Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Nov 26 2023 16:08:42 FOUS30 KWBC 261608 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1107 AM EST Sun Nov 26 2023 Day 1 Valid 16Z Sun Nov 26 2023 - 12Z Mon Nov 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS FAR NORTHEAST NEW JERSEY, FAR SOUTHWEST CONNECTICUT AND SOUTHEAST NEW YORK, INCLUDING NEW YORK CITY... 1600 UTC update Added a small marginal risk area across the urban areas just west and north of NYC, from far northeast NJ, far southeast NY State, including NYC, and far southwest Connecticut. The 12Z hi res runs all show increasing rainfall along the Northern Mid-Atlantic Coast this evening into Sunday night associated with surface low pressure deepening and moving northward just off the NJ coast and across Long Island into southern New England. The simulated hi res radars do show intensifying radar echoes in the 0300 to 0770 UTC time frame moving across the urban corridor from far northeast NJ into far southeast NY State, including NYC and far southwest CT in a region of strengthening boundary layer convergence along an inverted trof extending west northwestward from this low. Hourly amounts of .50-1"+ possible during this period, with isolated urban runoff issues the primary concern. HREF neighborhood probabilities for hourly amounts of 1"+ peak at 15-25% at 0500 UTC from far northeast NJ into NYC. .50"+ hourly rates are higher, 50-90%, in the 0400-0700 utc time period and across a broader region closer to the marginal risk area. Oravec Previous discussion As a shortwave trough swings through the base of a longwave trough becoming established along the East Coast today, it will become negatively tilted with a 150 kt jet streak at the trough base near 250 mb moving across the Mid-Atlantic states. A surface low is expected to deepen as it tracks through southeastern New England Sunday night at the same time that the left exit region of the Mid-Atlantic upper jet max sets up over coastal New England, supporting increased divergence aloft. While strong low level convergence following the surface low track will likely allow for a stripe of heavy rain from southeastern New England to the southern coast of Maine Sunday night, instability is expected to be negligible for inland locations which should limit peak rainfall rates to near 1 in/hr. While the more aggressive members of the 00Z HREF and the 06Z HRRR begin to approach the 2 inches in 3 hour FFG for southeastern New England, the heaviest rain should be only of a 2-3 hour duration for any given location with localized totals of 2+ inches possible for the event, limiting the threat for flash flooding. Given the recent lack of rain for this portion of the Northeast and coordinating with the affected WFOs, no risk area was introduced for southeastern New England. Otto Day 2 Valid 12Z Mon Nov 27 2023 - 12Z Tue Nov 28 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Locally heavy rain is expected to be ongoing within the Gulf of Maine and near the coast of New England at the start of the period (Monday morning). A deepening surface low is forecast to quickly track NNE from eastern MA at 12Z into eastern Maine through 18Z Monday with peak rainfall rates of 0.5 to near 1.0 in/hr to the immediate northwest of the surface low track. The low track and axis of heavy rain should be fast moving within a region lacking appreciable instability, limiting potential for higher rainfall totals along the coast. Therefore, while some of the more aggressive members of the 00Z HREF show 3-hr QPF near or locally in excess of 3-hr FFG for coastal portions of Maine, the probability of exceedance was considered too low for a risk area at this time. In addition, the speed of the surface low should limit maximum rainfall totals into the 1.5 to 2.5 inch range, not considered significant for flooding impacts given recent dry weather across the region and after coordination with the affected WFOs. However, newer model guidance for coastal Maine will have to be monitored for increases and possible changes to the Monday to Tuesday morning Excessive Rainfall Outlook. Otto Day 3 Valid 12Z Tue Nov 28 2023 - 12Z Wed Nov 29 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4mC1d3P5hu7GSrD8oK8aS4OOm49CsATRS6yAj3HLqn1K= TyBHs6m3ziM8MPfvRpYpCjsteJ_RqmHMbEaAaaqY9wjMqO0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4mC1d3P5hu7GSrD8oK8aS4OOm49CsATRS6yAj3HLqn1K= TyBHs6m3ziM8MPfvRpYpCjsteJ_RqmHMbEaAaaqYJimDBxE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4mC1d3P5hu7GSrD8oK8aS4OOm49CsATRS6yAj3HLqn1K= TyBHs6m3ziM8MPfvRpYpCjsteJ_RqmHMbEaAaaqYC4MJxRo$=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) .