Extended Forecast Discussion NWS Weather Prediction Center College Park MD 310 AM 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... A broad blend of the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian along with ensembles and the compatible 01 UTC National Blend of Models seem to offer a reasonable forecast basis Sunday into early next week for most areas with detail consistent with a pattern with seemingly near average predictability. Forecast spread then increases next week as uncertain Pacific systems work into the West Coast and downstream within a southern stream flow. The overall larger scale pattern seems well established and the guidance signal remains on track for another impactful Californian focusing storm heading into next midweek. However, an eventual threat for a Gulf of Mexico to East Coast coastal low next week remains wildly depicted in guidance, with recent runs of the GFS showing a plausible but less likely much deeper/stronger system versus the other models. Prefer increased usage of the GEFS/ECMWF ensemble mean for these longer time frames given forecast spread, but the ECMWF seems to provide some added detail that seems best in line with the ensemble means. ...Weather/Hazards Highlights... A robust storm will exit the Northeast to the Canadian Maritimes by Sunday as cold Canadian high pressure settles into the central and eastern states. A trailing frontal passage will usher the settling of much below normal temperatures across the central to eastern U.S. this weekend into early next week before moderating. Gulf moisture return to the Southeast along/ahead of the trailing front will meanwhile fuel showers and thunderstorms, with a main focus over southern Florida where the wavy/stalling front may fuel some locally heavy downpours. Elsewhere, expect a break from the recent string of anomalously strong and wet storms over the West after Thursday and into Saturday before a series of uncertain upper-level waves/systems renews precipitation chances. Moderate lower-elevation coastal/valley rain and higher elevation snow will be possible along a southeasterly track from the West Coast across the Great Basin and into the Central/Southern Rockies Sunday-Monday. Lead cold airmass intrusion through Texas may lead to the potential for some light upslope snow/ice over the Southern High Plains. Expect organized rainfall downstream across the Gulf Coast/Southeast with frontal waves. Possible downstream Southeast U.S. coastal low genesis offers some risk of further development and subsequent lift off the East Coast, depending on the model. Models have been less than stellar from run to run, and recent guidance other than the powerhouse GFS have trended to suppress low development. WPC progs show a modest low. Upstream, a deep Aleutian 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 night. There is potential for additional deepening of the upper trough off the West Coast next Tuesday before slamming inland to drive another impactful heavy rain/snow/wind threat through midweek. There is a growing guidance signal that ample energies exiting the West will lead to well organized cyclo/fronto genesis over the Plains. An emerging/expanding lead and wrap-back moisture and precipitation shield offers some moderate snow potential over the cooled north-central U.S.. 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