Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Fri Feb 22 2019 Valid 12Z Mon Feb 25 2019 - 12Z Fri Mar 01 2019 ...Pattern Overview... Latest guidance maintains and to some degree strengthens the Rex block pattern that has generally been in place over the east-central Pacific for the past couple weeks. A significant aspect of the forecast evolution during next week will be the expected northward drift of a strong upper high likely to start the week near the southeastern coast of Mainland Alaska. As this occurs, the core of strongest positive height anomalies should lift into the Arctic. Teleconnections relative to such a northward anomaly center favor a mean ridge close to the West Coast and troughing over eastern North America. This trough position is an eastward adjustment from the recent pattern but the end result keeps a high likelihood of below normal temperatures anchored over the northern Plains and vicinity. Meanwhile progressive mid-latitude Pacific flow will favor potential for significant focused precipitation over parts of the West Coast and some inland terrain. Embedded energy in low amplitude flow continuing eastward may produce one or more areas of precipitation over the South and East--the latter in possible combination with northern stream flow. ...Guidance/Predictability Assessment and Preferences... Multi-day means into the D+8 time frame agree quite well with each other for the overall pattern, and aforementioned teleconnections with the positive height anomaly center lifting into the Arctic yield a favored pattern consistent with what these means are showing. Thus there is decent confidence that the general pattern should gravitate toward a solution close to what the means show by the end of next week. However confidence is significantly lower for day-to-day details. Upper highs over the northeastern Pacific/northwestern North America can cause havoc with forecast details over the eastern Pacific and the Lower 48, due to complex/high-sensitivity evolution for shortwave energy on the southern/southeastern side of these highs. Specifically for today's forecast, the complexities involve a shortwave just off the Pacific Northwest as of early Mon and an upper low approaching from the west along with their possible interaction and subsequent movement. Details of the upper ridge will also affect the ultimate shape of Canadian flow that will eventually affect the Lower 48. Currently it appears that the upper ridge will reach far enough north to have less influence on a possible Pacific system nearing the West Coast by next Fri. The updated forecast blend starts with a combination of 18Z GFS and 12Z ECMWF/CMC/UKMET model runs from day 3 Mon into day 5 Wed. As of early Mon there is good overall clustering for the strong storm lifting away from New England and low pressure nearing the north-central West Coast. There is a general theme in the guidance that features off the Pacific Northwest should evolve into an upper low but with a moderate amount of spread for position by Wed. Also by Wed there are hints in some guidance that northern stream shortwave energy may produce a weak surface wave reaching the east-central U.S. Latest GFS runs and 12Z ECMWF are on opposite sides of the spectrum while a blend yields a modest reflection similar to continuity. As operational runs further diverge detail-wise, the forecast rapidly incorporates/increases ensemble mean weighting (18Z GEFS/12Z ECMWF mean) so that the means comprise 2/3 of the blend on day 6 Thu and 100 percent on day 7 Fri. On a relative basis the most agreeable features by the end of the week appear to be the system coming into the picture off the West Coast and southern Canada low pressure whose trailing front would bring a reinforcing push of cold air to the northern states. ...Weather Highlights/Hazards... The pattern over the West will favor an axis of significant precipitation (rain and higher elevation snow) from the central/northern West Coast into the northern half of the Rockies. The best probability for highest 5-day totals currently covers northern California/Sierra Nevada range and western Oregon with at least 5-10 inches liquid possible at some locations. The precipitation axis should extend into favored terrain of Idaho/Wyoming with somewhat less extreme totals. Farther eastward expect strong winds to persist over the northeast quadrant of the country into Mon as the deep weekend storm tracks away from New England. Also anticipate some lake effect/terrain-enhanced snowfall early in the week. At least one area of moisture may spread mostly light-moderate snow from the northern Plains into the Northeast during the week, but with low confidence in specifics due to uncertainty over dynamics aloft and associated surface reflection. Meanwhile progressive mid-lower latitude shortwave energy may periodically enhance rainfall along and north of the Gulf Coast region but guidance spread/continuity changes decrease confidence in details. Depending on stream interaction there is the possibility that some of this moisture could reach farther north over parts of the East but the signal is not very pronounced at this time. Expect very cold conditions to continue over and near the northern half of the Plains with temperatures at least 15-20F below normal most days. Some areas in the extreme northern Plains/eastern slopes of the Rockies may see highs 30-40F below normal early in the week. A late week cold front could bring highs back down to 25-30F below normal on Fri. To a less extreme degree some of this chilly air should extend across the Great Lakes/Northeast. Pacific flow will tend to bring above normal mins to about the southern 3/4 of the West while daytime highs should be within a few degrees on either side of normal. The forecast looks similar over the southeastern states. 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