Extended Forecast Discussion NWS Weather Prediction Center College Park MD 330 AM EDT Sun Mar 12 2023 Valid 12Z Wed Mar 15 2023 - 12Z Sun Mar 19 2023 ...Nor'easter wind/wave threat to exit Wednesday along with wrap-back New England snow... ...Lingering atmospheric river fueled precipitation threat to from southern California to the south-central Great Basin/Rockies Wednesday into Thursday... ...Emerging heavy rains to spread from the south-central to eastern U.S. Thursday-Saturday, with snow potential across the northern tier states... ...Overview... A powerful coastal storm/Nor'easter lingering off the New England coast will present an impactful wind/wave threat to linger into midweek along with remaining wrap-back New England snows. Meanwhile, an anomalously wet winter pattern will continue over the next few days for the West Coast. Lingering atmospheric river moisture/rain and snow melt bringing some risk from southern California into Arizona/Utah into midweek. There is also a heavy snow threats from the Sierra through the south-central Great Basin/Rockies Wednesday into Thursday with overall system translation. A fairly progressive but with some amplitude flow pattern downstream in the upper levels an deepening surface based low pressure/frontal systems could lead to emerging precipitation across the eastern Plains/Mississippi Valley states onward across the South then East later next week, with increasing snow/ice threats in the northern tier into the Upper Midwest/Great Lakes into the Northeast with low/system organization and Gulf moisture/instability return fueled enhanced rain and thunderstorms expanding to the South along/ahead of a wavy trailing cold front. ...Guidance/Predictability Assessment... Guidance starts off decently clustered with near average predictability with this active and stormy pattern Wednesday and Thursday, but model spread increases by late week. The ensemble means maintain overall structure, reflective of their respective parent model, but with less noise. Following a similar preference from previous cycles, the WPC forecast suite was comprised of a composite blend of best clustered guidance from the 18 UTC GFS, the 12 UTC ECMWF/UKMET/Canadian and the 01 UTC National Blend of Models Wednesday/Thursday. The composite was then transitioned to a best clustered blend of the GFS/ECMWF and GEFS/ECMWF ensemble means Friday and for next weekend amid growing uncertainty. Latest 00 UTC guidance remains in line. ...Weather/Hazards Highlights... A powerful coastal storm/Nor'easter lingering off the New England coast will present an impactful wind/wave threat to linger into midweek along with remaining wrap-back New England snows. A potent short range atmospheric River directed toward the West Coast and highest moisture anomalies and precipitation will shift into southern California and the south-central Great Basin/Rockies Wednesday into Thursday. The experimental WPC Day4 Excessive Rainfall Outlook (ERO) shows "Marginal Risk" areas from southern California to north-central Arizona and southern Utah terrain as lingering/anomalous atmospheric river moisture and southern stream upper trough energy works into the region. There are also terrain focusing heavy snow threats overtop for the south-central Great Basin/Rockies. Later downstream, there is a growing guidance signal that favorable cyclo/fronto genesis across the central U.S. and lead/wrapping moisture return to the eastern Plains and Mississippi Valley and onward across the South and East later week will fuel an emerging/expanding area of showers and thunderstorm driven rains. The Day5 ERO is set to show a "Marginal Risk" from the eastern southern Plains to the lower Mississippi Valley. A northern stream energy component may also produce Central Plains to Upper Midwest/Great Lakes to Northeast snows this period and latest guidance preferences would increase wrap-back snow potential as per the WPC Winter Weather Outlook (WWO) in a pattern with the stark digging of much below normal Canadian airmass into much of the lower 48 in the wake of the system. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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