Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 PM EST Wed Dec 04 2019 Valid 12Z Sat Dec 07 2019 - 12Z Wed Dec 11 2019 ...Moisture spreading through the West this weekend with heaviest rain/snow over northern California/Sierra Nevada... ...Precipitation to increase in coverage and intensity over the East early next week... ...Much below normal temperatures settle into the northern Plains/Upper Midwest next week... ...Pattern Overview and Guidance/Predictability Assessment... Zonal flow aloft to start the medium range period is expected to transition to a more amplified pattern through Day 7, featuring longwave troughing over the central U.S. and a narrow but highly amplified ridge along the western U.S. coast. Model guidance this cycle still exhibited slightly above average agreement and and consistency from previous cycles. As initial West Coast energy and amplifying Canadian flow head eastward, expect a cold front to drop south from Canada during the weekend and a wavy front to cross the eastern half of the U.S. during the first half of next week. Toward the end of the period next Wed expect some approaching eastern Pacific shortwave energy to push into the western ridge but with models/individual ensemble members diverging quite rapidly for specifics. For details, the 00Z CMC remained a faster outlier with the initial West Coast trough progressing across the southern/central Plains and was not utilized in the model blend preference after Day 3. The 00Z ECMWF remained on the slower end of the remaining guidance but between the 03.12Z-04.00Z ECMWF and 04.06Z GFS, the differences were decreasing. Overall, the day 3-4 forecast was primarily a blend of the deterministic guidance (ECMWF/GFS/UKMET) followed by increasing percentages of the ECENS and GEFS means by Day 6-7. The operational ECMWF/GFS was usable to some degree through Day 7 however. ...Weather Highlights/Threats... The system beginning to affect the West Coast late in the week will spread its rain and higher elevation snow across the West during the weekend. Expect highest totals with this event to be focused over northern California/extreme southwest corner of Oregon and the Sierra Nevada. Favored terrain from the Pacific Northwest to the northern/central Rockies will likely see significant but less extreme amounts. Southern parts of the West may see some lighter activity with greater sensitivity to exact details of flow aloft for which models are still attempting to resolve. The West will be relatively dry early next week while some moisture may begin to reach the West Coast around midweek, with low confidence in precipitation coverage/intensity. The eastern half of the country should see a fairly broad area of precipitation develop early next week ahead of a wavy cold front. Overall not a lot has changed with respect to forecast details compared to 24 hours ago. There is still potential for enough low level Gulf inflow to enhance rainfall over some areas but important shortwave specifics aloft are small enough in scale to lower predictability. For the time being the most favored axis for heaviest rainfall extends from the Tennessee Valley/southern Appalachians northeastward across the northern Mid-Atlantic and southern/eastern New England. There has been some latitudinal differences in the axis of heaviest rainfall, particularly between the 00Z ECMWF and 12Z GFS. A blended approach weighted on the ensemble means favors the most rainfall from portions of the Tennessee Valley, southern Appalachians, and portions of the Ohio Valley and Mid-Atlantic. East Coast states will see a chilly start to the weekend with highs in the Northeast up to 10-15F below normal, followed by a warming trend. Warm flow ahead of the ejecting West Coast system and upper trough amplifying into the central U.S. will bring well above normal temperatures (though mainly just for morning lows over the West) across the lower 48 from west to east during the period. Some areas may see a day or so with morning lows 10-25F above normal between Sun and Tue while highs should be a little more moderate, 10-15F above normal. Cold Canadian air feeding into the northern tier behind the front will bring readings down to 10-25F below normal over the northern Plains/Upper Midwest Mon-Wed. Rausch/Taylor 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 California, the Central Great Basin, the Pacific Northwest, and the Southwest, Sat, Dec 7. - Heavy rain across portions of the Northeast, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Ohio Valley, Mon-Tue, Dec 9-Dec 10. - Flooding occurring or imminent across portions of the Northern Plains. - Much below normal temperatures across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Mon-Wed, Dec 9-Dec 11. 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