Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/21/21

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
847 PM CDT Fri Aug 20 2021 .UPDATE... 847 PM CDT No significant changes to going forecast this evening. Did tweak overnight mins up just slightly in a few spots based on higher afternoon dew points, but otherwise going forecast is in good shape. Warm and somewhat muggy conditions can be expected overnight, as the forecast area remains in a region of weak southerly flow. Surface low pressure of 998 mb along the ND/MN border will lift northeast into Ontario through Saturday in association with a fairly deep negative-tilt short wave, while a trailing cold front currently across the mid-Missouri Valley moves slowly east. Height/pressure falls to our north will induce a 15-20 kt south-southwesterly 850 mb jet which will maintain warm/moist advection into the region. While some patchy haze/light fog is likely in a few spots, significant visibility reductions are less likely than the last few nights thanks to the stronger winds above the boundary layer and light south-southeast surface winds. Broken line of thunderstorms across MN/IA and KS are progged to move east overnight, weakening with time as they move toward the mid- level ridge axis to the east, through some weakening showers will likely reach the I-39 corridor by Saturday morning. Better thunderstorm potential looks to be late morning (west) into the afternoon hours as increasing low level moisture and approaching cold frontal boundary coincide with diurnal heating/destabilization. Ratzer && .SHORT TERM... 235 PM CDT Through Saturday Night... A quiet night ahead with emphasis on Saturday afternoon`s thunderstorm potential. Agitated cumulus this afternoon was unable to penetrate through a warming and already dry layer above 700 mb as noted on local aircraft soundings. A stray shower or two is still possible this afternoon along the confluence of the lake breeze or into northwest Indiana but that should be the extent to it. Convection is beginning to initiate over southeast South Dakota and should continue to increase in coverage across the Nebraska/Iowa border area into early evening. This is ahead of a cold front extending nearly due south from a deep 999 mb low in eastern North Dakota. Convection is likely to become organized enough to outpace the front late evening into overnight in Iowa, aided by a short wave impulse forecast to move eastward across southern Minnesota/northern Iowa. This convection will also outrun some of its better forcing and deep layer convergence for ascent, so should both slow and gradually weaken into the morning. The approaching primary upper wave and surface cold front will bring a renewed chance for showers and thunderstorms late Saturday morning into Saturday afternoon, although capping, convective inhibition (CIN), and a fairly dry atmosphere above 700 mb should keep the morning storm activity lower in coverage. Chances for showers and storms increase into the afternoon as the mid-level moisture increases, CAPE increases (>1,500 J/kg), and capping weakens. Storm coverage is expected to be scattered in nature, with chances for showers and storms throughout northern Illinois and northwest Indiana, mainly between noon and late afternoon. With the previously mentioned decent CAPE, high PWATS, weaker deep layer shear, a scenario for some storms developing strong downdrafts exists, with isolated gusts above 30 mph possible. The surface front moves through in the late afternoon / early evening, shifting any lingering showers into mainly northwest Indiana by mid-evening. A slight warming at 925/850 mb temperatures would indicate slightly higher high temperatures on Saturday, but increased cloud cover may balance out or even keep highs a bit lower than today. With continued southerly flow, expect dewpoints to increase into the mid 70s, creating muggy conditions and adding to instability. Outside of storms we can still see some gusty winds as we mix into stronger flow off the deck, with gusts of 20 to 25 mph possible in the late morning and early afternoon. Cloud cover will decrease Saturday overnight, while dewpoints will drop slightly behind the front into the 60s and even some 50s in the northwest. Low temperatures Sunday morning will drop into the 60s. MTF/BKL && .LONG TERM... 139 PM CDT Sunday through Friday... In the wake of the cold front responsible for scattered showers and storms Saturday afternoon, Sunday continues to look pleasant with "relatively" low humidity levels, plenty of sunshine, and highs in the low to mid 80s (upper 70s along the lakeshore). Forecast guidance has ever-so-slightly slowed on the timing of the frontal passage, which may help clouds overstay their welcome in northwest Indiana. However, Sunday afternoon looks great areawide. With healthy pressure rises of 3-5 mb/6 hours (highest across northern Lake Michigan) and a long northerly fetch, waves may become choppy enough for at least a Moderate swim risk along Lake Michigan beaches especially during the morning hours. So, it may be a good idea to keep an eye on the forecast if planning to head to the beach. The reprieve from hot and humid conditions will be short lived as an upper-level ridge builds across the central United States and a surface high pressure system across the southeast reestablishes unimpeded moist low-level flow from the Gulf of Mexico to the Great Lakes. Accordingly, muggy conditions will return Monday and last at least through the middle of next week. Forecast ensemble mean 850 mb temperatures of +20 to 21C will be supportive of daily highs around 90F with surface dew points in the low to mid 70s making it feel closer to 100F. Overnight lows will drop no further than forecast surface dew points -- generally in the lower 70s. Of course, building heat and humidity levels will also increase the chance for waves of gusty thunderstorms, each individually tied to subtle upper-level impulses as well as remnant mesoscale convective vortices from prior convective episodes. Accordingly, timing out when and where thunderstorm chances will be highest can be a futile effort at this range forcing a forecast with nearly continuous but low chances for precipitation in spite of the expectation for many hours to remain dry. Ensemble model guidance appears to favor Tuesday night into Wednesday as the time period of peak thunderstorm coverage in the Great Lakes, but really anytime Monday onward will carry a threat for thunder. Should thunderstorms move through some or all of our area during daylight hours, observed temperatures may end up much lower than advertised. Altogether, the pattern next week looks similar to last week and the message of increasing heat, humidity levels, and thunderstorm chances remains appropriate. Borchardt && .AVIATION... For the 00Z TAFs... Aviation Forecast Concerns: * Breezy south-southwest winds Saturday. * Scattered thunderstorms in the area Saturday afternoon. * Cold frontal passage Saturday evening with a wind shift to the northwest. Southerly winds late this afternoon were just strong enough to push a northwest-southeast oriented lake breeze boundary north of ORD and MDW. Some moderate TCU along the boundary produced a few isolated showers, though these have largely dissipated and diurnal cu should continue to fade with sunset this evening. Surface winds should settle to a light south-southeast this evening/overnight. While some haze remains across the region, not expecting a repeat of overnight fog development as we have seen the past few nights. Stronger south winds aloft should aid in mixing drier air into the top of the boundary layer and high clouds will likely increase later tonight, both of which should help limit fog development in addition to slightly stronger surface winds. To our west, a cold front was pushing east across the upper/mid Missouri Valley early this evening, with a broken line of thunderstorms within a relatively narrow pre-frontal moisture axis. Guidance continues to indicate that these will weaken as they move east of the Mississippi River Saturday morning and out run the frontal forcing and low level instability. Thunder chances diminish eastward by this time, with mainly an area of decaying showers expected to move into the KRFD area after sunrise. Low level dew points and instability will increase later in the day however as the front continues to move east into IL, and this should set the stage for at least scattered thunderstorm development by early-mid afternoon across the forecast area. Window of thunder potential may be relatively short-lived for KRFD as the front looks to pass by 23-00Z. Frontal approach and associated convergence and instability in the moist axis ahead of it will line up better with peak diurnal temps for the Chicago terminals however, with 19-24Z looking to be the time of highest TSRA threat for the metro terminals. Some differences, both in timing and coverage, exist in CAM guidance. Reasonably high confidence in some convection around the area though have maintained a VCSH/prob30 for thunder mention at this distance for uncertainty in coverage. While coverage may decrease, a few showers or isolated TS could linger into the evening prior to the passage of the cold front. Winds ahead of the front are expected to be from the south- southwest during the morning/early afternoon hours, with some gusts near 20 kts likely. Lower confidence in wind directions later in the afternoon/early evening in convection, though west- southwest is most likely outside of any immediate outflow. Surface cold front looks to pass KRFD around 23-00Z and KORD/KMDW 02-03Z which should produce a wind shift to the northwest. Ratzer && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at Follow us on Facebook, Twitter, and YouTube at: