Several clusters of downpour-generating showers and few isolated thunderstorms are being tracked as we post this at 3:50pm Wednesday--one on the Kane/DuPage County line and still another producing lightning north of McHenry over Wisconsin's Walworth County. The most extensive collection of cells runs from southern Boone into DeKalb and LaSalle Counties. All of these showers and thunderstorms been developing farther inland with time. It's a trend being promoted by westward moving cool air off Lake Michigan which has tended to stabilize the air and limit rainfall in lakeside counties. As of this posting, the tallest and most prolific rain-generating cells are located about 75 miles west of Chicago from near Rochelle to near Compton in Ogle and Lee Counties. Lightning has really been quite limited and had been associated almost exclusively with the Walworth County storm until a series of cloud to ground strokes began in the Ogle/Lee county thunderstorms. Cooling temperatures as we approach and pass sunset will lead to a rapid demise of these cells--with the likelihood scattered thunderstorms will flare again over sections of the Chicago metro area Thursday afternoon with daytime heating.
Track these storms here through this University of Wisconsin-Madison satellite imagery:
http://www.ssec.wisc.edu/~tomw/wisgifloop.html
Several cold air funnels have been reported within showers and thunderstorms which have been forming northwest of Chicago in Wednesday afternoon's unstable atmosphere. Cold air funnels tend not to be damaging--often spinning beneath towering cumulus clouds without touching the ground. Thunderstorms--some towering as high as 31,000 ft.--but topping out at 29,000 ft. as of this 2:45pm posting---have been developing and dissipating across sections McHenry County--some producing nickel size hail. Doppler radar indicates there has been slow southward development---even as remnant showers attempt to move southeast with with northwesterly upper steering winds. Thunderstorms weaken as they move into counties immediately adjacent to Lake Michigan as they encounter cooler air there. Weather Bug sensors are indicating temperatures are in the mid to upper 60s near the lake and in the vicinity of this afternoon's showers and thunderstorms--but in the low 80s at the warmer inland locations which haven't been subjected to cooling rainfall and storm outflows. A funnel cloud was reported around 1:55 pm near Woodstock also in Mc Henry County--something which may happen from time to time with any of these thunderstorms. The situation doesn't support damaging or an organized outbreak of tornadoes.
Temperatures are falling with height at a faster than normal pace which means the atmosphere is unstable. The set-up is just the latest ramification of the cooler than normal air which northwest upper steering winds have continued to deliver and replenish so often this summer. Daytime heating encourages air near the surface of the earth to warm, rendering it buoyant. This heated air then ascends and cools prompting the thunderstorm development we've been observing much of the afternoon. It's a situation likely to spawn additional (though scattered) thunderstorms over at least sections of the Chicago area into evening. The thunderstorm development is being further enhanced by the convergence of ground level winds along inland-moving easterly winds off Lake Michigan. The cooling these winds promote across the counties closest to the lake has been--and is likely to continue--inhibiting thunderstorm development by stabilizing the air. Only sprinkles or light showers--remanants of the west and north suburban thunderstorms---are able to make it into these lakeside counties.
Indications are areas west and north of the Chicago--including DeKalb, Kane, McHenry, Boone, Ogle and Lee Counties will be affected from time to time by these thunderstorm clusters--any of which may spawn some cold air funnels and produce hail and some downpours.
We'll have more on WGN News at 5:30 pm and 9PM tonight--and also a look at a surge of warmth Friday which appears poised to bring us July's warmest temperatures to date. That warmth may be followed by vigorous if not some possibly severe thunderstorms later Friday night into a portion of Saturday.
Tom Skilling
Here are some unofficial Chicago area rainfall tallies from this morning's pre-dawn rains from our Weather Bug observation network:
O'Hare 0.68"
Lake Geneva, WI 0.66
Valparaiso, IN 0.65"
Chicago (Robert S.
Abbott School) 0.60"
Darien 0.58"
Rockford 0.55"
Highland, IN 0.54"
Wilmette 0.54"
Wadsworth 0.52"
Trevor, WI 0.47"
Kenosha 0.47"
Carpentersville 0.47"
Long Grove 0.46"
Park Ridge 0.45"
Itasca 0.45"
Munster, IN 0.42"
Glenview 0.42"
Niles 0.41"
Lombard 0.40"
Des Plaines 0.40"
River Grove 0.39
Lake Villa 0.39"
Alsip 0.39"
Chesterton, IN 0.39"
Morton Grove 0.39"
Gary, IN 0.38"
Henry, IL 0.38"
Marseilles 0.37"
LaGrange 0.37"
Waukegan 0.37"
Janesville, WI 0.35"
Burr Ridge 0.35"
Oak Lawn 0.35"
Westchester 0.35"
Lombard 0.35"
Elgin 0.33"
Mundelein 0.33"
Lansing 0.33"
Lombard 0.33"
Elmhurst 0.33"
Naperville 0.33"
May-level temperatures arrive as a pool of cool mid-summer Canadian air settles into the area late this week into the weekend. We'll have more on that predicted cooling on our WGN News programs at 5:30 and 9pm. See you then!
Tom Skilling
Here's an update from the WGN
Weather Center on the latest thinking on the threat for severe
weather in the Chicago area. As indicated earlier, this threat
first materializes in the hours toward dawn Wednesday. The latest suite of
computer projections and our in-house analysis of the situation puts the focus
for any turbulent weather from 4 to 8 a.m.Wednesday morning.
Though clearly
outside the period of peak solar heating, an influx of warmer, more humid air
predicted to be ongoing by then will have the atmospheric moisture content (the
"precipitable water" values) surging to 1.84" of evaporated water at the same
time a pocket of especially strong jet stream winds approaches from the west.
Air sinks immediately below the front quadrant of such band of strong upper
winds commonly referred to by meteorologists as a "jet streak". But, this large
scale sinking of air actually enhances the upward motion of the air just ahead
of such a feature--which is where the Chicago area will be situated Wednesday
morning.
The 4 to 8 a.m. window would be the period in which the ascension of
air would be at its peak and, therefore, potentially the most supportive of
thunderstorm formation in the increasingly moist air expected to be pouring into
the area. In addition, the faster than usual vertical temperature decline--in
other words the instability of the atmosphere-- projected by computer models as
well as energy calculations (so-called "CAPE" values--an acronym for "Convective
Available Potential Energy") 2500 joules per kilogram--1000 is widely viewed as
the severe thunderstorm threshold--appear likely to be supportive too. Many of
the critical values we examine for severe weather generation are fairly
comparable to early this past Saturday when predawn thunderstorms roared across
part of the Chicago area generating 1"+ downpours and hail.
We'll be updating this on tonight's 5:30 and 9PM WGN News programs.
An
impressive line of thunderstorms developing from South
Dakota into Minnesota as we post
this at 3pm Tuesday appears to be the first stage of the thunderstorm cluster
expected to reach at least sections of the Chicago area early Wednesday and Doppler
scanned cloud tops there have already reached 48,000 ft. The weather system
producing these storms has a history of large hail production--having produced
4.25" diameter (softball size) hail in sections of Wyoming and South Dakota Monday evening.
Tom
Skilling
You keep looking as a forecaster for signs that heat's imminent. They just aren't there--at least at this time. That certainly doesn't mean we'll escape the summer with no hot weather at all. But the beautiful weather of late, free of a single spell of oppressive heat, is--putting it mildly---a bit remarkable! After all, if it's going to get hot here in the Midwest, this is the time of year it happens---and there are simply no obvious signs temperatures are likely to surge--other than a brief mid to upper 80 spell Wednesday. The heat continues in a pool hovering over the southern Plains and, at least up to this point, is showing no signs of staging a move on the Chicago area or this section of the Midwest anytime soon.
The stats on this summer's temperatures are really quite impressive. July's opening 12 days have seen half the number of 80+ highs of the same period a year ago and average 4.1-degrees below normal and 3.9-degrees behind the same period a year ago. While hot weather afficianados lament the cool summer weather, others relish it and appreciate the savings on summer air conditioning. Estimates based on temperatures put air conditioning use in July less than half normal (47%) and only 78% normal since June 1. The necessity for air conditioning usage this summer is estimated to be running 26% behind the same period a year ago.
Wet soils may be playing an important role in the cool summer pattern--though there are no doubt other factors at work as well. A feedback develops when soils are wet--and record spring rainfall here assured moisture levels were elevated heading into the warm season. As hot air makes a move on the region, moisture is returned to the atmosphere from the wet soils and transpired back into the air by plants with roots in these wet soils---a process which leads to clouds and thunderstorms which, in turn, mixes down cooler air and moderate the advancing heat. Our Frank Wachowski reported to us just last week that July's opening 9 days were the month's cloudiest in 40 years, confirmation that cloud cover has been way up and sunshine way down.
And, even as the southern Plains continues broiling in wilting triple digit heat, our section of the Midwest is not---staying instead emminently comfortable with easterly lake breezes to blow well into Tuesday and limiting shoreline temperatures to the 70s..
We're analyzing computer projections which include the onset of an unseasonably cool mid-summer air mass later this week into the coming weekend. We'll have more on that air mass and look the potential for some active or severe thunderstorms Tuesday night and possibly again midday Wednesday on our evening television shows (at 5:30 and 9 p.m.) and right here on our WGN weather blog.
Hope your Monday is going well!
Tom Skilling
The Chicago area has been cleared of the severe weather threat----morning and early afternoon showers and south suburban thunderstorms have stabilize the atmosphere by introducing rain-cooled air. An extended break in precipitation is underway which will last through the afternoon and evening, removing any threat of weather troubles for those with plans to be outdoors. But, thunderstorms may not be entirely over across the metro area. Scattered thunderstorms could may re-develop late Friday night in the humid environment predicted here night over 30 to 40% of the metro area. And, despite their limited areal coverage and the fact they are to occur beyond the period of peak heating, a few of the heavier thunderstorms may still be capable of localized downpours, gusty winds and even some hail. Forecasts of the atmosphere's ability to generate upward motion of the humid air predicted in the area, as reflected in predicted CAPE (Convective Available Potential Energy) values from various models plus predictions of late Friday night atmospheric stability, still appear supportive of at least some thunderstorm development. In addition, surface winds are to converge along a southeastbound cold front which enters the area toward morning. When winds converge at the surface, air if encourage to rise and cool---a development which fosters cloud and thunderstorm formation. If I had to put times on any thunderstorm threat, I would suspect a few isolated thunderstorms could bubble up over part of the metro area between 10 and 1 am--but that the period from 1am to 5 am is the one to be most closely watched for thunderstorms. Showers may extend beyond 5am into mid morning Saturday before clearing takes place and a rain-free period begins.
A short explanation of how we've gotten to where we are as we post this at 3 pm Friday. Among thunderstorms' functions in nature is to exercise a measure of control over extreme heat. As products of the tallest clouds on earth, thunderstorms are able to "mix" cooler air down to the ground from great altitudes quite efficiently with their rains and downdrafts. The process slashes the vertical temperature decline which so critical to additional thunderstorm development. There are instances in which this cooling can be overcome by daytime heating once storms clear an area. That had appeared a stronger possibility earlier today than it does now. New data makes clear the diminishing area of showers, some with thunder in some west and southwest suburbs, which passed between mid morning and 1:30 pm this afternoon, proved more effective at stabilizing the atmosphere than first predicted. The extended break in rainfall currently underway and predicted to continue through this evening over most of the area, is one result. The probability that organized severe weather won't develop through this evening and early tonight is another.
The rains which fell fairly lightly across the city earlier today were actually leftover from some impressive thunderstorms which moved out of Iowa overnight and this morning. Clouds tops were collapsing and the areal extent of the rain was shrinking by the time it arrived in the Chicago. But to our west, where the rains were heavier and better organized, there have been some impressive rain totals. Unofficial totals off our Weather Bug network indicate Marseilles had 1.02", Henry, Il 0.57", Rockford 0.52" Minooka 0.30", DePage Airport 0.29" , Sandwich 0.25" and DeKalb 0.22".
We'll have more on the thunderstorm threat later tonight and the full weekend weather outlook (a bit of instability could lead to isolated t-storms over several sections of the Chicago area Sunday afternoon) and a the longer range weather picture on tonight's 5:30pm and 9Pm News programs.
Severe weather threatens the Chicago area Friday and Friday night--though any storms which hit are likely to hit the area in fairly distinct clusters separated by MANY rain-free hours. At the moment, it's the late day/Friday night storm threat which concerns us most, since it will be able to tap daytime heating and is to take place in an atmosphere dripping with an impressive 2" of evaporated water. It's been my experience the 2" precipitable water values are often in place and have served as a proxy indicator of some of this area's most active severe weather outbreaks. That's A LOT of water to have floating around for developing thunderstorms to tap. Add to that a powerful, spring rather than summer intensity jet stream overhead, and the reason for some concern about the predicted atmospheric set-up is obvious.
The picture on just when thunderstorms might sweep the area Friday is becoming clearer and I thought I'd share with you some of our thoughts on this as we continue our afternoon/evening analysis. Two periods emerge as being at elevated risk--the first mid and late morning when a rapidly diminishing area of initially strong thunderstorms (strong to our northwest, to be precise) is expected sweep in from the west and northwest, exiting midday or shortly thereafter----the second and potentially most worrisome in terms of severe weather, could begin with a few t-storms erupting with peak heating later Friday then build into more organized storms Friday night. The University of Illinois WRF model runs suggest atmospheric energy as indicated by the CAPE index could reach or exceed 2000 joules/kilogram toward 9 to 10 pm Friday evening--well over the 1,000 joules per kilogram often viewed as the severe weather threshold. If true, the second potential squall line could be a nighttime affiar.
A noteworthy caveat on Friday's storm risk is revolves around the strength of the initial storm band--most likely a morning phenomenon. Late model trends suggest this cluster--or what remains of them---will be crossing the area mid and late morning in a far weaker state than the powerhouse complex of storms from which it is to originate overnight in Minnesota and far western Wisconsin. Weakening storms would NOT cut seriously into daytime warming and would therefore not hinder secondary storm development later in the day. But if any storms arrive here Friday morning stronger than currently predicted, they might well interfere with late day storm re-development. So, this will have to be watched.
We'll have much more on the Friday storm threat on our WGN weather segments at 5:30 and 9 pm, here on the wgntv weather blog and with Steve Cochran on WGN radio at 6:05 pm.
Tom Skilling
