WGN Weather Center Blog

Temperatures head to 70 for the first time in 6 weeks

2009-11-07T05:15:42Z

The warmest weekend in six weeks is underway. Chicago area temperatures broke above 60 degrees for the first time in a week Friday, reaching 62 at O'Hare and Midway airports, and 63 at the lakefront. Among the warmest area highs Friday were 65 degrees at Rockford; 64 at Aurora and DuPage Airport; and 63 degrees at La Grange, Wheaton, Lansing and Plainfield.

Weekend readings look even warmer: A pair of 70-degree highs (or readings awfully close) remain a good bet Saturday and Sunday -- temperature levels not seen here since late September.

Powerful south winds reached speeds of 43 mph in gusts at building-top levels on LaSalle Street Friday and 36 mph at Rockford, Lincolnwood, Burlington and north of the Wisconsin state line in Racine -- and have gusted to 30 mph at times overnight, mixing the air and preventing the usual nocturnal temperature drop. So Saturday's highs build from a higher starting temperature.

The air mass which dominates Midwest weather was so warm to the west of the city Friday it produced a second day of record-breaking highs in the Plains including 81 degrees at Valentine, Neb., 80 at Yankton and 77 at Rapid City -- both in South Dakota. Add to the air mass' inherent warmth the broad subsidence of air which is to occur Saturday beneath the nose of a powerful jet stream -- a process which helps heat the air as the sinking air is compressed in the higher pressures found near Earth's surface -- and there can be little question why weekend temperatures here are headed to levels 20 degrees above normal.

Warm spell to extend to 4 days -- not common this late in the season
With high temperatures exceeding 60 degrees predicted through Monday, this warm spell has only a comparative handful of peers over the term of Chicago weather records. Four days of temperatures 60 degrees or higher have occurred beyond Nov. 6 only once every six years on average. Records at the South Side site record only 13 comparable late-season warm spells in 81 years since 1928.

Strong winds ease overnight only to resurge with more warmth Sunday

2009-11-07T05:14:17Z

Frost when temperatures are above freezing

2009-11-07T05:12:42Z

Dear Tom,
I have seen frost on garage rooftops at sunrise when temperature reports are 38 or 39 degrees. What gives?
Robert Wolfson
Dear Robert,
Frost forms when the shallow layer of air at the ground (or on any other surface such as a rooftop) cools below its saturation temperature (and below freezing) by contact with a cold surface that itself has cooled to a subfreezing temperature. Water vapor in the air condenses directly, in ice-crystal form, onto the cold surface: frost forms.
Under clear, calm nighttime conditions, surfaces like blades of grass (but it works for roofs, too) radiate a great deal of heat and their temperature drops sharply. The cold-air layer can be very shallow, often only a fraction of an inch. Air temperatures in National Weather Service reports are taken by temperature sensors a few feet above the ground and sheltered from radiational cooling.

Warm 30+ mph gusts set stage for mildest weekend temps since Sept

2009-11-06T04:38:32Z

The Chicago area appears headed for its mildest weekend since September--one which may include high temperatures within striking distance of 70-degrees. The eastbound mild air behind the predicted weekend warm-up sent temperatures Thursday soaring to near 80-degrees in the western Plains. In Denver, where up to 4 feet of snow fell in the mountains to the west of the Mile High City less than a week ago, the temperatures soared to 77-degrees with 81-degree readings at La Junta and Springfield--both in Colorado. Highs in nearby Nebraska included 81 at Sidney, 80 at Imperial and 78 at Chandron while Elkhart and Dodge City in Kansas topped out at 80.

The warming there was produced by air sinking from the mountains into the Plains where it was compressed and warmed as it descended into the higher pressures found at lower elevations. The process is commonly referred to as the Chinook effect.

As that mild air of Pacific origin continues eastward, it won't be quite as warm by the time it arrives in Chicago. But, it is likely to introduce a 15-degrees temperature increase by Saturday afternoon over the levels observed Thursday. Accompanying that warming will be an influx of Gulf moisture in the lowest several thousand feet of the atmosphere--enough to contribute to the "warmer" feel of the air, but, based on present indications from computer models, not deep enough to produce layer of the atmosphere to produce the kind of cloud cover which would block the sun and thwart warming. Arrival of more significant moisture supporting more extensive cloud cover appears the only means of sidetracking what seems on track to become one of the nicest weekends of the fall season--and that's not expected to happen at the moment.

Low clouds forming over western Illinois late Thursday evening were predicted to expand into the Chicago area Friday morning. But, powerful winds Friday are expected to mix down to the surface in the late morning and afternoon allowing clouds to break, some sun to emerge and temperatures to head toward 60-degrees. Gusts in excess of 30 mph appear a good bet once this happens---and, with gusty winds expected to continue Friday night and Saturday, overnight readings should be significantly milder than those of recent nights.

A fascinating weather scenario may unfold in the Caribbean and Gulf of Mexico in coming days and into next week. Minimal Hurricane Ida--with 75 m.p.h. top winds--went ashore in Nicaragua Thursday. The storm, downgraded to a tropical depression late Thursday, threatened torrential rains totaling 15 to 20 inches at higher elevations. But, computer models suggest the storm's northbound remnants are likely to sweep out over the bathtub warm waters of the Caribbean, allowing the system, in the absence of strong winds aloft to regenerate. The re-energized system is then likely to spread north into the Gulf of Mexico where it may threaten sections of the coast--- potentially including Florida with downpours and wind mid and late week.

Thursday's 100 percent cloud -free skies the sunniest in nearly 2 months

Thursday gorgeous, completely cloud-free skies produced Chicago's first 100 percent sunny day in the nearly two months since September 2 and 11.

Only unexpected clouds could interfere with weekend warmth

2009-11-06T04:15:42Z

Explaining wind direction

2009-11-06T03:50:40Z

Dear Tom,
What is the meaning of "northwest winds"? Wind coming from the northwest or blowing toward the northwest?

Ralph Bellendir

Dear Ralph,

By international convention, wind direction always refers to the direction from which the air is moving. The proper interpretation of, say, a northwest wind of 10 mph is that the wind is blowing from the northwest to the southeast at a speed of 10 mph.

While we're on the subject, here's an unusual bit of weather trivia: You'll never hear a reported wind speed of 11 mph. Never. Here's why: Wind speeds are always measured in knots (nautical miles per hour). That's the way it's done, worldwide, and then in the United States that value is converted to statute miles per hour. One knot is 1.1508 statute mph. A value of 9 knots converts to 10 mph and 10 knots becomes 12 mph after rounding to a whole number.

Spectacular fall colors in the Hinsdale area

2009-11-05T23:10:20Z

Thanks to Kelly Anbach of Bensenville for these spectacular fall color shots taken in Hinsdale. They're beautiful! THANKS for sharing them with us, Kelly!

Tom Skilling

Photos courtesy: Kelly Anbach of Bensenville, Illinois

Mushroom-shaped cloud on the east side of Resurrection Bay in Seward, Alaska

2009-11-05T23:07:31Z

This remarkable picture comes to us from John and Andrea Skillman of Gurnee. It was taken while vacationing in Alaska on the east side of Resurrection Bay in Seward. The Skillman's first spotted this cloud while eating dinner and aptly describe it as a mushroom cloud. It's more technically referred to as an orographic cloud---one which is induced as terrain forces air to rise---a process which leads to cooling and condensation of airborne moisture produces clouds such as this. They report it dissipated a short time later as the sun set. John and Andrea add that Alaska is a great place for seeing weather----which is absolutely true! THANKS SO MUCH to the Skillmans for such a fascinating photo!

Tom Skilling

Photo courtesy of John and Andrea Skillman, Gurnee

Morton Arboretum fall colors update

2009-11-05T22:50:58Z

Allison Phelps, public relations coordinator for The Morton Arboretum in Lisle, give us an update on the fall color display at the arboretum and sent us these great pictures. Allison tells us:

Just to let you know, fall isn't quite over here at The Morton Arboretum. There are still colors to see. Thanks to our expansive collection of exotic trees, visitors can stop in and see the red and yellow leaves of the Persian Ironwood tree or the Paper-Bark Maple with it's bright red leaves accented by the cinnamon-red exfoliating bark!

Thanks again Allison!

-Tom Skilling

Tim's Weather World: Mile High City & Mount Kilimanjaro

2009-11-05T14:55:23Z

Hard to believe that just a week ago Denver was covered in more than a foot of snow. Today they will see sunshine and a high near 80. Snow is also melting on top of Mount Kilimanjaro but at a slower pace. It could be all gone in as little as 22 years though according to a new study.

As the ice melts, more of the darker colored surface is exposed which absorbs more sun and leads to further melting. The people near Mount Kilimanjaro depend on that meltwater for drinking water. Check out this article for more information.

My daughter wants to climb Mount Kilimanjaro with me. She was party inspired by a year of study there and a new book, "Don't Let Her Go". We have a little more than 20 years to make the climb if we want to see the snow on top.

Speaking of snow... We are less than two weeks away from the average date of our first measurable snow, November 16th. No snow in the forecast through the weekend though as we warm into the 60s on both Saturday and Sunday.

It can hit 70 this time of year--it did a year ago

2009-11-05T04:32:55Z

Shorter days and weaker, low angle sunlight make it harder to warm this time of year--but that doesn't keep mild temperatures from happening. The atmosphere just has to work harder to produce such "warmth"--by, for instance, generating stronger winds which blow from warmer regions into the Midwest before the incoming air can cool---or by minimizing cloud formation capable of blocking sunlight. It also doesn't hurt to sit beneath the nose of a pocket of powerful jet stream winds, where air sinks, compresses and warms on a broad scale---a setup predicted to fall into place this weekend. That warmth can occur this time of year was evident a year ago. The area was in the midst of a three day 70-degree spree. Last year's 71 degree high on this date was 18-degrees warmer than the 53 predicted Thursday.

Barring more extensive cloud development Saturday than is currently predicted, readings then could reach 70-degrees for the first time since Sept. 27. Late season 70-degree temperatures aren't common, but they have occurred beyond Nov. 5 an average of one year in three since records began at Midway Airport 80 years ago.

The coming warm-up---likely to produce the first set of back-to- back weekend 60s here since late September---may well be part of a three day spell of 60-degree-plus highs extending from Saturday through Monday. Three consecutive 60s have occurred here this late in the season in 44 percent of years on record.

Frigid arctic air remains trapped in northern North America where temperatures in recent days have dropped as low as 20-degrees below zero. In stark contrast, the tropics remain active. Tropical Storm Ida---with 65 mph winds and gusts of hurricane strength--formed Wednesday off the coast of Nicaragua. The system will lose strength as it punches into Central America on a northward trek. But, it's possible we've not heard the last of the system. It could emerge into the southern Gulf of Mexico off the Yucatan Peninsula in the next week where reorganization would occur.

Humidity surge this weekend will add to mild "feel"---over-development of clouds would thwart warming

Dew points, which reflect atmospheric moisture and have resided in the 30s in recent days, are to take off this weekend as Gulf moisture mixes with mild but comparatively dry Pacific air moving in from the west. They are to reach the 50s which will lead to a noticeably more "humid" feel to the air Saturday and Sunday just as temperatures surge. This should lead a mild feel to the air not experienced here since September.

Mildest weekend since late September ahead; a late season 70 possible

2009-11-05T04:12:37Z

How rare are big storms on Lake Superior?

2009-11-05T03:49:41Z

Dear Tom,
We are approaching the 34th anniversary of the sinking of the Edmund Fitzgerald. How rare is it to have a storm of that magnitude on the Great Lakes?
Mike Long

Dear Mike,
The legendary storm of Nov. 9-10, 1975 had a central pressure of 28.95 inches when it crossed Lake Superior, equivalent to a Category 1 hurricane. At its peak, it had sustained winds of nearly 80 mph with gusts above 90 and produced giant waves 25-30 feet high.

Though stronger storms have battered the Great Lakes, this storm was certainly on the high end of severity scale. The term "Gales of November", popularized by mariners long before it was made famous by Gordon Lightfoot, refers to the peak of the Great Lakes' storm season, when late-fall storms fueled by clashing warm and cold air masses gain extra energy from the residual summer heat stored in the waters of the Great Lakes.

It's chilly now--but 9 of 10 years produce more 60s

2009-11-04T04:28:03Z

November is Chicago's fastest cooling month, a point driven home by Wednesday morning's chilly temperatures. Cloud cover and showers overnight prevented a repeat of the mid 20s recorded across a number of far western suburbs Tuesday morning. But, 30s were widespread and clouds and lingering sprinkles and light showers Wednesday will prevent daytime readings from rising out of the 40s ---the chilliest high temps here of the coming 7 days. Normal November highs tumble from 55-degrees on the first to 40-degrees on the 30th.

However, the month's tendency to cool hardly means all mild weather is behind us. A year ago, the Chicago area logged three consecutive 70-degree days (73, 71 and 71 on Nov. 3, 4 and 5.) And weather records reveal an average of one in three years has produced at least one additional 70-degree beyond this date and 93 percent of all years have added additional 60s. The 139-year average is four 60+degree highs past Nov. 4.

All signs point toward such a warm-up this weekend---a mild spell likely to extend into early next week. Late season warm-ups are extremely sensitive to cloud cover. Too much cloudiness or an outbreak of precipitation can take a serious toll on late year warm spells. So can a wind-shift off Lake Michigan. But, barring more cloudiness than is currently foreseen, the Chicago area could be in for a string of four 60-degree daytime highs from Friday afternoon through Monday. It would make the upcoming weekend this area's first since late September to host back to back 60s.

Computer models do indicate a weakness in the pressure field Saturday night into Sunday morning which has to be monitored. This may well allow a wind-shifting front to sag southward across the area. Such a development would permit southeast winds to take hold reaching lakeside counties after a trip over cool lake waters a portion of Sunday, lowering shoreline temperatures. But these same models take the front back north of the area and strengthen south winds Sunday afternoon and evening. Such a scenario would put 60s within reach of most of the Chicago area Sunday. And well organized south winds Monday and indications that most cloudiness is to remain to the west of the area until Monday night and Tuesday appear to favor more 60s Monday. Temperatures at such levels this time of the year are 15 to 20-degrees above normal.

Southwest sizzles in hottest late season air on record

Record warmth across the Southwest U.S. Tuesday produced a 96-degree high in Phoenix---the hottest temperature there ever so late in the season--- and a peak reading of 93 in Tucson.

Late-season warm-up; mildest weekend temperatures since September

2009-11-04T04:08:25Z