Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 PM 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. Regarding this potential system that may reach over or just offshore the Northeast by midweek, the multiple pieces of energy that may be involved contribute to some depth/track/timing differences among model/ensemble solutions, and thus somewhat lower than average confidence for specifics. One influence may be energy over Lake Superior and Ontario at the start of the period on Mon with upstream energy and another that provides the primary dynamic support for surface development. The 00Z and 06Z guidance have started to show more agreement in the overall evolution with less spread than earlier runs. A blended solution was preferred, near the 06Z GFS/00Z ECMWF. This yields a potentially rapidly deepening area of low pressure over southern New England into the Gulf of Maine and then New Brunswick/Bay of Fundy on Wednesday. Elsewhere, a blended solution provided a reasonable starting point (00Z GFS/ECMWF/Canadian/UKMET). 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. Ensembles show a large amount of timing spread as the GEFS and ECMWF EPS became out of phase. The combination of detail uncertainties over both areas would recommend an increased tilt toward ensemble means late in the week (about 50%). ...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, but would favor snow to the north/northwest of the low track it addition to the leading edge to the northeast. Currently the best potential for light to modest snow extends from the Lower Great Lakes/Ohio Valley/south-central Appalachians into the Northeast. Maine would stand the highest probability of more significant snow. 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 with possible embedded convection. Wintry weather is possible over northern latitudes well to the north. Precipitation may become modest to locally heavy late in the period (next Friday and perhaps Saturday, just beyond this forecast) as the warm front lifts through the Southeast. 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 and even into Florida Wednesday behind a cold front. Precise evolution of the midweek Northeast system will influence temperatures as well. Fracasso/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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Mon-Tue, Jan 6-Jan 7. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Southeast, the Southern Plains, and the Ohio Valley, Fri, Jan 10. - Heavy snow across portions of the Northeast, Wed, Jan 8. - Flooding possible across portions of the Southeast and the Tennessee Valley. - Flooding occurring or imminent across portions of the Northern Plains. - Flooding likely across portions of the Southeast, the Lower Mississippi Valley, and the Tennessee Valley. - High winds across portions of the Northern/Central Plains, and the Northern/Central Rockies, Mon-Tue, Jan 6-Jan 7. 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