Extended Forecast Discussion NWS Weather Prediction Center College Park MD 207 PM EST Tue Nov 30 2021 Valid 12Z Fri Dec 03 2021 - 12Z Tue Dec 07 2021 ...Overview... During the Friday-Tuesday period expect an eastern Pacific mean ridge aloft to support varying degrees of cyclonic flow across southern Canada and portions of the lower 48, with the overall mean trough axis tending to extend from central Canada into the Great Lakes/Northeast. Upper troughing should be fairly shallow late this week and then become somewhat more amplified after Saturday. This pattern will carry along a series of surface waves and frontal systems, most of which should produce light to moderate precipitation. A front reaching the West early next week may support a period of heavier activity from the Pacific Northwest into the Rockies. Meanwhile one or more pieces of energy over the eastern Pacific/far southwest corner of the U.S. may consolidate into an upper low that settles near Baja California. This feature should not produce much significant weather during the period. ...Guidance/Predictability Assessment... The most prominent uncertainties over the lower 48 seem to arise from question marks with a couple streams of flow within a general trough to the south of Alaska. A leading shortwave reaches the Pacific Northwest by early Saturday and there are already amplitude differences at that time with more significant spread for the corresponding surface pattern off the Pacific Northwest coast. A preferred intermediate solution would have a moderate wave not far from the 00Z ECMWF mean position but somewhat weaker than it or the 00Z ECMWF/12Z CMC. The latest GFS/UKMET are quite weak and suppressed while the 12Z ECMWF is weaker/south versus the prior run. Early differences with this shortwave and then differences in how streams interact within the trough south of Alaska lead to rapidly increasing spread for low pressure tracking from the Plains into eastern Canada/Northeast U.S. by Sunday-Monday. Latest GFS/GEFS runs and the 12Z UKMET dig the incoming shortwave energy rather strongly into the east-central U.S., leading to strong low pressure over the Great Lakes/southeastern Canada by early Monday. In contrast the 00Z ECMWF and latest CMC runs are more progressive and less amplified, leading to weaker and farther east/southeast low pressure. Highlighting the uncertainty, individual ensemble members are very diverse and GFS/ECMWF runs have occasionally switched places in recent days. For example a number of recent ECMWF runs had some similarity to the GFS but the 00Z ECMWF happened to be similar to yesterday's 00Z GFS that was on the weaker/suppressed side versus most GFS runs. The 12Z ECMWF trended much stronger and a little northwestward with intermediate timing. Prefer to maintain a compromise approach that is fairly close to continuity. Additional energy should drop into the western half of the country by next Tuesday but with considerable differences for specifics. The new 12Z GFS is on the deep side of guidance by early Tuesday. Also note that 06Z/12Z GFS runs are on the eastern side of the full guidance spread for the upper low near Baja California by Sunday. A blend of the 00Z/06Z GFS runs and the past two ECMWF runs through the 00Z cycle, along with modest 00Z CMC/UKMET input, provided a good representation of forecast preferences during the first half of the period. The forecast transitioned to a mix of the aforementioned GFS/ECMWF/CMC runs and 06Z GEFS/00Z ECMWF means thereafter. ...Weather/Hazards Highlights... A wavy front progressing from the West/Plains through the East along with another wavy front farther north will bring relatively light precipitation to the Midwest, Great Lakes, and Northeast late this week into the weekend. Moisture increasing ahead of the cold front extending into the southern Plains should produce scattered showers and a few thunderstorms from the Deep South to the Ohio Valley over the weekend, but amounts are not likely to be excessive. A shortwave and frontal system may brush the extreme northern Pacific Northwest/Rockies with some precipitation during the weekend. This system could then produce a band of precipitation from the northern Plains through the Northeast into early next week. However confidence is low for exact timing/track/depth of the surface low, so it will take additional time to resolve precipitation type and amounts across this broad area. A shortwave and cold front reaching the Pacific Northwest by early next week should bring an episode of moderate to locally heavy rain and mountain snows to western Washington and Oregon. Expect some of this moisture to reach farther inland through the West, with best potential for heavy snow over the northern Rockies--including the higher elevations of northern Idaho, western Montana, and western Wyoming. Temperatures will be above normal for early December across most of the nation through early next week. The greatest positive anomalies should be over the central and southern Plains on Friday with highs forecast to be 15-25F above average. Another episode of well above normal temperatures is likely for parts of the Plains next Monday-Tuesday ahead of the next cold front with a broad area of highs and morning lows at least 10-20F above normal. A few locations may approach daily record highs with these mild temperatures. The Northeast U.S. will be an exception to the generally warm theme as mean troughing aloft will keep temperatures fairly cold with highs running up to 10 degrees below normal through the weekend. However, no major arctic airmass intrusions are expected during this forecast period. Rausch/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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml