Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Thu Apr 11 2024 08:00:00 FOUS30 KWBC 110826 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 AM EDT Thu Apr 11 2024 Day 1 Valid 12Z Thu Apr 11 2024 - 12Z Fri Apr 12 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PART OF THE OHIO VALLEY EASTWARD INTO PORTIONS OF THE CENTRAL APPALACHIANS AND MID-ATLANTIC... A complex and broad mid-latitude cyclone will produce a broad shield of rainfall across the eastern United States...with several regions where rainfall rates/amounts will threaten (if not exceed) flash flood guidance. One area is over portions of the eastern Ohio Valley into parts of the central Appalachians and adjacent Mid- Atlantic region where the 11/00Z HREF shows in excess of 30 percent neighborhood probability of 1 inch per hour rainfall rate and even a 5 percent neighborhood probability for 2 inch per hour rates by late afternoon. Synoptically, there is strong moisture transport into the region and precipitable water values approaching 2 standardized anomalies greater than climatology for this time of year. Flash flood guidance was lowered in parts of the area recently...making the possibility of flash flood exceedance a possibility. Maintained the previously issued Marginal Risk area without too many changes given the sensitivity of urban areas. Main question was how much convection will be lingering in the Southeast U.S at the beginning of the Day 1 period. Models appear to be underdone with amounts and too fast here...so will maintain the risk area primarily for the first few hours of the morning. Bann Day 2 Valid 12Z Fri Apr 12 2024 - 12Z Sat Apr 13 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHERN NEW ENGLAND... Made only minor modifications to the previously-issued Slight and Marginal Risk area. There was a bit of an eastward expansion based on the latest WPC deterministic QPF and the NBM. But the northern and western periphery remained in place without any real changes where the risk of flooding was due to a combination of rainfall and snow melt. There has been some run to run variation but the overall idea of strong moisture transport of deep moisture and higher dewpoints at low levels from the Atlantic that gets directed normally to the terrain has remained fairly consistent. Thus saw little or no reason to make any meaningful removal from the Slight/Marginal Risk areas due to the low-end excessive rainfall threat across northern New England. Bann Day 3 Valid 12Z Sat Apr 13 2024 - 12Z Sun Apr 14 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG A PORTION OF THE CALIFORNIA COAST... Generally light to locally moderate rainfall should gradually be working its way southward from southwest Oregon into northern and central California by the time the Day 3 period begins on Saturday morning as low pressure over the eastern North Pacific eases southward. On-shore flow ahead of the low and associated upper level trough will support some one quarter- to isolated on half- inch per 6 hours along the coastal ranges. Thinking is that the on-shore flow will be shunted southward with time, but there could be an up-tick in rainfall rates as cold mid-level temperatures near the center of the upper low results due to steepening low level lapse rates at time of maximum daytime heating. Bann = = = --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .