Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 PM EST Sat Jan 4 2025 Valid 12Z Tue Jan 7 2025 - 12Z Sat Jan 11 2025 ...Arctic blast affects central/eastern U.S. next week... 19Z Update: The 12Z guidance, including the CMC, has trended towards the cut-off upper low solution near the Southwestern U.S., whereas yesterday it was still more disagreeable with how it would evolve. At the time of fronts/pressures preparation, the 00Z CMC still had some differences with the model consensus with the Arctic surface high displaced and also a slower northern stream upper trough passage across the Great Lakes and Ohio Valley region late in the week. Therefore, the WPC forecast was hedged more in the direction of the ECMWF/UKMET/GFS, along with some previous continuity for the Tuesday-Thursday time period, and then gradually increased contributions from the GEFS and EC means to about half by Friday/Saturday. The 13Z NBM, similar to yesterday, is much too light with expected lake effect snows for the middle of the week, so the QPF was mainly based on the GFS/ECMWF with some upward adjustment from there. Elsewhere across the Continental U.S. the NBM was more reasonable and blended with some operational GFS/ECMWF and a little less of the CMC. It is important to note a general upward trend in QPF this cycle for the end of next week with a potential low pressure system developing over the northern Gulf, and this could be the next impactful weather system to monitor if trends continue upward in future model runs. Stay tuned for additional information as the event becomes more resolved in the days ahead. The previous forecast discussion is appended below for reference. /Hamrick --------------------------- ...Overview... The weather pattern for next week across much of the nation will quiet down in terms of precipitation aside from periodic Great Lake effect snow enhancement with reinforcement of widespread Arctic flow and some areas of mainly light precipitation in the western and central U.S., but much colder than average temperatures will persist in most areas from the Rockies eastward. There is also an uncertain signal for extent of far south-central U.S. rain/winter precipitation development into later next week to monitor. ...Guidance/Predictability Assessment... Guidance forecast spread and uncertainty overall seem less of an issue than normal well into med-range time scales, including for the latest 00 UTC cycle. A composite of best clustered GFS/ECMWF model and ensemble solutions along with the National Blend of Models (NBM) for the most part seems to provide a consistent and reasonable forecast basis. This solution maintains great WPC product continuity and remains best in line with machine learning guidance. Alternately, less supported but still plausible recent Canadian runs have been the least aligned with this composite with development of a less amplified/more progressive southern stream and a more amplified/less progressive northern stream in split flow as systems dig to the lee of full latitude upper ridging built along/off the west coast of North America. ...Weather/Hazards Highlights... Great lake effect snows will be periodically rejuvenated with deep cold surges next week given ample flow amplitude. The NBM runs remain characteristically underdone with this potential, leading to manual corrective adjustments. Light to modest precipitation including elevation snow will spread focus down through the Rockies next week, and eventually the north-central U.S. with an emerging surface system. Farther south, there are some chances for precipitation from the Southwest to Southern Plains and Lower Mississippi Valley and vicinity dependent on uncertain timing/ejection of energies in a separated southern stream. An emerging precipitation pattern may unfold into later next week, and with cold air in place from Texas eastward, some areas may need to monitor any onset of wintry precipitation. In terms of the excessive rainfall outlooks, no risk areas are warranted for either Day 4 or Day 5 given a paucity of deep moisture across the country. Colder than normal temperatures are likely across the central and eastern U.S. underneath rounds of upper troughing and with cold surface high pressure systems. Notable cold is possible in the far northern tier, with high temperatures barely reaching 0F and lows in the -10s for some areas. This should not be record breaking, but the cold could still be hazardous as wind chill values may dip into the -20s and -30s. Temperature anomalies of 10-20 degrees below normal are also likely from the central Plains and Mid-Mississippi/Ohio Valleys through the Appalachians next week, especially as temperatures remain quite cold for snow covered areas from a major lead short range winter storm. Meanwhile, upper ridging in the western U.S. will offer slightly above normal temperatures. Schichtel/Hamrick 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