. And a Destructive tag is for a severe thunderstorm capable of producing 80+ mph wind gusts and/or baseball sized hail. A Destructive severe thunderstorm will trigger the wireless emergency alerts (WEA) on your cell phone, like all tornado warnings do regardless of strength.

A "tornado possible" tag is also utilized within a severe thunderstorm warning when there is broad rotation, conditions favorable for rotation, or the storm has had a history of producing tornadoes. That "tornado possible" tag is a way to alert the public that a quick spin-up tornado can occur but is not imminent at that time.

You will often hear our weather team say "treat this severe thunderstorm warning as if it were a tornado warning." That is partly because of the larger coverage of damage than can be done, but also because the intensity of some severe storms can be equivalent to that of a weak tornado. Severe thunderstorm warnings will NOT automatically trigger your cell phone unless they reach 80 mph winds.

This is why it is important to have many ways to receive those warnings. Weather apps on your phone, including the "First Warn Weather App", can send push notifications for all severe thunderstorm warnings. Weather radios will broadcast warnings, watches, and other important weather information. Simply relying on your phone and tornado sirens will not alert you for all hazardous weather.

Hopefully this article has given some context behind why some storms have tornado warnings and others don't, the strength of some severe storms, and additional resources you can remain weather aware and stay safe during the storm season and the rest of the year.

Patchy fog likely tonight as isolated showers/storms come to an end

 

A mix of sun and clouds Thursday afternoon allowed temperatures to warm into the low to mid 80s with a heat index reaching the low 90s. The majority of the afternoon has been dry, outside of a few showers that moved across northern Illinois and southern Wisconsin earlier in the day.

We will see a gradual increase in showers and isolated thunderstorms through the evening as the combination of a lake breeze from Lake Michigan and cold front move across southern Wisconsin and northern Illinois. The biggest impacts from any shower/storm that develops will be periods of heavy rainfall. There may also be some gustier winds through sunset. The chance for rain will last through Midnight, perhaps even as late as 3am/4am Friday.

As the showers come to an end the light northeast wind and increased low level moisture could lead to some areas of patchy fog through Friday morning. While I am not anticipating anything too widespread or dense, locally dense fog will be possible for some during the morning commute.

Similar to Thursday, temperatures Friday will warm into the low to mid 80s. We may see an isolated shower/storm, or two, Friday afternoon but the majority of the area will remain mostly dry. Temperatures through the weekend will warm into the mid and upper 80s before reaching the 90s next week.