Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Apr 08 2023 20:07:35 FOUS30 KWBC 082007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 406 PM EDT Sat Apr 08 2023 Day 1 Valid 16Z Sat Apr 08 2023 - 12Z Sun Apr 09 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FROM THE CENTRAL GULF COAST TO THE CENTRAL CAROLINAS... 16Z update...With showers and thunderstorms diminishing this morning across the Mississippi Delta and parts of the central Gulf states the risk for excessive rainfall and flash flooding has greatly reduced, therefore the western bounds of the Marginal Risk area was removed from Louisiana, Mississippi and the western third of Alabama. Scattered showers will continue to progress eastward this afternoon and evening across the Southeast and the Carolinas bringing the potential for hourly rates above 0.50 inch/hour. The most sensitive areas remain to be areas with poor drainage and/or urban locations so kept the rest of the Marginal Risk area intact. Campbell The potential for excessive rainfall lingered across parts of the Gulf Coast this morning as an area of showers and thunderstorms continued to make its way east in association with a broad mid- and upper-level trough. It appears that the back-edge of the area of rainfall will still be lingering over portions of Mississippi at the beginning of the Day 1 period...so adjusted the position a bit westward from where it was depicted as yesterday's Day 2 Excessive Rainfall Outlook. Despite some reports of 1+ inch per hour rates still being reported during the overnight hours...much of the instability to support intense rainfall rates has been shunted off-shore ahead of a cold front. Considering that a surge of instability back into the region is not expected...opted to maintain the outlook area as a Marginal with the expectation that any flooding concerns would be confined to urban areas and regions of poor drainage. Focus then shifts into the Carolinas where latest numerical guidance was consistent with previous runs showing some 2 to 2.5 inch rainfall amounts in the Day 1 period.=20 Felt a Marginal was still adequate here given the prolonged period during which that amount of rain falls...implying that any flooding would be more urban and/or areal flooding in nature and less flash flooding. In addition...the 40 km neighborhood probabilities did not indicate even low probabilities for rainfall rates of 1 inch per hour or greater in the area of greatest QPF.=20 Latest guidance suggests most of the rain should be pushing off the Carolina coast line very late in the period. And the latest suite of CAM guidance still supported the idea that thunderstorms over northern Florida may produce rainfall rates at or even slightly greater than 0.5 inch per hour between 20Z and 01Z...so did not make any changes there. Bann Day 2 Valid 12Z Sun Apr 09 2023 - 12Z Mon Apr 10 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE PACIFIC NORTHWEST... 21Z... The latest guidance has trended the QPF higher over central portions of the Cascade Range with the onshore flow, with consensus showing 1 to 3 inches reaching further inland. In coordination with the local forecast offices the Marginal Risk area for excessive rainfall was expanded east to include parts of Lewis, Skamania, Clark and Cowlitz Counties. There may be isolated areas where flash flooding arises. Campbell After a relative lull on Saturday...another round of rain should be ramping up on Sunday as on-shore flow of deep-layered moisture develops ahead of a low pressure system over the Gulf of Alaska/eastern North Pacific ocean. Instability is modest...at best...but models still show onshore flow of 30 to 45 kts developing behind a cold front. The magnitude of the moisture transport results in rainfall amounts of 1.75 to 3.0 inches or so in the coastal ranges...most of that being focused in a 12 to 15 hour period. For this reason...feel there is a low-end risk of flooding focused in this area. Concern would be higher if there was more instability to support higher rainfall rates or a greater magnitude of moisture transport into the complex terrain. Bann Day 3 Valid 12Z Mon Apr 10 2023 - 12Z Tue Apr 11 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6IQSPYlWHiQ-ivusgYrDFkg1UoYl1iazAokBIlcQFjm_= 2iCbtzDZWWinBQlMZdVIioVg2gRQV7VQPVfBmTxT2afNLWA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6IQSPYlWHiQ-ivusgYrDFkg1UoYl1iazAokBIlcQFjm_= 2iCbtzDZWWinBQlMZdVIioVg2gRQV7VQPVfBmTxTkLEfRwo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6IQSPYlWHiQ-ivusgYrDFkg1UoYl1iazAokBIlcQFjm_= 2iCbtzDZWWinBQlMZdVIioVg2gRQV7VQPVfBmTxTGRJ_yck$=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) .