Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Fri Dec 22 2023 Valid 12Z Mon Dec 25 2023 - 12Z Fri Dec 29 2023 ...Heavy Snow Threat over the North-Central Plains on Christmas Day... ...Heavy Rain Threat from the Southeast Christmas Day then into the Mid-Atlantic by Tuesday... ...Overview... Most guidance suggests that an initial Central Plains upper low will drift northeastward early next week and then northern stream energy may dig into what should remain a fairly amplified upper trough reaching the eastern U.S. by late week. The system associated with the Plains upper low will spread a broad area of rainfall over the East early to mid-week with some locally heavy, and portions of the north-central Plains could see significant snowfall on the back side of the system on Christmas Day. Pacific shortwaves pushing into a persistent western U.S. mean ridge aloft will bring some episodes of precipitation to the central and northern West Coast. ...Guidance/Predictability Assessment... The guidance continues to show fairly good agreement on the clustering of the initial Plains/Midwest upper low and surface system early in the week. The 00z CMC was a eastern outlier with the GFS, ECMWF, and UKMET all showing better agreement with the ensemble means on upper and surface low placements Monday-Tuesday across the Upper Midwest region. For this reason, the CMC was not used in todays blend at all. By Tuesday and beyond though, the forecast continues to become more and more uncertain with regards to how digging northern stream energy interacts with the initial low. Previous runs has suggested it may act more to kick the original energy into the Northeast, but now seem to be trending towards more interaction and some general combination of the two energies. The ensemble means provide decent confidence in an eastern U.S. upper trough late week but the full array of guidance continues to offer little agreement on how energy may be distributed within the trough. Farther west, there is reasonable agreement on a shallow Pacific shortwave brushing the Pacific Northwest by early Tuesday and then a somewhat more amplified one crossing the West Coast by Thursday. Again, the 00z CMC is the outlier here with the Thursday trough deeper and slower to move through the West, while the rest of guidance shows a building ridge by Friday ahead of another trough in the Eastern Pacific. The WPC blend for today favored the GFS, ECMWF, and UKMET early in the period, trending more towards the ensemble means day 6 and 7. Was able to use some modest amounts of the deterministic GFS/ECMWF as they were most consistent with the ensemble means. Overall, this approach maintained good continuity with the previous WPC forecast. ...Weather/Hazards Highlights... The system affecting the central and eastern U.S. early-mid week will spread precipitation across the north-central Plains and most locations from the Mississippi Valley to East Coast. The north-central Plains will be on the cold side of the system and guidance does suggest some decent snowfall (potentially several inches or more) across eastern South Dakota and Nebraska on Christmas Day. Some areas across the far northern tier may also see some wintry weather but with more uncertainty in specifics. Meanwhile expect the potential for locally heavy rainfall ahead of the low pressure/frontal system in the East that should draw upon moisture from the Gulf and then the Atlantic. The Day 4 Excessive Rainfall Outlook valid Monday/Christmas Day maintains a Marginal Risk area over the Southeast with an embedded Slight Risk area centered over and near the Florida Panhandle where the best combination of moisture and instability should exist. The Day 5 outlook valid on Tuesday has a Marginal Risk area over the Mid-Atlantic as the moisture axis progresses northeastward. On both days, southeasterly low level flow may help to enhance totals over favored terrain along the Appalachians. Precipitation will continue into the Northeast by midweek. Some lake effect precipitation will be possible after system passage as well. Along the West Coast, the Day 4 Monday/Christmas Day Excessive Rainfall Outlook maintains a Marginal Risk area over the northern Pacific Northwest coast with a consistent signal for a defined atmospheric river, albeit with some continued spread for exact rainfall totals. Some potential for mixed precipitation or freezing rain in some of the lower elevations/valleys too. There is more guidance spread by Tuesday so no area is depicted in the Day 5 outlook at this time. A somewhat more amplified shortwave approaching/reaching the West Coast should help to focus moderate to heavy precipitation along the central West Coast by Wednesday. Then confidence decreases regarding coverage/amounts of any additional precipitation around the end of the week as guidance solutions diverge for flow aloft. Temperatures will be quite warm ahead of the system affecting the central/eastern U.S. with the greatest anomalies from the Midwest into the Northeast. Morning lows should be up to 20-30F or so above normal while highs will tend to be 10-25F above normal. Most daily records will likely be for warm lows over the Mississippi Valley into Great Lakes/Ohio Valley early in the week, with a few still possible farther to the east thereafter. Cooler air behind the system will still be on the mild side, with only slightly below normal readings over the central-southern Rockies/High Plains on Monday and then the Gulf Coast/Florida by later in the week as the West and High Plains trend gradually warmer. Santorelli/Rausch 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, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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/#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