...Another strong storm with heavy rain/snow and wind threats to impact California... ...Overview and Guidance/Predictability Assessment... Latest guidance agrees fairly well upon the idea of the large scale upper pattern featuring a mean trough along or a little inland from the West Coast. The most prominent system within this pattern should reach the West early-mid week with an atmospheric river containing some moisture of tropical origin, and then track across the Plains/east-central U.S. later in the week with an expanding area of precipitation over that part of the country. Another system dropping into the mean trough may reach the West Coast by next Friday. The main focus forecast-wise during the period will be on the transition into a more energetic and amplified pattern over the CONUS as at least a couple upper-level waves enter the picture. An upper-level low over the Northeast will be departing over the Atlantic with energy lingering to the Southwest as a persistant upper-level low remains in place upstream over the Gulf of Alaska at the start of the period. A weak southern stream shortwave will be crossing the Southern Plains and moving into the Southeast, with the 00Z CMC still a bit ahead of the other guidance, similar to previous runs. The prior few runs of the GFS continue to show this wave deepening near the Atlantic Coast, and this is even hinted at in recent runs of the ECMWF. However, the 00Z and particularly the 06Z GFS runs have once again backed off this trend. On the other hand, the guidance has come into better agreement with an initial batch of energy ejecting from the low over the Gulf of Alaska as a compact shortwave over the Pacific Northwest early in the forecast period. Attention then turns to the Gulf of Alaska low dropping southward and moving over the West by mid-period, sweeping up a surge of moisture over the Pacific to instigate another Atmospheric River even for California in the process. The guidance is in fairly good agreement on the evolution of this pattern on a larger-scale, but potentially significant disagreements pop up in the handling of embedded energy, specifically a shortwave/compact upper low translating northward over the central U.S. that has implifcations for surface cyclo/frontogenesis over the Plains. The 00Z CMC is a bit more agressive overall in depiciting a much stronger deepening low as opposed to a weaker low/open wave in recent runs of the ECMWF and GFS. The GFS and ECMWF also show a bit more amplified ridge downstream ahead of the troughing over the West. Details become even more cumbersome by the end of the period as another trough begins to dig in the West. This general pattern is captured in the lateast 00Z/06Z GFS and 00Z ECMWF runs as well as the 00Z GEFS and ECens means, while the 00Z CMC is less agressive and only hints at a weaker shortwave over the Southwest. More specific details in the evolution of split-stream energy downstream over the East is not consistent either run-to-run within the individual guidance or in comparison between different models. Even the 00Z GEFS/ECens means show potentially significant differences with the GEFS mean stronger with both features late in the period. The WPC forecast blend begins with contributions from the 06Z GFS and 00Z ECMWF/UKMET and a smaller influence from the 00Z CMC given differences with respect to the southern stream shortwave. Contributions from the 00Z GEFS/ECens means are added as the influence of the 00Z CMC is removed in the middle of the period as this solution diverges more significantly in the handling of the evolution of the pattern over the central U.S. mid-late period compared to the other guidance. The 00Z GEFS/ECens means are relied on more heavily towards the end of the period given the rapidly increasing uncertainty with respect to smaller-scale shortwaves in the flow. The means help to capture the broader pattern between the initial troughmoving from the central to eastern U.S. and the next trough digging in the West. ...Weather/Hazards Highlights... A fairly weak leading system will support areas of rain and higher elevation snow that should be in progress across a decent portion of the West at the start of the period on Monday. Then a stronger upper trough digging toward the West Coast, along with associated low pressure/frontal system, will direct a plume of enhanced moisture into parts of California and southwestern U.S. around Tuesday. Expect a period of moderate to heavy lower elevation/coastal rain and heavy high elevation snow especially in the Sierra Nevada. Rainfall totals should be less extreme than this past week's events while snow levels remain lower, somewhat reducing the flooding/runoff risk in comparison. However, the forecast rainfall totals along with existing wet ground conditions do appear to support a Marginal Risk in the day 5 (12Z Tuesday-12Z Wednesday) Excessive Rainfall Outlook for areas from the lower elevations of the Sierra Nevada to the southern California coastal ranges, as well as favored terrain over central Arizona. Eastward progression of this system should then bring significant snowfall into parts of the Great Basin and central Rockies Tuesday-Wednesday. This system should then emerge over the Plains and Great Lakes by Thursday-Friday, with some snow possible over far northern areas and rain/thunderstorms over the eastern Plains/east-central U.S. Currently expect the heaviest rainfall to extend from the ArkLaTex region into the Lower Ohio Valley. Another Pacific system may bring rain and mountain snow to the Pacific Northwest and parts of California toward the end of the week. The early part of the week will see well below normal temperatures across the southern tier with some localized readings of 20F or so below normal. Near or below freezing lows over some areas may be harmful to any new spring vegetation. There will be a steady warming trend over the eastern half of the country to above normal temperatures by the latter half of the week, ahead of the system tracking out of the West. The mean trough aloft over the West will maintain a broad area of below normal highs across that region, with minus 10-20F anomalies most common Tuesday onward. The far northern Plains will also see below normal temperatures for most of next week, especially on Monday-Tuesday. Putnam/Rausch 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 $$