Cool Friday – Weekend Warm-up ahead

Cool Canadian high pressure will settle over the Chicago area Friday. A northeast breeze will keep readings along the lake front in the 40’s while farther inland afternoon temperatures will reach into the lower or middle 50’s. Easterly flow will persist into the morning hours Saturday and then as winds become more southerly in the afternoon temperatures will peak in the middle 60’s- some 10 to 12 degrees warmer than Thursday.

Southwesterly flow will strengthen Saturday night and continue Easter Sunday. Despite increasing cloudiness, Sunday’s afternoon highs will reach the middle 70’s –  one of the warmest readings so far this year. With the approach of a cold front from the west, showers and thunderstorms could spread into the Chicago area late in the day and continue Sunday night. Periodic showers and thunderstorms and mild temperatures will probably continue Monday before the cold front finally passes through our area Monday night.

Potential for Severe thunderstorms over Chicago area this afternoon/tonight

Strong southwesterly low-level flow will pull warm moist unstable air into Illinois and southern Wisconsin today. There is a threat of severe thunderstorm development later this afternoon over the Chicago area with the best chance this evening into the overnight hours. Large hail and strong damaging winds will accompany heavy downpours in the stronger storms.

Low pressure will move northeast through Iowa today into central Wisconsin tonight. The associated cold front will move out of eastern Iowa into northwest Illinois in that same time frame, triggering showers and thunderstorm development in the warm moist air mass over northern Illinois and southern Wisconsin.

The National Storm Prediction Center outlook for severe thunderstorms Saturday and Saturday night is depicted in the yellow-shaded area on the map below.

Warmest day of the year brings severe storm potential

Southwesterly winds gradually strengthen during the day Saturday, pulling warm moist air into the Chicago area. Winds will be gusting over 25 miles per hour in the afternoon, as temperatures peak in the middle 70s – easily the warmest day so far this year, the second consecutive 70-degree day (Friday’s official high was 70-degrees), and the warmest temperature since 76-degrees was recorded back last October 11. Dew-points will be on the increase, and the added low-level moisture combined with developing wind shear, a cold front approaching from the west and an increasingly more unstable air mass – all point toward a good chance of strong severe thunderstorm development over Illinois into southern Wisconsin later in the afternoon and Saturday night.

Waves of showers and occasional thunderstorm downpours should occur from Saturday night, through Sunday into early Monday. During this period rainfall could total well over two-inches at many northeast Illinois locations – perhaps even exceeding three-inches in spots. Much colder air will follow on the wings of strong north to northeast winds later Sunday into Monday – temperature drops of 25 to 30-degrees will be common.

Chicago area in line for potential severe weather later Saturday – possibly Sunday

Strong southwesterly low-level flow will pull warm moist air from the Gulf of Mexico north up the Mississippi River Valley into Illinois Saturday. As temperatures rise into the 70s here Saturday afternoon, dew points will jump into the 50s. With a cold front approaching from the west and an upper–air disturbance aloft, atmospheric conditions will be ripe for strong thunderstorm development across northern Illinois and southern Wisconsin Saturday night.

Sunday we could be on the northern edge of potentially severe storms later in the afternoon and at night. If the warmth can persist into the evening hours Sunday, the moisture-laden air would be primed for a second round of severe storms that might include central and northern Illinois. The preliminary map below show the severe storm potential just reaching into southwest Illinois Sunday.

The maps below issued by the National Storm Prediction Center outline the preliminary outlook area included under a slight chance of severe storms in yellow for Saturday (top map) and Sunday (bottom map).

Slight chnce of severe weather Saturday…

Slight chance of severe storms Sunday…

 

 

 

Light rain/drizzle and fog to be followed by shifting/strengthening winds this morning  

Update 8:15AM CDT

The occluded front is through the Chicago area (note north-south oriented arcing cloud/precipitation band shown on weather radar picture above). The low pressure center is moving out of southeast Wisconsin headed over Lake Michigan – pulling away from our area. Visibility is on the rise and winds are picking up out of the southwest.

The winds will continue to increase and become more westerly and then west-northwesterly with time today – gusting over 40 miles per hour at their peak this afternoon. Winds will gradually diminish tonight.

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Update 7:15AM CDT...…

At 7AM CDT the center of low pressure is just along the Illinois-Wisconsin line north of Rockford continuing an eastward movement. The associated occluded front will be swinging through the Chicago area from the west during the next hour. As the front passes a given point, visibility will slowly improve and winds will shift to the southwest. As the morning progresses, winds will become more westerly and pick up in speed. This afternoon west to northwest winds will be gusting at times over 40 miles per hour.

Map of 7AM CDT Chicago area visibility

vsby 4

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The slow-moving center of low pressure and associated occluded front will slowly drifts east through northern Illinois this morning. Light rain and drizzle along with foggy cool conditions will prevail across the Chicago area. Visibility will be lower than a quarter mile in spots as easterly winds will eventually shift to a strengthening west wind this morning. Once winds shift, visibility will pick up along with the wind speed and rain/drizzle will diminish. The day’s high temperatures will probably not  be much more than the low 40s experienced this morning.

Below is a map of visibility as of 4AM.

vsby 3

As severe weather threat wanes – formation of fog increases

Most of the storm system energy seems to be increasingly concentrated on southern Illinois and western Kentucky as the evening progresses. Thus the threat of severe storms approaching the Chicago area has diminished significantly. Rain with a chance of thunder is still expected to develop across our area later tonight as the low pressure center and warm front approach from the southwest.

However the lack of strong convective activity this far north has allowed cooling to occur with temperatures dropping into the middle and upper 30s, while a slow low-level moisture increase continues. As a result the approach of the warm front is triggering fog formation across our area. Note on the map below, visibility at O’Hare Airport was down to ½ mile at 9PM CDT.

vsby 2

Severe thunderstorms still a possibility tonight across the Chicago area – strong winds tomorrow

A center of intensifying low pressure will move northeast out of Missouri through northern Illinois tonight. The associated warm front will be carried north with both the low center and warm front approaching and moving through the Chicago area after midnight – during the pre-dawn hours Friday.

With warm moisture-laden low level southerly flow streaming up the Mississippi River Valley into Illinois and a 100 mile-per-hour jet stream aloft, conditions are ripe for severe thunderstorm development to our south with the possibility that clusters of strong potentially severe storms with large hail, and damaging winds could erupt across the Chicago area as well later tonight.

Heavy 1 to 2-inch rains occurred across the southern portion of the Chicago area this morning and another round of more widespread thunderstorm downpours is likely overnight. Local flooding in already primed areas may be an ongoing problem into Friday morning.

Friday with the center of low pressure to our east, winds will shift to the southwest, and high pressure building in behind the departing low pressure will tighten the pressure gradient, resulting in very strong winds gusting between 40 and 50 miles per hour. Winds will become more westerly later in the day. High profile vehicles/trucks traveling on north-south highways will be especially susceptible, and affected drivers should be prepared to fight these turbulent winds tomorrow.

Heaviest 1 to 2-inch rains mainly south of Interstate-80 this morning

It was still raining at many observation points when the 8AM CDT reports came in this morning, but it was evident early that significant rainfall between 1 and 2-inches fell over much of the area south of Interstate-80 early this morning. Rainfall amounts tailed off significantly to the north with generally less than a tenth of an inch a short distance north of north of Interstate-90. Northernmost counties received little or no rainfall.

Following are some of the greater rainfall amounts reported at 8AM CDT…

Location             Rainfall (inches)

Marseilles                    1.50

Ottawa                         1.49

Aurora                         1.20

Pontiac                        1.73

Kankakee                    1.49

Morris                          1.09

Homewood                 1.07

Gibson                         1.83

Manteno                      1.00

Bonfield                       1.15

Seneca                        1.15

Streator                       1.14

Momence                    1.83

Oak lawn                     1.15

Chenoa                       2.14

Chatsworth                  1.05

Plainfield                     1.50

Joliet                            1.49

LaSalle                        1.36

Peru                            1.25

North Utica                  1.43

Oglesby                       1.13

Minooka                      1.07

Buckley                       1.98

Sheridan                      1.52

Mendota                      1.07

Homer Glen                1.04

Bradley                        1.97

Lisbon                          1.56

Sublette                       1.01

Naperville                    1.05

Coal City                     1.39

Manhattan                   1.18

Northwest Indiana

Chesterton                  1.80

Lake Village                1.80

Demotte                      1.50

Rensselaer                  1.00

Hobart                         1.13

Fair Oaks                    1.81

Wheatfield                   1.11

Hanna                         1.39

Mt. Ayr                         1.13

Munster                       1.40

Lakes of 4 Seasons    1.38

Valparaiso                   1.43

Hebron                        1.83

Risk of severe thunderstorms in Chicago area today and tonight

The National Storm Prediction Center has issued a severe storm outlook (see map below) that includes the southern half of the Chicago area in a slight risk (yellow-shaded area) of severe thunderstorms primarily later this afternoon and tonight. Southern Illinois is included in a moderate risk (red-shaded) area with the possibility of tornadoes that extends south including much of Arkansas and Missouri as well as portions of Kentucky, Tennessee, Mississippi and Louisiana.

Chicago will be located on the northern edge of the severe weather threat, with our best chance of severe thunderstorms with large hail and damaging winds occurring just south of Interstate-80. However as the storm system moves through northern Illinois tonight, the northern portion of the Chicago area could be at risk – mainly after midnight into the pre-dawn hours Friday.

An intensifying center of low pressure will track northeast out of Kansas through Missouri today, crossing northern Illinois into southeast Wisconsin into Lower Michigan later tonight. Strong low-level southerly flow will tap the Gulf of Mexico, pulling moisture-laden air up the Mississippi River Valley into Illinois as an upper-level impulse moves over the area. The associated warm front will lift north into central Illinois today – perhaps briefly reaching the Chicago area late tonight/early Friday morning. The center of low pressure system should be east of Chicago Friday morning with colder westerly flow over our area.

Map of Severe storm threat today and tonight