Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 09 2023 08:31:44 FOUS30 KWBC 090831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Sun Apr 09 2023 Day 1 Valid 12Z Sun Apr 09 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... The latest guidance trended QPF upward once again over central portions of the Cascade Range with the onshore flow, with consensus showing 1 to 3 inches reaching farther inland and extending farther northward than before. Overall, the big picture remains the same...with yet another round of rain should be ramping up later today as on-shore flow of deep-layered moisture develops ahead of a low pressure system over the Gulf of Alaska/eastern North Pacific ocean. The magnitude of the moisture transport results in rainfall amounts of 1.75 to 3.0 inches or so in the coastal ranges and in the west aspect of the northern Cascades. Given the fairly high snow levels expected...pulled the Marginal Risk a northward. Overall, though, the forecast reasoning remained similar to previous issuances. ....Southern Florida Peninsula... Introduced a Marginal Risk area mainly for concerns about locally heavy rainfall occurring in the urbanized corridor that leads to isolated flooding or run-off problems. A cold front will continue to make its way southward across the Florida peninsula during the day...with low level winds becoming east to southeast in the wake of the frontal passage. Given precipitable water values that approach 2 inches...the localized QPF maxima from the NAM and GFS later in the afternoon do not seem unreasonable (although exactly where remains open for debate). Given the amount of uncertainty...felt no more than a Marginal was warranted. Bann 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 SOUTH FLORIDA... Pacific Northwest... 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. ....Southern Florida Peninsula... 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... 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!-GByRBzXI40TQgWu_Vp2ORhUyMtcR2Krr_a_8H6Q3Cq_= eHdubmPNaZ7tfsCGNzbBToPYV4_YcW6grfbjKhyWln-5V-E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-GByRBzXI40TQgWu_Vp2ORhUyMtcR2Krr_a_8H6Q3Cq_= eHdubmPNaZ7tfsCGNzbBToPYV4_YcW6grfbjKhyWZCNbeGw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-GByRBzXI40TQgWu_Vp2ORhUyMtcR2Krr_a_8H6Q3Cq_= eHdubmPNaZ7tfsCGNzbBToPYV4_YcW6grfbjKhyWoX9v9ZE$=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) .