460 FXUS63 KILX 211619 AFDILX Area Forecast Discussion National Weather Service Lincoln IL 1119 AM CDT Sat Aug 21 2021 .SYNOPSIS... Issued at 333 AM CDT Sat Aug 21 2021 Scattered showers and thunderstorms will occur into this evening as a cold front moves east through the area. Severe weather is not expected, though a few thunderstorms will produce gusty winds and locally heavy rains in the tropical air mass. Highs today and Sunday will be in the mid 80s. Mostly sunny and dry on Sunday and not as humid as today. Heat and humidity return for first half of the upcoming work week, with temperatures in the lower 90s and heat indicies topping over 100 degrees during the afternoon hours Monday through Wednesday. && .UPDATE... Issued at 1110 AM CDT Sat Aug 21 2021 Updated the forecast to adjust chances of showers and thunderstorms into this evening, extending slight pops further west longer today and keeping chances pops this afternoon and early evening along and east of Peoria to Taylorville line. Latest radar mosaic shows a developing band of showers with some heavier rains along a Lincoln to Taylorville to just east of Hillsboro line and tracking eastward. SPC precipitable water values show a very moist 1.8-2.2 inch PW from highway 51 west. KILX 12Z sounding has PW of 1.8 inches and trending upward. ILX will be doing another 18Z special sounding today for Hurricane Henri support on the East Coast. SPC shows 20-35 kt wind shear in eastern IL, highest in southeast IL, while ML CAPES were 500-1000 J/kg over much of CWA and higher in southern IL from Litchfield and Taylorville south. Surface analysis shows weak cold front pushing slowly east into west central IL nearing a Galesburg to Mount Sterling (Brown county) line at 11 am. Very moist dewpoints of 72-78F over much of central/se IL late this morning supporting the higher PW levels. Temps have risen into the lower 80s in eastern IL where its been dry so far today and a more peaks of sun east of I-57. Also getting some breaks in the low clouds over the lower IL river valley from Havana sw. Temps were in the mid to upper 70s from Bloomington to Taylorville west where more low clouds and isolated to scattered showers. Weak cold front to push into eastern IL early this evening and continue chances of showers and isolated thunderstorms over central and especially eastern CWA. Diurnally enhanced convection to diminish after sunset this evening in eastern IL. Highs this afternoon in the low to mid 80s in central IL and upper 80s east of I-57 where more sunshine into early afternoon. Dewpoints in the 70s much of the day to give heat indices peaking in the low to mid 90s again in central IL and some upper 90s over counties along the IN border. && .SHORT TERM...(Today and tonight) ISSUED AT 333 AM CDT Sat Aug 21 2021 An upper level trough is in the process of sweeping across the northern Plains, and a 992-mb surface low was located near Winnipeg at 2am/07z Saturday morning with its associated cold front extending southwestward through Minnesota into central Iowa. A broken line of convection developed along the cold front Friday afternoon and has progressed eastward overnight. The two main areas of convection that developed yesterday are both on track to miss the ILX CWA, with one decaying across northeastern Iowa and the other surging southeast as an MCS in central Missouri. However, elevated convection developed along the Mississippi River Valley in recent hours. It is unclear just how far east these storms will track before dissipating. 06z soundings from ILX and DVN revealed 800 J/kg of MUCAPE, which is sufficient to sustain storms. Most CAMs have a poor handle on this region of convection. The WRF-NSSL, which seems to capture that convection the best, dissipates the line by 12z. Infrared satellite imagery showed cloud tops warming slightly at 3am/08z, indicating less vigorous updrafts and perhaps the start of a weakening trend. The weak cold front will push into the CWA later this morning, and should be near I-55 by midday. Additional convective development is likely along the front near and east of I-55 this afternoon, but storms will be scattered in nature, so maintained PoPs 50% or less. The modeled instability ahead of the front has trended downward, with models now indicating 1000-2000 J/kg of surface- based CAPE in eastern IL this afternoon. An isolated strong storm remains possible, but with weak deep layer shear and less instability than previously anticipated, severe weather is unlikely. The ongoing convection complicates today's high temperature forecast, and guidance seems aggressively cool as it keeps dense cirrus overhead through the day. Given the uncertainties as to where and when convection dissipates/redevelops today, aired a few degrees warmer than guidance with highs in the mid-80s for most of the area. Models continue to trend slower with the progression of the front, now pushing it out of eastern Illinois at 1am/06z Sunday, bringing precip chances to an end at that time. High pressure will build over the upper Midwest overnight, resulting in northerly flow across central Illinois. Temperatures will range from the low 60s northwest of the IL River to the upper 60s south of I-70. .LONG TERM...(Sunday through Friday) ISSUED AT 333 AM CDT Sat Aug 21 2021 Sunday's forecast remains little changed from previous days, and overall a pleasant day is expected with mostly sunny skies. Temperatures will be a touch cooler, still in the mid-80s, while dewpoints several degrees cooler will provide some short-lived relief to the heat and humidity. As upper level ridging builds over the southern Plains, the first of many shortwave disturbances will sweep across the northern Plains Sunday afternoon. This system produces another round of storms over eastern Nebraska/Dakotas which could eventually reach the ILX CWA by Monday morning. Similar occurrences will result in periodic precip chances through the week. Persistent southwesterly flow will return heat and humidity to the area through at least the first half of the week, with high temperatures expected to be in the low 90s and heat indices between 100-105. With periodic thunderstorm activity progged to track towards the area at times early next week, and models propensity to struggle in capturing activity of this nature, it is worth noting that the high heat forecast is conditional upon mostly sunny skies materializing each day. && .AVIATION...(For the 12Z TAFS through 12Z Sunday morning) Issued at 603 AM CDT Sat Aug 21 2021 A decaying line of rain and showers is progressing eastward into the area. Upstream observations show cloud bases are MVFR in the ongoing precip activity. It seems likely this precip/MVFR cloud cover will reach PIA/SPI around the start of the period, but uncertain how much further east it tracks beyond there. Did not carry it all the way to CMI. Additional scattered storm development is expected this afternoon along a weak frontal boundary. Light winds of 6-10 knots will shift from southerly ahead of the front to northwesterly behind it. && .ILX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ UPDATE...07 SYNOPSIS...Erwin SHORT TERM...Erwin LONG TERM...Erwin AVIATION...Erwin