Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Wed Dec 04 2019 Valid 12Z Sat Dec 07 2019 - 12Z Wed Dec 11 2019 ...Moisture spreading through the West during the 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... In most respects guidance is consistent and agreeable in showing an evolution toward fairly deep upper troughing over the central-eastern part of North America, downstream from a narrow but very amplified upper ridge that should drift to the West Coast by next Tue. The one notable trend in the guidance since a couple days ago--first showing up in yesterday's 00Z cycle--has been toward a slower and sharper ridge that allows for greater westward extent of cyclonic flow over the continent. 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. As for guidance details, the CMC continues to be the fast extreme with the initial West Coast trough starting fairly early in the period so it was not used in the forecast beyond day 3 Sat. The UKMET has been on the amplified side with the shortwave as it reaches the southwestern U.S. and northwestern Mexico. The 12Z version also had a central U.S. surface evolution that differed from consensus by day 5 Mon--eliminating that run from the forecast after day 4 Sun. The new run looks better in that regard. The 12Z ECMWF was slowest to eject the West Coast trough energy and multi-day trends in some guidance allowed for partial inclusion of the ECMWF in the forecast. Faster trend in the new 00Z ECMWF seems to validate the blend approach though. By mid-late period a combination of the 18Z GFS/12Z ECMWF and their ensemble means represented the large scale pattern well while toning down smaller scale details that have lower confidence several days out in time. Along the West Coast late in the period, recent trends have favored slower arrival of incoming Pacific energy but the sharpness and northern extent of the ridge do favor some undercutting flow to extend into the West around the end of the period. An average of latest solutions (minus the CMC that is again fast) appeared reasonable given the detail spread and lower predictability existing at that time. ...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. Locations mainly in the northern half of the Great Lakes should be cold enough to see snow from a combination of the synoptic system and lake effect/enhancement. Some snow will also be possible over northern/interior New England. 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 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