Point Forecast:

O’Hare

Today:  85/63, Mostly Sunny, Breezy.

Tomorrow:  81/63, Partly Sunny, Breezy, Thunderstorms Possible during later afternoon and overnight.

Thursday:  79/58, Partly Cloudy, Thunderstorms in the morning, more Thunderstorms during afternoon and evening/overnight.

Friday:  71/50, Rain/Thunderstorms in the morning, clearing skies by late afternoon.

Saturday:  70/48, Mostly Sunny.

 

Rockford

Today:  85/63, Mostly Sunny, Breezy.

Tomorrow:  80/61, Partly Sunny, Breezy, Thunderstorms Possible during later afternoon and overnight.

Thursday:  77/55, Partly Cloudy, Thunderstorms in the morning, more Thunderstorms during afternoon and evening/overnight.

Friday:  73/48, Rain/Thunderstorms in the morning, clearing skies by late afternoon.

Saturday:  73/46, Mostly Sunny.

 

Quad Cities

Today:  85/63, Mostly Sunny, Breezy, Chance of Rain/Thunderstorms later afternoon/overnight.

Tomorrow:  80/63, Mostly Cloudy, Breezy, Thunderstorms Possible during later afternoon and overnight.

Thursday:  79/54, Mostly Cloudy, Thunderstorms in the morning, more Thunderstorms during afternoon and evening/overnight.

Friday:  73/48, Partly Cloudy.

Saturday:  73/48, Mostly Sunny.

 

Forecast Discussion:

I hope everyone is enjoying this weather!

Yesterday we saw temperatures over achieve versus some forecasted highs, but high resolution guidance like the HRRR and RAP provided excellent coverage for northern IL, and were right on par with the actual high temperatures yesterday.  Today, I will again side with those two models.  For a good portion of northern IL we should see temps climb into that 85-87 degree range.

For the northwestern and western portions of northern IL, we could see cloud cover and decaying convection limit the high temperatures some.  It appears that there are some better dewpoints forecasted to advect into that area and would support some rain.  Coverage per current radar our of KDMX and KDVN appears to have less coverage than convection-allowing high-resolution guidance does.  Even with that known, I would not discount some rain being possible early this afternoon for the western and northwestern portions of northern IL.  For our area, dewpoints will struggle today, and despite the insistence of guidance to bring some of that decaying convection into our area, our dewpoints and dewpoint depression could be a hindrance in rain reaching the surface.  It may in fact just be a virga fest this afternoon in our area (rain evaporating prior to reaching the surface).  For any areas east of I-39, I will mention only a slight chance of rain.  Cloud cover would be more possible.

During the overnight hours tonight, we should see some better dewpoints advect into eastern portions of northern IL.  By tomorrow afternoon most of us should notice a more “muggy” feeling at the surface as guidance suggests mid-60’s dewpoints creeping into all of northern IL.  During Wednesday there could be an early round of thunderstorms which would still favor more northern and northwestern portions of IL as the best forcing, dewpoints, and instability favors those areas.  But, as we progress through the day, there’s a chance that we could see better forcing move further east.  Guidance is relatively consistent on the better forcing arriving after 7pm for the majority of northern IL, which would favor thunderstorms being more nocturnal in nature.  There’s also some chance that the warm front could sink south a little bit during the later portion of the day tomorrow, which would also provide a lifting mechanism for thunderstorms.  This is more likely to affect northwestern IL.  We will have to monitor conditions as they get closer, but as of right now, some guidance (NAM and 3KM NAM) would favor areas near I-80 and a little bit north (near I-88) as the favored corridor for severe weather.  During the overnight hours guidance has depicted a scenario of ample instability, decent speed and directional sheer, and moisture pooling.  Couple this with the fact that the warm front appears to be draped across northern IL, it could provide an area where thunderstorms could become rooted in the boundary layer and result in the possibility of severe thunderstorms.  Lapse rates at the surface are rather benign, but mid-level lapse rates are quite pronounced.  I caution anyone jumping to the conclusion that this is an absolute, there is still many details to be worked out.  But, there is potential.

This is the NAM NEST sounding for overnight Wednesday. Circled items: instability still available at 07z Thursday, lapse rates, speed shear (wind speed increasing with height), directional shear (winds veering with height), and SRH values at 1 and 3km. All indicate the possibility of severe potential ranging from hail, to tornadoes. But, this is one run, from one model. We will definitely be keeping tabs on this.

Thursday looks like a crap shoot.  With the warm front position, the surface low pressure should pass across portions of northern IL (north of I-88) late Thursday, but there is some spread in guidance in terms of overall temperatures, precipitation chances, and timing of precipitation.  The NAM would lead you to believe that a scenario of off and on thunderstorms is possible, whereas the GFS would indicate another warm day following morning convection with afternoon/evening thunderstorms.  The variance in guidance really does not help pinpoint one exact scenario and offers a scenario for convection depending on guidance.  Despite this, I will hedge my bets on a solution closer to that of the GFS due to the NAM possibly having ongoing convective feedback issues, especially at the latter portions of its run.  The greatest consistency in guidance is afternoon thunderstorms.  Mode, and severity will be determined as we get closer.

This is the GFS for Thursday afternoon. Note the surface temperature. Also, speed shear is more prominent on Thursday.

This is the NAM for Thursday. It is noticeably cooler at the surface (probably convective feedback related), but it depicts a similar scenario for CAPE, and shear.

Friday through midday should feature some leftover rain, and possible thunderstorms.  Severe threat would be almost non-existent.  We should get a cold front passage sometime during Friday, and I would favor the beginning half of the day.  By evening, the GFS shows northern IL clearing out nicely.  Even with the FROPA, temperatures should still moderate nicely into the low-mid 70’s.

Saturday looks to be a great day to be outside as well.  All signs at this time point to mostly sunny skies, with temperatures hovering in the mid-upper 70’s.  Although, areas near the lake may see temps a little cooler as guidance favors a NE wind off of the lake.