Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Dec 16 2024 Valid 12Z Thu Dec 19 2024 - 12Z Mon Dec 23 2024 ...Overview... The pattern should be turning more amplified during the medium range period as mean troughing becomes established over the East, and strong ridging builds over the West through next weekend. An initial East trough will shift off the coast on Thursday in favor of a reinforcing shortwave to amplify Eastern troughing again by next weekend. This shortwave should bring with it snow potential across the Great Lakes and a fresh blast of arctic air to the East by next weekend, while the West turns progressively warmer. An atmospheric river may bring a period of heavy rain/mountain snows to parts of the West Coast by the weekend as well. As this shortwave pushes inland early next week, it should help push the Eastern U.S. trough eastward. ...Guidance/Predictability Assessment... Models continue to show good agreement in the overall synoptic pattern across the CONUS during the medium range period but plenty of uncertainty still in the timing and details of individual systems. Sufficient model agreement the first half of the period allowed for a blend of the deterministic guidance as a starting point for the WPC forecast. By Day 5/Saturday, there is some uncertainty in the timing of the shortwave and AR as it reaches the West Coast. Yesterday's 12z run of the CMC was notably faster than the GFS/ECMWF and ensemble means, but the new 00z run tonight (available after forecast generation time) was slower and more in line with the consensus. Sunday into Monday, there are questions on amplitude of this feature as it progresses across the West, as well as the details on another weaker shortwave upstream into the Northwest at that time. In the East, models continue to resolve how quickly the amplified trough will exit the East Coast early next week. For day 5 and beyond, the WPC blend incorporated more of the ensemble means to account for the increasing uncertainty. ...Weather/Hazards Highlights... The shortwave into the Great Lakes region could bring some light to moderate accumulating snow across parts of the Upper Midwest to Great Lakes on Thursday, with lake effect snow lingering into next weekend in the favorable locations downwind of Lakes Michigan, Erie, and Ontario. This shortwave will eventually amplify troughing across the Eastern third of the country ushering in an arctic airmass supporting daytime highs by next weekend as much as 15 to 20 degrees below normal for especially parts of the Northeast. Out West, conditions should begin dry later this week except for some increased precipitation associated with weak shortwaves into the Northwest. Increasing precipitation looks likely by next weekend with the next Atmospheric River into the region with heavy rain/mountain snows possible across coastal Ranges of the Northwest into Northern California. Temperatures across the West should generally stay above average through the period underneath of ridging. Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw