Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Thu Mar 16 2023 Valid 12Z Sun Mar 19 2023 - 12Z Thu Mar 23 2023 ...Another strong storm with heavy rain/snow and wind threats to impact California... ...Guidance/Predictability Assessment... The model guidance at the beginning of the period depicts an upper-level low departing the East Coast with lingering trailing energy to the southwest, a shortwave moving across the southern tier of the U.S. in the southern stream, ridging in the northern stream over the Northwest, and a closed low upstream over the Gulf of Alaska with a gyre-like rotation of embedded energy in the surrounding flow. Subtle differences expectedly quickly arise with the more small-scale features embedded in the flow, including the shortwave over the southern tier. The 00Z CMC was more of an outlier with a shallower wave compared to the other guidance. The GFS has backed off this wave deepening over the Southeast and is now more in line with the other guidance. The 00Z ECMWF is also a bit more aggressive with a compact shortwave over the Northwest as a piece of energy from the upper-level low over the Gulf of Alaska ejects downstream, although the trend in the last few runs of the GFS is towards this solution. The focus of the mid- to late period turns to the more impactful evolution of the energy with the upper-level low over the Gulf of Alaska translating downstream as a trough digging over the West, and subsequent implications on surface frontal systems/cyclogenesis over the West and Plains late in the period. This also includes the arrival of yet another stream of tropical moisture/Atmospheric River to the West Coast. Individual ensemble members show significant uncertainty in both the timing and amplification of the flow. The last few runs of the ECMWF and GFS have trended more aggressive earlier with the timing and deepening of the flow off the West Coast mid-period. The 00/06Z GFS and 00Z UKMET are on the more aggressive side compared the the 00Z ECMWF, with the 00Z CMC lagging the most compared to the other guidance. The trend in the GFS away from deepening the shortwave over the Southeast has brought the subsequent evolution of the western trough over the West more in line with the ECMWF mid- to late period. Both models have also trended away from depeciting a stronger embedded shortwave ejecting over the Plains, most notably by recent ECMWF runs. The 06Z run of the GFS shows increasing subtle ridging immediately downstream over the Midwest with the progression of the trough slowing, which overshoots this trend, with both models most aligned at the 00Z run. The updated WPC forecast blend initially relied on contributions from the 00Z ECMWF/GFS/UKMET with only a small contribution from the 00Z CMC given noted differences compared to the other guidance, similar to the prior WPC forecast. A small contribution from the 00Z GEFS/ECens means increases through the early to mid-period given notable similarity in even more subtle details of the pattern to the deterministic guidance, and the increasing differences with respect to the initial evolution of the trough digging over the West. The contribution of the 00Z CMC is removed mid-period with the 00Z UKMET toned down as it was a bit more aggressive with respect to the trough, especially compared to the 00Z ECMWF and 00Z ECens/GEFS means. The contribution from the means increases in the late-period as more uncertainty arises with the evolution of the potential shortwave over the Plains, with the ensemble means showing the greater compromise in retaining a subtle depiction of height falls over the Plains as both the deterministic 00Z GFS/ECMWF have trended slower and weaker with the feature. ...Weather/Hazards Highlights... A trailing frontal passage and the settling in of Canadian high pressure will usher in much below average temperatures across the southern tier of the central to eastern U.S. this weekend into early next week before moderating. Portions of Texas will see highs 15-30 degrees below average with the chance some light snow might mix in with showers in the Southern High Plains. Morning lows across much of the South will also hover near or just below freezing, potentially harmful to any new spring vegetation. Meanwhile, gulf moisture return to the Southeast along/ahead of the trailing front will fuel showers and thunderstorms, with a main focus over southern Florida where the wavy/stalling front may fuel some locally heavy downpours. In the West, uncertain upper-level waves/systems will steadily increase precipitation chances across the region overall through the weekend and into early next week as a deep upper-level low is slated to descend to the East Pacific and tap an anomalous plume of subtropical moisture to generate another Atmospheric River event across California beginning Tuesday. Moderate to heavy lower elevation coastal/valley rain and heavy high elevation snow can be expected, particularly for the Sierra. Rainfall totals do not look to be as significant as this past week's events and snow levels should remain lower, keeping the threat for flooding/runoff issues much lower. Better model agreement on the axis of highest moisture may eventually lead to higher rain totals and a more focused threat for localized flooding. A strong, organized frontal system pushing eastward through the West ahead of the deepening trough will expand heavy snow chances into the Rockies Tuesday-Wednesday. High temperatures will be running 10-15 degrees below average with the broad upper-level troughing in place. There is a growing guidance signal that ample energies exiting the West will lead to well organized cyclo/fronto genesis over the Plains. Widespread showers and thunderstorms, including the potential for some locally heavy rainfall and severe weather, will be possible across portions of the Midwest and Plains potentially beginning Wednesday with increasing chances Thursday. An emerging/expanding lead and wrap-back moisture and precipitation shield is also increasing the potential for at least some light-moderate snowfall over a cooled north-central U.S.. Putnam/Schichtel 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