Extended Forecast Discussion NWS Weather Prediction Center College Park MD 137 AM EST Fri Dec 13 2024 Valid 12Z Mon Dec 16 2024 - 12Z Fri Dec 20 2024 ...Overview... The latest medium range forecast continue to show a significant pattern shift next week, transitioning from a series of progressive and weaker shortwaves to amplified western U.S. ridging and amplified mean troughing over the central-eastern states. A shortwave into the West early week should bring some additional precipitation, but conditions should trend drier and warmer mid to late week. A couple of cold fronts will march eastward from the central U.S. to the East Coast, with some potentially heavy precipitation to accompany and noticeably cooler temperatures by the second half of the week. ...Guidance/Predictability Assessment... Besides some relatively minor timing/surface detail differences, the models are still in good agreement the first half of next week as a shortwave moves through the midwest into the Northeast, and another shortwave enters the West. The UKMET was slower with the Western feature and was not used in tonight's blend, although the new 00z run did trend faster. By next Wednesday, the trough over the West will amplify and there is quite a bit of uncertainty on stream separations and amplification across the Southeast late week. There seems to be better support for a more progressive trough into the East, including the ensemble means, but some guidance, like the ECMWF continues to show a cut off closed low lifting from the Southeast U.S. next Thursday-Friday. At this time, this solution is a strong outlier and is not preferred. The CMC does show some amplification of the southern stream as well, but not nearly to the degree of the ECMWF. The WPC forecast trended more towards the progressive and less separated GFS and CMC, along with the ensemble means for the second half of the period. ...Weather/Hazards Highlights... A cold front into the Ohio Valley and East Monday-Tuesday should be accompanied by some modest precipitation. There is potential for heavier rain and 1+ inch totals in spots, but given the progressive nature of the front and relatively dry antecedent conditions, it does not appear at this point to warrant even a marginal risk on the Day 4 (Monday) Excessive Rainfall Outlook. However, some rainfall expected in the short term may temper the initial dryness and improved clustering toward heavier rainfall would increase the potential for runoff issues. The northern half of the West Coast should see another episode of rain and higher elevation snow on Monday, with moisture extending east through the Rockies by Tuesday. Details of the surface pattern and corresponding precipitation coverage and amounts over the central/eastern U.S. become increasingly uncertain by next mid next week, but there is some signal for another round of moderate to heavy rainfall from the mid-South into the East, with some wintry weather possible on the north side of wherever the low pressure actually tracks. Within an initial generally mild pattern across the lower 48, warmest anomalies should extend from the eastern slopes of the Rockies to the East. Expect a decent area of highs 10-15 degrees above normal and anomalies up to plus 15-25 degrees in places for morning lows. After Wednesday, temperatures should trend back to normal across the Central-East with some below normal readings possible across the northern Plains as arctic high pressure nudges into the region. Out West, expect temperatures to trend warmer underneath persistent amplified 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