Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sun Nov 12 2023 Valid 12Z Wed Nov 15 2023 - 12Z Sun Nov 19 2023 ...Heavy rainfall potential for the Southeast into Wednesday/Thursday... ...Moisture directed into California could lead to heavy rain and snow into Wednesday/Thursday... ...Pattern Overview and Guidance Evaluation/Predictability Assessment... The medium range period from midweek through next weekend features an upper trough moving through the Southeast, with heavy rainfall potential into the Southeast/Florida Wednesday into Thursday. This may lead to a potential western Atlantic low to lift northward off the East Coast heading into next weekend. Meanwhile, East Pacific troughing looks to split, with an amplified northern stream shortwave moving eastward through the full U.S. northern tier, from around the Northwest midweek to the Northeast by next weekend. Meanwhile, a dug/closed southern stream deep upper low looks to meander off the California coast until about Friday, eventually ejecting inland over the Southwest to possibly the southern Plains next weekend. Guidance from the 18 UTC GFS/GEFS mean and 12 UTC ECMWF/ECMWF ensemble mean seems reasonably well clustered overall with the main weather features and overall pattern evolution for much of the lower 48 and in line with WPC continuity overall, albeit with these models somewhat less progressive with the main southern stream features than these ensemble means. This seems more reasonable considering lead-in closed/amplified nature of the separated stream. In the northern stream these models have trended less progressive than continuity as well as the 12 UTC Canadian/UKMET and NAEFS mean that combines 12 UTC GEFS/Canadian ensembles. These timing differences seem most sensitive to the ultimate mid-later week upper ridge amplitude upstream from the Gulf of Alaska into western Canada that remains less certain than average. Accordingly, an overall composite blend of the GFS/GEFS, ECMWF/ECMWF ensembles and NAEFS seems to best capture favorable guidance trends in the southern stream and accounts in part for lingering timing uncertainties in northern stream flow. Guidance/WPC progs have trended in support of developing a Bahamas frontal wave later this week as supporting energies shift out from the Gulf of Mexico/Southeast, but subsequent guidance spread is ample. The 12 UTC ECMWF has the strongest low and most weekend interaction northward to off New England while others are less defined/more offshore, sensitive to northern stream timing/amplitude variance and any phasing potential. ...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 lower latitude/Gulf of Mexico and easterly fetch Atlantic moisture inflow to fuel protracted rainfall potential through the Southeast/FL into Wednesday/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, WPC Day 4/5 Excessive Rainfall Outlook Marginal Risk areas continue 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 late week/next weekend into the West Coast/California in separated flow. There has been some recent trends away from widespread impactful rainfall amounts, but it's likely that moisture should slowly spread organized and periodically moderate to locally terrain enhanced rains into the central to south-central California coast, and inland mountain snows for the Sierras Wednesday and Thursday. Rainfall amounts have increased a bit with this cycle and now do seem just enough to add WPC Excessive Rainfall Outlook Marginal risk areas to highlight for coastal terrain areas for both the Day 4/5 EROs. This activity should lessen but eventually expand inland over the Southwest into next weekend. 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