Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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452 FXUS63 KLOT 210824 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 324 AM CDT Fri Jun 21 2024 .KEY MESSAGES... - Potential for strong to severe storms with damaging wind, large hail, and localized flooding late Saturday afternoon through Saturday evening. - A brief reprieve in heat and humidity is expected Sunday and Monday, before it returns (with chances for thunderstorms) on Tuesday. && .DISCUSSION... Issued at 324 AM CDT Fri Jun 21 2024 Through Saturday night: Ongoing isolated showers primarily north of I-88 should persist for at least a few more hours in response to weak isentropic ascent in the 5-7kft layer. Cannot completely rule out lightning as marginal lapse rates yield up to 500J/kg of MUCAPE. Meanwhile, low-level dry-air advection has limited the fog potential across much of the area early this morning, though haze or patchy and shallow fog are still expected through sunrise. Also, persistent fog and low stratus advecting inland of Lake Michigan will continue well into the morning from the far south side of Chicago around the northwest Indiana shore. Turning toward today through tonight, we remain precariously close to a favorable axis of deeper low-level moisture and correspondingly more unstable airmass on the northwest flank of a persistent ridge. Upstream dew points in the mid 60s yesterday amid weak S to SSW low-level flow today and added evapotranspiration should yield afternoon dew points today in the mid to upper 60s across much of the forecast area. These values will be insufficient to erode a residual cap while also limiting MLCAPE into the area, especially in the absence of any appreciable forcing. The one exception will be where an axis of low 70s dew points should begin to filter into the far northwest CWA this afternoon and especially tonight as low-level flow begins to veer. Isolated to widely scattered thunderstorms are possible roughly northwest of a McHenry to Sterling line this afternoon and across the counties adjacent to Wisconsin tonight where weak WAA and any potential outflow boundaries would support enough forcing to generate convection. Saturday`s forecast has the potential to become rather muddled as an abundant amount of upstream convection across the central Great Plains tonight has the potential to eject a series of convective waves into the weakening ridge before a main mid- level wave crosses the area Saturday night. Conditional on gradual low-level moistening Saturday morning as well as a low- level focus of any outflow boundary from convection across southern Wisconsin, regenerating scattered severe convection with large hail and damaging winds as well as a low-chance (10-20%) of flooding is possible north of I-88 during the afternoon. 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 most of the remainder of the forecast area Saturday evening. Only modest deep-layer shear may limit the overall severe threat, but storm mode does favor the potential for some discrete supercell structures eventually congealing into a gradual SSE moving line with locally damaging winds. Veering deep-layer and backward propagating shear vectors do yield some minor flash flooding concerns later Saturday evening, though the overall convective threat should remain progressive enough to limit a more formidable flood threat. Kluber Sunday through Thursday: In the wake of the cold frontal passage responsible for thunderstorms Saturday evening, a welcome break in the heat and humidity is expected to start next week. Highs on Sunday should range from the low to mid 80s, while highs on Monday will range from the mid to upper 80s. Locally cooler temperatures in the low to mid 70s can be expected near Lake Michigan (mainly northwestern Indiana shore on Sunday, and the entire shoreline on Monday). While an upper- level shortwave will race through the Great Lakes on Sunday, chances for rain locally appear scant (<15%) owing to limited moisture. As a result, the start of next week is shaping up to be pleasant. Tuesday into Tuesday night, an upper-level shortwave will propagate across the northern United States. The leading "ridge" portion of the shortwave will arrive on Tuesday and allow for aggressive low- level temperature and moisture advection into the lower Great Lakes. Rising 850mb temperatures to +21 to +22C will support highs in the low to mid 90Fs. If the moister side of guidance verifies (e.g. GEFS, CMCE), heat indices may very well push above 100F. When considering expected partly cloudy skies, a nearly maximum annual sun angle, and southwest winds of only 10 to 15 mph, forecast max wet-bulb globe temperatures top off in the mid to upper 80s, as well. Now, the EPS remains steadfast that mixing downward of low-level moisture will keep heat indices in check, much like the past week. Regardless, Tuesday will be a day to watch for potentially needing a Heat Advisory for part of our area. Now, anytime we`re talking about a potential need for heat headlines this time of year, we often have to consider convection. While it`s a little early to dig into the details for a Day 5 forecast, the idea of an upper-level wave providing a glancing blow to a plume of steep mid-level lapse rates (>8 K/km from 700-500mb), a deep layer of moisture (PWATs approaching 2"), and an associated extensive reservoir of instability (MUCAPE >3000 J/kg) raises alarm bells for MCS activity in the general region. With the upper-level shortwave poised to propagate well to our north near the US/Canadian border, areas from Wisconsin through Lower Michigan should be the favored zone for MCSs (starting Monday night, lasting through Tuesday). However, this is the time of year when convection can verify farther south than advertised in model guidance. Even if the most robust thunderstorm activity remains to our north throughout the day on Tuesday, at least scattered thunderstorms may accompany a cold front dropping south through our region Tuesday night. 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. Borchardt && .AVIATION /06Z TAFS THROUGH 12Z SATURDAY/... Issued at 1224 AM CDT Fri Jun 21 2024 Aviation Key Messages: - 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. Updated Discussion (324 AM): Low-level stratus has been eroding quicker than expected this morning, so was able to cautiously bring most terminals to VFR with unscheduled AMDs. The clear exception is GYY where marine fog remains entrenched. Otherwise, with residual low-level, nearly calm winds, and now clearing skies, areas of shallow fog or fine mist may develop by sunrise (2-4SM visibility). Finally, the lone thunderstorm that developed in the vicinity of RFD should continue moving northeastward, and the remainder of the morning is now expected to be dry (at all terminals). Borchardt Previous Discussion (1228 AM): 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 Here are the daily high and warmest low temperature records for Chicago and Rockford for today and tomorrow: Chicago ----------------------------- Day: 6/21 6/22 ----------------------------- Record High: 101 97 Record Warm Low: 74 76 Rockford ----------------------------- Day: 6/21 6/22 ----------------------------- Record High: 100 97 Record Warm Low: 71 73 NWS Chicago && .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