Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Oct 18 2025 15:51:55 FOUS30 KWBC 181551 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1151 AM EDT Sat Oct 18 2025 Day 1 Valid 16Z Sat Oct 18 2025 - 12Z Sun Oct 19 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM FAR NORTHEASTERN OKLAHOMA AND NORTHWEST ARKANSAS UP THROUGH THE MID- MISSISSIPPI VALLEY... The current convective organization from the Red River of TX/OK up through MO lends credence to a continued scattered flash flood threat over the course of the afternoon and evening from eastern OK up into south-central IL and points in-between. Thunderstorm=20 genesis over a zone of modestly unstable air and anomalous moisture running between +2 to +3 deviations this morning will continue its progression to the east-northeast with a cold front progression trailing back into the Missouri Valley and the adjacent plains. A shield of moderate to locally heavy rains will allow for spots of 2-3" of rainfall in its wake with maxima closer to 4-5" most likely near the mid-Mississippi Valley around the St.Louis metro and surrounds. This has been a strengthening consensus within the CAMs this morning, aligning very well with the ML output via the AIFS and AIFS ensemble, as well as the First Guess Fields from the past series of runs. This is likely in part to the overlap of multiple waves of heavy precip in this area of the CONUS as a trailing wave will migrate northeast out of TX and impact areas of southern MO down through the Lower Mississippi Valley later in the period. The flow across MO has a bit more deep layer uniformity with a speed shear axis between 45-60kts analyzed across MO extending into IL. This will lead to a greater alignment of thunderstorms with embedded stronger cores due to the enhancement of the shear present.=20 The limiting factor into a greater risk is forward propagation speeds will be modest in nature with lower likelihood of back- building within the setup. The antecedent soil moisture also favoring parched grounds will lead to less of a threat of run off initially, unless approached by significant rates >2"/hr. This is why the urbanization factor will play a greater role in the threat as impervious surfaces already account for the full run off capabilities. 12z HREF neighborhood probs for >3" continue to run between 60-90% over a large area extending from south-central MO into south-central IL with the bullseye situated near and just south of St Louis. Lower, yet modest probabilities between 30-45% exist near the intersection of MO/AR/OK as well allowing for a minor expansion southwest of the SLGT risk inherited.=20 Across OK, a stronger than forecast 2nd wave is making headway through the state currently with trends indicating some 2-3" totals expected over eastern and southeast portions of the state. To account for a low-end threat of flash flooding, there was enough of a signal to expand the MRGL risk into these locations as well. The rest of the forecast into the Great Lakes was relatively unchanged outside some minor adjustments on the northeast flank of the MRGL into MI.=20 Kleebauer Day 2 Valid 12Z Sun Oct 19 2025 - 12Z Mon Oct 20 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE EASTERN GREAT LAKES INTO THE CENTRAL APPALACHIANS... The amplifying mid to upper level trof across the mid section of the nation day 1, will continue to accelerate eastward day 2, becoming increasingly negatively tilted as it moves through the OH Valley, Central Appalachians, Mid- Atlantic and eastern Lakes. The fast eastward movement will continue to be a limiting factor for very heavy rainfall amounts and any widespread runoff issues. While FFG values are much lower across the eastern Lakes into the Upper OH Valley and Central Appalachians than areas farther south and west during the day 1 period, the aforementioned progressive nature of the day 2 system should preclude most areas exceeding FFG values. Only some small changes made to the previous marginal risk area, aligning it more with the lower FFG values across northern PA, northern NJ and eastern NY State. Oravec Day 3 Valid 12Z Mon Oct 20 2025 - 12Z Tue Oct 21 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS NORTHERN NEW YORK STATE INTO WESTERN NEW ENGLAND... The negatively tilted upper trof moving into the Lower Lakes/Central Appalachians/Mid- Atlantic region late day 2 will continue to be very progressive as it swings northeastward into Southern New England day 3. While there will be an axis of above average PW values, 2+ standard deviations above the mean and anomalous 850-700 mb moisture flux anomalies, 2 to 4+ standard deviations above the mean accompanying this upper trof day 3, the expected overall progressive nature of the precipitation area should preclude any widespread runoff issues. There is model consensus for max precip areas of 1.5-2"+ across northern NY State, Vermont, western MA, central NH and western ME. The previous marginal risk area was decreased in size and oriented to fit the above mentioned areas where isolated runoff issues in the higher terrain may occur. Oravec Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4WBhb1ec_iymxqZPoqNlGzeTta5xPNJdZXm9E9NezIk-= -I-qQ845ftVJwlJ0QIpf7sLuuaZtb_FXOuHy-UblIekAPOE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4WBhb1ec_iymxqZPoqNlGzeTta5xPNJdZXm9E9NezIk-= -I-qQ845ftVJwlJ0QIpf7sLuuaZtb_FXOuHy-Ubl7gS-oEY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4WBhb1ec_iymxqZPoqNlGzeTta5xPNJdZXm9E9NezIk-= -I-qQ845ftVJwlJ0QIpf7sLuuaZtb_FXOuHy-UblcEDPGGA$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .