Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Thu Jun 29 2023 15:51:00 FOUS30 KWBC 292031 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 PM EDT Thu Jun 29 2023 Day 1 Valid 16Z Thu Jun 29 2023 - 12Z Fri Jun 30 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL HIGH PLAINS AND THE LOWER OHIO VALLEY... 16z update An MCS currently propagating across southern Iowa is expected to continue focusing around a warm front that exentds from southern Iowa down through Illinois, and western Kentucky/Indiana this afternoon/evening. A line of storms currently diving south through western Kentucky as well as developing backing convection on the backside over southwestern Indiana and southeastern Illinois will prime soils for repeated rounds of heavy rainfall/flash flooding from the Iowa cluster later on. The Front Range slight risk area was trimmed a bit on the western edge to account for initiation farther east and potential overperformance across the southern periphery. Minor adjustments were made to other areas. Kebede ....Northeast... One more day of scattered showers and storms, with locally heavy amounts possible, is expected for portions of northern New England as a slow moving upper trough finally exits to the northeast. While the threat will likely be on the downward trend as forcing weakens and drier air enters the region, additional localized rainfall of 2 inches or more is still possible. The HREF continues to highlight the White Mountains of New Hampshire to northern Maine as having the greatest threat for additional heavy accumulations. ....Florida... An axis of 850mb convergence off the southeast coast of Florida, combined with upper level shortwave activity entering from the north will combine with sea breeze activity to produce potentially heavy rain over the urban corridor of southeast Florida. PWs near 2 inches will provide enough moisture for the slow-moving storms to possibly produce up to 3 in/hr rainfall rates, but storm organization should become outflow dominant rather quickly and may contain cell mergers over southern Florida. This leads to the potential for over 3 inches of rain along the sensitive urban areas along the southern half of the Gold Coast, prompting the Marginal Risk. ....Northern Rockies and Central Plains... A slow-moving trough moving across the Intermountain West into the Rockies is expected to encourage additional showers and storms across portions of northern and central Rockies into the High Plains later today. Pooling moisture along a lingering boundary draped along the central High Plains, interacting with larger scale ascent afforded by the approaching trough, is expected to raise the potential for showers and storms capable of producing heavy rainfall. A Slight Risk was added to portions of southeastern Wyoming, northeastern Colorado, and the Nebraska Panhandle. HREF neighborhood probabilities indicate accumulations of 1-2 inches are likely across this area. This includes areas that are currently being impacted with heavy amounts and where flash flood guidance is already low. As storms develop, they are likely to be slow-moving at first, increasing the threat for heavy accumulations. Then as cold pools form, storms are expected to propagate more steadily to the east, moving into less prone areas, including the Nebraska Sandhills. ....Mid-Mississippi to Lower Ohio Valley... Convection developing overnight is likely to continue through the morning, with many of the 00Z CAMs showing convection beginning to blossom early in the period along a low level warm front extending from Iowa into western Kentucky. Strong westerly inflow will support PWs surging to at or above 2 inches along the boundary. A series of shortwaves moving over the top of the upper ridge will encourage redeveloping storms and heavy accumulations. While confidence that heavy amounts will occur has increased overnight, confidence in the details is limited by the typical model spread. The introduced Slight Risk highlights an area from southeastern Illinois to western Kentucky, where the HREF indicates that accumulations of 2-3 inches are likely, with much of that expected to occur within the first 6-hours of the period. Pereira/Snell Day 2 Valid 12Z Fri Jun 30 2023 - 12Z Sat Jul 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE CENTRAL ROCKIES TO THE CENTRAL/SOUTHERN APPALACHIANS AND ACROSS PORTIONS OF THE NORTHEAST... 2030z update Very few changes were made to the current day 2 ERO. The marginal risk area appears to capture the scattered convection that will occur reasonably well with some increased potential for higher amounts over the North Carolina piedmont. PWs between 1.75 and 2 inches, coupled with CAPE over 1500J/Kg and saturated soils in some places could support a targeted slight risk area over portions of the Middle Mississippi Valley. Kebede ....Central High Plains to the Appalachians/Northeast... Mid-level energy will continue to move across the top of the ridge centered over the lower Mississippi Valley and the Southeast, likely to help initiate additional rounds of showers and storms from the Ohio Valley eastward as they interact with an axis of deeper moisture. Meanwhile, a more defined upper trough moving out of the West is likely to generate additional storms back to the west from the foothills of the central Rockies into the Plains. Given the typical uncertainty and anticipated fast flow aloft, a broad Marginal Risk was maintained for now. However, embedded upgrades to a Slight Risk may be forthcoming, especially if the models begin to agree on successive days of heavy rain across the central Plains or Midwest. A Marginal Risk was maintained to highlight an increasing threat for heavy rains to spread back into portions of the Northeast that were recently impacted by heavy amounts. Mid-level energy and deeper moisture will be directed northeast into the region ahead of a low centered north of the Great Lakes and usher in renewed PWs near 1.75 inches. Although confidence in the details is limited, there is at least some potential for moderate to heavy amounts to begin to develop across some parts of Pennsylvania and New York that were recently impacted with heavy amounts and where FFGs remain relatively low. Pereira/Snell $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .