Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1108 AM EST Mon Dec 03 2018 Valid 12Z Thu Dec 06 2018 - 12Z Mon Dec 10 2018 ...Cold pattern over the contiguous U.S. with a significant storm system likely to affect at least the southern half of the U.S. late week into the weekend... ...Overview... The upper level flow across the CONUS during the medium range period continues to be in a transition zone from an amplified/blocked flow across the Pacific to progressive/zonal flow in the Atlantic. A ridge building across the western U.S. will assist in transporting cold/arctic air in Canada to the central and eastern U.S. especially during the second half of the medium range period. With the mean trough centered over the northern tier states/Great Lakes region, a number of short waves will move across this region. Farther south, an active southern stream will carry Pacific energy from near California eastward across the southern states. As this energy reaches the southern Rockies/Plains, a wave of low pressure is expected to develop along the polar front near the western Gulf Coast by Saturday. Models and ensembles continue to show varying degrees of phasing/interactive with this southern stream wave with the northern stream energy. However, all models and ensembles indicate a surface low developing in the western Gulf Coast region and moving along the East Coast by the end of the weekend. This will give way to an active, wet period for the southern Plains and Southeast throughout the weekend. By the end of the medium range period (early next week), guidance still suggesting the ridge across the western U.S. will shift eastward, giving way to height falls along the West Coast. This should usher in stormier conditions. ...Guidance Evaluation/Preferences... Models continue to handle the northern and southern stream features well during Days 3 -5 (Thu-Sat). The mean trough will center over the Great Lakes and northern tier states, with a trough digging across California and will move into the Southwest states. By Saturday, this trough will push across the south-central U.S. as an upper ridge builds across the western U.S. A blend of the 00Z ECMWF/00Z GFS was used during this time period. By Day 5 onward, differences especially with the detailing of the shortwave energy dropping into the northern U.S. begin to emerge and this has an impact on the evolution of the developing surface low in the southern stream. The 00Z ECMWF and the 00Z/06Z GFS show a short wave sliding down the upper ridge over the western U.S. However, the ECMWF is more aggressive with digging this short wave across the Mississippi Valley. Downstream, the 06Z GFS begins to close off an upper level low across the lower Mississippi Valley and is also faster with the progression of the surface low on Sunday. By Monday morning, the ECMWF shows a closed upper low over the Mid-Atlantic with the GFS showing an open wave. This is reflected at the surface--with the ECMWF showing the surface low closer to the East Coast and much deeper. Because of these differences, increased the usage of the ensemble means (ECENS/GEFS). With the positioning of the surface low by Monday morning, compromised between the 00Z ECMWF and 06Z GFS--which puts it just off the North Carolina coastline. The ensemble means also agree with showing a deepening trough along the West Coast. ...Weather Highlights/Hazards... With the occluded front reaching California on Thursday and moving inland on Friday, expect locally moderate to locally heavy precipitation (rain and high elevation snow) across central and southern California and into the Southwest. Southern California and favored terrain in Arizona should see the highest totals with this event. By the weekend, as the upper trough moves farther east toward the Plains, expect heavy rainfall to develop across central Texas into the lower Mississippi Valley. Cold air will also be in place along the northern periphery of this precipitation shield--thus wintry precipitation could impact areas from the northern portions of the southern Plains eastward into the Tennessee Valley and ending across the Appalachians and East Coast by the end of the weekend. Meanwhile, the heavy rainfall will also shift eastward with time--impacting the Southeast as well. Northern stream systems affecting the northeast quadrant of the nation will be relatively starved for moisture but lake effect enhanced snowfall will be possible. See the latest Day 4-7 winter weather outlooks for details on areas expected to see the greatest winter weather threat in the medium range. Below average temperatures will prevail across much of the CONUS during the medium range. The most extreme anomalies should be over the central U.S. behind the front that drops southward mid-late week. Some locations may see highs at least 15-20F below average. The eastern U.S. will generally see highs 5-15F below normal with some day-to-day variability. The West will begin the period Wed with highs 5-10F below average, but temperatures will moderate through the weekend as upper ridging slowly shifts east. Temperatures may reach several degrees above average across portions of the central/northern Rockies and the northern High Plains by Sun-Mon. Reinhart/Ryan 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