Extended Forecast Discussion NWS Weather Prediction Center College Park MD 203 AM EST Sun Dec 29 2024 Valid 12Z Wed Jan 01 2025 - 12Z Sun Jan 05 2025 ...Overview... A broad upper trough is forecast to set up atop the central and eastern U.S. for the latter half of this week, cooling temperatures and allowing for lake effect snow to form behind an exiting low pressure system accompanied by precipitation in the Northeast on Wednesday. Upstream, an upper ridge will build over the West Wednesday-Friday, though moist inflow into the Northwest could maintain precipitation chances there. Eastern Pacific energy/troughing looks to move inland Friday and quickly east across the West, which by next weekend could support a developing low pressure system over the Plains with possible north-central U.S. snow and south-central U.S. rain. ...Guidance/Predictability Assessment... Model guidance remains agreeable that upper troughing will be in place across the central and eastern U.S. for the latter half of next week. A surface low in/near the Northeast on Wednesday continues to show minor spread. The 12Z ECMWF switched to a solution where the main low was farther out to sea compared to consensus of GFS runs and the ensemble means atop Connecticut or so at 12Z Wednesday, and the newer 00Z run has jumped even farther inland than that. Regardless the low should track north into southeastern Canada Thursday-Friday with good confidence for mean cyclonic flow in its wake. Behind this system, over the past day or so guidance has been gradually improving its signal for a weak wave progressing from the Plains through or near the Tennessee Valley and into the western Atlantic Thursday into Saturday. Most dynamical support for this feature comes from shearing Pacific energy which looks to have low predictability by 5-7 days out in time, so changes in specifics are certainly possible in future runs. In the West, the upper ridge axis seems well represented in the guidance Wednesday-Friday before approaching eastern Pacific troughing. But the surface details and small impulses remain quite uncertain, affecting frontal positions and the axis of the atmospheric river and QPF. Then the eastern Pacific trough continues to show spread in its timing and depth, affecting the West downstream. The 12Z CMC was slower than consensus with the trough even by Friday and shallow with the troughing and was not favored. ECMWF runs on the other hand continue to show the deepest amplitude with the trough. GFS runs from 12/18Z were shallower than the EC with the trough and the 00Z GFS has become even shallower (and faster as a result). These differences naturally affect any surface low and frontal development and QPF in the central U.S. next weekend. A blended approach favoring the ensemble means seemed best at this point awaiting further clustering. The WPC forecast was based on a blend of the 12Z ECMWF, 12/18Z GFS, and 12Z CMC early in the period. Eliminated the CMC from the blend and decreased the EC and GFS proportions gradually, in favor of the ensemble means. The means reached over half the blend by the end of the period. Future changes to the forecast are likely especially by Days 6-7. ...Weather/Hazards Highlights... A surface low pressure system in the Northeast midweek will allow for precipitation there. Expect coastal/lower elevation rain, while higher elevation areas of the Interior Northeast such as the Adirondacks and the Green and White Mountains will see accumulating snow. In the wake of this system, expect multiple days of cold and brisk flow across the Great Lakes and Northeast, with lake effect snow persisting through the latter half of the week, perhaps extending into Saturday over some locations. Meanwhile shortwave energy and a weak frontal wave may produce a band of light snow late this week somewhere within an area encompassing the Midwest/Ohio Valley/central Appalachians. Moist inflow into the West should get renewed by Wednesday and lead to additional rounds of precipitation through late week in the Pacific Northwest 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. Model guidance is now suggesting that some heavy precipitation is possible across Oregon in particular on Wednesday, but placement has been waffling and the uncertainty continues/increases into Thursday. The combination of forecast totals and guidance spread do not seem to warrant risk areas in the Days 4-5 EROs yet. However this region will be sensitive due to recent/ongoing rainfall and will monitor guidance in future cycles for any trends toward clustering in the wetter end of the envelope. Even with a better defined feature arriving around Friday and pushing into the West on Saturday, there are differences with the southward extent of moisture focus due to spread for surface low track over the Pacific on Friday and then the amplitude of the supporting upper trough by Saturday. As upper troughing pushes east, low pressure will develop over the Plains in response. While the details remain nebulous, precipitation chances look to increase generally across the Plains and Mississippi Valley on Saturday. This precipitation would be snow in the north-central U.S. given the cold temperatures, with rain in the south. There may be a transition zone of ice in between the rain and snow, and some locations could transition from snow to rain or vice versa; the Middle Mississippi Valley appears most likely for transitioning precipitation types at this point. Keep an eye on future forecasts as details are likely to change. Generally, the mean upper ridge over the West will promote above normal temperatures from the Great Basin and Southwest to the southern/central Plains through the period, and the mean trough in the central/eastern U.S. will lead to cooler than normal temperatures from the north-central U.S. into the Midwest to Eastern Seaboard (after one more warmer than average day in the Northeast on Wednesday). The northern Plains is forecast to see the most anomalous temperatures, around 10-20F below normal for highs and around 5-15F below normal for lows. This translates to highs barely reaching 0F and lows between -10F and -15F in some areas. Meanwhile highs will reach well into the 70s in the Desert Southwest, and temperatures could warm into the low 80s in southern Texas by next weekend. Tate/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 (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