Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 AM EST Sat Nov 11 2023 Valid 12Z Tue Nov 14 2023 - 12Z Sat Nov 18 2023 ...Heavy rainfall potential for the central Gulf Coast into Tuesday... ...Moisture directed into California could lead to heavy rain and snow Tuesday-Thursday... ...Pattern Overview and Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite blend of best clustered guidance from the 18 UTC GFS/GEFS mean and 12 UTC ECMWF/ECMWF ensemble mean and to a lesser extent the 12 UTC Canadian, but also with significant input from the 01 UTC National Blend of Models (NBM) and WPC continuity for Tuesday into Wednesday. The 12 UTC UKMET offers more northern and southern stream system phasing into a eastern Pacific/West Coast upper trough position in this period compared to the aforementioned preferred blend. This seems a less likely scenario given the latest 00 UTC UKMET has significantly backed off from this scenario, trending more toward the other models. Building forecast spread increases through later next week, further lowering confidence with systems embedded within the establishing split flow pattern over southern Canada and the lower 48. However, recent runs of the ECMWF seem to offer a bit better run to run continuity and seem more in line than the other models with the 18 UTC GEFS/12 UTC ECMWF ensemble means and WPC continuity in both main flow streams. Accordingly, blended the 12 UTC ECMWF/18 UTC GEFS mean with the ECMWF ensemble mean and NBM through these longer time frames, albeit with emphasis on the NBM and ECMWF ensemble mean whose slightly less progressive/amplified southern stream seems more reasonable given flow separation. The 00 UTC ECMWF remains in line overall for much of the period and the 00 UTC GFS has trended strongly toward ECMWF ideas, bolstering forecast confidence to a degree. In contrast, recent runs of the Canadian show less than stellar run to run continuity. ...Weather/Hazards Highlights... The slow and steady progression of a leading/amplified southern stream upper trough and associated wavy surface front has increasingly shown a signal supportive of some eastern Pacific and enhanced Gulf of Mexico moisture inflow to fuel protracted rainfall potential from the central Gulf Coast states Tuesday through the Southeast/FL with added Atlantic inflow through Wednesday, exiting Thursday. The soaker may lead to some coastal/local and urban runoff issues given potential for several inches of frontal overrunning rains and a threat for some heavier local repeat/training duration amounts. Accordingly, plan to shift a WPC Day 4/5 Excessive Rainfall Outlook Marginal Risk area across these areas. Meanwhile upstream in the active southern stream branch of flow, recent guidance indicates that ample upper trough/closed low development over the eastern Pacific by early next week may prove slow to work until later next week into the West Coast/California in separated flow. However, latest progression trends and lead energies/moisture seem sufficient to slowly spread organized/periodic moderate to locally terrain enhancing rains, especially for the central California coast and inland spreading elevation/mountain snows, espcecially for the Sierra, into California Tuesday/Wednesday, eventually expanding inland over the Southwest later next week. In the northern stream, lead/amplified upper troughing is slated to increasingly shift from the Northeast to the northern Atlantic Tuesday onward as primarily a maritime low developer with an increasing offshore weather focus. High pressure will linger over much of the East in the wake of this system. Well upstream, upper trough and surface system energies are expected to work across the Pacific Northwest/Northwest through the northern Rockies early-mid next week then again late week to support modest to terrain enhanced moderate rains/snows across the region. Schichtel 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