Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 05/03/21

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
912 PM CDT Sun May 2 2021 .UPDATE... 912 PM CDT Minor tweaks to the going forecast for the rest of tonight, with the message remaining showers mainly overnight for areas along/east of I-55, and a chance for a few showers or a storm to sneak into far northern Illinois. The severe threat is low if that were to happen. Confidence has further increased in convection, some probably on the stronger to possibly severe end with gusty winds, moving into northern Illinois mid to late afternoon Monday. First for the rest of tonight, the system responsible for the rain in the southeast CWA -- an upper low over eastern Arkansas -- has its broad upper diffluence creeping northward across the southern halves of Illinois and Indiana. Light showers in that area will continue to slowly expand north, favored to reach southern parts of the Chicago metro after 2 A.M. This rain should primarily be light, lingering for a few hours after sunrise before departing. To the north, convection lit up along the frontal zone from northern Iowa into central Wisconsin. Convective prorogation will continue to favor more of an eastward movement, especially with the offsetting strength of the southerly low-level winds. However, there are multiple southward-moving outflows seen on radar, satellite, and surface obs in southern Wisconsin, and scattered convection on this is probable at least for the next few hours. This should result in the outflow getting close to the state line after midnight. While MUCAPE tapers quickly with southern extent (a stretched out 200 J/kg on the 00Z DVN sounding), there is enough low-level moisture transport into these outflows to possibly force a few showers or weak storms along/north of I-90 late. Looking ahead to Monday, the below AFD remains representative to expectations and the potential for some severe storms. One key thing we have been seeing with incoming observational data and CAM solutions is the likely presence of an MCV currently along the Nebraska/Kansas border that several high-res solutions pick up on and track into/across northern Illinois Monday mid-afternoon into mid-evening. With little in the way of convection expected to our south (upstream in the low-levels) through peak heating Monday, this more convectively-enhanced wave than it previously looked should have a decent air mass to work with. A convective uptick is favored during peak heating in eastern Iowa to north central Illinois mid to late afternoon. While morning clouds look like a good bet, RAP soundings continue to support the afternoon cap eroded with dew points peaking into the mid 60s up to I-80 and possibly I-88. Mid 60s dew points are presently up to the Missouri/Arkansas border, so that`s definitely doable. The overnight shift will assess convective evolution further, and try to lock down timing that still has 3 to 4 hour differences in various NWP solutions. MTF && .SHORT TERM... 242 PM CDT Through Monday night... Another day of breezy and warm conditions here, with temperatures early this afternoon generally in the low to mid 80s north of an encroaching region of higher cloud cover. Regional visible satellite loops show a bit more in the way of lumpy cumulus development ongoing north of I-88 where surface dewpoints are in the upper 50s with gradually eroding convective inhibition. Currently, things are still quite capped to surface-based parcels with a lingering pocket of +8 to +9 C air around 750 mb per recent AMDAR soundings. With surface temperatures rising into the mid 80s, it`s possible we erode enough of the lingering MLCIN before sunset to pop a few showers--maybe a thunderstorm--north of I-88. Any activity that does develop shouldn`t be too long for this world with the loss of daytime heating, but 30 degree dewpoint depressions, 0-3 km lapse rates approaching 9 C/km, and DCAPE values near 900 J/kg could support some gustier winds should any deeper showers/storms manage to develop. Additional--more organized/vigorous--convection is expected to develop farther to our north and west through this afternoon and evening ahead of a quasistationary front which stretches across central Wisconsin and back into southern Minnesota. This activity will build south and eastward with time, but is expected to wane with time. It`s possible some of this activity (in a very weakened state) presses into northwest Illinois later this evening, and have left some low-grade PoPs in place there to capture this potential. To the south, light showers associated with the divergent upper-level flow ahead of a strung-out shortwave will push into our forecast area, with the most likely areas to receive some light precipitation those that are south and east of I-55. This activity will peel east of the area through the mid-morning hours on Monday, leaving behind generally dry conditions over the region but with plenty of low and mid-level cloud cover which will stunt our temperature climb and also the destabilization process. However, model guidance is in pretty good agreement driving low to even mid 60 degree dewpoints into north-central Illinois through the afternoon hours ahead of a pretty vigorous shortwave. There remains some degree of disparity regarding the location of the northern terminus of the advancing EML plume tomorrow, but if anything, guidance has trended a bit farther north into the CWA with this. What this means for us is a greater degree of capping through the morning/early-mid afternoon hours, but a somewhat greater supply of instability ahead of the aforementioned shortwave, especially as you work farther south of I-80 and into central Illinois. Deep layer shear will remain in short supply through much of the morning hours on Monday, but looks to increase rapidly with the approach of the aforementioned shortwave through the late- afternoon. As a result, a gradual intensification of convection along an incoming cold front is possible as it approaches our region from the west. Thunderstorms appear likely with this feature as it traverses our forecast area, and some could become severe should MLCAPE values indeed claw their way towards 1000-1500 J/kg as some guidance suggests into an increasingly-sheared environment. Largely boundary-orthogonal shear may help keep some degree of discrete-ness going with a gradual clustering of cells with time yielding a damaging wind threat given the fairly fast H7 flow and some degree of mid-level drying with time. The steepened mid-level lapse rates may also afford storms the potential to produce some large hail, and, as with any system sporting a sub-1000 mb surface reflection traversing eastern Iowa, the tornado potential will be non-zero. Do think the tornado potential is probably on the lower side at this time given the sufficiently-veered look to our surface winds and overall pretty small hodographs in the 0-3 km layer, but still something to keep an eye on. The main window of concern is a later one with everything mentioned, closer to 5-9 PM or so. The main threat axis may align mainly south of I-80 (into the corridor of better instability), but will extent across the entire area and may increase north towards the I-88/I-90 corridors if instability increases sufficiently. Activity will clear the area through late Monday evening. Carlaw && .LONG TERM... 217 PM CDT Tuesday through Sunday... A cooler stretch of weather is expected to starting Tuesday with generally dry conditions expected outside of a couple systems that may bring precip chances. On Tuesday, a cold frontal boundary will be positioned across central IL with the low/mid level baroclinic zone still across the local area. A longwave trough will move across the area into Tuesday evening while a surface low tracks up the Ohio Valley ahead of a more amplified portion of the upper trough. As a result, Tuesday appears to be a showery day, especially across the southern part of the area closer to the front and better larger scale forcing. It will be cooler with highs in the mid to upper 50s north to around 60 south, with winds off Lake Michigan keeping lakeshore areas in the lower 50s. High pressure moves across Wednesday leaving dry conditions and highs in the lower 60s for all but lakeshore areas. Light winds will likely support lake breeze development so afternoon temps will likely only be in the low to mid 50s lakeside. By this time, large scale ridging will have developed across western North America with a broad upper trough over the eastern half. A shortwave trough is expected to traverse the flow and cross the area Thursday bringing chance for showers and maybe a few thunderstorms. The wind field should remain light enough to support another lake breeze. High temps should range from the upper 50s north to lower 60s south, with mid 50s near the lake. The pattern begins to shift east Friday allowing for somewhat milder air to return, except for areas near the lake once again. While the upper flow temporarily flattens into Saturday another trough is expected to pivot southward toward the region with a second trough ejecting from the southwest triggering surface low pressure which will move east or northeastward across the Ohio Valley. This would support another chance for showers/thunderstorms some time Saturday into Sunday. MDB && .AVIATION... For the 00Z TAFs... 636 PM...Primary forecast concerns include... Scattered thunderstorms Monday afternoon/Monday evening. Ifr cigs Monday morning. Gusty southwest winds tonight. Chance of showers early Monday morning. Southwest winds will remain gusty this evening, into the mid 20 kt range and will slowly diminish after midnight. Winds will turn more southerly early Monday morning and may briefly drift to the south/southeast after sunrise, but will turn back to the southwest in the 10-12kt range by late Monday morning. An area of rain across central IL and central IN will slowly drift to the northeast later tonight and a period of showers is possible during the predawn hours. Confidence is low for how widespread this activity will become as well as its duration and its possible that the bulk of this shower activity will remain east of the Chicago area terminals with a few lingering showers possible through mid morning Monday. A large area of mvfr cigs will spread north across the area early Monday morning and there is good agreement that these will lower to ifr around or just after daybreak for a few hours before then lifting back through mvfr by late morning and then a low vfr deck by early afternoon. Though its possible high mvfr cigs may remain prevailing across northwest IL and at rfd in the afternoon. Scattered thunderstorms are expected to develop across eastern IA and northwest IL Monday afternoon and quickly move east across the terminals through early evening. Models have been handling this system and these convective trends well for several days. Still some uncertainty on timing, but current tempo timing covers the expected time period well, with some timing adjustments possible as trends emerge. Left thunder mention out of the gyy taf for now, as best current timing could be at or just after the end of this forecast. Expect some weakening of this activity as it spreads east into the Chicago area and for now have only included higher wind speeds/gusts in the rfd tempo but these may be needed at the rest of the terminals as winds will likely shift from the prevailing southwest to a northwest direction as the storms move through. Prevailing winds expected to remain north/northwest after this activity shifts to the east. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory...nearshore waters until 3 AM Monday. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at: