Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 AM EST Fri Dec 27 2024 Valid 12Z Mon Dec 30 2024 - 12Z Fri Jan 03 2025 ...Overview... Progressive flow with multiple shortwaves rotating around the southern side of a south-central Canada upper low will be in place early next week, producing many low pressure/frontal systems across the lower 48. One system is forecast to exit the Northeast by Tuesday after producing some precipitation there. Another low pressure system will be deepening over the central Plains Monday, with windy conditions behind it and causing precipitation in the vicinity of the low track, including possible snow in the north- central Plains. This low should push into the east-central and eastern U.S. Tuesday-Wednesday for another round of precipitation that should overall be lighter than this weekend's precipitation. Expect a trend toward a larger scale western ridge/eastern trough pattern by mid-late week. Warmer than normal temperatures for most of the lower 48 early next week will moderate closer to normal, and even slightly below in some spots, from west to east during the period. ...Guidance/Predictability Assessment... Models continue to show some detail differences that impact sensible weather despite generally agreeable pattern evolution from a shortwave-dominant pattern early next week to a more amplified pattern with a western ridge and eastern trough later next week. Low pressure and upper trough differences linger in the Northeast on Monday, impacting timing of precipitation. GFS runs remain on the faster side of ending precipitation across the region. Meanwhile a shortwave producing Plains low pressure starts off in good agreement Monday, with models even converging better on the low position into the Ohio Valley early Tuesday. As the features move east midweek, spread increases somewhat. GFS runs continue to be faster than the consensus of ECMWF/ML runs at pivoting the shortwave and low northeastward, while the CMC is even slower than the EC. Another consideration with low placement is the potential for coastal low development in addition to the inland low. While the 12Z cycle of the ECMWF and ML runs were pretty consistent in producing a low near Long Island, the newer 00Z ECMWF favors one inland low rather than the coastal low developing, so this remains uncertain. At least by Thursday models are reasonably agreeable in a low position just east of Maine. The 12Z ECMWF did bump the surface low back west into Canada after that (by next Friday), which did not have much support. GFS runs/GEFS ensembles continue to be more pronounced than other guidance with shortwave energy that reaches the West by around Tuesday. Since ML models strongly support the non- GFS/GEFS guidance, continue not to favor the GFS/GEFS and its pattern of bringing southern tier precipitation into the central/eastern states by mid to late week. Models are agreeable that the pattern will amplify overall as the week progresses, with an upper ridge over the West to Rockies and troughing atop the eastern half of the lower 48. There is some model spread and run-to-run differences with the timing of eastern Pacific troughing approaching the Pacific Northwest by Friday, which will have to be resolved in future forecast cycles. The WPC forecast continued to favor the ECMWF in the forecast, with lesser proportions of other guidance. Gradually incorporated more ensemble means (especially the ECens) during the latter part of the period, reaching half by Day 7 as the control run of the ECMWF diverged westward with the eastern low. For QPF, reduced most GFS/GEFS influence in the south-central to southeastern U.S. (keeping the region drier), and enhanced lake effect precipitation compared to the NBM. ...Weather/Hazards Highlights... A low pressure system will move across south-central Canada into Monday with its fronts providing a focus for precipitation over the Northeast. There remains above average model spread with the exact evolution of the setup and associated precipitation placement, amounts, and type. Generally it seems that a warming atmosphere by Monday will allow for this to be mostly rain after some freezing rain Sunday/Sunday night, perhaps even in higher elevations like the White Mountains. Rain on cold hard ground or snow covered areas may lead to a nonzero chance of flash flooding, but given the uncertainties even in the pattern itself, held off on any ERO risk for Day 4/Monday. A low pressure system will develop in the Plains by Monday with a shortwave ejecting from the West. Strong and gusty winds are likely behind this low across parts of the Rockies and High Plains. Meanwhile north of the low track, snow is possible across the north-central Plains Monday with probabilities for at least 0.25 inch liquid in the form of snow maximizing at 30-50 percent over southern South Dakota and into northern Nebraska. A broader area of precipitation will spread across the East Tuesday-Wednesday, with some modest amounts but overall lighter than the precipitation this weekend. No areas are currently delineated in the Day 5/Tuesday ERO since rain should mostly be under excessive thresholds, but will keep an eye on it in future forecasts if amounts trend upward. Wintry weather should be confined fairly far northward as with the prior system, with snow possible across parts of the Great Lakes region Tuesday and into the Interior Northeast midweek. Highest probabilities for snow will be in higher elevation areas like the Adirondacks and the Green and White Mountains. Cold cyclonic flow behind the storm system will likely produce a period of lake effect snow into Thursday and Friday. The Pacific Northwest to northern Rockies can expect lingering moderate precipitation amounts on Monday with lower elevation rain and higher elevation snow. Precipitation chances will trend down Tuesday, but by Tuesday night moist inflow may get renewed in the Pacific Northwest and rain/snow chances could increase there once again. Precipitation is forecast to continue for the latter half of the week in the Pacific Northwest and at times farther east into the northern Rockies. Overall amounts are likely to be lower than the weekend system. What areas of enhanced activity that do occur are most likely to be over western Washington. Much of the country aside from the northwestern quadrant will be well above average temperature-wise into Monday. The most anomalous temperatures are forecast in the southern Plains, with highs in the 70s and 80s across Texas that could break daily record highs especially in southern parts of the state. The second most anomalous temperatures should be over the Northeast, with lows 20-30 degrees above average and highs 15-20F above average on Monday. As multiple cold fronts track across the nation, more seasonable cool air should gradually take over as the week progresses. Expect the northwestern to central U.S. to see near normal temperatures Monday-Tuesday, and cooler temperatures will spread across the eastern U.S. Wednesday-Thursday. By later week, the amplifying upper level pattern with ridging over the West will promote temperatures of 5-10F above normal especially over the Southwest, while conversely upper troughing aloft will spread cooler than normal temperatures across the north-central to east- central and eastern U.S. Thursday and Friday. 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