Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sat Dec 14 2024 Valid 12Z Tue Dec 17 2024 - 12Z Sat Dec 21 2024 ...Overview... The medium range period begins Tuesday with a shortwave shifting through the Northeast as another enters the West Coast. The latter will be responsible for a significant pattern shift as it amplifies across the Central-Eastern U.S. mid to late week and ridging develops upstream over the West. This should promote warmer and drier conditions across the West, and cooling temperatures and precipitation for the East, with a potential emerging nor'easter threat late week, but with low confidence. ...Guidance/Predictability Assessment... Besides some relatively minor timing/surface detail differences, the models are still in good agreement on Tuesday as a shortwave moves through the Midwest into the Northeast, and another shortwave enters the West. The pattern grows more amplified after Tuesday but much more uncertain with the details as troughing becomes established over the East. There is uncertainty in the timing of the trough/shortwave but also with splitting of streams and a possible low pressure system near the Mid-Atlantic later in the week. The GFS was noticeably slower with the trough through its 18z run tonight, but the new 00z run was slightly faster. The ECMWF had the best support from other guidance (including AI/ML models) but was strongest with a southern stream upper low over the Southeast and a more classic nor'easter type feature up the East Coast. However, the new 00z run trended much weaker with southern stream energy and overall more progressive with the front and a more northern surface low. So it remains to be seen what kind of trend may be emerging, so late week details are very uncertain. There has been general agreement though for an overall more amplified pattern as this first system exits and additional shortwave energy reloads over the East into next weekend. The WPC blend was able to utilize a general model blend the first day or two of the period, but shifted quickly towards more ensemble means (with minor contributions from the ECMWF and CMC) to account to increasing spread and very low confidence in any solution at this time. Generally maintained good continuity with the previous WPC forecast, but was overall faster with the system through the East. As for the QPF forecast, diverged completely from the NBM for the first couple of days as the 18z GFS was an outlier in the front and precipitation setup farther north and the NBM appeared very influenced by that solution. The QPF for days 4 and 5 was based on the ECMWF, the EC Mean, and the EC AI model. Was able to lean more heavily on the NBM later in the week. ...Weather/Hazards Highlights... A lingering surface front across the Mid-South should be accompanied by some modest precipitation, but doesn't at this time appear hazardous. Details of the surface pattern and corresponding precipitation coverage, amounts, and timing over the central/eastern U.S. become increasingly uncertain by Wednesday though, but models do continue to indicate another round of moderate to heavy rainfall emerging in the south-central U.S. to Mid-South. There remains enough uncertainty to preclude any sort of risk areas on the Day 5/Wednesday Excessive Rainfall Outlook at this time, but there is some general agreement on enhanced rainfall across portions of the southern-central Appalachians where soil moisture is above normal. Precipitation may spread across the Eastern Seaboard on Thursday- Friday, with some wintry weather possible on the northwestern side of the low track. This forecast indicates some 10-30 percent probabilities of 0.25" QPF in the form of snow in the Interior Northeast especially in higher elevations, but continue to monitor forecasts because snow amounts and placement will be quite sensitive to the uncertain eventual low track. The northern half of the West Coast and into the Northwest should see another episode of rain and higher elevation snow lasting into Tuesday. After that, the West should see drier conditions underneath ridging, other than perhaps some moisture into western Washington rounding the ridge. Within an initial generally mild pattern across the lower 48, warmest anomalies should extend from the Southwest/Rockies into to the East on Tuesday. 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- eastern U.S. with a warming trend out West. As troughing gets established across the East late week, an arctic high will spread below normal temperatures from the Northern Plains/Upper Midwest into much of the East by next weekend. 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