Extended Forecast Discussion NWS Weather Prediction Center College Park MD 147 AM EST Sat Mar 11 2023 Valid 12Z Tue Mar 14 2023 - 12Z Sat Mar 18 2023 ...Atmospheric river fueled excessive rainfall/flooding threat for the West Coast, particularly California, through early to mid next week along with heavy snow threats from the Cascades and Sierra inland across the Intermountain West/Rockies to extend into late next week... ...Nor'easter wind/wave threat early-mid next week along with Great Lakes through Northeast heavy snows and coastal rain/flooding threats... ...Overview... A potent coastal storm/Nor'easter set to track off the New England coast will likely present a impactful wind/wave threat early-mid next week along Northeast heavy snows and maritime to coastal rain/flooding threats. Meanwhile, the anomalously wet winter pattern will continue for the West Coast through early-mid next week with a series of frontal systems focusing deep layered and long fetch moisture plumes/atmospheric rivers and to cause continued flooding concerns. The timeframe around Monday-Tuesday looks to see the highest chances heading into the medium range period for precipitation causing flooding, with lingering atmospheric river moisture and snow melt bringing some risk from southern California into Arizona/Utah into midweek. There are also concurrent heavy snow threats from the Cascades and Sierra inland across the Intermountain West/Rockies to extend into late next week with overall system translation. A fairly progressive, but embedded system uncertain, pattern downstream in the upper levels with surface based low pressure/frontal systems could lead to emerging precipitation across the eastern Plains/Mississippi Valley states onward across the South into mid-later next week, with increasing snow/ice threats in the northern tier into the Upper Midwest/Great Lakes 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... A composite blend of best clustered guidance from the 18 UTC GFS and 12 UTC ECMWF/UKMET along with the 01 UTC National Blend of Models and WPC continuity seems to offer a good forecast basis for Tuesday into Wednesday across a stormy nation in a pattern with near average predictability. Forecast spread steadily increases mid-later next week, mainly with varied system evolutions from the Pacific and Canada onwar into/across the lower 48, but recent model differences have decreased some over the past day worth of runs. Opted to blend the GEFS/ECMWF ensemble means for this uncertain period to mitigate some of the variance. WPC product continuity was best maintained with greater blend emphasis on the ECMWF ensemble mean. ...Weather/Hazards Highlights... The anomalously wet pattern will renew through early-mid next week as another atmospheric river looks to focus into the West Coast. Highest moisture anomalies and precipitation is expected to steadily shift southward through California Tuesday. This is leading to a continuance of a "Slight Risk" area across the region with embedded "Moderate Risk" areas along the central coast and Sierra foothills in the WPC experimental Day 4 Excessive Rainfall Outlook (ERO). Substantial impact focus of this precipitation event could be dependent on what issues and hazards evolve in the short range period, but include more flooding. The WPC Day 5 ERO is planned to show "Marginal Risk" areas from north-central Arizona into southern Utah terrain as lingering/anomalous atmospheric river moisture and southern stream upper trough energy works into the region by midweek. Additionally, rounds of precipitation including heavy terrain focusing snows will impact the Sierra/Interior West to Rockies with multi-faceted system inland translation next week. Later downstream, there may be some moisture/instability return to the eastern Plains and Mississippi Valley and onward across the South mid-later next week, but this remains less certain. Uncertain northern stream energy component may also produce Upper Midwest/Great Lakes snows and latest guidance preferences would increase wrap-back snow potential. Meanwhile, there is a still growing guidance signal that as low pressure systems likely consolidate into a deepening coastal storm/Nor'easter into Tuesday/Wednesday off New England, heavy snow will spread across the Northeast. There is also a risk for enhanced winds/waves along with an early period ERO "Marginal Risk" for southeastern New England coastal rain/flooding threat given system potential intensity and proximity. Cold flow in the wake of the system will also yield some lake effect snows, with windy flow lingering into Wednesday/Thursday. 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