Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 16 2025 15:37:44 FOUS30 KWBC 161537 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1137 AM EDT Sun Mar 16 2025 Day 1 Valid 16Z Sun Mar 16 2025 - 12Z Mon Mar 17 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC, AS WELL AS FOR PORTIONS OF NORTHERN CALIFORNIA=20 AND SOUTHWEST OREGON... ....16Z Update... ....East Coast... Very few changes were needed to the inherited Marginal Risk area, while it's a low-end Marginal for the DC-Boston corridor, it remains a more substantial Marginal Risk area for the Carolinas and eastern Virginia. All of the instability remains over the Southeast and Midwest, with very little to none for most of the Mid-Atlantic and Northeast. Thus, the strongest storms will likely remain from the eastern Carolinas to eastern Virginia, with only elevated convection making it to New England. While this raises the flash flooding potential in the Southeast, this would conversely also=20 substantially cap rainfall rates into the Northeast. Urban=20 concerns and upslope into the terrain of southern New England and=20 eastern Pennsylvania should still bring the flash flooding threat=20 to Marginal levels, so no changes were needed there. ....West Coast... No changes were needed to the ongoing Marginal Risk area. A long fetch of persistent southwest to westerly flow will continue to pump plentiful Pacific moisture into the west coast, with rates exceeding a half inch per hour at times. Any resultant flooding would likely be larger-scale river flooding since the rain rates don't necessarily support flash flooding, but rapid rises in streams and creeks could locally lead to out-of-bank flooding. Wegman ....Previous Discussion... ....East Coast... A progressive cold front and negatively tilted longwave trough will traverse much of the East Coast today, as the upper-level pattern becomes a bit less amplified (shortwave over the Southeast cutting off from northern/polar stream with ridging building over central CONUS). Convection in association with the front looks to generally result in areal average totals of 1.0-1.5" (per latest WPC QPF), but the latest CAMs (00z HREF suite) suggests relatively high odds (30-50%+) for localized totals in excessive of 2" (per 40-km neighborhood probabilities). The inherited Marginal risk area was expanded a bit to generally encompass where 2" exceedance probs are 30%+ (but excluding FL and much of southeast GA, as FFGs are particularly high here). While this is a lower-end Marginal risk for most places, the risk is maximized over coastal NC where 3" exceedance probs are 40-60%. ....Southwest Oregon into Northern California... A relatively weak and narrow atmospheric river (AR) will continue to bring moderate to heavy rainfall to the OR/CA coastal border region with additional forecast localized totals today of 2-5". Most of this rainfall will be spread out over a 12-18 hour period with peak rates staying under 1"/hr due to a lack of instability (though 1"/hr rates may be reached for a brief period near the CA/OR border along the coast where the HREF indicates MU CAPE up to nearly 500 J/kg). Maintained the inherited Marginal risk, though a future targeted upgrade to Slight risk remains possible (depending on the evolution of rainfall this morning and any upticks in QPF in the 12z HREF suite). Churchill Day 2 Valid 12Z Mon Mar 17 2025 - 12Z Tue Mar 18 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Churchill Day 3 Valid 12Z Tue Mar 18 2025 - 12Z Wed Mar 19 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6DtrR0urkFQ5UU0BOKkjllczoiDXgR8kNUYIAdgh-LvF= p1ePTE6oydMidfGWXnCVbJRSOXimy8_Qckto8lTDxUdaatY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6DtrR0urkFQ5UU0BOKkjllczoiDXgR8kNUYIAdgh-LvF= p1ePTE6oydMidfGWXnCVbJRSOXimy8_Qckto8lTDi45FU64$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6DtrR0urkFQ5UU0BOKkjllczoiDXgR8kNUYIAdgh-LvF= p1ePTE6oydMidfGWXnCVbJRSOXimy8_Qckto8lTD135REkE$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .