Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Mon Dec 16 2024 Valid 12Z Thu Dec 19 2024 - 12Z Mon Dec 23 2024 ...Overview... Expect the upper pattern to become more amplified during the late week through weekend time frame as mean troughing becomes established over the East and a strong ridge builds over the West. An initial East Coast trough will depart on Thursday in favor of an amplifying shortwave reaching into the East Friday-Friday night. This shortwave and its surface system should bring snow potential across the Great Lakes and a fresh blast of arctic air to the East by next weekend. Trailing energy will likely reinforce the trough during the weekend. In contrast, the West will be quite warm due to the mean ridge aloft. Pacific shortwave energy may finally push into/through the mean ridge by the weekend, with an atmospheric river potentially bringing a period of heavy rain/mountain snows to parts of the West Coast. Additional moisture should reach the West Coast early next week. Current forecasts show continued eastward progress of the leading Pacific shortwave into the Plains by early next week helping to push the Eastern U.S. upper trough eastward. ...Guidance/Predictability Assessment... Overall predictability is above average due to the general dominance of medium to larger scale features, though some typical smaller scale uncertainties exist within the deepening mean trough over the East as well as with West Coast details plus the better defined shortwave that may reach the central U.S. by next Monday. With the average of most operational runs fitting within the template of the ensemble means, a general model blend represented consensus well for most of the period, with the 00Z GFS preferred over the 06Z version by next Monday. The forecast included a light touch of the ensemble means (06Z GEFS/00Z ECens) after conclusion of the 00Z UKMET. Over the East, typical detail differences persist for the surface wave tracking into or near the Lower Great Lakes by early Friday. The machine learning (ML) model average compares well to the dynamical average at that time while the new 12Z GFS appears to be a bit on the weak/suppressed side. A trailing upper shortwave may produce some weak waviness over the Plains around Saturday. ML models and the new 12Z GFS show a better signal for this than most other dynamical model runs. By next Monday the GFS/ECMWF have been inconsistent with how much troughing may linger near the East Coast, with ensembles and ML models also displaying a fair amount of spread in the latest cycle. Majority clustering says the trough axis should be offshore by then. Detail differences along the West Coast have come into better agreement aside from very small-scale aspects that are not easily resolved several days out in time. Solutions diverge some for details of the Pacific shortwave crossing the West during the weekend and reaching the central U.S. by next Monday. As of Monday the most common theme is for the northern part of the shortwave to be a little weaker and/or slower than the 06Z/12Z GFS, with the 00Z GFS more in line with consensus. ...Weather/Hazards Highlights... The amplifying northern Plains into eastern U.S. upper shortwave late this week and associated surface system tracking into the Great Lakes region by early Friday could bring 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. Relatively highest totals are possible over parts of the Upper Midwest and northwestern Great Lakes region where probabilities of at least 0.25 inch liquid in the form of snow are 30-50 percent. A brief period of brisk to strong winds behind the surface wave may lead to some blowing snow. By next weekend the trailing arctic airmass should bring daytime highs down to as much as 15 to 20 degrees below normal for especially parts of the Northeast and close to that in the Mid-Atlantic. Over the West, expect dry conditions to prevail over most areas aside from the Northwest which may see periods of precipitation associated with shortwaves/moisture on the far eastern periphery of Pacific systems. Upper dynamics and surface systems will start to push farther eastward by next weekend, with the corresponding atmospheric river increasing precipitation totals and southward coverage along the West Coast. Heavy rain/mountain snows will be possible across coastal ranges of the Northwest and extending into northern California. This enhanced activity will likely continue through next Monday. As the leading shortwave reaches the central U.S. by next Monday, precipitation may develop over parts of the Plains/Midwest but with low confidence in specifics at this time. Mean ridging aloft will generally keep temperatures over the West into High Plains above average through the period with broad coverage of plus 10 to 20 degree anomalies. A few daily records (mainly for daytime highs) will be possible over the West late this week. Rausch/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