By Richard Koeneman, WGN Weather Center Meteorologist
Monday is likely to be best weather day of the next seven across northern Illinois -- pleasantly cool temperatures, moderate humidity and light winds. It's a welcome interlude following a three-day period of turbulent weather across the north third of the state. But active weather is set to return to the area by mid-week. Periods of strong thunderstorms arrive Tuesday and Wednesday, precursors to 90-degree heat and tropical-level humidity by the weekend.
Phet hits Pakistan
Tropical Storm Phet brought torrential rains and severe flooding to Karachi on Sunday -- and a break in that city's sweltering tropical heat. Despite damage and several deaths, residents welcomed the cooling rains. Resident Saeed Ali commented, "We rarely get rain in summer. This is a golden moment."
Thunderstorms that swept across the Chicago area Monday were a hit-and-miss affair. Residents in many western suburbs experienced only weak thunderstorms, but damaging thunderstorm winds toppled trees in and around Antioch. And a genuine two-hour deluge swamped Chicago's South Side, the southern suburbs and much of Lake County in Indiana with localized 3-inch totals.
More storms on way
Two storm systems have northern Illinois in their cross hairs this week, and both carry the threat of locally severe thunderstorms and heavy rainfall. Thunderstorms associated with the first system arrive in Wednesday's predawn hours and continue intermittently into early Thursday. The next system arrives Saturday and preliminary indications are that it, too, could be potent.
Monday's expected thunderstorms, triggered by the arrival of cooler air, are but the initial salvo in a cycle of active weather that threatens at least a few hours of thunderstorms on four of the next six days.
Storms should move out of the area later Monday, but expect the next wet weather Wednesday and Thursday -- possibly with heavy rainfall -- followed in quick order by another round of thunderstorms late Friday into Saturday.
Agatha triggers landslides
Agatha, the season's first named tropical cyclone in the Pacific basin, brought only 45 mph winds at landfall on Guatemala's Pacific coast late Saturday, but has generated 15-30 inches of rain that is triggering deadly flash floods and landslides in the mountainous interiors of Guatemala, El Salvador and Honduras. Rains are forecast to diminish today.
Memorial Day weekend weather in Chicago has been known to be finicky. Over the past 137 years, half of the holidays have produced measurable rain and only a quarter have managed highs of 80 degrees or higher. This year's holiday weekend may include rain on Monday.
Near perfect weekend.
That rain aside, Chicago's weather over the majority of the weekend will be as close to perfection as late May weather gets here. The higher-than-normal temperature trend that has dominated much of meteorological spring will be very evident in coming days.
Though humidities will increase in the next few days, the rise isn't likely to become evident until Sunday afternoon when the year's second official 90-degree high is predicted.
Memorial Day weekend weather in Chicago has been known to be finicky. Over the past 137 years, half of the holidays have produce measurable rain and only a quarter have managed highs 80 or higher. This year's holiday weekend may indeed include rain--some of it thundery on Monday. While those rains aren't likely to be continuous, our latest forecast has extended their stay in the area in response to the predicted slowdown of a cold front expected to produce the precipitation.
That rain aside, Chicago's weather over the majority of the weekend will be as close to perfection as late May weather gets here. The warmer than normal temperature trend which has dominated the majority of the meteorological spring season to date will be very evident in coming days.
Though humidities ease higher in the next few days, the increase isn't likely to become evident until Sunday afternoon when the year's second official 90-degree high is predicted to occur. Dew points, a measure of atmospheric moisture, are to reach the mid 60s--a bit below the Gulf-Coast level low 70s recorded during last week's warm spell--but noticeable nonetheless.
While 90-degree readings aren't unheard of at this point in the year, only 28 of the 82 years since 1928 at Midway Airport--34 percent of them--have generated two 90-degree or warmer readings before May's close.
Through Friday, meteorological spring temperatures since March 1 have averaged 51.9-degrees--nearly 5 degrees above the long-term average and more than 3 degrees warmer than the same period last year. The season, which ends midnight Monday night, is well on its way to becoming the area's 4th warmest of the past 140 years.
Summers after warm springs often big 90-degree producers
An analysis of summer 90s in years with warm springs indicates a link in many years. Of the summers which have followed Chicago's seven warmest springs, five recorded 90-degree tallies well above the 23 days which occur here on average. One year--1955--produced 46 days with 90-degree or higher temperatures--just two 90s shy of the record of 48 days of 90s at the South Side site in 1988.
11th day above 70 Saturday, something which didn't happen until mid June last year
Friday's 79-degree high at O'Hare was the 10th consecutive high above 70 degrees. It wasn't until mid-June--nearly three weeks later--that last year's first 10-day stretch of 70s occurred.
Late-season snows whiten mountaintops in the West
While the nation's Heartland was bathed in warm early season weather, up to 4" of snow whitened the Lemhi Ridge observation site in western Montana Friday. The measurement occurred at an elevation of 8,100 feet.
Sun-drenched holiday weekend to foster a rebound in warmth and humidity; some storms possible
The Chicago area heads into this Memorial Day weekend positioned for warm weather befitting the holiday which marks the unofficial open to the summer season while honoring those who have served this country. For a holiday juxtapositioned with the year's warmest temperatures, it's ironic how often it's weather has been anything but summer-like.
While it's true sections of the area could see several thunderstorms before the holiday weekend comes to an end, most hours will be rain-free. The best thinking now is that any rain which might flirt with the area would do so late Sunday night, perhaps lingering into early Monday. Rain coverage with those storms is likely to be scattered, involving perhaps 40 to 50 percent of the metro area. But by in large, the weather in coming days looks likely to live up to the highest expectations of those hungry to get outdoors.
With the exception of the 77-degree high predicted Friday, official highs away from Lake Michigan are likely to reach or exceed 80 Saturday, Sunday and Monday. Only in lakeside areas, where northeast breezes are to blow off cool lake waters Friday and Saturday, will residents have to wait until Sunday and Monday to reach 80.
Temperatures which flirt with 90 Sunday?
The possibility that air sinking and compressing beneath the nose of an incoming jet stream wind max Sunday could nudge temperatures toward 90-degrees appears real. It would mark only the second time this year a temperature that warm has occurred. Warming occurs as air sinks on a broad scale and this meteorological setup has been enough to boost temperatures close to 90 in the past.
Before that happens northeast winds off Lake Michigan are likely to keep lakeside readings a bit chilly.
Memorial Day weather has not always cooperated with outdoor enthusiasts in the past
Memorial Day weather has run the gamut in Chicago; as cool as 32 degrees in 1992 and as warm as 93-degrees more than a half century ago in 1953. A 91 degree Memorial Day high occurred as recently as 4 years ago.
Rain has fallen on just under half (46 percent) of the 137 Memorial Days for which we have official records. And rains has been no stranger to the holiday in recent years. Seven of the past ten holidays have recorded at least a trace of rain.
In terms of temperature, only a quarter of the Memorial Days on the books here since 1873 have managed an 80-degree high. There are indications this Memorial Day will join the ranks of the 80-degree producers.
NOAA joins other tropical forecasters in issuing ominous 2010 Atlantic basin hurricane season forecast
Forecasters at the National Oceanic and Atmospheric Administration's Climate Prediction Center, citing the warmest ocean temperatures in 50 years in the Atlantic basin's hurricane breeding ground, as well as citing the demise of the ongoing El Nino in the equatorial Pacific, issued a sobering forecast Friday for 2010 Atlantic basin hurricane season. The number hurricanes and tropical storms is likely to exceed the averages---and possibly exceed them significantly. The agency's director Dr. Jane Lubchenco didn't mince words. "If this outlook holds, this season could be one of the more active on record."
The forecast comes in an especially challenging year. Thousands in Haiti---a country prone to the ravages of hurricanes ---remain in tent cities in the wake of the past year's devastating earthquake. At the same time, a catastrophic oil spill is underway in the Gulf of Mexico---a development which could be severely impacted by any hurricanes or tropical storms.
In issuing such a forecast, NOAA forecasters have joined other tropical weather researchers and forecasters at Colorado State and the UK Met Office in predicting what many fear may become an exceptionally active 2010 season.
The coolest, driest air in a week rides gusty north to northeast winds into the Chicago area Thursday bringing to an end the warmest May 23-26 period in 140 years of record keeping. By day's end, dew points---which reflect atmospheric moisture levels---will have plunged to the low and mid 40s, far from the muggy Gulf Coast-level low 70-degree readings recorded just days ago. The past week has played host to the most humid weather to occur so early in a warm season here in Chicago over the last six years.
The drying process Thursday leads to the demise of the occasionally sprinkly overcast with which the day opens. Clouds are predicted to diminish from the north during the afternoon and skies should be cloud-free by late in the day. Lakeside areas from southeast Wisconsin through northeast Illinois and northwest Indiana will be on the receiving end of a full-fetch northerly wind flow traveling the length of Lake Michigan's chilliest surface waters Thursday. Those areas are to bear the brunt of the day's cooling, with readings struggling to get out of the low to mid 60s--- while inland locations reach the mid 70s.
Chicago temperatures this past Sunday through Wednesday (May 23-26) averaged 77.8-degrees---a stunning 17-degrees above the long term average of 60.8-degrees and nearly 15-degrees warmer than the same period a year earlier. In only twelve other years since 1928 have four consecutive days with highs of 85-degrees or warmer occurred at the South Side site. The last time that occurred this early in the season was eight years ago in 2002. Interestingly, the summer which followed went on to produce 32 days with highs of at least 90-degrees.
Warm weather enthusiasts may be interested to learn that the summer months which have followed early-season strings of four or more days with mid-80s have generally gone on to produce an average of 31-days of 90s. That tally is well above the long term average of 23 at Midway Airport.
If this summer ends up producing a large number of hot days, it will end up far different than its four predecessors, each of which has produced only a comparative handful of 90s. Of the 11 years with four or more days in the mid 80s this early in the season, 9 of the 11 summers which followed (82 percent of them) ended up producing more than the average number of 90s at Midway Airport.
Gusty downpours, hail accompany Wednesday afternoon t-storms over 40 percent of metro area
Thunderstorms erupted over portions of the Chicago metro area Wednesday afternoon, at one point prompting urban and small stream flood advisories for sections of Cook and DuPage counties. Doppler scans put cloud tops as high as 46,000 feet and hail---a half inch to an inch in diameter---crashed to earth in wind-driven, lightning-punctuated downpours at a number of locations. Peotone was hit with 1-inch diameter hail.
Lightning data indicated more than 1,800 cloud to ground strokes occurred in a single ten minute period at the height of Wednesday's storms. Heavy rain totals included 1.68 inches at Bartlett in northern DuPage County and 1.03 inches in only 30 minutes near Inverness, located in northern Cook County not far from Schaumburg and Hoffman Estates.
Downstate cloudburst unleashes 4-inch+ rains just east of state capital
Heavy as rains were in some Chicago area storms, several totals downstate were eye-catching. Just a mile east of Springfield, 4.43 inches of rain was reported to have fallen in 90 minutes Wednesday afternoon. A full May in the state's capital typically produces 4.06 inches!
Daytime heating is likely to coax thunderstorms to life again Wednesday over 40 to 50 percent of the Chicago metro area. Exceptionally light winds above ground suggests these storms aren't likely to hurry along. The thundery deluge which hit Rockford and sections of Wisconsin and Minnesota late Tuesday were products of slow-moving thunderstorms which towered at times as high as 51,000 feet into the atmosphere.
The downpours of slow-moving thunderstorms can really add up---and quickly. Rainfall in Tuesday afternoon's heaviest thunderstorms fell at rates of up to 1 to 2 inches per hour. Rockford was hit with 1.29 inches of rain and powerful 58 mph gusts. Damage was reported just a mile southeast of the Rockford airport in New Milford Illinois. There, trees and power lines came down in the thundery downpours. Lightning flashed frequently. At the height of Tuesday afternoon's thunderstorms, lightning occurred at the rate of 2,000 discharges in just ten minutes time. A trained spotter reported standing water several feet deep.
At Ogle County's Polo, to Rockford's southwest, street flooding was reported with the same storm complex.
Daytime heating provides the perfect environment for thunderstorm development
As sun heats air, it expands and becomes buoyant. Ascending air cools and condenses into cottony cloud masses. These cumulus clouds which become cumulonimbus once the cloud begins precipitating, can grow to altitudes of tens of thousands of feet. Thunderstorms emanate from the tallest clouds on earth. The unique mix of liquid and ice which occurs in these towering clouds contributes to lightning formation by separating charges.
Wednesday set-up appears one which could support downpours in portions of the Chicago
The set-up predicted Wednesday will be monitored closely. With light steering winds aloft, the same moisture-rich pool of air in place here since Sunday remains available to meet the voracious moisture needs of the towering cumulonimbus clouds---or thunderheads---predicted to develop. Projections put the air's water content Wednesday at 1.50 inches---slightly more than was available to the downpour-generating cluster of storms in the west and northwest suburbs Tuesday.
Storms Wednesday are likely to be slow-movers raising the possibility their downpours could again provoke flooding in locations hit by the heaviest rains.
Temp/humidity rebound later this Memorial Day weekend; 80s to stage comeback
A break in the recent heat and humidity is on the horizon. By Thursday, north to northeast winds are to travel the length of Lake Michigan into Chicago. Satellite measurements of the Lake Michigan water surface temperatures along the path these winds are to follow will range from 47 to 51-degrees. With winds in contact with such chilly water over the 250+ mile trek from near the Door Peninsula in Wisconsin to Chicago---the air which reaches the city may only be in the 60s---markedly cooler than the string of four consecutive mid 80-degree days which will be on the books by then.
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