Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM 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 will 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 event. 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. The West may see a modest warming trend late in the week underneath the stronger upper ridge. ...Guidance/Predictability Assessment... Latest guidance maintains good agreement with the large scale pattern evolution while spread is gradually narrowing some for embedded shortwaves/systems. For the leading system affecting the Northeast U.S. and southeastern Canada as of Monday, guidance trends have been in favor of the slower side of the spread per recent ECMWF runs and most machine learning (ML) guidance. Latest GFS/CMC/UKMET runs have been closing in on this scenario and in fact the 12Z CMC/ECMWF have nudged a bit slower with the surface low and/or trailing cold front. Behind this system, dynamical and ML guidance is converging on tracking Monday low pressure over the Plains into the Ohio Valley by Tuesday, followed by a continued split over low pressure emphasis between the eastern Great Lakes and northern Mid-Atlantic/southern New England coast as of early Wednesday. Latest ECMWF runs have nudged toward a consolidated inland system over southern New England while other guidance remains mixed, though with a coastal tendency toward northern New England. Dynamical/ML models maintain good continuity in showing a path over or northeast of Maine by Thursday. Deeper trends by ML models over the past couple days and trimming from the deep side of the spread in the dynamical guidance have led to better consensus for storm depth. The 06Z GFS looked too weak near Maine though. Farther west, GFS/GEFS runs have still been on the strong side with shortwave energy reaching the West around Tuesday. The 06Z GFS was most prominently overdone, while the 00Z GFS/06Z GEFS mean were at least tempered enough to include a small weight in the forecast. The new 12Z GFS has dampened this wave and is now closer to remaining guidance than most prior runs. Meanwhile guidance agrees on an eastern Pacific pattern that will have multiple features brush the Pacific Northwest but with a decent amount of spread for individual wave/frontal specifics during Wednesday-Thursday. An approaching larger scale upper trough should improve predictability some by Friday, aside from timing differences. Model/ensemble consensus suggests the 06Z GFS may be too fast with the Friday system (00Z and 12Z GFS comparing better to other solutions), though the ML model spread is wide enough to include the faster scenario. The amplifying nature of the pattern seems to favor leaning away from fast solutions though. Based on 00Z/06Z guidance, the first half of the forecast started with half 00Z ECMWF and the rest comprising the 00Z GFS/CMC/UKMET in order of more to less weight. Thereafter the forecast incorporated 35-45 percent ensemble means (06Z GEFS and 00Z ECens/CMCens) along with more 00Z ECMWF input relative to the 00Z GFS and CMC. ...Weather/Hazards Highlights... A low pressure system will move across southeastern Ontario/western Quebec by Monday with its fronts providing a focus for precipitation over the Northeast. Recently above average model spread with the exact evolution of the setup and associated precipitation placement, amounts, and type has shown some narrowing today. 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. Strong southerly flow and anomalous moisture ahead of the cold front may promote a band of briefly heavy rain passing through northern New England. This rain falling on cold hard ground or snow covered areas may lead to a nonzero chance of flash flooding or at least some nuisance issues in areas that have poor drainage. However per WFO coordination this potential still seems to be below the Marginal Risk threshold, so the Day 4/Monday Excessive Rainfall Outlook continues to depict no risk area. 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. Accumulations could reach heavy criteria within a narrow band, but the small scale nature of such a band yields high sensitivity with respect to exact totals at any specific location. Continue to monitor forecasts as finer scale details become better resolved. Precipitation will then spread through the Midwest/Ohio Valley/Lower Great Lakes, mostly in the form of rain aside from perhaps some snow on the northwest periphery of the moisture shield. Precipitation will likely reach the Mid-Atlantic and Northeast by Tuesday-Wednesday. The Day 5/Tuesday ERO depicts no risk areas since rain should mostly be under excessive thresholds, but will keep an eye on it in future forecasts if amounts trend upward. Farther east, wintry weather should remain confined fairly far northward as with the prior system, with what snow occurring in the Great Lakes on Tuesday extending into the Interior Northeast by midweek. Highest probabilities for snow (currently 30-60 percent probability for 0.25 inch or more in liquid) 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. Expect precipitation 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 somewhat lower than with the weekend system, though some localized areas of enhanced rain/mountain snow are possible. There is a fair amount of spread for system/frontal specifics before a better defined feature arrives around Friday--albeit with some timing differences. Highest totals should be over Washington with meaningful precipitation possibly reaching south into Oregon at times. 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 or so 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. Rausch/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