Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Sat Mar 18 2023 Valid 12Z Tue Mar 21 2023 - 12Z Sat Mar 25 2023 ...Another strong storm with heavy rain/snow and wind threats to impact California... ...Overview and Guidance/Predictability Assessment... Guidance is still showing broad cyclonic mean flow aloft with an overall axis along and then inland from the West Coast, while an upper ridge meanders from the northwestern Caribbean into the Gulf of Mexico and then over Cuba and the Bahamas. The system forecast to head into the West on Tuesday-Wednesday with an atmospheric river containing some moisture of tropical origin has held fairly steady in the guidance over recent runs, aside from detail differences within the typical guidance spread for a few days out in time. However the latest guidance behavior for the system's evolution east of the Rockies as well as southern Canada/northern U.S. flow and associated surface reflection have lowered confidence in forecast specifics over the eastern two-thirds of the country compared to yesterday. Still, it appears likely there will be heavy rainfall over some areas along with snow potential over some northern locations. The next Pacific system dropping into the western mean trough Friday-Saturday should be weaker and less amplified than leading one. Guidance has come into better agreement at the start of the period with the initial phasing and strength of an upper-level low over the Gulf of Alaska descending southward over the eastern Pacific and eventually translating over the West. A surge of embedded upper-level energy is expected to dive southward over southern California/the Southwest and far northwest Mexico. Differences quickly arise as this energy/shortwave translates downstream over the Plains ahead of the main trough, with the CMC (and 00Z UKMET) continuing to be more aggressive and faster in comparison to the the GFS/ECMWF. The GFS/ECMWF show greater height rises preceding the the wave as ridging amplifies downstream over the Caribbean/Gulf of Mexico/southeastern U.S. The last few runs of both the GFS and ECMWF have trended further in this direction. Additionally, the last few runs of the GFS/ECMWF as well as the latest 00Z UKMET have started to show a northern stream shortwave dropping southward over the northern tier of the CONUS immediately preceding this system moving over the Plains, which will have implications for potential surface cyclogenesis and front evolution over the Midwest/Northeast. Predictability deteriorates quite rapidly in the mid- to late forecast period with respect to the evolution of the trough over the central/eastern U.S. and the potential for another trough/surge of upper-level energy to dig southward over the West. Individual members of both the 00Z GEFS/ECens show a notable increase in spread over the same time period. The GFS in particular has shown poor run-to-run consistency with the phasing of the trough over the Plains mid-period, although at least the latest 00/06Z runs has come more into line with the ECMWF, which has been a bit more consistent. Earlier GFS runs were much more progressive and weaker with the trough. The 00Z CMC is also in general agreement with the latest guidance mid-period. However, the guidance still disagrees on the strength of the wave and the potential for a compact closed low to form as it moves over the eastern CONUS at the end of the period, with the CMC a bit more aggressive than the GFS, which is then a bit more aggressive than the ECMWF. Additionally, the ECMWF, and the GFS through 00Z, have trended less amplified with a second trough digging over the West. This lines up with recent runs of the CMC as well. The 06Z run of the GFS did reverse this trend and now shows a much stronger closed low dropping southward over California/the Southwest. The 00Z GEFS/ECens means are more similar to the ECMWF and 00Z GFS, although it is expected that the means may lose more subtle details at this range with respect to the potential for a more amplified pattern over the West. The WPC forecast began with a blend of the deterministic guidance, with less emphasis on the CMC given the earlier differences related to the northern stream short wave. The 00Z GEFS/ECens means are slowly increased as part of the blend in the mid- to late period as uncertainty increase with respect to the phasing and details of the evolution of both the trough over the Plains/eastern CONUS and the potential for another trough over the West. The means help to capture the broader evolution without emphasizing any of the more specific details in the deterministic guidance that show increasing differences. The means replace the 00Z UKMET and contributions from the 00Z GFS/CMC more so than the 00Z ECMWF given that the ECMWF showed a bit more run-to-run consistency. The 00Z CMC is kept as part of the blend despite earlier differences because of its better agreement in the mid- to late period. ...Weather/Hazards Highlights... A strong upper trough digging toward the West Coast, along with an associated low pressure/frontal system, will support a plume of enhanced moisture and yet another Atmospheric River event into parts of California and the southwestern U.S. around Tuesday. Expect a period of moderate to heavy lower elevation/coastal rain and heavy high elevation snow expanding across central/southern portions of the West, with precipitation trending lighter to the north. Heavy snow is most likely for the Sierra and ranges of the central Great Basin/Rockies. 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, forecast rainfall totals over favored terrain along with existing wet ground conditions still support a Marginal Risk in the day 4 (12Z Tuesday-12Z Wednesday) Excessive Rainfall Outlook for areas from the lower elevations of the Sierra Nevada to the southern California coastal ranges, in addition to favored terrain over central Arizona. Farther east, a leading system supported by northern stream energy may spread some light to moderate precipitation from the Plains/Midwest into the Great Lakes/Northeast Tuesday-Wednesday. The trailing front that stalls may interact with moisture arriving from the ejecting western system to begin generating heavier rainfall over the Midwest around Wednesday night per some guidance, but other solutions have very different ideas. This area may require monitoring for future trends/guidance consolidation but the current guidance spread and neutral to dry soil conditions over the region of interest favor no Excessive Rainfall risk area for now in the day 5 (12Z Wednesday-12Z Thursday) period. Right after this time frame, there should be a broader area of moderate to heavy rain and thunderstorms developing over areas from the south-central Plains east and northeast into the lower Great Lakes, Ohio Valley, and Tennessee Valley. There continues to be uncertainty in the guidance as to where a potentially significant axis of heavy rainfall may set up, how widespread it might be, and how high totals may get. However, a signal remains for heavy rainfall totals which will likely raise some flash flooding concerns, especially late Thursday night and into the day Friday. The potential for severe weather will also ramp up ahead of the system, and the Storm Prediction Center has already highlighted portions of the Southern Plains for this threat on Thursday. Northern areas may see one or more opportunities for snow, but again with moderate or lower confidence in specifics, and currently not a strong signal for any particularly notable snowfall. Rain chances look to continue in the Northeast Thursday and spread south/eastward into the Mid-Atlantic and Southeast Friday-Saturday ahead of the advancing system. The next arriving Pacific system should bring the best focus for rain and mountain snow to the Pacific Northwest with somewhat lesser totals reaching California and eastward into the northern-central Rockies. The mean trough aloft over the West will keep the region chilly through the period, with fairly broad coverage of highs at least 10F below normal. On the other hand, most of the eastern half of the country will see a warming trend to above normal temperatures after the Southeast starts out with lows 10-15F below normal early Tuesday. By late in the week many locations in the East should be at least 10-20F above normal, with locally higher anomalies for morning lows. Frontal progression will trim western/northern portions of this warmth by Saturday. 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