Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Apr 10 2023 00:33:56 FOUS30 KWBC 100033 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 833 PM EDT Sun Apr 09 2023 Day 1 Valid 01Z Mon Apr 10 2023 - 12Z Mon Apr 10 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE PACIFIC NORTHWEST AND SOUTH FLORIDA... ....Pacific Northwest... Steady rainfall will continue through tonight across the region associated with a plume of deeper moisture oriented into the Oregon and Washington coasts. Rainfall amounts of 1 to locally 2 inches will be common, highest across the favored upslope regions of the Cascades and Oregon coastal ranges. 18Z HREF probabilities suggest some moderate threat of 0.5" hourly through 06Z then conditions begin to improve through 12Z. Until then, the persistence and magnitude of the moisture and associated rainfall suggests an isolated potential for flooding through the overnight. ....Southern Florida Peninsula... As a cold front slips southward along the Peninsula this evening, onshore northeast flow in its wake will help sustain the threat of isolated to scattered convection right along the eastern coasts of the Peninsula in the presence of slightly anomalous moisture. 18Z HREF and recent runs of the HRRR show potential for isolated but potentially intense convection, affecting possible urban areas. Ahead of the front, deeper moisture and instability exists which may provide a boost in the rain rates associated with any convection and possible flooding, particularly over the urban corridors. Taylor Day 2 Valid 12Z Mon Apr 10 2023 - 12Z Tue Apr 11 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE PACIFIC NORTHWEST AND FLORIDA... Pacific Northwest... 21Z update... Incrementally, QPF has continued with its an increased trend further inland across the Pacific Northwest and into the northern fringes of the Intermountain West. With snow levels remaining higher in some locations, and factoring in multiple days of recent/active precipitation, the risk for excessive rainfall and associated flooding concerns the Marginal Risk was upheld along with a few adjustments. The area along the West Coast was expanded further east across western Oregon and southwest Washington. The Marginal Risk area from northeast Washington to northwest Montana was broadened in an east-west fashion to account for the new QPF and additional snowmelt. Campbell Little change in the large scale picture...although persistent on-shore flow of moisture has had a chance to extend farther inland. Maintained the low-end risk of excessive rainfall along the coastal ranges and Cascades where an additional 1 to 2 inches of liquid precipitation are possible as the moisture plume makes its way southward during the period. Introduced a second Marginal Risk from northeastern Washington into northern Idaho where precipitation...mainly in the form of rain due to fairly high snow levels...occurs on Monday. An added concern is that snow water equivalent is expected to drop more than an inch during the same 24 hour period...although the greatest snow water equivalent is forecast in northeast Washington and not in the northern Idaho panhandle where the QPF is maximized. Felt a Marginal was sufficient for the time being. ....Florida Peninsula... 21Z update... With a slowed frontal boundary near South Florida, northeasterly flow of 20 to 30 knots will transport PW values of 1 to 1.5 inches onshore the eastern side of the Florida peninsula. This will support showers and thunderstorms capable of producing periods of higher intensity rainfall, especially some of the same areas that have moderate to heavy rainfall on D1 which will keep an elevated risk for isolated areas of flash flooding/flash ponding. The Marginal Risk area over southeast Florida was expanded a bit north/south, as well as, westward in Miami-Dade County. Additionally, convection is expected to persist from day 1 and continue to focus higher amounts over portions of the Nassau, Duval, Clay, St John, Putnam, Flagler and Volusia Counties. A Small Marginal Risk area was raised for this part of the coast where additional 1 to 3 inches will be possible. Campbell Continued the Marginal Risk area mainly for concerns about locally heavy rainfall occurring in the urbanized corridor as northeasterly low level flow continues across the same portion of the Florida peninsula and keeps maintain a risk of the showers and thunderstorms. Given the amount of uncertainty...felt no more than a Marginal was warranted. Bann Day 3 Valid 12Z Tue Apr 11 2023 - 12Z Wed Apr 12 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG AND NEAR THE CENTRAL GULF COAST REGION... 21Z update... The uncertainty with QPF placement persists with the latest guidance, however there is some semblance of consensus across portions of southeast Louisiana. The latest WPC forecast has areal averages up to 2.75 inches for the southern most Parishes. The Marginal Risk area that was inherited continues to encompass much of the varied QPF placement and where there may be an increased threat for isolated flooding concerns, therefore no adjustments were made with this update. Campbell There is much model uncertainty regarding the potential for heavy rainfall from Tuesday into the pre-dawn hours on Wednesday across the states which border the northern Gulf of Mexico...with the main problem involving the amount of surface/low-level reflection there will be underneath a mid- and upper-level cyclone. At 500 mb at 250 mb...shortwave energy starts to drop southward across the Southern Plains on Monday evening in response to ridging from Arizona to South Dakota. This results in the formation of a positively tilted trough over the Gulf Coast states within which an upper level cyclone forms on Tuesday...with the 250 mb closed low being similarly located by the NAM/GFS over eastern Texas while the ECMWF low was positioned over northwest Louisiana at 12/00Z. The GFS surface reflection in the form of an inverted trough is notably stronger than the other models...which results in a wetter solution. A spaghetti plot of 3 inch rainfall contours from the 09/00Z GEFS shows the potential exists from just east of Houston eastward to just south of Lake Pontchartrain while the 09/00Z SREF had just 2 NMM core members with 1 inch of rainfall that barely reached the Louisiana coastline at the same time (a decrease from the 08/21Z run). Given the genesis of the upper system being over land...do not want to totally dismiss the GFS idea but confidence in its solution is reserved at best.=20 Considering the recent soaking to excessive rainfall over portions of Louisiana within the past couple of days...this system has the potential for impacts if the wetter solutions verify. Given the probabilistic nature of the ERO, will issue a Marginal Risk area despite some of the deterministic guidance...including the WPC QPF...that would typically not warrant one. It is hoped that additional runs bring clarity to the low level response and any associated rainfall hazard. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7hQ2bcqmFLxqlQ9OHhRqiylt-8z3MucYqqb61lotjarT= 9biJy885zqPppNH2ty4RXhhWtorhhTK3yRKTDcdxxOl90nI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7hQ2bcqmFLxqlQ9OHhRqiylt-8z3MucYqqb61lotjarT= 9biJy885zqPppNH2ty4RXhhWtorhhTK3yRKTDcdxCyEpS8I$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7hQ2bcqmFLxqlQ9OHhRqiylt-8z3MucYqqb61lotjarT= 9biJy885zqPppNH2ty4RXhhWtorhhTK3yRKTDcdxgbxpb0c$=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) .