Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Feb 23 2019 Valid 12Z Tue Feb 26 2019 - 12Z Sat Mar 02 2019 ...Pattern Overview... The expected mean pattern and embedded uncertainties look fairly similar to 24 hours ago. A Rex block configuration should persist over the east-central Pacific with a strong ridge/closed high meandering over eastern Alaska and northwestern Canada while features dropping into the mid-low latitude mean trough progress toward the West Coast. In the multi-day mean perspective teleconnections relative to the ridge's positive height anomaly continue to favor the consensus idea of a modest mean ridge close to the West Coast by late period and troughing over eastern North America. There are multiple significant uncertainties with embedded details though. The most common themes in guidance continue to be the potential for significant precipitation from the central and northern West Coast into the northern Rockies and persistence of very cold temperatures over and near the northern half of the Plains. There will likely be multiple precipitation areas over the eastern half of the country during the Tue-Sat period but specifics will be sensitive to the uncertain details at the surface and aloft. ...Guidance/Predictability Assessment and Preferences... The difficulty that an upper high over the northeastern Pacific/northwestern North America tends to create for the forecast is evident again today, with significant spread quickly developing for features to the south and southeast of the high. Already from very early in the period there are meaningful differences for flow details over and offshore the Pacific Northwest as well as for downstream flow over the northern U.S./southern Canada. Regarding the former, there is agreement in principle that initial energy should close an upper low while mid-latitude energy approaching from the west is incorporated into the southern side of the circulation by day 4 Wed. However there are significant position differences for this upper low as of early Wed with further divergence thereafter. Latest GFS runs including the 00Z version are particularly fast with this system versus most other guidance, ultimately leading to another very deep Great Lakes storm by Sat. The 18Z FV3 GFS clusters toward the other slower solutions. Overall preference leans increasingly away from the GFS by mid-late period. Meanwhile out-of-sync shortwave details from the northern Plains eastward (with sufficiently small scale leading to fairly low predictability) affect possible surface waviness and snowfall amounts/coverage. The GFS continues to be on the strong side of the full spread with any low pressure while ECMWF/ECMWF mean runs persist with no reflection at all. Taken as a whole there seems to be more potential for modest waviness than indicated by the ECMWF scenario but it should be weaker than in the GFS--favoring an intermediate solution. Later in the period there is a reasonable degree of persistence--though with a somewhat slower trend--for the system likely to track toward the West Coast. This slower trend argues for at least intermediate timing between the faster 12Z ECMWF-ECMWF mean/18Z FV3 GFS and other somewhat slower solutions. By Sat ECMWF/ECMWF mean scenario of keeping the early period upper low offshore Vancouver Island would lead to some influence on the system while other guidance shows inland ejection albeit slower than in the GFS. Farther east one notable divergence is for the path of a southern Canada upper low, for which GFS runs lean to the southern part of the spread. Even if the sequence of events leading to the deep eastern storm by next Sat currently looks questionable, there is a signal for some amount of surface development (such as in the past two ECMWF runs and 12Z/18Z GEFS means. Albeit with very uncertain latitude there is somewhat more agreement toward the idea of low pressure just off the East Coast during the day Sat as height falls aloft approach. The updated forecast starts with a multi-model (18Z GFS/FV3 GFS and 12Z ECMWF/UKMET/CMC) blend during the first half of the period to reflect a consensus or intermediate solution as appropriate for individual features early in the period. By mid-late period the forecast includes 30-60 percent total input of the 18Z GEFS/12Z ECMWF means, with some ideas from the 12Z ECMWF/CMC and 18Z FV3 GFS for localized detail. ...Weather Highlights/Hazards... Over the West some of the day-to-day details have varied but guidance is consistent in principle with the potential for significant precipitation--in the form of rain and higher elevation snow--from the central/northern West Coast into the northern Rockies. There may be a brief break in activity around late week between initial moisture flow and the arrival of another system by Sat. Best probability for highest 5-day totals still covers northern California/Sierra Nevada range and southwestern Oregon with at least 5-10 inches liquid possible at some locations. Expect meaningful but somewhat lower max totals over the northern Rockies. Precipitation coverage and intensity from the Plains to East Coast continue to be difficult to pinpoint due to significant differences in shortwave energy and surface evolution. From Tue into Wed night an area of mostly light snow may progress eastward from the Plains. Meanwhile southern stream impulses may promote periods of rainfall across the Gulf Coast states over the course of the period. Toward Fri-Sat there may be an expansion of precipitation coverage over the eastern states (snow north and rain south) in association with some degree of surface development over the East and/or extreme western Atlantic. Within the area of much below normal temperatures centered over the northern half of the Plains, coldest anomalies (at least 30F below normal at some locations) should exist early and late in the period. Some of the cold air will extend eastward early-mid period followed by a moderating trend. Flow from the Pacific will keep morning lows above normal over all but the extreme northern parts of the West, as well as over the southern tier, while clouds and precipitation will tend to keep highs closer to normal. Rausch WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation and winter weather outlook probabilities are found 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/wwd/pwpf_d47/pwpf_medr.php?day=4