Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 21 2023 20:27:57 FOUS30 KWBC 212027 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 326 PM EST Tue Feb 21 2023 Day 1 Valid 16Z Tue Feb 21 2023 - 12Z Wed Feb 22 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Local radar imagery over Mississippi and Alabama this morning showed a couple of narrow axes of higher reflectivity, oriented west to east, correlating to MRMS-derived and at least one gauge confirmed (Wunderground) hourly rainfall total in excess of 1 inch. Cloud tops on infrared imagery and 12Z soundings from JAN and BMX indicated the showers were largely warm rain based with little lightning driven by shallow instability predominantly below 500 mb. Low level convergence near 925 mb was helping to focus these showers. Short term training could conceivably support 2 inches of rain in 1 to 2 hours, which given above average soil moisture across Mississippi and Alabama, could result in very localized flooding, including any overlap with any urban areas. This threat is too low to include a risk area on the outlook and the showers/localized thunderstorms are expected to translate eastward into Georgia (drier soils) by late afternoon or early evening, coinciding with weakening low level winds and convergence. Otto Day 2 Valid 12Z Wed Feb 22 2023 - 12Z Thu Feb 23 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEASTERN MISSOURI TO SOUTHWESTERN MICHIGAN... A closed upper level low over northwestern Mexico this afternoon is expected to weaken into an open wave tonight and eject northeastward across the southern/central Plains and Mississippi Valley as a negatively tilted trough axis Wednesday morning. At the surface, a Pacific cold front will advance eastward from Texas/Oklahoma into the middle Mississippi Valley through 18Z followed by weakening and slowing or stalling beyond 18Z. To the immediate north of the weakening Pacific cold front will lie a strong warm front that will lift north through the Ohio Valley during the day. Moisture return will be robust ahead of the weakening Pacific front which will be linked more to a dewpoint gradient than temperature. 850 mb winds are expected to peak in the 50 to 70 kt range via the 12Z model consensus from the west-central Gulf Coast into the Lower Mississippi Valley. Tuesday afternoon Layered PW Imagery showed a moisture connection to the tropical eastern Pacific located in the middle and upper levels of the atmosphere, ahead of the closed low over northwestern Mexico, extending into the Southern Plains. Precipitable water values are forecast to peak in the 1.3 to 1.5 inch range from Missouri into Illinois and Indiana on Wednesday, representing standardized anomalies of roughly 3 to 4+ for late February. Forecasts of instability from the 12Z NAM and ARW showed 500-1000 J/kg MUCAPE across Illinois and west of the Mississippi River which should support hourly rainfall of 1 to 1.5 inches. Weaker instability is expected east of Illinois. Heavy rain should be ongoing at the start of the period from eastern Kansas and Missouri, southwestward into northern Texas. Any flash flood threat across eastern Oklahoma into southwestern Missouri should be in the morning and fairly short-lived, prior to what will be a fairly steady eastward translation of the heavy rain threat, following the passage of the negatively tilted 500 mb shortwave trough axis. The greatest risk for excessive rainfall is aligned with the Slight Risk, extending from northeastern Missouri into north-central Illinois, northwestern Indiana and far southwestern Michigan. It is here that an expanding area of showers/thunderstorms will align with both the initiating frontal boundary and unidirectional southwesterly steering flow, supporting axes of training. To the north, from locations in the vicinity of southern Lake Michigan and western Lake Erie, 850-700 mb frontogenesis will aid in some banded west-east precipitation but southwesterly steering flow should limit the duration of any training. Despite a relative minimum in precipitation over many of the areas contained in the Marginal and Slight Risk, flash flood guidance values are fairly low at 1 to 2+ inches for 1 and 3 hours for most of the region outlooked on the Excessive Rainfall graphic. Within the Slight Risk, there will be a good chance of peak total rainfall of 2 to just over 3 inches with localized 1 to 2 inch totals across the remaining areas within the Marginal Risk. Otto Day 3 Valid 12Z Thu Feb 23 2023 - 12Z Fri Feb 24 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-9fwhblgxJVOotxmNMBLL1RLdpoVztkrQdEQPhT7GJc1= XLClMZ-Cr8s7-tOY-nE6RUhivnQxdPm8yhZAeYa69HpzlEM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-9fwhblgxJVOotxmNMBLL1RLdpoVztkrQdEQPhT7GJc1= XLClMZ-Cr8s7-tOY-nE6RUhivnQxdPm8yhZAeYa6pRMCa40$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-9fwhblgxJVOotxmNMBLL1RLdpoVztkrQdEQPhT7GJc1= XLClMZ-Cr8s7-tOY-nE6RUhivnQxdPm8yhZAeYa68Nbp6kg$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .