Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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747 FXUS63 KLOT 210528 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1228 AM CDT Fri Jun 21 2024 .KEY MESSAGES... - Isolated to scattered storms into early evening. Locally gusty winds and torrential downpours possible with these storms, but organized severe weather is not expected. - Period of dense fog possible near the Lake Michigan Shore tonight. - Hot and muggy conditions will prevail through Saturday, although cooler conditions are expected near Lake Michigan today again on Friday. - Highest chance of thunderstorms over the next 7 days late day Saturday and Saturday evening, with the next chance of thunderstorms probably on Tuesday. && .DISCUSSION... Issued at 258 PM CDT Thu Jun 20 2024 Through Friday Night: The threat of widely scattered thunderstorms will continue across parts of northern IL through late afternoon before waning into this evening. Most of this activity is expected to remain focused in northern IL west of a lake breeze boundary in close proximately to a west/southwest-to-east/northeast oriented surface frontal boundary currently sagging southward near the I-88 corridor. Severe weather is not anticipated with these storms, owing to the overall weak deep layer shear. However, the potential exists for some isolated wet microbursts, which could thus result in some localized instances of strong gusty winds with any of the stronger storms. Otherwise, the very moist airmass in place (featuring near 2" PWATs) will also support torrential downpours with these storms trough early evening. Cooler onshore flow off of Lake Michigan will continue into tonight as high pressure remains in place across the northern Great Lakes. This could allow low clouds and dense marine fog to bleed inland across some of the collar counties of southern Lake Michigan after sunset this evening. If this does occur, a short fused dense fog advisory could be needed, especially for areas within a few miles of the lakeshore. Confidence on the inland extent of the fog remains low at this time, though I did opt to add areas of fog to the forecast database near the lake for tonight. Low clouds and fog may persist along the lakeshore areas into early Friday morning before conditions diurnally improve through the morning. Otherwise, a frontal boundary stalling out south of I-80 tonight will begin shifting northward again on Friday as a surface wave of low pressure shifts east-northwestward into the Mid-Missouri Valley. Because the front may get hung up along the southern shores of Lake Michigan for several hours, the surface flow may remain onshore for a majority of the day across parts of far northeastern IL, thus resulting in another day of cooler weather along the IL shores. Farther inland, however, another hot and humid day is expected. Inland high temperatures are expected to top out in the low to mid 90s amidst dewpoints well into the 60s. This will result in heat indices up to near 100. The primary threat for thunderstorms on Friday is expected to largely remain north of the area into southern WI, though some chance for a few scattered afternoon storms will exist across far northern IL (north of I-88). KJB Saturday through Thursday: A developing surface low to our west/northwest on Saturday will yield a tighter surface pressure gradient across the region. This will afford us breezy southwesterly winds (gusts up to 30-35 mph) that will help facilitate warm air advection and push the warmth all the way up to the lakeshore for the first time in a few days. Saturday`s highs are pegged to top out in the lower to mid 90s (locally upper 90s) across the area, and with dew points generally expected to span from the mid 60s to upper 60s F, this will translate to another day of heat indices peaking in the 95-100 degree range at most locations. The cold front associated with the aforementioned surface low will press southeastward on Saturday as the low center tracks toward the northern Great Lakes, and thunderstorms will likely develop along it as it does so. These storms will enter our forecast area from the northwest as early as the mid-late afternoon, but more likely during the evening, and continue southeastward into the night, likely weakening and diminishing in coverage with time as diurnal instability is lost. Deep-layer shear will be better than it has been the past several days given the presence of enhanced flow through the tropospheric column on the southeastern periphery of the deepening low pressure system, and with pre-frontal MLCAPE potentially pushing 2000-2500 J/kg, the overall kinematic and thermodynamic parameter space will likely allow for some of the frontal convection to become severe. Whether any severe thunderstorms occur in our forecast area will largely hinge on the timing of the cold frontal passage. An earlier frontal passage while diurnal instability is still plentiful would be more favorable for severe weather prospects here, while a later frontal passage would see convection arrive as the boundary layer stabilizes and our supply of instability has been depleted to some degree, thus keeping the potential for severe weather minimal. The level 1/5 (Marginal) risk introduced into the northwest half or so of our CWA in SPC`s Day 3 Convective Outlook early this morning still seems appropriate for now. However, there has been a recent trend in the guidance toward a bit faster cold frontal approach. Considering that we will have the latest sunsets of the year through July 2nd (and associated diurnally higher instability until past sunset), this would likely put a level 2/5 severe threat in play for portions of the area and extend low severe probs farther southeast. A slower frontal passage could also see showers linger around into Sunday morning in our southeastern counties, but otherwise, no additional precipitation is expected on both Sunday and Monday as surface high pressure and a drier post-frontal air mass settle into the region. Averaged across our forecast area, both days should generally be cooler and less humid than the past few days, but high temperatures on both days still look like they`ll end up being slightly above normal for late June, coolest near the lake. Towards mid-week, a flattened ridge should build back into the southern Great Lakes and allow for 90+ degree air temperatures to make a return to the area on Tuesday. The presence of a baroclinic zone in the region could also present us an additional opportunity to see showers and storms at times Tuesday-Tuesday night. The cold front passage Tuesday night will probably shunt the baroclinic zone and any meaningful convective chances safely south of the area on Wednesday and Thursday. High temperatures on Wednesday will be cooler than Tuesday, but likely still above normal away from the lake (mid-upper 80s in many areas), with seasonable lower to mid 80s on Thursday. Lakeshore highs are forecast to be in the upper 70s to around 80F on Wednesday and lower to mid 70s on Thursday. Ogorek/Castro && .AVIATION /06Z TAFS THROUGH 12Z SATURDAY/... Issued at 1228 AM CDT Fri Jun 21 2024 Aviation Key Messages: - Marine stratus and fog will continue at DPA/ORD/MDW/GYY through mid-morning. Occasional beaks to VFR within otherwise prevailing LIFR to IFR cigs may occur from time to time. - Light and variable winds overnight will become southwesterly after sunrise and increase in magnitude this afternoon. A lake breeze may stall or oscillate in the vicinity of ORD/MDW/GYY this evening. - A few thunderstorms may develop in the vicinity of RFD this evening (20-30% chance), with higher coverage expected in Wisconsin. Discussion: An area of low-level stratus and fog originating from Lake Michigan and near a remnant frontal boundary remains relatively stagnant near and DPA/ORD/MDW/GYY. With little change in the surface pressure pattern expected over the next 12 hours, the stratus should more or less stay in place with LIFR to IFR conditions prevailing. A few holes in the stratus should ebb and flow through the morning thanks to a very modest increase in 925-850mb flow (which will facilitate occasional mixing of dry air into the top of the thin stratus layer). Otherwise, stratus should hold steady until both upward- and down-ward mechanical mixing of relatively drier air commences after sunrise. Meanwhile, the aforementioned modest increase in 925-850mb flow will continue to support scattered showers, particularly in the vicinity of RFD at press time and perhaps DPA/ORD later this morning. A strike of lightning cannot be ruled out overnight. After sunrise, any lingering stratus near Lake Michigan should erode giving way to a scattered high-based cumulus deck. Light and variable winds will also gradually become southwesterly after sunrise and lift the remnant frontal boundary northward into southern Wisconsin. With a weak pressure gradient initially in place through early afternoon, a lake breeze appears poised to develop and move inland. However, it may stall near (or oscillate between) ORD/MDW/GYY this afternoon as opposing southwesterly flow increases in magnitude. For this reason, confidence is lower than average in the wind direction forecast this evening. Finally, mixing of air from a pocket of dry mid-level air into the boundary layer as well as low-level capping should limit convective instability across northern Illinois and northwestern Indiana this afternoon. However, locally deeper low-level moisture and weaker capping in the vicinity of the frontal boundary in Wisconsin may allow for thunderstorms to develop as far south as the Wisconsin state line. For now, felt a PROB30 for thunder was justified at RFD this afternoon keeping with the expectation that it very well may not earn an upgrade to a TEMPO in later TAF packages. Borchardt && .CLIMATE... Issued at 337 AM CDT Wed Jun 19 2024 After breaking a couple of temperature records this week, the high and warmest low temperature records for both today and Friday are unlikely to be broken, though Saturday`s temperature records for both Chicago and Rockford could potentially be threatened. Here are the daily high and warmest low temperature records for Chicago and Rockford for each day through Saturday, June 22nd: Chicago ----------------------------- Day: 6/20 6/21 6/22 ----------------------------- Record High: 104 101 97 Record Warm Low: 78 74 76 Rockford ----------------------------- Day: 6/20 6/21 6/22 ----------------------------- Record High: 101 100 97 Record Warm Low: 73 71 73 Ogorek && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002- INZ010-INZ011. Air Quality Alert until midnight EDT tonight for INZ019. LM...Dense Fog Advisory until 11 AM CDT this morning for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago