Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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721 FXUS63 KLOT 220835 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 335 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 /06Z TAFS THROUGH 12Z SUNDAY/... Issued at 1252 AM CDT Sat Jun 22 2024 Aviation Key Messages Include: - Scattered showers and a few thunderstorms will move near RFD/DPA/ORD this morning (high confidence). - Breezy southwest winds with gusts of 25-30kt will prevail from mid-morning through the evening (high confidence). - A line of thunderstorms will sweep across the terminals this evening (high confidence) accompanied by northwesterly winds gusting 25-35kt (medium confidence), and trailed by MVFR cigs (medium confidence). Discussion: Through 12Z: Ongoing scattered showers and thunderstorms over northwestern Illinois will continue moving slowly northeastward throughout the night. Opted to introduce targeted TEMPO groups between 07-09Z at DPA/ORD for TSRA. The showers and thunderstorms may graze the 10-mile vicinity ring of MDW, but for now, opted to keep all precipitation out of their TAF this morning. Meanwhile, at RFD, will go out the gate with VCTS through 09Z as the current cells appear to be on a trajectory to just miss the airfield. Outside of showers and thunderstorms this morning, a slowly- moving outflow boundary with a northeasterly wind shift is in the process of stalling along a line from Elgin to MDW. Confidence is low on whether it will actually reach MDW, though if it does, speeds should be less than 5 kt (given a forward motion of the boundary of a mere 3 kt). Finally, will have to keep a close eye on an area of marine mist and LIFR/IFR cigs lagging behind the outflow boundary near PWK. The forward progress of the BR/IFR cigs is slowing, so confidence is cautiously growing that it will not reach ORD, let alone MDW. Regardless, trends will be monitored throughout the night (visby may lower to 2-3SM and cigs may lower to 400-500ft if the bank does reach ORD). After 12Z: Southwest winds will become increasingly breezy after sunrise as the low-level pressure gradient gradually packs overhead. Confidence is high in gusts over 25kt, and medium in a few gusts nearing 30kt, particularly this evening. The expectation is that all terminals should be dry from daybreak through at least mid-afternoon. However, will have to keep an eye on ongoing thunderstorms in northwestern Iowa for signs that they build southward into the feeding LLJ and roll along the Wisconsin state line from mid-morning through mid-afternoon. In fact, there may be a somewhat continuous west-to-east line of convection near the Wisconsin state line at some point this afternoon. All things considered, felt introducing a 6-hour PROB30 for TSRA at RFD from 16-22Z was appropriate given a low confidence but high impact event. This evening, a line of thunderstorms is then expected to sweep southeastward across the terminals (generally between 23Z to 05Z across the airspace). Confidence is high enough in thunderstorms to convert the inherited PROB30 groups into targeted TEMPO groups, albeit maintaining longer than needed 4-hour long windows to account for continued wiggle room in the time of arrival of the line. Later TAF packages should offer refinements in the timing (to be 2 hours or so) at all terminals. A northwesterly wind shift with gusts of 25 to 35kt appears likely with the line of storms. Gusts may even locally exceed 40kt (particularly at RFD). In addition, visibility may drop to 2 miles or lower in the most vigorous convective cells given very high environmental moisture content. Confidence is high that there will be thunderstorms this evening, and medium in the magnitude of wind gusts and drops in visibility as they arrive. Finally, cigs will attempt to build downward into MVFR behind the line of storms this evening and prevail through the remainder of the TAF period. In fact, pockets of IFR may develop, as well. Confidence is medium on cig trends after 00Z. 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