Explainer

Chicago's wet spring has grown even wetter. The season’s precipitation ranking surged two slots in just hours late Thursday with the onset of the area’s latest downpours.

Spring 2013’s ranking jumped from 10th to 8th-wettest and the season’s 11.13" total through 11 pm Thursday evening made it Chicago’s soggiest on record since the 11.29" fell over the same period in  2009.

It’s the area’s highest spring precipitation tally in four years

A “normal” spring typically produces just 6.92” by this date. The 2013 tally means rainfall is now 164% the seasonal norm---well over one and a half times normal!

Longest 70-degree spell here in over 8 months is over for now; chilly air’s arrival slashes Friday highs 27-degrees from Thursday's 75---a late March level

Thursday’s 75-degree high marked the fourth consecutive day temperatures here reached or exceeded 70---the longest such streak in the more than 8 months since 35 consecutive 70s occurred between August 14 and  September 17 last year.

The latest spell of warmth is over. A windshift to the northeast sent temperatures crashing overnight and Friday’s thick, sprinkly overcast promises little prospects for a rebound. In fact, lakeside temperatures may hover in the 40s through the day thanks to northeast winds---March-level readings.

Jet stream diving into Lower 48 from Canada to deliver second, reinforcing cool air outbreak this weekend

A second, reinforcing surge of cool air is to ride powerful northwesterly jet stream-level steering winds into the Lower 48 from Canada setting the stage for a cooler than normal Mothers Day weekend.

A brief surge in temperatures immediately behind the  second cold front Saturday---a front which may initiate spotty morning showers---will be short-lived. Temperatures are predicted to drop slowly to the mid or upper 50s by nightfall Saturday evening.

Sunday Mother’s Day 2013 temps far from last year's 72; day to feature an early April-level chill

The 72-degree high last Mother’s Day is to be but a distance memory this coming Sunday. That's because brisk northwest winds are to continue transporting cool air into the area restricting highs to the 50s---early April-level temperatures.

The arrival of widespread cloudiness Sunday after mixed morning sun promises to further compromise the daytime warming which the presence of May sunshine might have produced.

A late season frost threat looms Sunday night

Lighter winds and the development of breaks in the clouds late Sunday night is to allow temperatures to settle into the 30s, flirting by Monday morning with the 1996 record of 34-degrees for the date.  The chill also puts temperatures close to if not “at” frost producing levels.

Stunning temp rebound could take Chicagoans from a frosty Monday morning to a summerlike near-90-degree Tuesday temp

A dramatic temperature rebound follows next week.  Southwest winds, due to put in an appearance here Monday afternoon, are to begin the warming process----pumping readings into the low 70s after a morning of 30s and patchy frost. But that warming's just the beginning. All indicators continue to point toward temperatures moving well into the 80s Tuesday and Wednesday---a period which will also see mid and upper 60-degree dew points arrive! Dew points provide a measure of atmospheric moisture and readings at those levels indicate perceptibly more humid air is to accompany the year’s warmest temperatures to date.

A 90-degree reading isn’t completely out of the realm of possibility on either day. And powerful thunderstorms could sweep the area late Wednesday or Wednesday night with a new cold front’s approach and passage.

Thundery downpours swept the Plains late Wednesday, products of a developing storm system headed for the Midwest. It’s a storm likely to bring the Chicago area its heaviest rainfalls in 3 weeks.

The warm, dry weather this past week has finally allowed Midwest farmers to get into the fields and begin planting the 2013 crop---albeit very belatedly.

The rains and thundery downpours on the way later Thursday and Thursday night are likely to put planting on-hold again the next several days.

The wet storm system is to play another important role in shaping the weather of the upcoming Mothers Day weekend. It is to drag an unseasonable April-level chill into nation’s mid-section---a development which could even allow patchy frost  Saturday and Sunday nights.

Mother's Day (Sunday) is likely to produce a high temperatures in the 50s---readings which fall far short of last year’s 72-degree high.

Storm responsible for the new rain threat in Chicago hammered the Plains Wednesday afternoon and evening, producing severe weather

Reports of severe weather Wednesday flooded in from Texas north to Oklahoma and Kansas. Five reports of tornadoes had been filed with the Storm Prediction Center late Wednesday. There were also 102 reports of hail and damaging winds.

Pollen counts remain elevated; miserable allergy season of 2013 continues

The 2013 allergy season is off to a miserable start for too many Chicago area residents.

Pollen counts out of Gottlieb Memorial Hospital Wednesday continue to show tree pollen values which are through the roof. The tree pollen count came in at 600 Wednesday compared to 60 a year ago.

The area’s pollen values surged Tuesday to the highest levels of the year, the latest development in a allergy season which has brought misery to many across the Chicago area. Dr. Joseph Leija, allergist at Loyola Gottlieb Memorial Hospital and provider of this area’s pollen counts, lamented in his daily report that the slow pace of spring warming has combined with the season’s huge rain tallies to bring high tree counts at the same time high grass and weed levels are present. The result, says Dr. Leija, has been to make allergy sufferers miserable despite the lovely weather of late. Swollen and itchy eyes were among the reasons many patients visited his office as well as the need for medication adjustments

Tree, grass and weed pollen were ALL reported as “high” Tuesday.  Tree pollen reached 1,000 grains per cubic yard---just 500 below the “alert level” of 1,500.

Tuesday's 75-degree O’Hare high the city’s  3rd warmest temp yet this year; 80s recorded farther inland 

Temperatures, which have been slow to warm this spring, put on quite a show away from Lake Michigan Tuesday. While O’Hare and Midway recorded 75 and 76-degree highs respectively, Lake Geneva, WI topped out at 81 while Rockford and Morris each recorded 80-degree temperatures.

Area temperatures Tuesday varied 25-degrees thanks to lake breezes which move into a 7th consecutive day Wednesday

Easterly winds off Lake Michigan’s abnormally chilly spring waters are sending Chicago temperatures lower along and near Lake Michigan for a 7th consecutive day Wednesday.

Those same east winds helped generate a 25-degree east/west temperature spread across the metro area Tuesday---from 57-degrees at Wilmette to as warm as 83-degrees at Lake Geneva.

New rains---some thundery---loom; incoming rainfalls could approach or top 1 inch---the heaviest here in 3 weeks

The heaviest rainfall in nearly 3 weeks may be on its way. The first scattered of showers, possibly a thunderstorm, may arrive toward daybreak Thursday after a night of increasing cloudiness. Heavier and more numerous showers and thunderstorms spread into the area Thursday afternoon and night.

By the time all of the rain the system has to offer has fallen, an average of 0.98" water-equivalent precipitation may be in the books---with some totals potentially exceeding 1 inch.

Blustery cool snap to take Mother's Day temperatures back to early April levels

A chill hits this weekend--and it's badly timed. It threatens early-April-level temperatures here Sunday for Mother's Day. Peak readings are predicted to hit the low or mid 50s. It’s what may follow the weekend chill which could prove most interesting.

Strong, wind-driven warming could lead 80-deg or higher temps next Tuesday/Wednesday

It appears we have quite warm-up on deck which we expect to follow the Mothers Day chill. Temperatures could be flirting with 80-degrees next Tuesday with mid 80s not out of the realm of possibility by  Wednesday next week.

There’s not a month of the year more likely to produce cool easterly lake breezes near Lake Michigan than May. Easterly winds have historically occurred on 45% of May days---and Tuesday will be no exception. In fact, Tuesday’s easterly flow will mark the 6th consecutive day onshore winds have been a part of the Chicago weather scene.

The lake winds have, in recent days, been stronger than those predicted Tuesday and Wednesday,  and therefore in a position to deliver their lake-cooling to more of the metro area than will be the case the next few days.

An impressive “cut-off" low, responsible for drenching sections of downstate Illinois and Missouri with more than 4" of rain over the weekend, has contributed to the strength of the Chicago area's easterly flow in recent days. But, its influence over Chicago’s weather, is fading. That’s why Tuesday’s east to northeast winds will be lighter and, therefore, able to send lake-cooled air little more than several miles inland---unlike Monday when they propelled lake-cooled air as much as 15 miles inland.

Afternoon temperatures may hold to the low and mid 60s Tuesday on area beaches---a little warmer than the widespread 50s observed there Monday. Inland areas, on the other hand, are to see readings flirting with 80-degrees Tuesday and into the low 80s Wednesday from DuPage county westward to the Fox Valley.

Warmth in coming days to boost 2013's lagging 70-degree count

May's opening 6 days have averaged 6.7-degrees cooler than the same period a year ago. The lower temps have impacted the area's 70+degree count as well.

Monday's 71-degree high at O’Hare and Midway brought to just 6 the total number of 70s on the books in 2013. That's just over half the normal 11 days of 70s by this date and only a fraction of last year's nineteen 70s up to May 7.

Rains, some thundery, may total one-half inch---locally more---later this week

A few isolated showers may build in the afternoon warmth Tuesday to the south and east of Chicago.  But it's late Wednesday night through Thursday night which have the best chance of producing rain this week.

An average across a suite of computer model rain projections puts potential late week rainfall here around 0.70"---though the range in predictions from which that average was derived, runs from as little as 0.38 inches to as much as 1.17 inches. That spread may more accurately reflect the range in potential late-week rainfall which is likely to affect the area.

Sharp weekend cooling could produce Mothers Day temps at early April levels Sunday

A sharp temperature pullback from the 70s predicted in coming days takes place this Mother's Day weekend. The cooling is likely to begin Friday. Daytime highs near 60 Saturday will trend lower with the arrival of strong northerly winds Saturday afternoon and night.

Peak readings Sunday (Mother's Day itself) may rise no higher than the 50s.

Season’s widest Lower 48 warming to-date indicated by models next week

It's next week that forecast trends suggest may bring the widest warming of 2013 yet seen across the Lower 48. Chicago temperatures amid gusty SW winds Monday may resurge to within striking distance of 70-degrees while the large area of 80s predicted in the Plains and western Midwest as the next week gets underway on Monday, may offer a clue to what's ahead for Chicago.

Chilly east winds off Lake Michigan are as much a part of spring in Chicago as the increase in daylight for which the season is known.

Warmer air has actually made an effort to back into Chicago from the east overnight—a development most evident in areas and west and south of the city which are NOT immediately downwind of Lake Michigan.

With the latest wet spring storm tracking south of Chicago and pulling some of the system’s deeper moisture along with it, the chances for breaks in the clouds will increase Saturday afternoon and it’s likely sunshine may become quite abundant here Sunday.

Lake fog a possibility as moist easterly winds travel out over cold lake waters then ashore

The potential for patches of fog is real along Lake Michigan Saturday.  The day’s east to southeast winds originate in Indiana and Michigan—areas expected to warm into the 70s—but even more importantly, a region in which dew points, a measure of moisture in the air, are in the low 50s. All that's needed to encourage fog formation is to find a way to cool that air to the low 50s. The fog and haze forms as moisture in the lake-cooled air begins to condense into water droplets.

Of course, with winds flowing out over the chilly 40-degree lake waters, cooling this air to its dew point will occur, a development which may well lead to fog formation Saturday.

Pollen count worsens; tree pollen soars to 2013 high Friday

Allergy sufferers are well aware that the 2013 season is off to a bad start, a situation underscored by Friday’s high counts in the Chicago area.  Especially troublesome was the day’s tree pollen count which reached its highest level of 2013, reports Gottlieb Memorial Hospital’s Dr. Joseph Leija.

Heavy April rains led Dr. Leija to predict the potential for a  “miserable” allergy season back on the 12th of last month—a forecast which has verified as pollen counts have worsened.

Friday’s 47 was the chilliest May 3 temp here in 56 years and  equal to March 16’s normal high

A chill, in stark contrast to the past week’s 80s on Tuesday and Wednesday, continued Friday.  The day produced a high of only 47-degrees making it the coldest May 3 in 56 years—since a 47-degree high on the date in 1957.  The reading was not only 19-degrees below normal, it equaled the normal high temperature which occurs on March 16.

Wet storm being deflected south; rain slated for the Kentucky Derby Saturday evening

The sluggish-moving "cut-off" upper atmospheric storm system, a truly prolific rain producer to Chicago’s west and south Friday, continues its slow east/southeast drift Saturday and is threatening Saturday evening's Kentucky Derby with rain.

The storm generated huge rain tallies over Missouri and sections of downstate Illinois Friday.  Festus, Missouri was hit with 3.60” of rain while 3.10” fell at Sullivan, MO; 3.07” came down at Medora while 3.02” was measured at Beverly—both in Illinois.  Other totals Friday included 3.00” at Oakville, MO; 2.90” Mt. Sterling, IL; and 2.71” at Weldon Spring, MO.

Heaviest Chicago-area rains topped 0.60”

Rainfall Friday in the Chicago area wasn’t as extreme—but was significant nonetheless. 0.74” fell at Algonquin; 0.71” Union; 0.68” Harvard; 0.62” South Elgin; 0.60” DeKalb and 0.48” at Crystal Lake. Streator picked up 0.65”.

System’s backside snows continued to set records Friday; Minneapolis’ 0.5” tally ranks as that city’s biggest late-season accumulation in 67 years

The snowfalls recorded in recent days in a corridor from Tulsa, Oklahoma north to Rice Lake and Montreal, WI, have been May record-breakers.

Minneapolis’ 0.5” accumulation Friday was added to the list of cities in which the snow produced new records. That amount is the greatest to occur in the Twin Cities area in the 67 years since 2.8” fell on May 11, 1946.

The contrast couldn't have been more stark! Chicagoans, who reveled in July-level 80+ degree warmth Wednesday, had the meteorological rug pulled out from beneath them Thursday, experiencing a 29-degree 2-day temperature drop! It was the city's largest official day-to-next May temp plunge to take place here in the past 5 years,  and one of only six Mays since 1871 to experience a temperature pullback of that magnitude!

The arrival of northeast winds off Lake Michigan delivered a chill which would have right at home in March.

While Wednesday chalked up an 84-degree high, temperatures once the clock ticked past midnight and into Thursday featured readings in free-fall.

The upper 70s which were in place as many Chicagoans turned in Wednesday night had turned into 40s by the time they rose Thursday morning.

A March-level chill, 16-degrees below normal, extends into a second day Friday

Friday marks the second day of unseasonably cool weather. The day's predicted 50-degree high is 16-degrees below normal and equals the normal high on March 24th.

Showery spells could turn thundery Friday afternoon

Showers, expected to remain scattered Friday morning, are likely to increase in number Friday afternoon and evening as daytime “heating” proceeds.

Though a shallow layer of chilly air hugs the surface, the air mass above is warm and unstable. As this warmer air bubbles upward through the atmosphere, several thunderstorms may take shape, much as they did Thursday.

Blocking pattern to direct “cut-off” upper air low’s heaviest rains across Missouri and downstate Illinois and Indiana; Chicago area still subject to rain

Rain is to arrive in Chicago in clusters or waves the next two days. But the core of the cold upper air system responsible for the uptick in precipitation chances here, is being shunted farther south than first forecast. This appears likely to target sections of Missouri, downstate Illinois and Missouri with the system's most prolific rainfall.

Chicago is still subject to some rain. But it's looking increasingly likely the rain-making system is to be shunted to our south by a large dome of warm air aloft developing over southeast Canada.

System’s southerly track to graudually decrease weekend precip coverage here while threatening the Kentucky Derby with rain

The Kentucky Derby in Louisville may be wet Saturday! More than three-quarters of an inch of rain may fall by race-time threatening a wet track and limiting temperatures to the 50s.

May records fall as snow accumulates across sections of 5 states; portions of Wisconsin, Minnesota and Iowa hardest hit

Big snows which hit sections of the Upper and Western Midwest Thursday are raising eyebrows. They have occurred in quantities never before observed there in May. Some of the heavier totals Thursday included 18” at Hayward and 17” at Rice Lake---both in Wisconsin.

Warming ahead next week; return to 70s a good bet—especially Tuesday through Thursday

Temperatures are to stage a nice comeback next week with readings soaring into the 70s Tuesday, Wednesday and Thursday.