Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Fri Jan 03 2020 Valid 12Z Mon Jan 06 2020 - 12Z Fri Jan 10 2020 ...Overview/Guidance Assessment... The latest model/ensemble consensus displays a progressive mean pattern aloft with embedded features of varying amplitude extending from the short range time frame into the medium range forecast. Then by the later half of next week guidance indicates development of a large scale west-central U.S. mean trough as one ridge builds over the Pacific around 140-160W and another builds off the southeastern U.S.. This pattern is likely to extend well beyond the end of the period in light of the CPC 8-14 day forecast. For the medium range period there is reasonable agreement regarding the mean flow but discrepancies exist for some important shortwave details--in particular for a system expected to develop over parts of the East around Tue-Wed. Regarding this potential system that may reach over or offshore the Northeast by midweek, the multiple pieces of energy that may be involved contribute to significant depth/track/timing differences among model/ensemble solutions, and thus lower than average confidence for specifics. One influence may be energy over Lake Superior and Ontario at the start of the period on Mon. Recent GFS runs have tended to be the slowest and most closed with the energy though with the 00Z run nudging a little faster. Upstream energy that provides the primary dynamic support for surface development involves two streams with various ideas in the guidance for exact interaction/evolution. As a result models and ensemble members vary widely for the system's depth and track at any particular time. An intermediate solution appears best given the low confidence in any specific piece of guidance. The next meaningful shortwave in the series should approach the West Coast on day 4 Tue with most differences generally within the typical error range into midweek. This energy will likely support a wavy front crossing the central/east-central U.S. by Thu-Fri with details requiring more time to resolve. By day 7 Fri there is a signal in some guidance for additional shortwave energy to approach the Northwest. Latest GFS runs have been noticeably stronger than most other solutions (aside from a couple ECMWF runs from 1-2 days ago). The combination of detail uncertainties over both areas would recommend an increased tilt toward ensemble means late in the week. In order to achieve the desired forecast over areas of interest, the starting blend used a multi-model approach for about the first half of the period and then steadily increased ensemble weight (12Z NAEFS/ECMWF means) so that the means had 70 percent emphasis by day 7 Fri. GEFS means have been on the weak side versus other means for the northeastern system and CMC mean input helped to add some definition for the NAEFS mean. ...Weather Highlights/Threats... Flow coming into the Northwest will focus highest totals of rainfall/mountain snow over favored coastal terrain along the northern half of the West Coast and the Cascades. Some multi-inch liquid totals for the full five-day period are possible over the Pacific Northwest. Significant though somewhat less extreme amounts are possible over the northern Rockies. Expect lighter/more scattered activity over the southern half of the West. Cooler than normal highs should prevail over parts of the Rockies most of the period, with expansion of below normal anomalies over the West late in the week as mean troughing aloft becomes established. Precipitation will likely expand in coverage over the East during Tue-Wed as developing low pressure heads into or near the Northeast. Guidance spread thus far has made it difficult to resolve precise impacts, but some solutions would produce significant precipitation and a period of strong winds. Precip type is also uncertain at some locations. Currently the best potential for at least some snow extends from the Lower Great Lakes/Ohio Valley/south-central Appalachians into the Northeast. Areas over the east-central U.S. should see increasing precipitation by next Thu-Fri as the large scale pattern evolves toward a west-central U.S. trough aloft with a wavy surface front reaching the central U.S.. Low level flow from the Gulf may help to enhance rainfall totals in the warm sector. Wintry weather is possible over northern latitudes. A majority of the U.S. east of the Rockies will see above normal temperatures during the period but with some pockets of below normal readings such as in the Upper Midwest. Precise evolution of the midweek Northeast system will influence temperatures as well. 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