Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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305 FXUS63 KLOT 221126 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 626 AM CDT Sat Jun 22 2024 .KEY MESSAGES... - Potential for strong to severe storms with damaging winds, torrential downpours and localized flooding this afternoon near the Wisconsin state line and area-wide this evening. - Breaks in heat and humidity are expected Sunday and Monday, and again Wednesday through the end of the workweek. - Tuesday may feature dangerous heat and/or severe weather in the general region. && .DISCUSSION... Issued at 335 AM CDT Sat Jun 22 2024 Through Tonight: Ongoing small clusters of convection across northern Illinois early this morning have persisted for a few hours in response weak low-level WAA within a 5-7kft layer of higher RH. Given only a shallow layer of nocturnal stability, the strongest cores have managed to generate locally strong surface winds. Expect this activity to continue through around sunrise north of the I-90 corridor. In the absence of any appreciable convection into our area today, we are well in line to have another hot and breezy day. High temps in the mid 90s with SW gusts of 30 mph to possibly 35 mph are likely. We will need to closely monitor the trajectory of a well-formed MCV just north of Omaha early this morning as it tracks ENE toward far southern Wisconsin early this afternoon. The far northwest CWA will reside within a strong theta-e gradient east of a mostly uncapped and increasingly unstable airmass across eastern Iowa this morning. Broad mid-level steering flow from the WNW should keep the MCV north of the CWA, though a substantial swath of WSW to ENE convection over northern Iowa may focus a formidable boundary and turn the MCV more easterly along the strengthening theta-e gradient. Additionally, 70+ degree dew points across western Iowa will begin advecting into the northwest CWA this morning. So while the potential for convection across the northern CWA during the day is quite conditional, have opted to bring low-end (15-30%) thunder chances as far south as a Mendota to Chicago line in the 11am-3pm window. Less favorable lapse rates this far east will limit the severe potential, but sufficient effective shear will still support an isolated severe threat with large hail and gusty winds. To further add to this, stout SW low-level flow with moisture advection could result in continued generation of convection on the tail end of the MCV and lead to localized flooding mid to late afternoon across the far northwest CWA. Organized deep-layer forcing with the arrival of the mid-level wave and surface front should generate a more organized line of strong to potentially severe convection across the area this evening. Strong deep-layer shear along with increasing MLCAPE values and corresponding high DCAPE values will support some discrete supercell structures congealing into a gradual SSE moving line with damaging winds. Expectations are that the convection may become outflow- dominant by mid to late evening as low-level shear vectors become oblique to the line and consequently less favorable for balanced linear convection. With that said, outflow winds ahead of any convection would still be capable of producing very strong gusts. A corridor of impressive PWATs surpassing 2" and potentially as high as 2.3" will feed into developing convection across northern Illinois early this evening. While deep-layer and propagation vectors suggest that convection should be progressive this evening into the early overnight hours, intense rainfall rates over any period of training convection will be capable of producing local instances of flash flooding. A Flash Flood Watch may ultimately be needed for areas around and north of I-80 this afternoon and evening if convection north of the WI state line begins to drift southward (i.e. regeneration behind MCV) or the main line of convection is trending slower than currently forecast. Kluber Sunday through Friday: In the wake of a cold front, Sunday will be noticeably cooler and less humid with highs in the low to mid 80Fs. A sharp upper-level shortwave will dive southeast across the Great Lakes but should remain north of our area. Hence, Sunday looks dry. Upper-level ridging will start to build into the region Monday causing temperatures to rebound into the upper 80s to around 90F. A weak surface pressure gradient (thanks to the passage of a surface high) will allow for a lake breeze to surge inland, so lakeshore locations should be cooler with highs topping off in the low to mid 70s. All in all, Sunday and Monday continue to look pleasant, relatively speaking. Tuesday into Tuesday night continues to look like a period of interest with respect to both the chances for severe weather as well as a brief stint of dangerous heat. With the core of upper-level ridging and an associated pool of 850mb temperatures >20C moving overhead, temperatures are poised to skyrocket into the low to mid 90Fs. Ensemble model guidance also advertises a rapid increase in column moisture partly thanks to increasingly breezy southwest winds, with mean PWATs climbing above 1.75" and surface dew points rising into the low to mid 70Fs. Taken together, Tuesday appears to be the first day this summer where widespread heat indices of 100- 105F+ are possible, including up to the Lake Michigan shoreline. Now, anytime we deal with heat indices above 100F, we often have to consider the threat for severe convection. Big-picture wise, our area will be pretty far removed from the upper-level jet located along the US/Canadian border. However, upper-level shortwaves embedded in the jet to our north should sideswipe the deep reservoir of instability across the Upper Mississippi River Valley, which may provide the means for several outflow-dominant MCSs to parade into the Great Lakes (perhaps starting Monday night, lasting through Tuesday night). A southward-moving cold front may also act as a trigger for severe thunderstorms Tuesday evening, though our area may be fairly capped in the absence of large-scale forcing. (In other words, we may need an MCS with a deep cold pool to activate the deep reservoir of instability). Even with the questions of how and when thunderstorms may happen, it`s worth noting that nearly all the individual GEFS/EPS members show QPF sometime between Monday night and Tuesday night, which is a pretty strong signal for convection in this type of pattern. Moreover, GEFS-based ML severe guidance is fairly bullish with Level 3/5-caliber severe weather probabilities already advertised across our area. For now, our official forecast will feature broad-brushed 40 to 60% PoPs in the Tuesday to Tuesday night timeframe in favor of refinements going forward. Needless to say, any thunderstorms (or associated southward- moving outflows) on Tuesday would complicate the forecast for temperatures. Wednesday through the end of next week, ensemble model guidance favors a pool of relatively cool low-level air spilling into the Great Lakes region leading to lowering humidity levels and near seasonable temperatures. There is a weak but growing signal in ensemble guidance for another period of hot, humid, and stormy conditions next weekend. Borchardt && .AVIATION /12Z TAFS THROUGH 18Z SUNDAY/... Issued at 626 AM CDT Sat Jun 22 2024 Key Messages: - A band of showers may move across northern Illinois and impact RFD/DPA/MDW/ORD from late morning through early afternoon. Confidence is low that they will survive all the way to DPA/ORD/MDW (20-30% chance, compared to a 40-50% chance near RFD). - Southwest winds will gust 25 to 30 kt from mid-morning through early evening (high confidence). - An outflow-dominant squall line is expected to sweep across the terminals this evening (80% chance). - Cigs will gradually lower into MVFR and possibly IFR overnight as winds shift northwesterly (medium confidence). Discussion: Regional radar imagery depicts a mesoscale convective vortex (MVC) and an associated north to south-oriented band of warm air advection-driven showers and isolated thunderstorms in central Iowa. As the MCV lifts into southern Wisconsin early this afternoon, the band of WAA-driven showers may continue across RFD, and possibly DPA/ORD/MDW. However, coverage of showers may become increasingly confined to areas near the Wisconsin border going toward owing to less instability across northern Illinois. For now, will advertise a brief window of SHRA in the RFD TAF via a TEMPO group from 17-19Z and wait to add any mention of precipitation in the DPA/ORD/MDW TAFs for now. With forecast ELs near -20C, thunder is expected to be isolated within the band of showers. Southwest winds will increase in magnitude this morning with high confidence in gusts over 25kt and medium confidence in gusts near 30kt, particularly this afternoon. SCT to BKN VFR cloud decks are expected at times this morning and afternoon. This afternoon, thunderstorms are expected to develop in central Iowa and merge into a squall line while crossing the Mississippi River into Illinois this evening. As the line moves across northern Illinois, it should become outflow dominant. Confidence remains relatively high that thunderstorms will occur (>80% chance), so will continue to offer TEMPO groups at all terminals. The current 4-hour windows will be shortened in later TAF packages as confidence increases in specific timing. Even as thunderstorms begin to wane in intensity while crossing across northern Illinois, a high-DCAPE environment will support strong northwesterly winds with gusts of 25-35kt common along the gust front (locally stronger near RFD). Behind the thunderstorms, winds will veer westerly and eventually northwesterly with occasional gusts up to 20kt. In addition, cigs will lower to MVFR and possibly IFR. Confidence is only medium in the occurrence of IFR to MVFR cigs. Borchardt && .CLIMATE... Issued at 335 AM CDT Sat Jun 22 2024 Here are the daily high and warmest low temperature records for Chicago and Rockford for today: Chicago ------------------------- Day: 6/22 ------------------------- Record High: 97 Record Warm Low: 76 Rockford ------------------------- Day: 6/22 ------------------------- Record High: 97 Record Warm Low: 73 NWS Chicago && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory from 10 AM this morning to 1 AM CDT Sunday for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago