Chicago O'Hare Weather Area Forecast Discussion Wednesday June 28, 2023

Area Forecast Discussion 
National Weather Service Chicago/Romeoville, IL 
637 AM CDT Wed Jun 28 2023

.SHORT TERM... Issued at 411 AM CDT Wed Jun 28 2023

Through Friday...

The primary forecast concerns revolve around:

* Continued low visibility due to stagnant near-surface wildfire   smoke today with signs of improvement this afternoon/evening in   experimental smoke model output

* Low (10-20%) chance for an isolated shower or storm from late   morning to early afternoon

* Beginning of the "Ring of Fire" pattern tonight with low inherent   forecast confidence and corresponding high bust potential. The main messages are that the threat for severe weather starts   tonight and may last through Friday. In addition, Thursday may be hot and humid with heat indices at or above 100 for part of our   area.

Technical Discussion:

Our area remains entrenched in low visibility due to stagnant near- surface wildfire smoke with widespread visibilities of 1 to 3 miles. Experimental HRRR, RAP, and CMC smoke forecasts suggest the smoke will only slowly ooze northward throughout the day as low-level flow adopts a southeast wind component. So, today looks to at least start similar to yesterday with low visibility and very poor air quality with only a gradual improvement this afternoon. Given just how unusual it is to see such low visibility due to smoke in our area (and hence our limited experience forecasting such), our confidence in how fast conditions improve today is pretty darn low.

Meanwhile, a convectively-generated MCV is swirling eastward into western Minnesota with bands of showers and thunderstorms extending southeastward into central Iowa along the northern edge of a northeastward-drifting EML plume. Evidently, there`s just enough low- level moisture at the base of the EML to support convection, and forecast soundings in central Iowa look similar to northern Illinois from late morning to early afternoon. So while the ongoing activity in central Iowa is expected to decay as it approaches the Mississippi River toward daybreak, we cannot rule out (so a 10-20% chance) at least isolated showers and thunderstorms survive into northern Illinois and even northwestern Indiana throughout the day. The main threats would be lightning strikes and gusty winds given a residual dry low-level airmass.

Once we get through the smoke and low (10-20%) chance for an isolated shower or storm today, our attention turns firmly to the developing "Ring of Fire" pattern that is already ongoing in the Southern Plains. Ensemble model guidance remains locked in that the ridge facilitating the pattern will shift northward today and Thursday before stalling through Friday. Now, the "Ring of Fire" pattern is a notoriously difficult forecast regime characterized by episodic convective events where initial hail-producing supercells grow upscale into damaging-wind producing MCSs (often derechoes) that ride along the edge of a heat dome and a convectively-modified frontal zone. As each convective episode pushes the frontal zone further south, determining when and where the convective episode will occur can be a futile effort until, say, 12 hours beforehand when the new position of the front can be discerned. And, observed convective events often verify further south than advertised by model guidance. (In other words, there`s a play where our area misses out on the heat and storms entirely).

With all of that said, it appears that the next MCS in the northward- shifting pattern is now developing over central Nebraska, which will would be on track east/southeasterly toward St. Louis this afternoon. Assuming that occurs, the cadence of upper-level waves in anticyclonic flow aloft may favor the next MCS developing near the South Dakota and Nebraska borders tonight and racing into northern or central Illinois on Thursday. As the upper-level ridge will no longer be lifting northward, the convective event on Thursday should conceptually push the frontal boundary south for Friday giving way to a less hot and humid and dry on Friday. (Without putting all our eggs in one model basket, it sure looks like the HRDPS is performing well so far). So, even with all the aforementioned uncertainty in the paragraphs above, we`re cautiously maintaining the idea that Thursday is still the day to watch for heat and storms in our area. Needless to say, stay up to date on trusted weather sources the next few days in case our forecast, in the wise words of Johnny Cash, burns burns burns in the ring of fire.

Borchardt/NWS Chicago


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