WEATHER UPDATE

As low pressure over southeastern Iowa moves east across northern Illinois tonight, showers and thunderstorm downpours will continue to form and move east-northeast across the Chicago area. The National Storm Prediction Center has outlined the possible severe thunderstorm risk area to the south of Chicago (denoted on map below) with the highest potential for severe storms primarily in southern and central Illinois and central Indiana.

Scattered thunderstorm downpours could give upwards of an inch or even more rainfall in at many locations across northeastern Illinois and northwestern Indiana.

Strong thunderstorms are likely to develop to our west later this afternoon/evening and move east into the Chicago area. Large hail and damaging winds will occur with these storms.

Prior to that time, a warm front located far south along the Ohio River this morning is expected to surge north today – triggering bands of showers and thunderstorm downpours across Illinois. The warm front will eventually pass through northeast Illinois this evening. The combination of the north-moving warm front colliding with the storms tracking east out of Iowa and Missouri will enhance the severe storm potential later here today and tonight.

The map below depicting the potential severe storm area was issued by the Storm Prediction Center.

Scattered showers and a few small thunderstorms have bee tracking east along and just south of Interstate-80 this evening. Small hail has been reported at several observing points between 6 and 8pm.

Time    Location                 hail (diameter inch)

605     Homewood (Cook)          .50

730     Cissna Park (Iroquois)    .50

612     Valparaiso (IN)               .25

600     Mokena (Will)                 .25

530     Morris (Grundy)             .25

As a warm front surges north through Illinois Wednesday – likely passing through the Chicago area that evening, showers and thunderstorm downpours are expected to develop. Then as low pressure passes to our northwest and the associated cold front approaches and moves through chicago from the west, showers and thunderstorms are expected to continue a good part of Thursday.

At this time it appears the strongest storms with potential for damaging winds and large hail could occur later Wednesday through Wednesday night into Thursday morning (see two severe weather outlook maps below). The heaviest rainfall looks more and more like the Chicago area will be located in the center of heaviest rainfall Wednesday and Thursday (see forecast accumulated totals on map below where northern Illinois and Chicago lies in the 4-inch plus area.

                                                  Severe weather outlook Wednesday-Wednesday night

                                                       Severe weather outlook Thursday

                                                   Predicted rainfall (inches) Tuesday through Thursday night

A stationary front is oriented west-east across central Illinois (blue and red line on map 1 below) – well south of Interstate-80.  Triggered by an upper air impulse this morning, a band of showers and a few embedded thunderstorm downpours will spread east across the state and the Chicago area. Once this impulse moves east of our area later this morning, there will probably be a lull in the convective activity – then storms are expected to redevelop in Iowa and Missouri, ahead of a stronger upper-level impulse - moving east across Illinois later this afternoon and overnight.

Heavy rainfall from one to three inches could cause many already swollen rivers and streams to rise to flood levels. The area just south of Chicago – as depicted in yellow shown on map 2 below could be in line for severe thunderstorms – primarily associated with the second storm surge tonight.

                        Map 1

Issued by the Chicago National Weather Service Office

Northeast Illinois/northwest Indiana

10pm Sunday evening

Location                    snowfall (inches)

 WINNEBAGO                   2.5

MARSEILLES                   0.2

ROCKFORD                     2.0

MIDWAY AIRPORT          0.4

MARENGO                       0.3

BYRON                             2.0

OTTAWA                         1.3

WATSEKA                       3.0

DE KALB                         0.4

STREATOR                     1.0

LOWELL, IN                    0.5

GIBSON CITY                 4.1

ROSCOE                        2.0

CHATSWORTH              3.5

CROWN POINT, IN        0.2

JOLIET                         0.4

WHEATFIELD, IN         2.0

KANKAKEE                  1.3

CAPRON                     1.0

MANTENO                  1.5