Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sat Dec 28 2024 Valid 12Z Tue Dec 31 2024 - 12Z Sat Jan 04 2025 ...Overview... Mean upper troughing is likely to form across the central and eastern U.S. Tuesday-Wednesday and last through at least the rest of the week, with some embedded shortwaves producing low pressure/frontal systems. A primary surface low is forecast to track through the Ohio Valley Tuesday and across the Northeast Wednesday-Thursday accompanied by some modest precipitation. Upstream, an upper ridge will set up atop the Interior West, leading to temperatures warming above normal there while the East cools under the trough. Eastern Pacific energy/troughing that may move inland by Friday or Saturday will lead to increasing precipitation chances across the Northwest for the latter half of the week. ...Guidance/Predictability Assessment... Model guidance continues to show good agreement with the large scale pattern evolution with some lingering uncertainties on smaller scales. Models are well clustered with a surface low position in the Ohio Valley on Tuesday, with more differences by the time the low reaches the Northeast in emphasizing a coastal low or an inland low. Overall guidance trended away from showing a low as far west as the Lower Great Lakes, but the 12Z ECMWF and one AI/ML model (Pangu) were still farther inland than the consensus that is over NYC or so 12Z Wednesday. The new 00Z EC has trended east a bit, which fits with other guidance better, but there is still some minor timing spread. Models still indicate the low strengthening as it tracks north, reaching over or just east of Maine by 12Z Thursday. Farther west, after a few days of GFS/GEFS runs being on the strong side with shortwave energy reaching the West around Tuesday, recent runs starting with the 12Z GFS have been moderated with its energy and match the other models better. However, the 18Z GFS brought in energy just behind that shortwave, which produced an upper trough closing off a low just south/southwest of California Wednesday-Saturday. This seemed to be an outlier as other models maintained ridging there, so leaned against the 18Z GFS. After some impulses move into the Pacific Northwest Wednesday- Thursday, a larger scale eastern Pacific upper trough is forecast to approach Friday and come into the Northwest Saturday. The 12Z ECMWF and GFS and their means showed reasonable agreement on its timing, while the 12Z CMC was faster than preferred given the amplified pattern and the 18Z GFS perhaps too slow. The 00Z CMC has ended up flipping to an even slower solution than consensus despite a shallower trough. The timing of this trough will naturally impact the pushing of the upper ridge eastward toward the Rockies. The WPC forecast was based on a multi-model deterministic blend favoring the ECMWF early in the forecast period. Gradually eliminated the proportions of the 18Z GFS, 12Z UKMET, and 12Z CMC in turn, replacing them with the GEFS and EC ensemble means. By Day 7 the means comprised half the model blend with the remainder 12Z GFS/ECMWF. ...Weather/Hazards Highlights... A low pressure system tracking from the Ohio Valley into the Northeast will spread precipitation across those regions Tuesday- Wednesday. Precipitation should fall as rain for most of the Ohio Valley to the coastal areas/lower elevations of the Northeast. The rain rates and rain amounts look to stay mainly under excessive thresholds. Some moderate to locally heavy precipitation may fall in the urban corridor (most likely Philadelphia to New York City) later Tuesday to Tuesday night though, and will continue to monitor since urban land use can be vulnerable to flash flooding. Snow is likely in the higher elevation areas of the Interior Northeast like the Adirondacks and the Green and White Mountains, and probabilities for 0.25 inch or more of liquid equivalent in the form of snow remain 30-70 percent there on Wednesday. Then cold cyclonic flow behind the storm system will likely produce a period of lake effect snow into Thursday and Friday. The Pacific Northwest is forecast to see a relative break in precipitation on Tuesday after a steady train of atmospheric rivers into the region. But by Tuesday night or Wednesday moist inflow may get renewed there and rounds of precipitation are likely to continue through late week and at times farther east into the northern Rockies. Precipitation amounts from day to day and in total vary considerably in models as there is a fair amount of spread for system/frontal specifics before a better defined feature arrives around Friday--albeit with some timing differences. Most guidance agrees that highest precipitation will occur in Washington/northern Oregon rather than California. The eastern U.S. can expect one more day of above average temperatures (by 10-15F) on Tuesday, before upper troughing pushes a series of cold fronts that gradually cool temperatures to near normal on Wednesday and gradually below normal into late week. High temperatures by Saturday are forecast to be around 10-15F below normal for the Ohio Valley to Appalachians to Mid-Atlantic while lows should be 5-10F below average. Colder than normal temperatures will also impact the north-central U.S., and lows could reach -10 for northern North Dakota and Minnesota on Thursday-Friday. Meanwhile, the amplifying upper ridge over the West will promote warming, with temperatures generally 5-10F above average increasing in coverage by the second half of the week. Locally higher anomalies are likely in the Southwest and highs could reach well into the 70s. Tate 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