Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Thu Nov 30 2023 12:24:00 FOUS30 KWBC 301601 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Thu Nov 30 2023 Day 1 Valid 16Z Thu Nov 30 2023 - 12Z Fri Dec 01 2023 ....THERE IS A SLIGHT RISK AREA FOR PORTIONS OF THE CENTRAL GULF COAST... ....16z update... Synoptic setup, thermodynamic and moisture ingredients remain mostly the same from overnight issuance. Minor adjustments made to the Slight Risk area, but the rest of the outlook remains mostly untouched. HREF neighborhood probabilities of 24hr QPF exceeding 3" have steadily climbed from an initial 15% area inland over southwestern Louisiana yesterday to a more focused 35% area from the eastern Texas coast to the central Louisiana coast with the latest 12z issuance. This steady upward trend the guidance supports a higher end Slight Risk (at least 25% chance) of Excessive Rainfall leading to Flash Flooding over the aforementioned areas. Mitigating factors for a higher risk outlook include relatively dry soil moisture (ongoing exceptional drought) and some variance in the CAMs as to where exactly the different rounds of convection will setup and propagate. Kebede ....Previous discussion... A Slight Risk area was raised for portions of the Southeast Texas Coast to south-central Louisiana. The latest guidance has persisted with the focal point of convection and flash flood concerns across east Texas into the western half of Louisiana where instability maximum for the period will be co-located within the best ascent from our passing shortwave trough to the north and surface convergence within the confines of a propagating cold front. Hourly rates of 1 to 2.5+ inches/hour is projected to focus across this stretch of the coast and lifting north/northeast through the entirety of this period. Parts of this region have been drier of late and rain will be beneficial, at least initially. Rainfall efficiency ramps up by 00Z and with multiple hours with rates over 1.5/2 inches/hour could quickly reach or exceed local flash flood guidance leading to isolated/scattered areas with flooding concerns. The prospect for supercell generation across east Texas given the deep shear profiles and backing low-level wind field off the Gulf will yield some enhanced rainfall within the Storm Prediction Center's Enhanced/Slight severe storm risks respectively. Later in the period, convergent signature along the leading edge of an advancing cold front will act as a focal point for a line of heavy thunderstorms to move eastward across the Louisiana/Texas border into the central portion of Louisiana. Further north some shallow convection will develop across the Ozarks region however amounts will largely be in the 0.75-1.5" territory, and likely over an extended period of time. Isolated areas could develop flooding concerns therefore maintained the Marginal Risk for this part. Campbell/Kleebauer Day 2 Valid 12Z Fri Dec 01 2023 - 12Z Sat Dec 02 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL/EASTERN GULF COAST AND THE SOUTHEAST... The area of convection from day 1 will shift east along the Gulf Coast into western portions of the Southeast U.S. Convergence along an approaching cold front within the confines of a anomalously moist environment will generate locally heavy rainfall with flash flood concerns over any urbanized settings. The primary focus has been over southern Alabama into the Florida Panhandle where the instability axis will be greatest and convective regimes could produce 1-2 inches/hour rainfall with locally up to 3 inches/hour near the coastal areas. The latest guidance suggested a small shift in the QPF axis northward further into central Alabama which in turn caused an uptick of QPF for eastern Louisiana. A growing consensus of somewhere within the Marginal bounds could pick up 3-5 inches of rainfall over the course of the period. The Marginal Risk that was already in effect largely covers the area of concern. With the northward trend in the latest guidance and WPC forecast, the western bounds of the Marginal was expanded to include parts of southeast Louisiana and northward across eastern Alabama/western Georgia. Campbell/Kleebauer Day 3 Valid 12Z Sat Dec 02 2023 - 12Z Sun Dec 03 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL/EASTERN GULF COAST AND THE SOUTHEAST AND FOR THE OREGON AND NORTHWEST CALIFORNIA COAST... ....Gulf Coast and Southeast... The low pressure system will continue to eastward due this period with the warm front lifting through the Southeast and into the Mid-Atlantic region. Showers and thunderstorms will persist across this region as anomalously moist Gulf air continues to advect over this boundary. Additional 1 to 2+ inches forecast for the Southeast which will keep the risk for locally excessive rain/flash flooding elevated for this period. A Marginal Risk is in effect from southeast Louisiana to the Florida panhandle and north to South Carolina. ....Northwest coastline... A plume of deep Pacific moisture will stream onshore the Pacific Northwest with a series of systems tracking onshore. Rain will be nearly widespread across western Washington, Oregon and northwest California will amounts generally ranging from 1 to 4 inches. The best orographically enhanced rainfall is expected to focus along the Oregon coast and into far northwest corner of California. The inherited old day 4 Marginal Risk was expanded to span the entirety of the Oregon Coast and northwest California. Campbell $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .