Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Thu Dec 01 2022 Valid 12Z Sun Dec 04 2022 - 12Z Thu Dec 08 2022 ...Below normal temperatures over the West with rain/mountain snow generally trending lighter... ...Heavy rainfall potential for portions of the eastern and central U.S. next week... ...Overview... Expect a majority of the lower 48 to see fast flow aloft between a mean low over central Canada and a ridge over the Gulf of Mexico. Guidance has trended towards a positively-tilted upper-level trough persisting over the eastern Pacific and western U.S. with only a gradual eastward drift during the early-mid week time frame. An upstream ridge should move into the Northwest U.S./western Canada by midweek or so, followed by a shortwave trough approaching the Pacific Northwest next Thursday. This pattern should promote below normal temperatures over most of the West and northern Plains on most days, while the rain and mountain snow affecting some parts of the West should steadily decrease in coverage and intensity with time. On the other hand it looks to be a favorable setup for heavy rainfall over portions of the east-central U.S. early next week as multiple moisture sources interact with a couple fronts. The far southern tier states should see mostly above normal temperatures. ...Guidance and Predictability Assessment... While the large-scale pattern tended to remain relatively consistent across the guidance, significant differences in the more fine scale details run-to-run continue to prove challenging. The main features to account for were a positively-tilted upper-level trough progressing slowly over the eastern Pacific along the West Coast and shortwaves moving through southern Canada/the northern U.S. in the northern stream. The WPC forecast followed a general trend of incorporating mainly the deterministic guidance early in the period to blending in more of the ensemble means later to hopefully best capture the broader pattern while accounting for small-scale shortwave energy that proved to have an impact on the evolution of systems at the surface. Early in the period, the large-scale flow was generally consistent across the guidance except for subtle differences in strength of the closed low off the West Coast and the timing of shortwave energy in the northern stream over south-central Canada. Both the 06Z GFS and 00Z Euro were trending towards agreement on the timing of this feature while the 00Z CMC/UKMET were more progressive. The WPC forecast incorporated each piece of guidance with an emphasis towards the 06Z GFS And 00Z Euro as to not introduce too much of a bimodal phasing of the flow. Uncertainty in the forecast began to increase significantly during the middle of the forecast period in terms of the evolution of both the western trough and energy in the northern stream. This uncertainty was highlighted by the rapidly increasing divergence in the solutions of the individual GEFS and ECens ensemble members. The latest few runs of the GFS wavered back and forth between retaining a closed low, but fell in line with the other guidance by 06Z in depicting an open wave. The WPC forecast continued with a blend of the latest deterministic guidance while gradually incorporating the 06Z GEFS and 00Z ECens means as to not put too much emphasis on any one solution given the smaller scale disagreement. The trough in the West has trended a bit slower compared to the prior WPC forecast. A similar pattern of significant run-to-run uncertainty continued late into the forecast period. The previous 11/30 00Z GFS/ECMWF runs were significantly different from eachother. The GFS was much more progressive with the trough along the West Coast, depicting it moving east quickly and blending into the large-scale flow. On the other hand, the latest 00Z GFS had a closed low off the coast. The ECMWF demonstrated a bit more consistency run-to-run with a less progressive open wave, and the CMC also had a similar solution. Given the GFS did trend this way by 06Z, felt confident enough to continue with a general blend of each of the deterministic guidance and means which resulted in an updated forecast that remained relatively similar to the prior forecast despite the lingering consistency issues. The 06Z GFS and 00Z ECWMF indicate this trough finally moves into the Southwest by the end of the period, while the 00Z CMC was much less progressive and an outlier. Completed the WPC forecast period with a blend of the 06Z GFS and 00Z ECMWF along with the GEFS And ECens means, with the deterministic guidance helping to better depict the trend of a deeper trough moving into the Southwest than the prior WPC forecast. Use of the means helped to continue to account for significant differences in the timing of shortwave energy moving through southern Canada and the northern U.S. with no clear evidence of a dominant solution or pattern to follow. ...Weather/Hazard Highlights... Low pressure near the West Coast through Sunday should produce some enhanced rain and mountain snow along favored terrain of the Sierra Nevada. Some of the moisture from this system will also extend into the Great Basin and central/northern Rockies with some locally heavier snow possible there as well. Moist southern stream Pacific flow aloft ahead of this system may continue to produce some rainfall over parts of southern Arizona and New Mexico through Sunday. Rainfall amounts have trended a bit drier, but given rainfall prior to the start of the period, an isolated instance or two of flash flooding remain possible. The southern half of the West should trend drier early next week as the West Coast system opens up before the system begins to move into the Southwest Thursday. Another frontal system approaching the Northwest may bring an increase of precipitation by next Thursday as well. Farther east, a front settling near the Gulf Coast on Sunday is forecast to return northward as a warm front early next week while a system (currently expected to be of moderate strength) emerges from the Plains with a trailing cold front. West-southwesterly upper level flow of moisture from the Pacific and a gradual increase of low level Gulf inflow may interact with the fronts to produce areas of heavy rainfall over the east-central U.S. early next week. Currently the signal is favorable enough to depict a Slight Risk area on the experimental day 5 Excessive Rainfall Outlook (valid Monday-Monday night) centered primarily over most of Tennessee and southern Kentucky. Fairly wet ground conditions will make this area sensitive to significant rainfall as well. Rainfall could persist over parts of the south-central U.S. through at least midweek if the wavy front crossing the region is slow enough. Any snow should be confined to far northern latitudes of the East. Expect below normal high temperatures to persist over much of the West through the Sunday-Thursday period, with most locations 5-15F below normal. The northern Plains will be below normal as well, with Monday-Wednesday having the coldest highs of at least 15-20F below normal and lows that may dip into the negative teens. The southern tier will tend to see above normal temperatures, and especially for morning lows that could be up to 20-25F above normal early-mid week. A few daily records for warm lows may be possible. System progression should bring variable temperatures to the northern half of the East. Putnam/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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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