Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 17 2023 15:54:28 FOUS30 KWBC 171554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1153 AM EDT Fri Mar 17 2023 Day 1 Valid 12Z Fri Mar 17 2023 - 12Z Sat Mar 18 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE LOWER MISSISSIPPI VALLEY/CENTRAL GULF COAST, TOWARD THE SOUTHERN APPALACHIANS... ....16Z Update... Very little has changed in the forecast with this morning's update. The squall line draped from southern LA to north GA is likely to consolidate and weaken over the next couple hours, with potential for a flare-up of convection on the eastern FL Panhandle this evening. In both cases, the line is progressive and rainfall totals have broadly hovered in the 1-2 inch range. A few notable bullseyes of over 3 inches of rain were noted from far east TX and south central LA overnight last night, and it's these kinds of local maxima of rain where the isolated flash flooding threat is greatest as the line continues east today. The northern and western end of the Marginal Risk area was trimmed to follow the line of convection. Wegman ....Previous Discussion... The strong cold front pushing south through the Southern Plains early Friday morning will continue to surge quickly southeastward through the Lower Mississippi Valley and into the north central Gulf of Mexico during the day on Friday and across the Florida Panhandle by Saturday morning. Showers should remain active along and ahead of this front where PW values will remain above average, 1.5 to 2 standard deviations above the mean, boundary layer convergence will be strong and upper difluence well defined. Similar to past model runs, the expected fast movement of the front and any associated convection will be a detriment to widespread runoff issues. The broad marginal risk area was drawn to fit where HREF neighborhood probabilities for 1"+ amounts are high during day 1. HREF probabilities for totals greater than 2" decrease significantly, indicative of the expected fast movement of the precip area. Stream flows across the marginal risk area are at or below normal levels. This and the expected fast movement of the front and associated precip warrants keeping the risk level at marginal. Oravec Day 2 Valid 12Z Sat Mar 18 2023 - 12Z Sun Mar 19 2023 ....THERE IS A MARGINAL RISK OF RAINFALL ACROSS NORTHERN FLORIDA... Model consensus is for a narrow axis of moderate to locally heavy rains early in the upcoming day 2 period across northern Florida along and ahead of the strong cold front that will be moving southeast from northern Florida at the beginning of the day 2 period. Similar to the day 1 period, the cold front is expected to be fairly quick moving, which should limit the time that heavy rainfall rates may occur. The heavy rainfall rates will likely occur at the beginning of the day 2 period in a narrow axis of instability along and ahead of this front. HREF neighborhood probabilities during the first half of day 2 are high for 1"+ amounts across northern Florida, 50-75%, and 20-50% for 2"+ amounts. The previous marginal risk area for this period was narrowed to better match the model qpf consensus axis, which does overlap where above average precip values have occurred over the past week or so. While soils are generally sandy in this area, the heavy rain potential area will also overlap more urbanized areas of northern Florida, where some urban runoff issues are possible.=20 Oravec Day 3 Valid 12Z Sun Mar 19 2023 - 12Z Mon Mar 20 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8BweIirTWju-36WPsWZCE0RMNfdRqftrppA1ewb4Sb-_= -KCMmdImP71LLibkqeedV_NzHw832-gc6Fnnbca8RIgXlJ4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8BweIirTWju-36WPsWZCE0RMNfdRqftrppA1ewb4Sb-_= -KCMmdImP71LLibkqeedV_NzHw832-gc6Fnnbca8a0S6vBc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8BweIirTWju-36WPsWZCE0RMNfdRqftrppA1ewb4Sb-_= -KCMmdImP71LLibkqeedV_NzHw832-gc6Fnnbca8DpWmveY$=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) .