Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 AM EST Wed Feb 27 2019 Valid 12Z Sat Mar 02 2019 - 12Z Wed Mar 06 2019 ...Pattern Overview... Broadly cyclonic mean flow across the Lower 48 at the start of the weekend should transition toward increasingly amplified troughing over the East by the middle of next week as an upper ridge reaches the Rockies and a trailing trough approaches the West Coast. Mean ridging aloft over northwestern North America (specifically an initial upper high dropping south over western Canada followed by another ridge building over eastern Alaska and the Yukon) will support a core of extremely low temperatures over the central U.S. with anomalies up to 30-45F below normal during Sat-Mon and likely widespread new daily records. Some of this cold air will extend into northern parts of the West and through much of the East. Two southern stream Pacific systems will bring more significant precipitation to parts of the West with heaviest focus over California, while there is still a signal for heavy rainfall potential over parts of the Southeast. Farther north over the eastern states there will be one or more possibilities for snow but with a fair degree of uncertainty over the specifics. ...Guidance Evaluation/Preferences... Early in the period a blend of operational model guidance through the 12-18Z cycles provided a good starting point for yielding the desired solution for systems of interest. There have been continued differences for the overall area of low pressure expected over the northeastern U.S./southeastern Canada and off the New England coast during the weekend. In the 12-18Z array of guidance there were essentially two clusters, the GFS/CMC and their means that were better defined along/inland from the New England coast and the ECMWF/ECMWF mean/UKMET that placed more emphasis on offshore low pressure. Dependence on low predictability small scale features (originating in part from the upper low now near the Pacific Northwest) favored a blended approach that kept the forecast close to continuity. There have been some trends toward a more fragmented ejection of that upper low energy so the weaker adjustment in the 00Z GFS is plausible. Meanwhile trailing waviness affecting the Southeast/Mid-Atlantic will be influenced by energy from the system initially nearing California (consensus mean still looking good for this system) and possibly interaction with northern stream flow. The multi-model blend yielded a forecast close to the 12Z ECMWF mean, a reasonable intermediate solution between the weaker GFS/GEFS mean runs and stronger/northward 12Z ECMWF/UKMET/CMC. A majority of new 00Z runs (minus the GFS) maintain support for significant waviness. During the latter half of the period, model/ensemble clustering has shown a pronounced deeper trend with the upper trough axis that crosses the Midwest/Great Lakes and vicinity. However latest operational runs have shown less evidence of a closed low than 24 hours ago. Recent GFS runs strayed a little faster than consensus with this trough over the East by Tue-Wed. Meanwhile by day 7 Wed the next system to affect the West Coast shows expected track/timing differences but in general the clustering is not bad for that time frame and an intermediate solution appears reasonable at this time. One complexity that will have to be monitored is potential for elongating southwestern Canada energy to close off an upper low off Vancouver Island and have some influence on the incoming system. Overall both the models and means appeared to provide some useful information to contribute to the days 6-7 forecast so the blend followed an even weight between the 18Z GEFS/12Z ECMWF means and 12Z ECMWF-CMC/12-18Z GFS. ...Weather Highlights/Hazards... Across the West the primary axis for heaviest rain and higher elevation snow will extend from California eastward through the central/south-central Rockies with the Sierra Nevada likely seeing the highest 5-day totals. Two systems will affect the region during the period, one arriving early in the weekend and spreading moisture rapidly eastward then followed by a second arriving Tue-Wed. As moisture from the second system spreads northward there may be potential for wintry precipitation at low elevations over interior portions of the Northwest given the cold air likely to be in place. The system affecting the Northeast during the weekend should bring some snowfall to the region but weakening and possibly offshore trends for low pressure as well as fast progression may limit accumulations. Southern stream energy interacting with the trailing front may produce a period of heavy rainfall over parts of the Southeast with the weekend into early Mon time frame the most likely period for this activity. Guidance has not yet fully converged for details so confidence is only moderate thus far. Snow will be possible in the northern part of the moisture shield--from the south-central Mississippi Valley into the Mid-Atlantic. Lighter rainfall could linger over parts of the extreme Southeast/Florida into next week. Within the widespread area of extremely cold air over the central U.S., expect the most extreme anomalies to be over the northern-central Plains during Sat-Mon with anomalies of 30-45F below normal. Especially on Sun-Mon many places over the Plains and Midwest should challenge or exceed daily record values--especially for cold highs but also for morning lows at some locations. Less extensive coverage of daily records may continue into Tue. The eastern U.S. will see a modified form of this cold air by the first half of next week with readings "only" 10-20F below normal. Localized parts of the interior Northwest may see temperatures 10-30F below normal. Relative warmth will be confined to the southern half of the West and more so for morning lows than afternoon highs. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml