Extended Forecast Discussion NWS Weather Prediction Center College Park MD 320 PM EST Sun Nov 12 2023 Valid 12Z Wed Nov 15 2023 - 12Z Sun Nov 19 2023 ...Heavy rainfall potential for parts of the northern/northeastern Gulf Coast Wednesday... ...Pattern Overview and Guidance Evaluation/Predictability Assessment... Upper pattern across the Lower 48 will feature a slowly-moving upper low off the coast of California that gradually works inland next weekend, outpaced by northern stream upper ridging moving into southwestern Canada. In the east, a trough in the Southeast will move eastward with its surface reflection carrying a low pressure center across Florida. This system may merge/interact with a frontal boundary moving through the Great Lakes to form a deeper area of low pressure near or just offshore the Mid-Atlantic/Northeast next weekend. Much of the rest of the middle of the country will be dry with mild temperatures for mid-November. The guidance has largely trended slower with the approach of the lumbering upper low off CA, with the deterministic models leading the way of the ensemble means. With a steady trend slower in the multi-day ensemble trends, went fully toward the slower solutions via a blend of the 00/06Z GFS and 00Z ECMWF/UKMET for much of the forecast. By next weekend, the agreement was actually better as the upper low opens up into a trough progressing across the Great Basin/Four Corners/southern Rockies. In the east, multi-model blend sufficed with the handling of the Gulf low, but spread increased rather smartly by later in the period as the two systems interacted along/off the East Coast. For now, ensembles keep such interaction along/east of 70W which would limit impacts to parts of New England. ...Weather/Hazards Highlights... The slower and drier trend in the guidance over California allowed for the removal of the Marginal Risk outline for day 4, but kept the day 5 area focused on parts of the Transverse Ranges north of Los Angeles where southerly flow into the terrain may enhance rainfall. Over the northern Gulf of Mexico, focused the day 4 (Wed) heavy rain threat around the FL Panhandle where favorable convergence may support heavier rain rates to the north of the low. Though rainfall is likely over much of the Sunshine State, the evolution of the system may translate the bulk of the heavier rain east of the Peninsula by Thursday. However, kept the Marginal Risk area over eastern FL for day 5 given easterly flow and a slow relaxation of well above normal precipitable water over the region until the system passes. For the West/Southwest, the slow approach of the upper low will only gradually bring in bouts of light to perhaps moderate rain and mountain snow that expand eastward in time. The delay in the height falls likely lessens the stronger moisture flux into the area as the system fills before moving ashore, but most areas will see at least measurable rainfall (especially coastal areas) outside some of the shadowed deserts. As the upper system finally crosses the Divide, some return flow out of the western Gulf will help increase moisture over the Plains with rain expanding in coverage late next weekend. At the same time, another Pacific system, but in the northern stream, will move into the Pac NW with some rain and mountain snow. Over the Southeast and East Coast, the approaching cold front from the north and Gulf low will spread rain along the I-95 corridor later this week into the start of the weekend, but the extent/amounts are still fairly uncertain and dependent on the track of the system and how far offshore it may be. Temperatures will largely be near to above normal for most of the CONUS, especially over the Upper Midwest where ridging will increase by next weekend amid dry conditions. Near to slightly below temperatures are expected for the Southeast and parts of the Southwest due to cloudier and wetter than normal conditions. Fracasso Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw