Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM 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 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 for at least another day in the West before a brief reprieve. A final wave of atmospheric river moisture/rain and snow melt will bring some flood risk to southern California into Arizona/Utah midweek. Snow is also expected from the Sierra through the south-central Great Basin/Rockies Wednesday into Thursday with overall system translation. Precipitation chances increase again by the end of the weekend. A fairly progressive but with some amplitude flow pattern downstream in the upper levels and deepening surface based low pressure/frontal systems will lead to increased precipitation chances across the eastern Plains/Mississippi Valley states onward across the South and East Coast later next week, with increasing snow/ice threats in the northern tier into the Upper Midwest/Great Lakes into the Northeast along the cold side of the low pressure system/storm track. ...Guidance/Predictability Assessment... The medium-range period begins on Wednesday (the 15th) with an overall low-amplitude but energetic upper-level pattern with an upper-level low over the Northeast and additional shortwaves crossing over the CONUS. The deterministic and ensemble guidance agree that the main feature to start, an upper-level low/trough over the Northeast associated with the early-mid week Nor'easter, will depart to the northeast over the open Atlantic over the next couple of days. Meanwhile, a shortwave over the West Coast will quickly amplify over the plains with deeper troughing in place but with disagreement in the guidance as to the location and number of additional waves of embedded energy, whether the streams tend to split, and whether a closed low forms. The deterministic GFS/ECMWF have come better in line with the timing/phasing of the trough axis with less of a tendency to evolve with a separate north/south split in embedded energy in the flow, though the deterministic 00 Z CMC/UKMET as well as the 00 Z ECens and GEFS means still hint at a bit of flow separation, which has downstream effects at the surface in helping to drive two frontal systems across the Plains. The trend in the ECMWF/GFS up until 00 Z was to develop a deeper closed low, with the UKMET also depicting a closed low, albeit weaker. However, the 00 Z CMC showed significant differences with a much less amplified trough, more in line with the UKMET trend than the GFS/ECMWF, and the 06 Z GFS did also move a bit away from the closed low scenario while still maintaining a generally deeper trough overall. Much of this finer detail is still lost in the GEFS/ECens mean solutions. Later in the period, all deterministic guidance does move towards a deepening closed low over the Midwest with the exception of the 00 Z CMC. The 00 Z ECens mean even moves towards a closed low solution at this point, suggesting most deterministic and ensemble guidance is in agreement on the deepening of this system with time during the forecast period. A look at the individual ensemble members does show there is increasing uncertainty with the exact evolution of the strength of the trough/potential closed low, but there at least is good agreement as to the phasing of the system's progression in the overall pattern. This general agreement devolves rapidly by the end of the period with significant differences in both the phasing and strength of the trough as it moves over the Northeast both between the latest individual runs of the GFS and ECMWF and well as between the ECMWF/GFS/CMC. Meanwhile, general phasing/timing differences with additional shortwaves to the west over the Southwestern U.S. and ahead of a closed low over the northeast Pacific also increase. The GEFS/ECens means at least somewhat capture these features in the flow so not all detail is lost despite significant differences in the deterministic models. The WPC forecast suite blend began with a higher contribution from the 00 Z GFS/ECMWF over the 00 Z UKMET which is similar but will phase out quicker due to forecast length, and with all three contributions higher than the 00 Z CMC given the increased divergence from the other deterministic guidance by the middle of the period. The 00 Z GFS was chosen over the 06 Z given the similar trend/agreement between the GFS/ECMWF through 00 Z that remained close to continuity with less confidence in a change in the pattern, at least based on one additional run of the GFS. This blend continues through the early period with the noted concern that there may be some subtlety lost in the potential split flow pattern over the West, but with not enough agreement or distinction across the current runs of the deterministic guidance to best capture it. The 00 Z CMC is removed mid-period given the differences compared to the other guidance, while the influence of the UKMET is also slightly reduced as the forecast range of the model comes to a close. The inclusion of a small contribution from the 00 Z GEFS/ECens means given increasing uncertainty completes the blend. This blend keeps quite a bit of continuity with the prior WPC forecast in retaining a deepening closed low over the Northern Plains/Midwest. Contributions from the 00 Z ECens/GEFS means are increased for the late period as the ECens in particular still captures a closed low as in the deterministic guidance, and to help account for growing differences in the details of the more subtle shortwaves over the west in the deterministic guidance. As noted, the means help to capture their presence despite the increasing uncertainty. Increased reliance on the means also helps to account for the significant divergence between the GFS/ECMWF runs with respect to the evolution of the closed low over the northeast at the end of the period. ...Weather/Hazards Highlights... The lingering highest moisture anomalies and subsequent precipitation from a potent short-range Atmospheric River directed toward the West Coast will shift into southern California and the south-central Great Basin/Rockies Wednesday into Thursday. The experimental WPC Day 4 Excessive Rainfall Outlook (ERO) shows "Marginal Risk" areas Wednesday for southern California and upslope favorable terrain of northern/central Arizona and southern Utah as the anomalous atmospheric river moisture and southern stream upper trough energy works into the region. Moderate to heavy snow will also continue for the Sierra and mountain ranges of the central Great Basin/Rockies. There looks to be a brief reprieve from precipitation over the West after Thursday and into Saturday before another system may approach the region by Sunday/early next week, bringing more lower-elevation coastal/valley rain and higher elevation snow to the region. High temperatures look to remain around 5-10 degrees below average in general, with a pocket of colder temperatures 15-20 degrees below average lingering over the Rockies. Downstream to the east, a powerful coastal storm/Nor'easter lingering off the New England coast will present an impactful wind/wave threat through Wednesday along with remaining wrap-back New England snows. Following the departure of the Nor'easter, focus turns to 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 Day 5 ERO has a "Marginal Risk" Thursday from the eastern southern Plains to the lower Mississippi Valley where locally heavy rains may accompany thunderstorms ahead of and along an eastward moving cold front. Latest trends in the guidance have shown an increase in forecast rainfall, and the region will need to be monitored for a potentially higher rainfall/flash flood threat if confidence increases in the location and evolution of more widespread heavy rain producing storms. The heavy rain threat should shift to a localized region over the far Southeast/Florida Panhandle Friday where access to Gulf moisture remains highest. Otherwise, moderate rainfall chances will overspread the Midwest on Thursday and the East Coast into Friday/Saturday. Additionally, latest guidance preferences have continued to increase wrap-back snow potential from the Central Plains to the Upper Midwest/Great Lakes Thursday-Friday and eventually the Interior Northeast Saturday-Sunday following the surface low track as per the WPC Winter Weather Outlook (WWO). The highest chances for impactful snowfall currently look to be over the Middle Missouri Valley Thursday and portions of the Upper Great Lakes Thursday-Friday. High temperatures will tend to be cooler than average by 10-20 degrees across most of the central U.S. following the passage of the system cold front and a Canadian air mass settling in. Putnam/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