As a low pressure system approaches northeast Illinois, clouds thicken and scattered light showers are expected to break out Monday. Best chance of showers will be in northern sections of the metro area. The most significant impact from the low is expected to occur across Minnesota and northern Wisconsin, where up to 5 inches of new snow could accumulate Monday. Colder air will follow the front with strong northwest winds flowing over warmer southern Lake Michigan waters, triggering snow showers over northern Indiana and the southwest lower Michigan snow belt Tuesday. Temperatures moderate briefly Thursday before another cool-down next weekend.
Flooding along the Gulf Coast
A stationary front oriented west-east just south of the Gulf of Mexico coastline has dumped heavy rains over the southern portions of Louisiana, Mississippi and Alabama, causing widespread flooding. New Orleans reported rainfall of 5 to 7 inches Sunday. Another wave of heavy rains is expected to dump an additional 2 to 4 inches over that same area Monday and Tuesday. Dense fog has also seriously impacted driving conditions all along the Gulf coastline from Texas to the Florida panhandle.
Several sections of the metro area and far suburbs may have some slick spots to start this Sunday morning. A mix of rain and spotty sleet and snow spread into northeast Illinois and northwest Indiana Saturday evening, and was expected to diminish and end sometime after midnight. The precipitation started out as rain, but because the air was extremely dry, especially in northwest Indiana, a process called evaporative cooling took place as the raindrops fell through the very dry air, dropping air temperatures below freezing and resulting in a wintry mix of precipitation. It could be slick in some spots early this morning -- even without precipitation -- wherever surface temperatures remain at or below 32 degrees.
Next surge of cold air hits Tuesday
Southerly winds, high temperatures in the upper 30s and rain on Monday will work on melting the remaining snow cover deposited by last week's storm. But a southward shift in the polar jet stream will swing winds aloft around to the northwest, allowing a surge of cold air out of the Canadian Tundra to flow into the Midwest and and western Great Lakes Tuesday. After Monday, Chicago's high temperatures will probably not climb out of the 20s again until Thursday.
The last day of November will see Chicago's 27th record-setting consecutive day with above-normal temperatures. Warmer readings are likely Tuesday as southwesterly winds and abundant sunshine allow afternoon high temperatures to approach and even exceed 50 degrees at many observation points around the metro area. A cold front should pass through Chicago early Wednesday, followed by a sudden turn to colder weather with falling temperatures.
Winter hits Chicago full-force Thursday
Winter storm warnings are in effect today over the southeast corner of Arizona, the southern half of New Mexico and extreme western Texas. Snow accumulations of 5 to 10 inches or more are forecast for higher elevations with several inches of snow possible even over lower desert areas. Current forecasts have the storm tracking through southern Texas and then northeast through Louisiana, eventually connecting with the cold front in Ohio and western Pennsylvania Thursday. The intensifying storm will pull cold air from the Canadian Tundra south into the Midwest. Thursday into Friday Chicago will experience strong north winds, snow showers and wind chills in the teens.
Tuesday marks the first day of meteorological winter (Dec.1-Feb. 28), and the following day winterlike weather is forecast to hit the metro area. After the sunny and mild Saturday when Chicago hit a high of 54 degrees and St. Louis came in with a 71 (just a degree below Miami, FL), the first three days of this week here are expected to average out about 5 degrees above normal. But the long-running mild trend that began in early November (the streak of above-normal temperatures reached 25 Saturday--tying the all-time record for November) will come to an abrupt end next Thursday. Low pressure moving northeast out of the southern Plains will merge with a cold front charging through the Midwest into the eastern Great Lakes, leading to a wintry mix over Chicago Wednesday with light rain changing to snow or flurries as temperatures fall.
A cold last half of the week
The first arctic air mass of the season will ride a strong northerly flow out of the Canadian Tundra directly into the western Great Lakes Thursday and Friday. In Chicago temperatures will fall into the 20s Wednesday night and fail to warm out of the 20s for the next two days, dropping into the teens overnight. With temperatures in the 20s and strong northwesterly winds gusting as high as 30 mph, Chicago's daytime wind chills will probably be in the teens both Thursday and Friday.
Much of the work week looks to be on the cool side with periods of cloudiness as minor weather disturbances move through. The upper-air flow pattern becomes more northwesterly, steering cooler air into the Midwest and holding milder air well to the south of the Ohio River. The jet stream will shift north by Friday with a weaker westerly flow aloft over Illinois, allowing milder air to ride southerly surface winds into northeast Illinois. While showers will be in the forecast off and on this week, expected rainfall should be on the light side. Thus runoff into rivers and streams will be primarily from the saturated soils, allowing for at least a little drying as the week goes on. Temperatures in the 60s and gusty winds over the coming weekend will also help to dry the soils, but again be tempered somewhat by the possibility of scattered showers.
Flood advisories
Flood advisories remain up for many rivers in states bordering on the Mississippi River from Louisiana all the way north to Illinois and Iowa. Most rivers in northeast Illinois and northwest Indiana are running close to bankfull, but the flood threat is farther to the south, the nearest being at LaSalle on the Illinois River.
It's been a long time coming, but rain moved into northeast Illinois, southern Wisconsin and northwest Indiana Sunday afternoon, ending a dry period that started in late August. Rainfall totals by Sunday evening ranged from a quarter to a half inch over the metro area. Earlier on Sunday, heavy rains downstate caused flooding of creeks, streams and susceptible roadways. There were numerous reports of one and two inch rainfall, and several locations in central Illinois received well over three inches. Jacksonville, just west of Springfield, recorded 4.76 inches. More flooding and 3-inch-plus rains occurred along the Ohio River in northern Kentucky and southern Indiana. All this rain was actually the northern edge of the record rainfalls that have been occurring across the Gulf Coast states as well as Missouri, Arkansas and Tennessee.
Cloudiness and showers to dominate forecast into next weekend
A strong low pressure system to the lee of Rockies over the Central Plains is forecast to remain essentially stationary much of the week ahead, finally ejecting to the northeast next weekend. This will mean an almost continuous flow of warm, moist air north up the Mississippi Valley, occasionally reaching northern Illinois and resulting in extensive cloudiness and periods of showers/thunderstorms, rather high humidity, and mild temperatures for the Chicago area.
It's been two weeks since September's only rainfall was recorded at O'Hare International Airport, Chicago's official observing site. It won't take much rain to exceed our entire monthly total of 0.03 inch. The high pressure regime that has produced an almost steady east wind along with the dry conditions and mild temperatures is finally breaking down. A low pressure system is set to establish itself over the Rockies and influence weather over the Midwest this week. A southerly flow up the Mississippi River Valley should feed warm, moisture-laden air into Illinois, resulting in considerable cloudiness. Even though showers or thunderstorms will probably not occur every day, it will be difficult for meteorologists to leave rain out of the daily forecasts.
Eastern U.S: A land of contrasts
The upper-air blocking pattern has persistently held the center of cold high pressure over the Northeast. Sunday morning frost and freeze advisories were posted for most New England states as well as northern lower Michigan. Meanwhile, persistent low pressure to the south has resulted in record heavy rains and flooding over the Gulf Coast the past week. Atlanta was among the latest, reporting a record daily rainfall of 3.7 inches Saturday.
The upper-air pattern remains relatively unchanged with the stronger northern jet stream winds well to the north in Canada and the southern jet running from the Gulf of Mexico up the eastern seaboard. The light winds aloft leave little opportunity for any significant air mass changes here in the Midwest or western Great Lakes. As a result, the light easterly flow continues to allow fine particulate concentration to slowly increase, and hazy conditions will persist. In Wisconsin the Department of Natural Resources has continued an Air Quality Watch through Monday over southern and central portions of that state with concern that particle pollution and ozone concentrations might reach unhealthy levels for people in sensitive groups.
Rain/cooler conditions possible next weekend
Computer models indicate a cold front accompanied by showers and thunderstorms could bring cooler Canadian air into northern Illinois next weekend. Chicagoans may be in store for highs in the 60s, more like late September and early October.
