Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 07/23/21

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
936 PM CDT Thu Jul 22 2021 .UPDATE... 936 PM CDT For the rest of tonight, it appears the chances for any convection of note are diminishing. In the warm and moist air mass, we`ve seen widely isolated showers pop up at times, though as of this writing, only seeing a lone cell south of Pontiac. A weak short- wave centered near the southeast MN, northeast IA border will drift south-southeast through the night. Meanwhile, a slightly stronger wave over north central WI, responsible for higher coverage of showers and thunderstorms will give only a glancing blow of forcing to northeast IL and northwest IN overnight, with the stronger forcing passing into lower MI. 00z RAOBs showed a pronounced dry layer above 700 mb and most recent AMDAR soundings also indicated a weak inversion around 600 mb. Meanwhile, despite the very muggy boundary layer with low to mid 70s Td, lacking better saturation in the 900-800 mb layer on aforementioned soundings, where a theoretical elevated parcel could lift from to tap into MUCAPE, also appears to point toward a struggle to generate robust enough updrafts for lightning generation. Thus, with only modest forcing and rather unfavorable profiles, kept thunder mention at slight chance through the night. For this update, kept the 30-40% PoPs into northeast IL and far northwest IN, though looking like that may be a bit generous. Some widely scattered lighter showers may be all that ends up occurring, if that. Recent HRRR runs have fully backed off on activity, while 3km NAMnest has a few isolated cells over Chicagoland overnight. The only other forecast change of note was to have haze mention from effects of lower level wildfire smoke to areas where there were recent minor visibility reductions. On Friday, southwest flow looks to bring a higher concentration of low-level smoke to the region per the 00z HRRR surface smoke forecast, so added haze mention through Friday night to the grids. Subtle forcing toward mid day Friday into the afternoon and lake breeze convergence hugging the shore may be just enough to pop isolated to widely scattered showers and thunderstorms. Castro && .SHORT TERM... 248 PM CDT Through Friday night... Southwesterly flow is pumping in warm. moist air, with dewpoints jumping from the mid-60s this morning to the lower 70s as of writing. Clouds and haze from smoke has scattered out in many areas, allowing temperatures to climb into the low to mid 80s. Temperature differential has allowed the lake breeze to set up and move inland, just east of O`Hare at this moment, and is expected to move just west of the main airports, shifting winds to the southeast, and helping keep locations near Lake Michigan a bit cooler in the upper 70s. Increased heat and moisture is reflected in isentropic upglide, allowing for a slightly unstable atmosphere, as reflected with occasional showers moving through the forecast area. With CAPE values in excess of 1000 J/kg, the potential for isolated thunderstorms to fire this afternoon exists, although they should be isolated in nature. A short wave/vorticity max will move out of northern Iowa and may provide focus for increased convective activity Thursday night into early Friday morning. Guidance is not in agreement with the location of this increased chances, but presently appears to be focused along the Wisconsin/Illinois border, especially northeast Illinois. The short wave will move east Friday morning, but with continued southerly flow pumping in warm, moist air, instability will remain throughout Friday for a continued slight chance of isolated showers and maybe even a thunderstorm or two to fire Friday afternoon. High temperatures will be in the mid to upper 80s, although the heat indices will be in the lower 90s. Chances for precipitation decrease Friday overnight, but continued southerly flow will keep dewpoints in the lower 70s, with lows Saturday morning in the lower to mid 70s. BKL && .LONG TERM... 256 PM CDT Saturday through Thursday... No substantial changes to the forecast for this weekend into the middle of next week as seasonably summertime conditions with intermittent chances of thunderstorms continue. Broad ridging centered across the central and southern High Plains will remain established through the forecast period, keeping the western Great Lakes on its northeast fringe. Seasonably warm and humid conditions will be the rule for northern Illinois and northwest Indiana. While the greatest convective chances through the period will remain northeast of the forecast area, the most favorable period for storms to edge southwest into the area will be late Saturday afternoon and night, and possibly into Sunday. Saturday and Sunday: Low-level moisture will continue to increase through the day Saturday on southwest winds sporadically gusting to 25 mph. While some guidance may be overdone with forecast dew points into the upper 70s, widespread dew points solidly in the low 70s and edging into the mid 70s is likely. With these dew points, the environment should remain capped through much of the day under an 850 hPa warm nose. We will have to monitor any potential upstream convection on Friday night, however, as there are some signals a residual MCV may track within faster WNW mid- level flow just north of the area and bring either enhanced mid- level clouds/decaying showers (assuming the expected stronger cap) to northern Illinois late morning/early afternoon. The increase in dew points combined with temps potentially into the low 90s will result in heat index values around 100F in the afternoon. If mid-level cloud cover and any potential smoke aloft is minimal, slightly higher temps and heat index values nearing the local Heat Advisory criteria of 105F are possible. Capping may erode enough ahead of an approaching cold front to allow for isolated to scattered convection to develop over primarily northeast Illinois and northwest Indiana Saturday evening. A highly unstable environment with modest 0-8km shear (better metric due to potentially very tall storms) of 25-30kts will support explosive convective growth and a resultant severe thunderstorm wind risk with any convection that does develop. The southward push of the cold front or effective outflow boundary will depend on the amount of upstream convection on Saturday/Saturday night. While guidance has been fairly persistent on clearing the boundary through most, if not all, of the CWA by Sunday afternoon, any slower progression will support another afternoon of isolated explosive severe thunderstorm potential across southern portions of the forecast area. Additionally, heat index values will near or surpass 100F again around and even behind the boundary ahead of somewhat drier air advecting in from the north. Monday through Thursday: The latest GFS is a bit of an outlier in bringing additional rounds of convection across the area with WNW flow aloft on Monday and again on Tuesday, likely owing to unrealistic amounts moisture (dew points nearing 80F) in then lowest 500ft of modeled profiles. With surface high pressure setting over the region and the weekend`s boundary drifting south on renewed convection each day, Monday and Tuesday will likely remain dry and warm. A more pronounced wave should bring increasing chances for convection Wednesday into Thursday. Kluber && .AVIATION... For the 00Z TAFs... Aviation Forecast Concerns: * Some low VFR/high MVFR visibility in haze for metro terminals this evening. * Low probability for SHRA/TSRA late tonight/pre-dawn Friday. Somewhat ill-defined surface warm front extended from southern WI, southeast along the lake breeze boundary west of ORD/MDW and then southward near the IL/IN state line early this evening. This front will continue to evolve slowly northward overnight. While a few isolated showers had developed along this boundary across the Chicago metro this afternoon, a lack of stronger forcing should keep things quiet across the terminals through this evening. Various model guidance continues to depict the potential for some convective development primarily across southeastern WI later tonight along/north of the warm frontal boundary, on the nose of a modest southwesterly low level jet and in association with a weak mid-level short wave which will track southeast across the region. With a southeasterly propagation expected to any showers/storms which develop over southeast WI, the potential does exist for some of this to affect the Chicago metro terminals after midnight, generally in the 06-11Z period based on CAM solutions. Confidence in any significant coverage this far south across the terminals is fairly low, with better chances north and northeast of the terminals across WI/southern Lake Michigan on the nose of the jet and with a little better mid-level moisture profile. Felt it prudent to leave some mention for now though and since can`t use the inherited prob30 (only allowed beyond first 9 hours of TAF) have carried a VCSH mention at this time and will monitor for development across WI later this evening to assess the potential for our forecast area. On Friday, the mid-level wave will track off to the east and an upper level short wave ridge is progged to build across the western Lakes region. Warming mid-level temps due to subsidence beneath this ridge looks as if it will generally provide a capping inversion with respect to renewed convection. Some guidance weakens this cap a bit late in the day and suggests some isolated convective threat, though feel that this is a fairly low coverage and low probability occurrence at any particular terminial and thus will maintain dry TAF Friday/Friday evening. East-southeast winds for ORD and MDW will become light southeast later this evening and then light south-southwest overnight. Modest south-southwest winds Friday should keep lake breeze boundary east of ORD/MDW. Only other concern is with current patchy low-end VFR visibility in haze across parts of the Chicago metro. Expect this may improve as winds turn southerly overnight. Ratzer && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at: