Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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105 FXUS63 KLOT 041803 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 103 PM CDT Sat May 4 2024 .KEY MESSAGES... - A line of scattered thunderstorms is expected to cross the area late this afternoon into mid-evening, with locally strong to severe wind gusts to 60 mph possible. - Periodic shower and thunderstorm chances are expected next week. There is a potential for strong to severe storms along with a locally heavy rainfall threat on Tuesday. && .UPDATE... Issued at 921 AM CDT Sat May 4 2024 A recent hand surface analysis reveals a 1009mb surface low pressure system centered in central Iowa ahead of a cold front arcing southward across far eastern Kansas and central Oklahoma. An MCV is evident ahead of the front just to the northeast of Kansas City along the nose of a developing instability plume arcing northward along the backside of a surface high pressure system centered over southern Ontario. A narrow line of showers and storms approaching the Mississippi River continues to outpace the developing instability gradient and cold front to the west, and accordingly, should continue to decay. With that said, we can`t rule out a shower survives as far east as I-39 by early afternoon. The forecast for this afternoon and evening appears to be on track. Pockets of clearing ahead of the cold front and northward-progressing instability plume will allow for steepening low-level lapse rates and eroding inhibition. As a result, showers and thunderstorms appear poised to erupt along and ahead of the front this afternoon near the Mississippi River and spread eastward into northern Illinois and eventually northern Indiana this evening (reaching I-39 in the 5 to 7 pm timeframe, Chicago metro in the 6 to 8 pm timeframe, and northern Indiana in the 7-9 pm timeframe). With the upper-level jet displaced to our north, thunderstorms will be limited to "pulse" like single-cell behavior. However, somewhat linear forcing ahead of the cold front, as well as relatively dry low- level moisture profiles, will afford efficient amalgamation of surface outflow/cold pools and support "upscale growth" of initial single-cell thunderstorms into gusty multicell clusters. While the environment doesn`t appear supportive of widespread severe weather, prospects for a wall of 45-55 mph wind gusts along the leading edge of any consolidated thunderstorm outflows appears to be increasing this evening. Where thunderstorm pulses are most intense, locally damaging wind gusts of 60-65 mph may occur, as well. We`ll have to keep a close eye on the integrity of the aforementioned MCV as it tracks toward central Illinois this afternoon, as it may provide just the oomph needed for a more focused threat area for damaging winds. Updated products have been sent. Borchardt && .DISCUSSION... Issued at 314 AM CDT Sat May 4 2024 Through Sunday: A retreating surface ridge extending southwestward across the western Great Lakes will give way to ongoing lee cyclogenesis ahead of a mid-level trough entering the central Great Plains early this morning. Robust convection from northern OK into western IA has likely enhanced the trough, with multiple MCVs evident along this corridor. An associated cold front just behind this convection will become the main focus for storm potential into our area later this afternoon into this evening. Surface dew point trends today will be an important factor on the coverage and intensity of storms into the forecast area. Friday`s weak cold front has since stalled across central Illinois and will jump northward in response to diurnal warming and moistening today. However, the magnitude of moistening primarily from evapotranspiration remains less clear this afternoon. Current thought is daytime mixing into a very dry layer above the growing PBL will with offset rising dew points or at least reduce the depth of moisture sufficient for development and maintenance of convection. Have opted to use only scattered thunderstorm wording even as some CAMs depict higher coverage as a result of higher modeled dew points. While it is expected that a more prominent MCV currently near Omaha will lift NE within the broader mid-level steering flow this morning, the extreme southern influence of this MCV may clip the northwest CWA late this morning. Capping should remain far too high to support convection this far southeast, but isolated attempts at convection may reach as close as far northwest Illinois. Overall expectations are for a line of broken showers and storms to develop/intensify roughly along and around the Mississippi River by mid-afternoon within a pre-frontal ribbon of higher theta-e air. Coverage of thunderstorms should then begin a slow downward trend across the forecast area late afternoon through mid-evening as the drier pre-storm environment becomes less favorable with time. However, given an increasing reservoir of DCAPE with eastward extent across the CWA, the potential for strong to locally severe wind gusts may actually maximize across the west half of the CWA up to and through sunset. The antecedent dry air ahead of these storms combined with diurnal mixing of low-level moisture speaks to this wind risk, with the strongest winds likely occurring with any existing loosely organized multi-cell cluster or collapsing cores as storm intensity wanes with time. After the storms and cold front clear the forecast area by early Sunday morning, dry and cooler conditions are expected through the remainder of the period. Kluber Sunday Night through Friday: Quiet weather starts the period Sunday night, as surface high pressure is progged to be drifting east across the Great Lakes region. Farther south, guidance continues to depict mid-level short wave approaching IL late, though it is deamplifying as it moves into the short wave upper level ridge axis. Forecast soundings depict very dry low and mid-levels initially in place across the forecast area, though persistent isentropic ascent and associated warm/moist advection does eventually produce saturation and precipitation development into central IL/IN and the LOT/ILX border region after midnight. Mid-level lapse rates are unimpressive in forecast sounding guidance, suggesting little/no thunder threat. Showers are expected to persist into Monday across the southern parts of the cwa as the mid-level wave continues its transit of the area. NBM blended pop guidance appears to spread too far north in this scenario, and note that much of the EPS/GEPS/GFS ensembles are dry along/north of the I-88 corridor through the day across northern IL. Farther west, guidance continues to depict a deep upper level trough across the western CONUS, within which a strong, negative-tilt short wave develops a closed upper low over the northern high plains by Monday night. Along with a series of short waves propagating around the southeast periphery of the upper circulation and upper level diffluence ahead of a strong cyclonically-curved upper jet, this sets the stage for southerly low level warm/moist advection from the western Gulf into the mid-Mississippi Valley and upper Midwest Monday night, with a northward lifting warm front spreading into the area into Tuesday morning. Thus shower and thunderstorm chances look to ramp-up into the forecast area from the west especially after midnight and into Tuesday morning. While the primary upper/surface lows will be well to our northwest Tuesday, guidance continues to depict a strong mid-level jet is progged to spread east across the area during the day, with a surface wave developing along an occluding cold front approaching the Mississippi Valley later in the day. Combined with a warm, moist and conditionally unstable low- level air mass, additional thunderstorms (potentially strong to severe) are likely within the region Tuesday afternoon/evening. In addition to severe weather, high precipitable water values around 1.75 inches will support locally heavy rainfall will be possible especially if storms train over a particular area. SPC and WPC outlooks for Tuesday for both severe and heavy rainfall potential continue to appear appropriate at this distance. The upper trough is forecast to drift east across the upper Midwest and northern Great Lakes region during the mid and late week period, maintaining an active weather pattern across our forecast area. While the primary low-level theta-E axis will shift southeast of the forecast area Wednesday through Friday, a series of short waves rotating through the southern periphery of the upper Midwest upper low will likely produce additional periods of showers and thunderstorms, especially during the diurnally more favorable afternoon and evening hours. Temperatures will remain above average through most of the period, with daytime highs in the 70s to low 80s (Tuesday and Wednesday have the potential to be the warmest days). Cooler, more average temperatures are expected by Friday, with highs in the mid-60s. Ratzer && .AVIATION /18Z TAFS THROUGH 00Z MONDAY/... Issued at 101 PM CDT Sat May 4 2024 The primary aviation weather concerns through the 18Z TAF period include: * A line of gusty showers and thunderstorms this evening * MVFR cigs expected late this evening into Sunday morning A line of showers and thunderstorms will move into northern IL later this afternoon and into the Chicago metro around early evening. It`s expected to reach RFD mid-late afternoon and the Chicago sites closer to 00Z. The rain potential over any given area will last only a few hours or so clearing the Chicago sites sometime near 03Z. It`s possible that vsbys could get pulled down to MVFR for a brief period or two underneath any heavier rainfall. Additionally, gusty westerly winds will likely accompany the rain and storms. Gusts into the 30- 35 kt range look attainable while the rain moves in and it`s not out of the question that we could see an isolated higher gust or two. Cigs are expected to remain VFR ahead and during the bulk of the rain event. However, MVFR cigs should take hold this evening as the rain begins to move away. MVFR will persist through the night and much of Sunday morning over the Chicagoland sites. A period of IFR is possible overnight, through best guess is that we`ll remain MVFR. At RFD, the MVFR looks to lift much sooner, likely late this evening or early in the overnight. The MVFR will clear over the metro during the late morning and early afternoon. Meanwhile, SSW winds will continue to occasionally gust into the teens to near 20 kt this afternoon. Winds are expected to veer to westerly with the arrival of the rain. A couple of forecast models suggest we may see a backing in the wind field to SSE for an hour or two as the system approaches, through a lack of confidence left this potential wind shift out of the TAF. Winds at the time will be moving at near 10 kt with gusts as high as 20 kt. Any of those notably higher gusts just ahead of the system that were discussed will be out of the west. NW winds just over 10 kt will follow the rain out of the area this evening before flopping over to NNE early- mid morning. The majority of Sunday will see NE winds below 10 kt. Doom && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Dense Fog Advisory until 7 PM CDT this evening for Winthrop Harbor to Northerly Is. IL. && $$ Visit us at weather.gov/chicago