Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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105 FXUS63 KLOT 202358 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 658 PM CDT Mon May 20 2024 .KEY MESSAGES... - Scattered severe storms with large hail and damaging winds ending by evening, favored across far eastern IL and NW Indiana. - Lower chance for thunderstorms late tonight northwest of the Chicago metro - Another round of thunderstorms is expected as early as mid to late Tuesday afternoon, but more likely Tuesday evening into the nighttime hours. Severe thunderstorms with damaging winds and brief spin-up type tornadoes are the main hazards, though large hail may occur as well. && .DISCUSSION... Issued at 327 PM CDT Mon May 20 2024 Through tonight: The MCV has now shifted into southern Wisconsin, east of Janesville and west of Racine. With an unstable atmosphere ahead of the MCV, arcs of convection have shift mostly over Lake Michigan this afternoon. Additional showers and storms may occur across northern Illinois and even into northwest Indiana into early evening where the atmosphere is unstable, but this window will also be fairly short lived into the evening, and areas farther south are a bit more removed from the MCV influence. The severe threat is low (<10-15%), with hail to dime sized and isolated instances of minor tree damage remain possible. There is another subtle wave across SW Iowa that could trigger a few more showers or storms atop a shallow boundary layer through mid to late evening south of I-80, though confidence on this is only low-medium. There should be a period of ridging behind the MCV which should lead to a dry period later this evening and into the overnight hours, and the warm front that has shifted north today will likely get shunted back south, with some uncertainty as to where that will stall. This front in conjunction with additional upstream waves that may be embedded in the flow aloft may trigger some additional thunderstorms late tonight. Tough to say right now as these are not apparent on upstream water vapor satellite just yet. Tuesday and Tuesday Night... A complicated storm evolution is setting up for Tuesday. While there are two sharp upper troughs upstream, our feature of interest is the sharpening vort max nearing the desert Southwest. This feature is well agreed upon in guidance to strengthen as evidenced by becoming negatively tilted through the afternoon hours. With a fairly potent southern stream jet axis progged to shift into the central Plains, in addition to a secondary northern stream jet core, the surface cyclogenesis process will lead to deeply anomalous low pressure core that will likely drop below 990 mb by late afternoon and even deeper in the evening hours. The low strength will approach the bottom of the NAEFS climatology for this time of year. These all point to a significant severe weather event in the region. From a sensible weather perspective, there are several periods of concern. The first is along the advancing warm front ahead of the low on Tuesday morning on increasing low level southwest flow. The boundary will still be across northern Illinois during the morning. The aforementioned system will be advancing upon an elevated mixed layer, and thus any storms have the potential to generate mostly sub- severe hail. It will be a warm and breezy day following any morning activity, and a rapidly destabilizing warm sector through the day portends a concern for discrete supercells, with all hazards possible in the mid to late afternoon as this larger system approaches. This is currently favored across Iowa and into NW Illinois, but does extend to the I-39 corridor. By the evening hours the deep layer shear vector will become oriented more perpendicular to the surging cold front across the plains as the surface low will continue to shift north into Minnesota. Upscale growth into a mature wind producing system (MCS) appears the more likely mode in the evening, making damaging winds the highest concern, although some lingering low level shear will still support a tornado threat along the line. There is still some uncertainty as to how far south and east the severe threat will be into the evening, thus the highest severe probabilities do still remain favored west, but with the potential for a fairly organized MCS these hazards will extend across much of northeast Illinois and possibly into northwest Indiana as well. Wednesday and Wednesday night: There is still some signal for showers later Wednesday into Wednesday evening south and east of I-57, but most areas will have cooler conditions and a dry day. KMD Rest of the forecast discussion unchanged... Thursday through Sunday: Brief upper level ridging should allow for better height rises on Thursday for much drier conditions setting up. Winds should be out of the west and light with the occasional afternoon burst to 15 mph. Afternoon temperatures should warm into the 70s, with slightly cooler conditions in Northern Indiana along the immediate shoreline. The next upper level trough ejects out of the Rockies Thursday night and starts tracking toward southeastern Manitoba. Despite the system`s track being farther to the northwest than earlier in the week. As the reflected surface low passes through the Dakotas and Minnesota, a warm front will slowly enter Illinois on Friday. Southerly winds will slowly advect precipitable water amounts over an inch back into the area. The best instability will be in the warm sector behind the front in a Rockford to Fowler, IN line, but even then forcing does not look impressive. So PoPs for scattered showers were kept at the chance threshold. Overnight and into Saturday, the low exits into Canada as a secondary surface low strengthens over the Southern Plains. Friday`s surface front will be come a NW/SE oriented cold front and move over the region on Saturday. With it comes better forcing and instability to allow widely scattered thunderstorms to develop through the day. This far out, timing can be adjusted closer to the event. At this juncture, model soundings have weaker mid level lapse rates and deep layer shear to keep the word severe out of the forecast. There may be isolated showers that stick around through Monday, but there should be breaks in the rain. However, confidence remains to low this far out for specifics at this time. DK && .AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 657 PM CDT Mon May 20 2024 The primary aviation weather concerns through the 00Z TAF period include: * Gusty near-southerly winds during the day on Tuesday. * A line of strong to severe thunderstorms expected Tuesday evening. * At RFD, a chance for a few morning thunderstorms in addition to the evening potential. * While not expected, a small chance for some fog and low stratus at the Chicago sites late tonight into early Tuesday. Westerly winds will lose their gustiness shortly after 00Z and begin to veer in direction. By around 04Z, winds are expected to flop over to easterly while moving at around 5 kt. Some model guidance suggests that fog and low stratus could bleed inland off of the lake and bring MVFR vsbys and MVFR to high-end IFR cigs to the Chicago sites late tonight into early Tuesday morning. The signal for this appears too marginal to warrant mentioning such conditions in the TAF, but the potential is there. Winds will veer to SE by early morning before going near- southerly for the afternoon and evening. Gusts during the day should regularly approach 30 kt with occasional gusts to near 40 kt possible, particularly in the late afternoon and early evening. Direction is anticipated to be predominantly just west of south (190-200) although we could maybe see lengthy periods of gusty winds out of a 160-180 direction. A complex of showers and thunderstorms will move into the region tonight. While all of the activity is expected to stay west of Chicagoland until the evening, a few showers or storms could find their way over RFD during the morning. There is a chance that some of these storms could be on the stronger side and feature locally gusty winds, although the more appreciable severe threat comes with the evening activity. During the evening, a line of strong to severe thunderstorms is expected to push across northern IL. It`s forecast to move over RFD sometime in the early-mid evening and over the Chicagoland sites closer to late evening. These storms are likely to feature strong, if not damaging, winds in addition to heavy rain and possibly some hail or a few tornadoes. The line should work east of the terminals by or shortly after midnight with breezy but dry conditions in its wake. Doom && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago