Explainer

Only 21% of years since official weather records began here in 1871 have produced 85-degree temperatures by April’s close---and 2013 is among them!

Tuesday's 85-degree high at O'Hare and the 86 max at Midway were not only 20-degrees above normal and more typical of July than late April, they rank as the city’s warmest readings since the 86-degree peak on September 12 last year.

Much of the Chicago area has a good shot at recording a second consecutive 80+ degree temperature Wednesday. The exception will be lakeshore areas.  There, southeast winds slicing ashore off chilly Lake Michigan and its 45-degree waters (that's the average water temperature despite a 53-degree reading on Chicago’s lakeshore Tuesday), should limit air temps to the 65-72 degree range while setting the stage for even cooler readings by nightfall.

The cooler temperatures interacting with moisture in the air may lead to fog patches along the lakeshore as the day proceeds.

Monday’s mid 80s here even warmer than a number of the country’s usual warm locations

You know Chicago’s weather is in rare form when late April warmth produces temperatures higher than the nation’s perennial warm spots. For instance, the city’s official 85-degree high at O’Hare Tuesday was warmer than readings at Miami (82), Dallas (82), Daytona Beach (83), Key West (84), New Orleans (77), Atlanta (78), Memphis (83) and Nashville (80).

April closes in Chicago with high temps 44-degrees warmer than at the the 41 on Day #1 

Tuesday marked April 2013’s final day and the 85-degree high was 44-degrees lower than the 41-degree high with which the month opened on April 1.

The warmth was in stark contrast to much of the remainder of the month which saw 18 of its 30 days finish at or below normal.

The month closes as Chicago’s wettest April on record. It produced only 46% of its possible sun compared to the 52% considered normal and the 56% which occurred during the month a year ago.

Chicago area on the verge of a late week temp downturn and onset of a wetter pattern; rainfall approaching 1” possible over the coming 7 days

The entire Chicago area is to see sharp cooling in coming days, particularly in areas closest to Lake Michigan. But the arrival of precipitation, likely to arrive in waves, could produce unwelcome rainfalls approaching an inch over the coming 5 days as a complex weather scenario unfolds. At its heart is to be a slow-moving upper air storm expected only to crawl across the area because of a strong atmospheric blocking pattern predicted to organize to its east.

While the new pattern threatens renewed rainfall here, winter storm watches have been posted for possible accumulating snow in northwest Wisconsin. This development comes only a day beyond record-breaking warmth Tuesday in Milwaukee, Oshkosh and Green Bay where highs hit 85-degrees, 83 and 84 respectively.

Accumulating snow may extend southward into sections of Iowa, Nebraska and Kansas as well.

May is Chicago’s 3rd fastest warming month

Temperatures are to rebound into the 60s here over the weekend and into the 70s by Tuesday next week. May is Chicago’s third-fastest warming month with normal temperatures surging from 65 and 44 the first of the month to 75 and 53 on the 31st.

It's been 13 years since Chicagoans have waited so long for the year’s first 80-degree temperature. Eight 80s were on the books by this time a year ago.

Tuesday’s predicted July-level 84-degree high is to come in 19-degrees above normal. It means April, which opened with a chilly high of just 41-degrees, is to close with a daytime temperature 43-degrees warmer!

The July-level temperature on the way Tuesday is the area’s warmest weather in over 7 months, and a second 80+ degree reading appears headed this way Wednesday ahead of  cooler easterly winds and a wetter weather regime expected to take up residence here later this week.

Spring planting across the Midwest off to its slowest start in 20 years; just 1% of Illinois’ corn is in compared to 76% a year ago

The combination of wet weather and chilly temperatures has kept area farmers out of waterlogged fields in recent weeks. The USDA, in its weekly Crop Progress report Monday, indicates just 1% of Illinois, Indiana and Iowa corn has been planted. NO corn is in the ground across Wisconsin, Michigan or Missouri. It’s the slowest open to a planting season in Illinois in 20 years.

John Hazzard, who farms in the Wilmington area of Will County, tells us if rain holds off and the sun shines the next two days, area farmers may be able to get some critical planting done in drier fields.

By this time a year ago thanks to extraordinary spring weather, 76% of Illinois' corn had been planted. Indiana farmers had 67% of that state’s corn in the ground while Iowa farmers had planted 44% of their corn.

Monday’s 74-degree high only the second reading to top 70 this year compared to 15 a year ago

Monday’s 74-degree high at O’Hare and Midway blew past the year’s previous warmest temperature (70-degrees) and became only the second day in 2013 to produce a 70-degree reading. By comparison, 15 such days were on the books a year ago.

Warmth melting snow fast to the north in Duluth; 27” snowpack’s down to just 4”

The warmth has been broad-based in recent days. That's even been true in Duluth, Minnesota, where the year's snowfall to-date has established new April and February through April records. With the warm weather's arrival, snow there has been melting fast. The 27" snowpack there just a week ago is down to just 4 inches!

Phoenix, Arizona bakes at 102-degrees Monday; triple-digit heat running two weeks ahead of schedule 

The warmth which reaches into the nation’s mid-section emanates from a hot air mass which generated a 102-degree high temperature Monday in Phoenix, Arizona. The reading was that city’s second triple-digit temperature to date this year. That level of heat is running nearly two weeks early!

Wetter pattern looms later this week with a sluggish "cut-off" upper low predicted

All eyes in the meteorological community are on developments later this week. The predicted formation of a sprawling “cut-off” upper level low pressure---a system whose forward motion is to be stymied by a blocking dome of warm air aloft predicted to take shape over southeast Canada--- could mean significant new rains are on the way in the already waterlogged Midwest.

A survey of rainfalls since March over nearly every corner of the region indicates rainfall tallies are running far in excess of normal---in many cases as much as one and a half to two times the normal meteorological spring precipitation.

Models hint 3 to 4" of rain may target Iowa and that significant rainfall could extend eastward toward the Chicago area, still drying after record April rains which have pushed the 2013 precip tally over 17 inches here---the wettest year to-date total observed in over 143 years of official observations.

The chilliest spring here in 17 years, a season responsible much of the past week for March-level 40-degree highs, finally relented Friday allowing more seasonable 60+-degree temperatures to sweep in on 30+ mph wind gusts.

Friday afternoon temperatures hit 67 at O’Hare and Midway and 66 at the lakefront—readings 16-degrees warmer than the low 50s recorded the day before. It marked the first time since Monday anything close to a seasonable late-April-level reading had occurred here.

The mild air spills into the weekend, though lakeside residents may not share the thermal bonanza to the degree their inland neighbors will.  That’s because easterly winds blowing around the northern flank of a downstate weather disturbance have replaced Friday’s southwest winds which pushed the warmth into lakeshore areas, overcoming lake cooling.

Predicted wind velocities aren’t likely to be particularly strong Saturday so the inland penetration of lake-cooled air isn’t likely to cover much real estate. But, the flow off chilly lake waters, which currently average 45-degrees, is likely to limit immediate shoreline highs to the low and mid 50s while 70-degree highs occur to the west in the warmest sections of DuPage, Will and McHenry counties westward to the Fox Valley, on to DeKalb and Rockford, and north into Wisconsin near Lake Geneva.

Warm-up welcome in chilly spring 2013 which has produced half the typical 60+-degree temps

Friday’s warm-up was a welcome development in a season which has seen nearly three-quarters of its temperatures coming in “below normal” since March 1.

The 67 at O’Hare marked only the 8th time in 2013 a daytime reading has made it above the 60-degree mark.  That’s far short—less than half—of the eighteen 60s Chicago’s official observational record suggests have occurred on average by April 27.

Warmth not limited to Chicago; 70s reach Minneapolis while mild air melts a foot of snow at Duluth

Warming across the nation’s Heartland was hardly restricted to Chicago. Temperatures hit 72 at Minneapolis Friday—that city’s first 60 and 70-degree reading.

In Duluth where late winter and spring snows have broken nearly every snow record on the books, the warm-up is responsible for melting 12” of that city’s deep snowpack over the past three days leaving 15” on the ground.

Ominous signs wetter pattern to re-establish later in the coming week

Model forecasts have taken a disturbing turn in their predictions for later next week into the weekend and week which follow.  A peculiar mid-continent upper storm system is predicted to take shape the second half of next week—a system apt to only crawl across the region because of strong upstream atmospheric blocking over eastern Canada. By inhibiting this potentially wet new system’s escape route to the east, the blocking pattern means the upper storm expected to “spin-up” over the nation's mid-section could linger, generating waves of rainfall over a multi-day period.

Rainfall projections off Friday computer model runs suggest more than 1” of rain could fall in the coming 7-days alone—with several multi-week forecasts putting potential Chicago area rainfall in the 1 to 2.5” range.

Blocking patterns have often been involved in ratcheting up the intensity of storms and their impacts.  By slowing a system’s movement, as the predicted block will, the potential for big rainfall numbers rises. This is a development we’ll be monitoring in coming days.  Stay tuned!

Ever-more dramatic stats on our moisture situation in 2013 keep pouring in! Not only has this month's 8.58" of rain made it the wettest April on record in Chicago, 2013's tally of 17.24" to date is the wettest Jan. 1 through April 25 period recorded at any point over the 143 year observational record here.

The last time a year opened with anywhere close to the amount of precipitation we've seen this year was the 14.32" recorded up to this point in 1975. It's little wonder our rivers have so horrifically strained under the volume of water which has poured into them over the past three months in particular.

The wet weather’s sent Great Lakes water levels surging 

Many of you have inquired what impact 2013's wet weather has had on the Great Lakes. The answer is: A DRAMATIC ONE!

New measurements through Thursday by the U.S. Army Corps of Engineers indicate Lake Michigan has surged 9" higher since the first of the month.  A rise of that magnitude involves a phenomenal quantity of water! Each inch Lake Michigan’s level changes is the equivalent of 390-billion gallons of water!

The surge in the lake's level has more than halved the difference in water levels between this year versus last.  Only a month ago, Lake Michigan was running 17" lower when compared to a comparable period in 2012. The comparison has shrunk to just 7".

Lake Michigan’s not the only one which has rise. The other Great Lakes have posted water-level increases since April 1 as well. Lake Superior's up 4", Lake Huron's surged 9", Lake Erie's risen 8" and Lake Ontario's up 10".

Does the wet pattern continue? Where do we go from here? 

So what happens from this point forward? Does history offer us any guidance? An average of precipitation forecasts off a suite of computer forecast models covering the next 2 weeks suggests more modest rain totals than we've seen of late. But these forecasts aren't without several spells of rainfall which may have to be monitored.

Averaged across a series of forecast cycles and different models, estimates of Chicago's potential rainfall comes to 0.44"--about half normal---(53%) the next 7 days while 8 to 14 day totals  come out to 0.93" which is 115% the normal of 0.81" for that period of time.

The transition from the warmth predicted to take up residence here early next week, could be a wet one and may end up generating rains which boost the predicted one-week total beyond the 0.43 inch tally just cited. But overall, the rains expected to occur in the short and medium term---i.e. over the coming 15 days---appear more modest than the deluge which brought on the record floods of the past week.

Looking into May and beyond, initial drying is followed by renewed rainfall

Historically, Mays which have followed wet opens to the year have often eased inly to be followed by wetter than normal summer weather. That’s the finding of an in-house analysis of precipitation patterns which have followed Chicago’s wettest periods through April 25.

My colleague Richard Koeneman, who conducted the analysis, has found more than half (6 of the 11) years since 1871 generated reduced rainfall in May. But the pullback was temporary and followed by a return to wetter than average weather over the meteorological summer months of June through August. Nine of the 11 wettest opening years on the books here went on to produce above normal June through August overall rainfall.

It's been a spring desperately short on warmth. 40 of meteorological spring 2013’s opening 55 days since March 1 have posted temperature deficits. That’s 73% of them!

March, 2013 finished 5.3-degrees below normal and April’s not been much better, averaging 3.2-degrees short of the month’s historic norm.

Spring 2013 is the coolest in 17 years; season ranks among Chicago's 17% coolest of past 143 years

Meteorological spring continues Chicago's coldest of the past 17 years, ranking among the 17% of coolest springs since records began in 1871.

The season’s average temperatures to date of 37.4-degrees is 3.4-degrees below the long-term average---cold enough to rank 29th chilliest of the past 143 years.

Wednesday’s lackluster 50-degree high made it the coolest April 24 here in 9 years.

Temperatures surge well into the 60s Friday and to 70 Saturday---but weekend lake winds to restrain warming on area beaches

Warmer days are on the way with temperatures jumping nearly 16-degrees from Thursday afternoon’s predicted cool high of 53-degrees well into the 60s Friday afternoon. A strong southwest wind is expected to push the “warmth” up to the lakefront Friday.

Lake breezes a part of Chicago's spring; could play important role in this weekend's shoreline temperatures

Cooler lake breezes develop whenever organized winds across the area drop off allowing the colder, denser mass of air which hugs chilly Lake Michigan's surface water to charge inland as a “lake breeze”, significantly lowering shore line temperatures in the process.

On Saturday, as an example, while inland temperatures are predicted to surge to within striking distance of 70-degrees in the afternoon, lakeside residents can expect onshore easterly winds to develop, lowering temps into the mid-50 to low 60-degree range.

Stronger, better organized south/southwest winds winds are to drive temperatures into the 70s late this weekend and into early next week. There's still a chance more modest lake breezes Sunday could have some cooling effect on area beaches. Whether that occurs will depend upon the strength of the southerly winds predicted for Sunday.

Tuesday marked April 2013's 11th day of measurable rain. The day's 0.61" rain accumulation was enough to put this month's  8.45" tally (late Tuesday night, with rains still falling) into the record books as the wettest April to occur over Chicago's 143-year observational record.

The previous record for most April precipitation here --- 8.33"---was retired after a 66 year run dating back to 1947.

The new 8.45" monthly total is more than 9 times  (939%) the amount of rain which had fallen during April's opening 23 days a year ago (0.90") and 2.5 times the full month's 3.38” "normal" total.

April's trended cloudier and chillier than typical

Rain hasn't been April 2013's sole attribute. The month's been noticeably cloudier than many of its predecessors, having allowed only 41% of it possible sun. A "normal" April generates 52% of its possible sun.

It's also been a cool month averaging more than 3-degrees below normal.

From 60-degrees to snowflakes in a single day; Tuesday undergoes a  20-degree afternoon temp pullback  

Tuesday brought the weather of two seasons to Chicago. The day dawned mild and springlike---an anomaly in a month which has produced cooler than normal daytime temperatures15 of its first 23 days---65% of them. But, the day was to watch the "warmth" evaporate as a 20-degree temperature drop ensued in the wake of a cold frontal passage which shifted winds northerly and set the thermal pullback in motion.

Late-winter level temperatures had taken hold by nightfall and reports of wet snowflakes began arriving in our weather office.

Big changes to milder weather loom late week

A huge pattern change is to lift Chicago out of the throes of chilly air before this work week comes to an end.

Vigorous south/southwest winds Friday start the process and deliver temperatures which surge well into the 60s. But even warmer weather is to follow later in the coming weekend---including very possibly the first string of consecutive 70-degree temperatures which has been logged here in six months.

First series of consecutive 70-degree or warmer days to kick off Sunday

If atmospheric conditions unfold as expected, Sunday, Monday and Tuesday next week may each host 70-degree highs. Monday appears likely to be the warmest with mid 70s not out of the question.

Rain’s the last thing this area needs. The month’s rain stands at 7.84” at O’Hare ---2.3 times the area’s normal full-April tally and tied for second-wettest April of the past 143 years. Flood waters are finally lowering, albeit far too slowly. But, with the onset of new rains, even a few possible embedded thundery downpours Tuesday into Tuesday night, the downward trend in river levels may slow and possibly temporarily reverse in some areas.

Several key developments argue for the area’s major rivers not to resurge to the extent they did after last week’s deluge. For one thing, the rains predicted Tuesday---while possibly amounting to inch or more in some areas from Chicago south---aren't expected to fall with that kind of intensity over the headwater areas of key rivers like the Des Plaines or Fox. That means huge new quantities of water aren't likely to enter either river system.

Second, the amount of water expected to fall with the Tuesday/Tuesday night system won't be anywhere near as extreme as the drenching 5 inch-plus amounts which came down in last week’s deluge.

National Weather Service’s hydrologist Bill Morris, points out smaller creeks and streams can respond more quickly to rainfall because of the speed at which run-off gets into these waterways. This could mean more trouble for some smaller water ways than with the big rivers systems in the area.

Morris cautions the intensity with which Tuesday's rains end up occurring will have to be carefully monitored. Too much rain too quickly could lead to new flood troubles not currently expected.

Tuesday's wet weather may propel April 2013 rainfall to record heights, eclipsing old 8.33” benchmark set 66 years ago

Just 0.50” of rain---an amount entirely possible from Tuesday and Tuesday night's precip-system---will push April’s 7.84” tally to a new record high, surpassing anything that’s been observed here in April over the 143-year term of Chicago’s observational record.

Spring 2013 is the coldest to date of the past 17 years

Chicago's average temp since meteorological spring began March 1 has been 37.5-degrees, a reading 4.1-degrees below norma---but perhaps more significantly, a reading which qualifies as  Chicago's coolest spring period to-date since a 36.5-degree average in 1996.

Monday’s 67-degree O’Hare/Midway highs only the 7th this year to top 60-degrees 

Temperatures will take a big hit as Tuesday proceeds and a cold front passes, shifting winds to the north in the afternoon and evening.  When that happens, readings ranging from the mid 50s to near 60-degrees are expected to drop precipitously toward the low 40s by nightfall.

This follows 67-degree highs at both O’Hare and Midway Monday---only the year's seventh daytime readings to surge to or exceed 60-degrees.

Chill’s return to send temps tumbling to early March levels in coming days

The chill's that's on its way is to roll off an extensive snowpack well northwest of Chicago in the northern Plains and Upper Midwest, returning daytime temperatures here to early March levels in the low and mid 50s both Tuesday and Wednesday.

70s Sunday and Monday would be the first to occur back to back in 6 months!

Temperatures surge back to the 60s Friday and Saturday ---though a bit of an easterly component to the winds Saturday could deprive lakeshore areas of readings that warm. But more southerly winds Sunday and powerful south to southwest winds Monday may put together the first back to back 70-degree or higher temperatures which have occurred this year in Chicago. Two 70-degree highs in a row would be the first such pairing in 6 months here!

To have recorded only one 70-degree or warmer temperature by this late date is a true climatological rarity. Only 5 of the past 85 years at Midway Airport have reported so few 70s by April 23: 1984,  1944. 1950. 1951 and 1993.

New snows hit winter-weary upper Midwest; Duluth, MN sets a new April snow record and has been snow-covered 135 consecutive days

The snow story which has unfolded over sections of the northern Plains and the upper Midwest has been a record-breaker.

Rapid City, South Dakota’s 43.4” April snow total to date is nearly 7 times the normal to date (6.3”).

But it’s Duluth, MN where the season’s snow story is most remarkable. Snow’s been on the ground in that city each of the past 135 days since December 9. Snow sits 22 inches deep there and the city’s official seasonal snow tally since February 1 alone has reached 88.9”---a new February through April record!