Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EST Fri Mar 10 2023 Valid 12Z Mon Mar 13 2023 - 12Z Fri Mar 17 2023 ...Atmospheric river fueled excessive rainfall/flooding threat for the West Coast, particularly California, through early to mid next week along with heavy snows 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 deepening coastal storm/Nor'easter set to develop off the Mid-Atlantic to New England coasts will likely present a impactful wind/wave threat early-mid next week along with Great Lakes through 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 likely causing continued flooding concerns. The timeframe around Monday-Tuesday looks to see the highest chances in the medium range period for precipitation causing flooding. There is also a concurrent and prolonged 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 and vicinity into mid-later next week, with some snow/ice in the northern tier and Gulf moisture/instability return enhanced rain and thunderstorms to the South. ...Guidance/Predictability Assessment... A general model blend consisting of the 00z ECMWF/UKMET/CMC and 06z GFS were utilized on days 3 and 4 with more weighting going toward the EC after day 3. This was to account for a slowing trend that began in the 00z run of the GFS with respect to the EPac trough approaching the West Coast on day 4. 00z ECE and 06z GEFS were introduced to the blend on day 5 to mitigate outlier split flow pattern projected by the 06z GFS in the West-Central CONUS and slow trough axis in the West. Reasonable consensus with upper low moving through East so gmb is useful there. The southern stream trough in the GFS begins to align with the amplified EC trough over the Four Corners region on day 6. The ensembles are in reasonable agreement through day 7 which supported a heavier weighting toward them. ...Weather/Hazards Highlights... An active pattern across the West Coast will persist from the short range period into the medium range period. Model guidance indicates a relative break over the weekend in the heaviest totals. The pattern likely rejuvenates through early-mid next week as another atmospheric river looks to focus into the West Coast, despite some continued model variations in the timing and placement of heaviest rain. Highest moisture anomalies and precipitation forecast is expected to steadily shift from southwest Oregon through southern California mainly Monday and Tuesday. This is leading to "Slight Risk" issuance across the region with embedded "Moderate Risk" areas along the central coast and Sierra foothills in the WPC experimental Day 4/5 Excessive Rainfall Outlook (ERO). Impacts of this precipitation event could be dependent on what issues and hazards evolve in the short range period, but could include more flooding. Rounds of precipitation including heavy terrain focusing snows could impact the Interior West to Rockies with multi-faceted system inland translation all next week. Later downstream, there may be some moisture/instability return to the eastern Plains and Mississippi Valley mid-later next week, but this remains uncertain. Uncertain northern stream energy component may also produce modest Upper Midwest snows. Meanwhile, there is a growing guidance signal that as low pressure systems likely consolidate into a deepening coastal storm/Nor'easter Monday into Tuesday, enhanced snow may spread into parts of the Appalachians/northern Mid-Atlantic/Northeast. Confidence is increasing that portions of those areas should see enhanced snow, but amounts and type of the precipitation will be highly dependent on the exact low track, so the details will continue to be refined with time. There is also seems to be a risk for enhanced winds/waves along with a "marginal" Northeast coastal rain/flooding threat given system potential intensity. Expect cold flow in the wake of the system may also yield some lake effect snows, with windy flow lingering into Wednesday/Thursday. Kebede/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