Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 18 2024 15:42:53 FOUS30 KWBC 181542 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1042 AM EST Mon Nov 18 2024 Day 1 Valid 16Z Mon Nov 18 2024 - 12Z Tue Nov 19 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHEAST LOUISIANA INTO SOUTHERN MISSISSIPPI... 16Z Update... Only changes of note for this update were to trim back the Marginal Risk area across the central Plains to account for where the rainfall has ended or is about to come to an end. Elsewhere, only minor adjustments were made to the Marginal and Slight Risk areas near the central Gulf Coast where the latest hires model guidance continues to support a heavy and locally excessive rainfall threat. Orrison Previous Discussion... ....Central Gulf Coast... An axis of much above average tropical PW values expected to be drawn rapidly northeastward from the remnants of Sara, across the=20 central Gulf of Mexico and into the Central Gulf coast ahead of the negatively tilted closed low ejecting northeastward from the=20 Southern Plains into the Upper Mississippi Valley day 1. These=20 anomalous tropical PW values, 3 to 4+ standard deviations above the mean will enhance convection along and ahead of the cold front=20 pushing eastward across the Gulf coast, primarily during the second half of day 1 from southeast Louisiana into southern Mississippi.=20 A slight risk area was maintained where the latest HREF=20 neighborhood probabilities are highest for 2 and 3"+ amounts. The=20 simulated hi res radars are in good agreement in showing potential=20 for a period of training of cells in this anomalous PW axis in the=20 0000-1200 UTC Tuesday time period across the slight risk area of=20 southeast LA into southern MS. In areas of training, hourly=20 rainfall amounts of 1-2"+ possible, resulting in flash flooding,=20 especially in the urbanized areas of New Orleans. ....Central Plains... Across the Southern to Central Plains toward the Upper MS Valley...there is fairly good agreement on an axis of heavy rain from northern OK, across much of KS, eastern NE, northwest MO, southeast SD, much of IA, southern MN into southwest WI in a region of strong isentropic lift ahead of the deepening surface low moving quickly northeastward across these areas. While widespread heavy amounts are likely, hourly rates will are mostly depicted as .25-50"+ in the latest hi res guidance. This should keep the threat of any runoff issues at marginal. The previous marginal risk area was trimmed on the eastern end, taking out much of IA, MO and eastern OK. The current marginal risk coincides well with the axis of the highest HREF 2"+ day 1 probabilities. Oravec Day 2 Valid 12Z Tue Nov 19 2024 - 12Z Wed Nov 20 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM SOUTHEAST LOUISIANA, ACROSS FAR SOUTHERN MISSISSIPPI, FAR SOUTHERN ALABAMA INTO THE WESTERN FLORIDA PANHANDLE... The convection that is expected to enhance along the Central Gulf coast late day 1, will continue into day 2 as the surface front continues to push east along the Gulf Coast. The tropical PW values entrained ahead of this front day 1, will continue to push across central to eastern Gulf coast day 2. Training of cells in a southwest to northeasterly direction likely to continue Tuesday morning from southeast LA into far southern MS, far southern AL and the western FL Panhandle. There is some timing differences as to whether heavy rains will continue in the vicinity of New Orleans early Tuesday, or have pressed just to the east. The New Orleans metro area was kept in the slight risk given potential for an overlap of day 1 and day 2 heavy rain areas. The latest HREF 12 hour neighborhood probabilities ending 00Z Wed show the highest probabilities for 2 and 3"+ amounts remaining across far southeast LA, far southern MS, far southern AL and the western FL Panhandle. This was the region depicted in a slight risk for day 2, with the previous risk area trimmed on the northern end across south central AL after collaboration with WFO MOB. The slight risk area corresponds with the hi res consensus for the greatest hourly totals of 1-2"+ in regions of training. ....Coastal Washington, Oregon and far northwest California... Models are in good agreement with the rapid cyclogenesis event across the northeast Pacific off the Pacific Northwest coast. The low level southerly flow along and ahead of the associated cold front will become very anomalous Tuesday night into early Wednesday, with 850-700 mb moisture flux anomalies of 3-4 standard deviations above the mean impacting the Pac NW/far northwest CA coast. A well defined area of frontal precip likely to push inland across coastal Pac NW/far northwest CA after 0000 UTC. Hourly rainfall rates in the .25-50"+ range possible, especially across southwest Oregon into far northwest CA where the strong southerly low level flow will have a better upslope component. For areas of northwest California into far southwest Oregon, this will be the beginning of a multi day Atmospheric River event. No changes made to the marginal risk area from the previous issuance. Oravec Day 3 Valid 12Z Wed Nov 20 2024 - 12Z Thu Nov 21 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS FAR Southwest OREGON INTO NORTHWEST CALIFORNIA... The multi day atmospheric river event the began late day 2 for far southwest Oregon into northwest California will continue day 3. The rapidly deepening day 2 low over the northeast Pacific will anchor a nearly stationary upper vortex off the Pac NW coast day 3. Strong persistent deep layered west southwest flow on the south side of this vortex will impact northwest California into far southwest Oregon day 3. with IVT values of 500-800 km/m-1 s-2. and 850-700 mb moisture flux anomalies of 2-4 standard deviations above the mean. There is fairly good model agreement with heavy precip totals of 3-5" primarily over northwest California, with isolated totals of 6-8" possible. No significant changes made to the previous slight risk area. Oravec Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4ABMDAC1NqfhtpEwAEoKsX1LnYmCIkCBifjaNBQMRn8V= 80dJWCP9h3Oh5MdNLAwe1IE5c-hukHtNSbnUETFb_jOwVgU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4ABMDAC1NqfhtpEwAEoKsX1LnYmCIkCBifjaNBQMRn8V= 80dJWCP9h3Oh5MdNLAwe1IE5c-hukHtNSbnUETFbIDGbfMQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4ABMDAC1NqfhtpEwAEoKsX1LnYmCIkCBifjaNBQMRn8V= 80dJWCP9h3Oh5MdNLAwe1IE5c-hukHtNSbnUETFbykEGcT8$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .