Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sun Dec 16 2018 Valid 12Z Wed Dec 19 2018 - 12Z Sun Dec 23 2018 ...Significant precipitation over the Northwest mid-late week... ...Confidence increasing for a late week storm over the East... ...Overview... Some details remain in question but there is good agreement/continuity with the general idea that a rapidly building/sharpening ridge over the West Wed-Thu will support downstream digging of shortwave energy into an amplified trough over the Mississippi Valley by Thu. Eastern trough energy, possibly closing off an upper low for a time, should support a fairly strong surface system that progresses north-northeast over the eastern U.S. into Canada Fri onward as incoming Pacific shortwave energy dampens the western ridge. By next weekend the overall mean flow becomes noticeably less amplified with the hint of a split flow pattern becoming evident along/just inland from the West Coast by next Sun. The progressive/low amplitude nature of flow in this time frame will likely create difficulties in resolving shortwave specifics. ...Guidance Evaluation/Preferences... The 12Z GFS/ECMWF and the 18Z GEFS/12Z ECMWF means provided the best clustering for the overall days 3-7 forecast. Thus they were used exclusively as the input for the surface/500mb progs, with primary emphasis on the operational runs early and the ensemble means late. The 18Z GFS was comparable for the eastern U.S. storm but posed questions for the shortwave energy reaching western North America around mid-period. Upper trough energy over the Pacific near 150W longitude as of early day 3 Wed is continuing to pose significant forecast difficulties beyond that time. Individual models and ensemble members still display much wider than desired spread for how the trough energy separates, with the northern portion serving to flatten the western ridge and the southern remainder lingering back farther westward. UKMET runs are very persistent in being the operational model extreme in being more progressive with the northern shortwave. On the other hand GFS/GEFS runs had been slowest with the northern shortwave but recent GEFS runs have been gradually trending toward a more subdued form of ECMWF-based solutions, and GFS runs seem to be heading into that intermediate scenario as well. The 18Z GFS had reverted back to the slower scenario but the new 00Z version is faster once again. This shortwave energy will determine the strength/track of low pressure expected to track over or just northwest of Vancouver Island around Thu. Thus far there is poor clustering for this wave so confidence is below average for a day 4 forecast. Then the wave will progress into western Canada and likely track near the U.S.-Canadian border. Toward the end of the period shortwave details become very ambiguous due to the progressive and low amplitude nature of flow. D+8 multi-day means show a developing area of positive height anomalies over the Pacific near 150W longitude, with teleconnections supporting a decent potential for west-central U.S. mean troughing. Smoothing out the smaller scale features, there is some comfort in that guidance seems to be evolving toward such a feature in the days 7-8 time frame. Farther east the guidance has shown considerable consolidation relative to the past 1-2 days, with seemingly less sensitivity to exact details of upstream flow providing some improvement to confidence as well. The latest majority cluster shows amplifying northern Plains energy on day 3 Wed rapidly amplifying and overwhelming an initial southern stream trough, with potential for an upper low to close off over or a little south of the Tennessee Valley by Thu-Fri, followed by north-northeast progression. As this occurs, consensus shows deepening surface low pressure tracking into the upper Ohio Valley/central Appalachians by early Fri and then the eastern Great Lakes later in the day before continuing into eastern Canada. Recent ECMWF runs were somewhat earlier to catch onto the degree of trough amplification/sharpening versus the GFS. So far it appears that the middle of earlier guidance spread is working out well for timing, with CMC mean runs trending slower (but still likely too fast) while latest GEFS mean runs are a little faster than 12Z/18Z versions from a couple days ago. ...Weather Highlights/Hazards... One wave of heavy rain/higher elevation snow over the Northwest during the short range period should be tapering off by next Wed. Then low pressure likely to track just offshore the Pacific Northwest into western Canada Thu-Thu night should bring another episode of significant precipitation. This event may have somewhat more limited extent of heaviest activity, most likely focused over the Olympics and northern Cascades, but still expect some moisture to reach into extreme northern California. There may be a period of fairly strong winds if the surface low is on the deeper side of the full guidance spread. Expect lighter and more scattered precipitation Fri followed by periods of mostly light-moderate rain/snow during the weekend. The storm system expected to affect the East should spread a broad area of moisture over most of the eastern half of the Lower 48 during the latter half of the week, with chilly cyclonic flow supporting Lake Effect/terrain-enhanced precipitation during the weekend. While some details of system evolution have changed over the past day, best guidance signals for highest rainfall totals for this event continue to exist over the Florida Peninsula and North Carolina coast. Other relative maxima may exist with Atlantic inflow across the northern half of the East Coast and to the northwest of the storm track. This should be a fairly warm system for the time of year, with most snow confined to the western periphery and then over the Great Lakes and vicinity after system passage. Expect the greatest temperature anomalies to be of the warm variety over the central U.S. during Wed-Fri, with some areas in the northern Plains seeing min temperatures in particular exceeding 20F above normal. Such anomalies may reach daily records for warm mins. Meanwhile the eastern U.S. storm may pull in a brief surge of warmth to the Mid-Atlantic/Northeast late in the week. The Pacific origin of supporting energy for this storm should keep cold sector temperatures from reaching much lower than normal, though some cooler readings may be possible over the Southeast around Fri-Sat. The overall flatter trend for flow aloft should promote moderation of temperatures toward normal over many areas of the country by next Sun. Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes 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 https://www.wpc.ncep.noaa.gov/heat_index.shtml